SNAP TECHNICAL BACKGROUND DOCUMENT:

                          RISK SCREEN ON THE USE OF
                            SUBSTITUTES FOR CLASS I
                          OZONE-DEPLETING SUBSTANCES:

                      REFRIGERATION AND AIR CONDITIONING

                     U.S. Environmental Protection Agency
                          Office of Air and Radiation
                       Stratospheric Protection Division
                                Mail Code 6205J
                              401 M Street, S.W.
                            Washington, D.C.  20460

                                  March 1994

	NOTICE

      This background document supports the U.S. Environmental Protection Agency's (EPA) rulemaking for controlling the replacement of Class I ozone-depleting substances (ODSs) with any substitute that presents unacceptable risk to human health or the environment where safer alternatives are available.  Specifically, this document examines the risk from the use of various refrigeration and air conditioning substitutes.  Similar documents address other industry sectors.

      ICF Incorporated provided support for the preparation of this document at the technical direction of the Stratospheric Protection Division, Office of Air and Radiation, EPA, Contract No. 68-D9-0068,  Work Assignments 3-23 and 3-69; and Contract No. 68-C2-0107, Work Assignment 39; and Contract No. 68-D3-0021, Work Assignments 1-11 and 2-18.  Meridian Research Incorporated also provided technical support under Contract No. 68-D9-0068, Work Assignment 3-68 and Contract No. 68-D3-0021 Work Assignment 2-18, particularly for the occupational evaluation.  ZBA, under Work Assignment 3-74, provided technical support for evaluating the feasibility of substitutes.

	TABLE OF CONTENTS

Section	Page

Notice		ii

Executive Summary	vi
      ES.1	The Refrigeration and Air Conditioning Sectors	vi
      ES.2	Risk Screening Methodology	viii
      ES.3	Risk Screening Results	x

1.	INTRODUCTION	1-1
      1.1	Background	1-1
      1.2	Evaluation of Substitutes	1-2
      1.3	Risk Screening Reports for Refrigeration and
            Air Conditioning	1-3

2.	OVERVIEW OF SUBSTITUTES AND END USES	2-1
      2.1	End Uses	2-1
      2.2	Alternative Refrigerants	2-2
            2.2.1	HCFCs	2-2
            2.2.2	HFCs	2-3
            2.2.3	Hydrocarbons	2-3
            2.2.4	Ammonia	2-3
            2.2.5	Chlorine	2-3
            2.2.6	Potential Substitutes Manufacturers Have Submitted as
                  Confidential Business Information	2-4
      2.3	Alternative Technologies	2-4
            2.3.1	Absorption Refrigeration Systems	2-4
            2.3.2	Evaporative Cooling	2-4

3.	TOXICITY REFERENCE VALUES FOR SUBSTITUTES	3-1
      3.1	Occupational (Inhalation) Exposure Limits	3-1
      3.2	General Population/Consumer (Inhalation) Toxicity Reference Values	3-6
            3.2.1	Reference Concentrations (RfCs)	3-6
            3.2.2	Cancer Slope Factors 	3-8
      Chapter 3 References	3-9

4.	ATMOSPHERICS MODELING	4-1
      4.1	Introduction	4-1
      4.2	Description of Refrigerator and Air Conditioning End Uses	4-1
      4.3	Control Options	4-4
      4.4	The Baseline for the Analysis	4-4
            4.4.1	Uniform Assumptions Made Throughout the Analysis	4-4
            4.4.2	Assumptions Made for All Other End Uses	4-5
            4.4.3	Assumptions Made Within the End Use of Concern	4-5
                  <IP_ADDRESS>  Ideal Substitute Baseline	4-5
                  <IP_ADDRESS>  No Substitution Baseline	4-7
      4.5	Overview of the Modeling Approach	4-8
            4.5.1	Chemical Use	4-8
            4.5.2	Chemical Emissions	4-8
            4.5.3	Atmospheric Effects	4-8
            4.5.4	Human Health Effects	4-9
      4.6	Model Results	4-10
            4.6.1	Description of the Results	4-10
            4.6.2	Interpreting the Results	4-10

      4.7	Global Warming 	4-92
            4.7.1	The Nature of the Global Warming Problem	4-93
            4.7.2	Properties of Greenhouse Gases	4-93
            4.7.3	Properties of CFCs and CFC Substitutes	4-95
            4.7.4	Indirect Effects from Changes in Energy Efficiency	4-98
                  <IP_ADDRESS>	Refrigeration and Air Conditioning End Uses	4-98
                  <IP_ADDRESS>	Foam Insulation	4-99
      Chapter 4 References	4-100

5.	OCCUPATIONAL EXPOSURE AND HAZARD ANALYSIS	5-1
      5.1	Introduction	5-1
      5.2	Description of the Model	5-1
      5.3	Discussion of the Results	5-4
            5.3.1	Presentation of Results	5-4
            5.3.2	Interpretation and Comparison of Model Results with Sampling Data	5-5
      5.4	Potential Workplace Flammability and Explosivity Risks	5-8
      Chapter 5 References 	5-10

6.	CONSUMER EXPOSURE AND RISK SCREENING ANALYSIS	6-1
      6.1	Home Appliances	6-2
            6.1.1	Risks from Long-term Exposure to Leakage and Servicing Events  Previous Estimates	6-2
            6.1.2	Risks form Short-term Exposure During Servicing and Accidental
                  Releases	6-3
            6.1.3	Summary of Modeling Results	6-10
            6.1.4	Safety Concerns:  HFC-152a in Household Refrigerators	6-10
      6.2	Mobile Air Conditioners (MACs)	6-11
            6.2.1	Approach Used in a Previous Consumer Exposure Report	6-11
            6.2.2	Comparison of Parameters Contained in Consumer Exposure Report
                  with Data from Other Sources	6-12
            6.2.3	Summary	6-13
      Chapter 6 References	6-14

7.	GENERAL POPULATION EXPOSURE AND RISK ANALYSIS	7-1
      7.1	HCFCs and HFCs	7-1
            7.1.1	Approach	7-1
            7.1.2	Results	7-4
            7.1.3	Caveats and Limitations	7-4
            7.1.4	Conclusions	7-7
      7.2	Other Alternative Refrigerants	7-7
            7.2.1	Background	7-8
            7.2.2	Approach	7-8
            7.2.3	Results 	7-9
      7.3	Chlorine	7-9
      7.4	Lithium Bromide and Ammonia in Absorption Systems	7-11
      Chapter 7 References	7-12
      Attachment 7-A:  Detailed Categorization of Refrigeration End Uses and
                      Equipment Types	7-A-1
      Attachment 7-B:  Assumptions for Ambient Air Release Calculations	7-B-1
      Attachment 7-C:  Methodology for Estimating General Population Exposures to Ambient  		    Air Releases of Substitutes for Class I Substances for SNAP Risk     Screens 	7-C-1
      Attachment 7-D:  Detailed Results	7-D-1
      Attachment 7-E:  Percent of Equipment with Assumed Charge Size	7-E-1

8.	VOLATILE ORGANIC COMPOUND ANALYSIS	8-1
      8.1	VOC Emissions from Substitutes to Class I ODSs	8-1

      8.2	VOC Emissions from All Sources	8-1
      8.3	Comparison of VOCs from Substitutes to VOCs from All Sources	8-1
      8.4	Caveats and Limitations	8-2
      Chapter 8 References	8-3
      Attachment 8-A:  Estimation of Total VOC Emissions from All Sources in 1995	8-A-1
      Attachment 8-B:  VOC Controls	8-B-1

                               EXECUTIVE SUMMARY

      This report presents a screening assessment of human health and environmental risks associated with the use of potential substitutes for ozone depleting substances (ODSs) in the refrigeration and air conditioning sectors.   ODSs are being phased out of production in response to a series of diplomatic and legislative efforts that have taken place in the past few years, including the Montreal Protocol and, more recently, the Clean Air Act Amendments of 1990.  The analyses presented in this report support the United States Environmental Protection Agency's Significant New Alternatives Policy (SNAP) Program, which is authorized by Section 612 of the Clean Air Act Amendments.  Implementation of this program involves: (1) developing, promulgating, and administering a regulatory program for identifying and evaluating substitutes, and (2) publishing a list of acceptable and unacceptable substitutes for specific end uses.

      The remainder of this summary provides an overview of ODS use in the refrigeration and air conditioning sectors, the methodology used to conduct the risk screen, and the key results.

ES.1	THE REFRIGERATION AND AIR CONDITIONING SECTORS

      ODS use in the refrigeration and air conditioning sectors accounted for almost 22 percent of the total use of Class I substances in the United States in 1990.  CFC-12 is the most widely used refrigerant, with applications in mobile air conditioners (MACS), household refrigerators and freezers, various appliances, chillers, retail food refrigeration equipment, cold storage warehouses, refrigerated transport systems, and industrial equipment.  CFC-11 is most commonly used to provide cooling for large buildings, while CFC-115, as a component in the refrigerant blend R-502, is used for low temperature applications in a variety of sectors.  CFC-113 and CFC-114 are used in special applications chillers.

      Of the Class II controlled substances, HCFC-22 is the refrigerant of choice in small to medium size air conditioning systems, and some types of retail food and industrial process refrigeration systems.

      EPA has divided the refrigeration and air conditioning sector into the following general end uses:

      	Retail Food Systems:  These include stand alone refrigeration cases found in small markets, convenience stores, restaurants and other food establishments, large systems found in supermarkets, commonly referred to as parallel systems, and HCFC-22 systems found in a wide variety of retail and service establishments;

      	Cold Storage Systems:  Public and private facilities used to store meat, produce, dairy products, frozen foods, and other perishable goods.  These systems refrigerate warehouses of varying sizes and include high pressure and low pressure systems;

      	Chillers:  These are large commercial air conditioning systems used to cool office buildings, shopping malls, and other large indoor spaces.  End uses include CFC-11 (low pressure) chillers, CFC-12 centrifugal chillers, CFC-12 reciprocating chillers, HCFC-22 chillers, CFC-114 centrifugal chillers, and CFC-500 centrifugal chillers;

      	Industrial Process Refrigeration:  Refrigeration systems used in the chemical, pharmaceutical, petrochemical, and other manufacturing and food processing industries.  These include both packaged and built-up systems used in manufacturing processes as well as extra-large ice makers;

      	Ice Skating Rinks:  There are several different types of ice rink systems, but all are grouped into one end use in this analysis;

      	Ice Makers:  These include small, medium, and large ice makers used by a number of entities, including restaurants and hotels;

      	Household and Light Commercial Air Conditioning:  These include window units, packaged terminal air conditioners, central air conditioners, direct expansion commercial air conditioners, and heat pumps used for space cooling and heating in homes and businesses.  All systems in this end use currently use HCFC-22;

      	Refrigerated Appliances:  These include household refrigerators and freezers, water coolers, vending machines, and dehumidifiers.  Most currently use CFC-12 as a refrigerant, but there are some CFC-502 units in existence;

      	Refrigerated Transport Systems:  These include systems designed to refrigerate materials in transit.  They are found in trucks, trains, and ships; and

      	Mobile Air Conditioning:  This end use includes systems installed in motor vehicles for the comfort of occupants.

      The following substitutes to Class I and Class II substances are examined in this report:

      	HCFCs:  HCFCs are chemically similar to CFCs except that they contain hydrogen in addition to chlorine and fluorine.  HCFCs analyzed as possible substitutes to the use of Class I chemicals in the refrigeration sector include HCFC-22, HCFC-123, HCFC-124, HCFC-141b, and HCFC-142b.

      	HFCs:  HFCs do not contain chlorine and do not contribute to destruction of stratospheric ozone.  They can, however, have high greenhouse gas potentials and may contribute to global warming.  HFCs analyzed as possible substitutes to Class I and Class II substances in the refrigeration sector include HFC-134a, HFC-152a, HFC-143a, HFC-125, HFC-227ea, and HFC-23.

      	Hydrocarbons:  Propane, ethane, propylene, and (to some extent) butane are used as refrigerants in specialized industrial applications, primarily in oil refineries and chemical plants.  Perfluoropropane and other perfluorocarbons (PFCs) also have potential applications in the refrigeration sector.

      	Ammonia:  Ammonia has been used as a refrigerant in vapor compression cycles for more than 100 years.  Its main application is in moderate to low temperatures systems.

      	Chlorine:  Chlorine has been proposed as a Class I substitute refrigerant for use in chlorine liquification, a processing step in the manufacture of the chemical.

      	Potential Substitutes Submitted as Confidential Business Information.  A few substitutes included in this analysis were submitted by their manufacturers as confidential business information.  As a result the identity of these substances are not revealed.  The results of the use analyses for these compounds are presented under the following identities:  R200b through 200i; and HCFC/HFC/Fluoroalkane.

      In addition to these chemical substitutes, there are a number of alternative technologies considered in this risk screening analysis:

      	Absorption Refrigeration Systems:  Absorption refrigeration systems are a major existing alternative to systems based on vapor compression cycles.  Ammonia and lithium bromide are used in absorption refrigeration and air conditioning systems.

      	Evaporative Cooling:  Evaporative cooling is another technology that could be an alternative to CFC- and HCFC-based vapor compression systems in some applications.  Contrary to vapor compression systems, evaporative coolers do not require compressors and consequently can offer considerable energy savings.

ES.2  RISK SCREENING METHODOLOGY

      To examine risks associated with the use of substitutes for ODSs in each sector, EPA examined many different types of exposure routes, receptors, and effects.  As discussed in more detail later, EPA used an atmospherics modeling framework to assess global ozone depletion and associated health effects.  To examine risks to workers, consumers, and the general population, EPA first conducted screening-level assessments using generally conservative assumptions to identify scenarios potentially of concern, and then conducted more in-depth assessments of those scenarios.  These risk screening assessments were conducted in three basic steps:

      (1)	Assess exposure.  EPA estimated levels of substitutes to which workers, consumers, the general population, and environmental receptors may be exposed, and over what period of time.

      (2)	Evaluate toxicity.  EPA compiled information on the toxicity of each substitute, including occupational exposure limits, reference concentrations, and cancer slope factors.  These values were identified from an appropriate source (e.g., Permissible Exposure Limits [PELs] from the Occupational Safety and Health Administration [OSHA]) or were derived for this report using extrapolation and other techniques.

      (3)	Estimate risk.  Exposure assessment results were combined with toxicity values to estimate risks.  Uncertainties in the risk estimates were analyzed qualitatively.

      EPA did not estimate risks for every possible combination of substitute, end use, and type of effect; combinations that clearly were not of concern were not analyzed.  For example, substitutes that volatilize quickly when exposed to the atmosphere were not examined for potential releases to streams and effects on aquatic life.  EPA believes that all of the significant human health and environmental exposure pathways and effects were covered in the risk screen.

      Below are brief descriptions of how the general approach discussed above was applied to the risk screen for the refrigeration sector.

      Worker Exposure and Risk.  EPA used a mass-balance model to estimate worker exposure to HCFC and HFC refrigerant and air conditioning substitutes for ODSs used in this sector.  Potential exposure levels to the substitutes were assessed during the manufacture, installation, and servicing of refrigeration and air conditioning equipment.  Estimated exposures were compared to OSHA and EPA occupational exposure limits to screen the potential risks to workers.

      Consumer Exposure.  Consumers may be exposed to ODS substitutes that could potentially be used in household refrigeration and air conditioning equipment and mobile air conditioners.  Mass-balance models were used to estimate the risks from short term and long term exposures to ODS substitutes used in these end uses.  To assess noncarcinogenic effects, estimated long term exposures were compared to reference concentrations.  Cancer risks were calculated using standard risk equations.  The estimated short term exposures were compared to interim emergency guidance levels (EGLs) developed by EPA for each substitute.

      General Population Exposure.   To screen risks to the general population, EPA considered releases from the following types of facilities:  manufacturing facilities that produce refrigeration or air conditioning equipment, operating facilities using refrigeration or air conditioning equipment, service stations (mobile air conditioners only), and recycling centers (household refrigeration and air conditioning end uses only) or salvage yards (mobile air conditioners only).  Model facilities were developed for each refrigeration and air conditioning end use.  For each type of facility, conservative release estimates were multiplied by dispersion factors to predict fenceline concentrations of HCFC, HFC, and other chemical substitutes downwind of these facilities.  These concentrations were then compared to estimated reference concentrations to screen noncancer health risks.  Cancer risks were calculated using EPA's standard risk equation.

      Atmospherics.  In this analysis, two different baselines are employed to present the atmospheric impacts associated with each of the potential substitutes in a given end use:  an "ideal" or "no-risk" baseline, and a "no-substitution" baseline.  These two baselines bound the range of health effects, since the first examines impacts from a risk-free scenario and the second presents effects assuming continued use of all CFCs.  These baselines can be summarized as follows:

      	"Ideal" Baseline:  This baseline presents the incremental ODP risks associated with the use each of the potential substitutes in a given end use relative to a hypothetical substitute with an ODP of 0.0.  In essence, this baseline presents the increase in ODP risks that could result from the use of the substitutes relative to a no risk (or ideal) baseline.

      	No Substitution Baseline:  Under this baseline, the incremental performance of each substitute within an end-use is judged against continued use of Class I and Class II substances.  In essence, this baseline presents the reduction in ODP risks associated with phasing out the use of the Class I compound in a given end use and switching to each of the potential substitutes.

      EPA used the Atmospheric and Health Effects Framework (AHEF) to model the effects of substitutes for ODSs in the refrigeration and air conditioning sector.  The framework employs the Vintaging Model for forecasting consumption and emissions of refrigeration and air conditioning substitutes.  The Atmospheric Stabilization Framework (ASF) uses these emissions as a basis for predicting changes in ozone levels.  Changes in UV-B flux are then estimated from the ASF output.  Finally, AHEF utilizes a human health effects model to calculate skin cancer cases and fatalities.

      VOCs.  VOC emissions from the refrigeration and air conditioning end uses were estimated under the conservative assumption that all ODSs currently used in refrigeration and air conditioning equipment would be replaced with VOCs.  These emissions were then compared to total VOC emissions from all sources at the national level.

      The use of substitutes for ODSs in the refrigeration and air conditioning sector were assumed to result in no releases of chemicals to surface waters that could result in potential risks to the general population or aquatic life, nor were the substitutes expected to result in the generation of solid waste that could potentially result in risks to the general population.

ES.3	RISK SCREENING RESULTS

      The results of EPA's risk screens are summarized briefly below and discussed in greater detail in the body of this report.

      Atmospherics.  Exhibits ES-1 and ES-2 summarize the impacts on skin cancers, fatalities, and cumulative Clx resulting from the use of the potential ODS substitutes in the refrigeration and air conditioning sectors relative to the ideal and no substitution baselines, respectively.  Exhibit ES-1 (ideal baseline) indicates that for the vast majority of end use and substitute combinations the increase in skin cancers is less than 10,000 cases and the increase in fatalities is less than 1,000, relative to the ideal baseline.  The greatest increase in skin cancers and fatalities results from certain substitutes examined in the MACs sector.  This is due to (1) the significant quantities of chemical use in the MACs sector; (2) the ODPs of some of the alternatives examined; and importantly (3) the timing of the availability of the chemical substitutes.  Exhibit ES-2 (no substitution baseline) indicates that virtually all of the substitutes analyzed are substantially less harmful to the ozone layer than continued use of CFCs.

      Worker exposure.  The results of EPA's analysis indicates that for the vast majority of end use, substitute, and operation combinations workers will not be exposed to potentially hazardous airborne gas or vapor concentrations.  Exhibit ES-3 summarizes the results for those scenarios where the model suggests that worker exposures exceed either the 8-hour or 30-minute occupational exposure limit (OEL) for the alternative refrigerant (Exhibit ES-4 summarizes the OELs for the potential alternative refrigerants).

      In considering the implications of the results presented in Exhibit ES-3, it is important to note the following observations that suggest that the analytical results will overstate likely occupational exposures to alternative refrigerants:

      	The emission estimates from the Vintaging Model used in the analysis are considered to be worst-case estimates.

      	The analysis does not assume the use of any local exhaust ventilation, which is likely to be used in manufacturing and disposal operations involving any significant releases.  The use of properly designed local exhaust ventilation would likely prevent gas or vapor concentrations from exceeding the OELs most of the time.

      	The analysis assumes for each release scenario that the entire daily release occurs over a single 8-hour workday at a single location.  This is not the case for many scenarios, and particularly for manufacturing operations, where the estimated daily release may occur over two or three workshifts or where the release may be spread over two or three production lines.

      	The vast majority of the exceedances over the OELs are the result of assuming that there will be no recycling of HFC refrigerants.  Given that such recycling is

es2

	EXHIBIT ES-3
	SUMMARY OF ESTIMATED WORKER EXPOSURE TO ALTERNATIVE REFRIGERANTS
	FOR SCENARIOS WHERE THE OCCUPATIONAL EXPOSURE LIMIT IS EXCEEDED[a]

                                 8-Hour TWA[b]
                                 30-Minute TWA
                                End-Use Sector
                                   Operation
                                  Substitute
                                  Material[c]
                                   Predicted
                                Exposure (ppm)
                                   Operation
                                  Substitute
                                  Material[c]
                                   Predicted
                                Exposure (ppm)
Retail Food (Parallel System)
Disposal

HFC-134a
HFC-125
HFC-143a
HFC-23
                                                                          1,579
                                                                          1,342
                                                                          1,917
                                                                          2,300
Disposal
HFC-134a
HFC-125
HFC-143a
HFC-23
                                                                          4,742
                                                                          4,030
                                                                          5,757
                                                                          6,909
Cold Storage Warehouses
Disposal
HFC-152a
HFC-23
                                                                          1,309
                                                                          1,234

Centrifugal Chillers (Low Pressure)
Manufacture
HCFC-123
                                                                          73[d]

Centrifugal Chillers (High Pressure)
Disposal
HFC-124
HFC-134a
HFC-227ea
                                                                          2,271
                                                                          3,050
                                                                          1,647
Disposal

HFC-124
HFC-134a
HFC-227ea
                                                                          6,819
                                                                          9,159
                                                                          4,928
Industrial Process (Low Pressure)
Manufacture
Repair
HCFC-123
HCFC-123
                                                                             54
                                                                             49

	EXHIBIT ES-3 (continued)
	SUMMARY OF ESTIMATED WORKER EXPOSURE TO ALTERNATIVE REFRIGERANTS
	FOR SCENARIOS WHERE THE OCCUPATIONAL EXPOSURE LIMIT IS EXCEEDED[a]

                                 8-Hour TWA[b]
                                 30-Minute TWA
                                End-Use Sector
                                   Operation
                                  Substitute
                                  Material[c]
                                   Predicted
                                Exposure (ppm)
                                   Operation
                                  Substitute
                                  Material[c]
                                   Predicted
                                Exposure (ppm)
Industrial Process
  (High Pressure          Build-Up)
Manufacture

Disposal
HFC-143a
HFC-152a

HFC-134a
HFC-152a
HFC-125
HFC-143a
HFC-23
                                                                          1,293
                                                                          1,016

                                                                         30,840
                                                                         47,661
                                                                         15,740
                                                                         18,375
                                                                         13,490
Manufacture

Disposal
HFC-134a
HFC-143a
HFC-152a
HFC-134a
HFC-152a
HFC-125
HFC-143a
HFC-23
                                                                          3,941
                                                                          7,751
                                                                          6,090
                                                                         92,608
                                                                        143,122
                                                                         47,266
                                                                         56,260
                                                                         40,509
Large Ice Makers
Repair
HFC-152a
                                                                          1,076

Ice Rinks
Disposal
HFC-134a
HFC-152a
HFC-23
                                                                          4,609
                                                                          7,123
                                                                          2,016
Disposal
HFC-134a
HFC-152a
HFC-23
                                                                         13,840
                                                                         21,390
                                                                          6,054
Central A/C and Home Heat Pumps
Manufacture
HFC-152a
                                                                          1,332

[a]	Exposure levels are rounded to the nearest whole number.
[b]	Time-weighted average.
[c]	One manufacturer's representative states that worker exposures to HCFC-123 during centrifugal chiller manufacture are typically 1 ppm or less, due to the use of exhaust ventilation.
[d]	The refrigerant blends analyzed for these sectors may be considered confidential business information (CBI) by the manufacturer.  As a result, the identification of the constituents of any blend and the proportion of each constituent contained in any blend are not presented in this exhibit.

	EXHIBIT ES-4
	OCCUPATIONAL EXPOSURE LIMITS FOR
	POTENTIAL ALTERNATIVE REFRIGERANTS*

                                  Substitute
                                   Material

                                   WGL (ppm)

                                   EGL (ppm)
HCFC-22
                                     1,000
                                     5,000
HFC-23
                                     1,000
                                     3,000
HCFC-123
                                        30
                                     2,500
HCFC-124
                                     1,000
                                     3,000
HFC-125
                                     1,000
                                     3,000
HFC-134a
                                     1,000
                                     3,000
HFC-143a
                                     1,000
                                     3,000
HCFC-142b
                                     1,000
                                     5,000
HFC-152a
                                     1,000
                                     4,000
HFC-227ea
                                     1,000
                                     3,000

            required under the Clean Air Act beginning in 1995, exposures should be below the OELs for these compounds.

      	The model's tendency to overstate worker exposures is further supported by sampling obtained for HCFC-123 refrigerants during repair operations conducted on low-pressure centrifugal chillers.

      Finally, the analysis did not consider the use of ammonia, butane, propane, or perfluoropropane as alternative refrigerants.  However, existing OSHA and ASHRAE standards governing the handling and use of these materials should serve to limit occupational exposures to them.

      Consumer exposure.  EPA's consumer exposure analysis of household refrigeration, household air conditioning, and mobile air conditioning equipment indicates that unless the entire refrigerant charge is released over a short period of time, none of the substitutes exceed EPA's threshold of concern for human health risk in any piece of equipment.  Routine leakage and servicing with recycling simply will not release enough refrigerant to pose significant short-term or long-term risks to human health.

      If the entire charge is released (due to an accident or during servicing without recycling of HFC-containing equipment), then the 30-minute average concentrations exceed the EGLs for substitutes in household air conditioning equipment only.  The following factors should be considered when judging whether these exceedances are cause for concern:

      	Recycling at servicing will be required for the HFCs by 1995.  Until then, EPA believes that most HFCs will be recycled anyway, because of their high cost relative to CFCs.

      	Accidental releases of an entire refrigerant charge occur infrequently.  The UNEP (1992) reports that less than 5 percent of the refrigerators and freezers have accidental releases of the entire charge.

      	Exposures are below levels that would raise concern for cardiotoxic effects.  This would be true even if higher, shorter-term TWA exposures (e.g., 15-minute TWAs) had been used as the basis for the assessment.

      General population exposure to ambient air releases.  EPA's general population exposure analysis indicates that even with conservative screening assumptions there are no end uses within the refrigeration and air conditioning sectors where the use of HFCs or HCFCs as substitutes for CFCs will result in exposure concentrations above the chemicals' health-based reference concentrations for non-cancer effects.  Similarly, there are no areas for which predicted cancer risks would be above the upper-bound cancer risk level of concern.  The analysis also indicates that general population exposures are not of concern for ammonia, butane, propane, and perfluoropropane.

      VOCs.  EPA's analysis indicates that potential VOC emissions from all substitutes for all end uses in the refrigeration and air conditioning sector are likely to be insignificant relative to VOC emissions from all other sources.

	1.  INTRODUCTION

      This report documents the results of the examination of human health and environmental risks associated with the use of substitutes for ozone depleting substances (ODSs) in the refrigeration and air conditioning sectors.  This introduction provides a brief background on the history and philosophy of the Significant New Alternatives Policy (SNAP) program, discusses in general terms the types of analyses performed to screen the risks from the substitutes, and provides a list of the chapters that follow.

1.1	BACKGROUND

      Over the past decade, a series of diplomatic and legislative efforts have taken place in response to growing concerns over the progressive depletion of the Earth's stratospheric ozone layer and its attendant health and environmental problems.  The first major step was in September 1987, when the United States and 23 other nations signed the Montreal Protocol.  The original agreement established a schedule for reducing by half the production and consumption of eight specific ODSs, including CFC-11, CFC-12, CFC-113, CFC-114, CFC-115, Halon-1211, Halon-1301 and Halon 2402.  When the parties to the Protocol met again in London in June 1990, they agreed to fully phase out the original eight ODSs, as well as methyl chloroform, carbon tetrachloride, and other fully halogenated CFCs.

      Congress incorporated the requirements of the London Amendments into the Clean Air Act (CAA) Amendments of November 15, 1990, and in some cases went beyond the London requirements.  Title VI of the CAA differs from the London Amendments by mandating a faster phaseout of methyl chloroform, a restriction on the use of hydrochlorofluorocarbons (HCFCs) after 2015, and a ban on the production of HCFCs after 2030.  When the Parties to the Montreal Protocol met in Copenhagen in November 1992, they further accelerated the phaseout dates for many controlled substances.  To implement the Copenhagen amendments and adjustments, EPA promulgated regulations in December 1993 that set the phaseout date at January 1, 1996 for Class I substances (as defined in the CAA), with the exception of halons which were subject to a phaseout by January 1, 1994.  The regulation also includes a phaseout of methyl bromide by the year 2001.

      Section 612 of the CAA requires EPA to develop a program to evaluate the risks to human health and the environment posed by alternatives to ODSs.  EPA is referring to this new program as the Significant New Alternatives Policy (SNAP) program.  The SNAP program is directed at fulfilling the general mandate in Section 612 of identifying acceptable and unacceptable substitutes for the Class I ODSs.  Initial SNAP implementation involves two key activities: (1) developing, promulgating, and administering a regulatory program for identifying and evaluating substitutes, and (2) publishing a list of acceptable and unacceptable substitutes for specific end uses.

      EPA took the first major step in implementing Section 612 of the CAA on January 16, 1992, with publication of a Request for Data and Advance Notice of Proposed Rulemaking (ANPRM; 57 Federal Register, p. 1984).  This notice, developed with the help of the Stratospheric Ozone Protection Advisory Committee (STOPAC), presented the initial plan for implementing the SNAP program and requested producers and formulators of substitutes for ODSs to provide EPA with information to facilitate review of the substitutes.  A Notice of Proposed Rulemaking was published in the Federal Register on May 12, 1993.  The proposed rule described the proposed structure and process for administering the SNAP program and presented initial determinations on the acceptability of key substitutes.  This Final Rule provides both the SNAP process and additional SNAP determinations on the acceptability of substitutes.  In the future, substitutes will be addressed on a case-by-case basis through individual notices.  In addition, EPA will accept petitions requesting that a substance be added or deleted from either of the lists.

1.2	EVALUATION OF SUBSTITUTES

      To develop the lists of unacceptable and acceptable substitutes, EPA conducted a comprehensive screening assessment of the health and environmental risks posed by various substitutes in the context in which they are used.  Substitutes are grouped into one or more of eight general sectors:  refrigeration and air conditioning; solvent cleaning; foam blowing; fire extinguishing; aerosols; sterilization; adhesives, coatings, and inks; and tobacco expansion.  Within each sector, substitutes are evaluated in the context of particular end uses.  Examples of end uses within the refrigeration and air conditioning sectors include cold storage warehouse refrigeration systems, household refrigerators, and mobile air conditioners.  EPA also determined the feasibility of using each substitute by assessing its production capacity and projected market share.  Based on these analyses, EPA will identify as "unacceptable" only those substitutes that pose significantly higher human health and environmental risks than other available substitutes.  EPA does not intend to restrict substitutes that are only marginally worse based on some criteria.

      EPA's evaluation of each substitute in each end use is based on the following types of information and analyses:

      	Atmospheric effects are assessed by using models to predict stratospheric ozone depletion.  Ozone depletion is measured in terms of cumulative Clx loadings and increased incidence of skin cancer cases and skin cancer mortalities.  Changes in global temperatures may result from releases of the substitutes themselves or from changes in fossil fuel use due to increases or decreases in energy efficiency.  These impacts are discussed qualitatively.

      	Exposure assessments are used to estimate levels of substitutes to which workers, consumers, the general population, and environmental receptors may be exposed, and over what period of time.  These assessments are based on personal monitoring data or area sampling data if available.  Otherwise, exposures are assessed using measured or estimated releases as inputs to mathematical models.  Exposure assessments may be conducted for many types of releases, including releases in the workplace and in peoples' homes; releases to ambient air and surface water; and releases from the management of solid wastes.

      	Toxicity data are used to assess the possible health and environmental effects from exposure to the substitutes.  If Occupational Safety and Health Administration (OSHA)-approved or EPA-wide health-based criteria such as Permissible Exposure Limits (PELs; for occupational exposure), reference concentrations (RfCs; for noncarcinogenic effects), or cancer slope factors (for carcinogenic risk) are available for a substitute, exposure information is combined with this toxicity information to determine whether there is potential for concern.  Otherwise, toxicity data are used in conjunction with existing EPA guidelines to develop health-based criteria for interim use in these risk screens.

      	Flammability is examined as a possible safety concern for workers and consumers.  EPA screens flammability risk using data on flash point and flammability limits (e.g., OSHA flammability/combustibility classifications), test data on flammability in consumer applications conducted by independent laboratories (e.g., Underwriters Laboratories), and information on flammability risk minimization techniques.

      	Some of the proposed substitutes are volatile organic compounds (VOCs), which are chemicals that increase tropospheric air pollution by contributing to ground-level ozone formation.  Local and nationwide increases in VOC loadings from the use of substitutes are also evaluated.

      In conducting these assessments, EPA made full use of previous analyses, including the 1990 interim assessments prepared by EPA's Office of Pesticides and Toxic Substances (OPTS) and the supporting documentation (full citations are given in appropriate chapters of this report).  These analyses were modified in some cases to incorporate more recent data or new field experience.  Where possible, EPA incorporated data submitted in response to the ANPRM and NPRM.  Finally, these analyses assume that the regulated community is in compliance with applicable requirements of other statutes and regulations administered by EPA (e.g., recycling requirements promulgated under the CAA) and other federal agencies (e.g., enforceable workplace standards set by OSHA).

1.3	RISK SCREENING REPORTS FOR REFRIGERATION AND AIR CONDITIONING

      The remainder of this package includes a series of chapters on the following topics:

      	Overview of substitutes and end uses;

      	Human health and environmental concern levels for substitutes;

      	Atmospherics modeling (ozone depletion, global warming);

      	Occupational exposure and risk screening analysis;

      	Consumer exposure and risk screening analysis;

      	General population exposure and risk screening analysis (ambient air); and

      	VOC analysis.

Flammability is addressed within several of the chapters.  Compliance costs are addressed in conjunction with other SNAP decisions in a separate package, as are regulatory burdens and effects on small businesses.

      Finally, three of the substitutes analyzed in this report, HCFC-123, HCFC-124, and HFC-134a contribute to or generate trifluoroacetic acid (TFA) as an atmospheric transformation product.  The potential ecological effects on aqueous and terrestrial ecosystems are not addressed in this report because of insufficient data on the toxicity of TFA and the level to which ecosystems may be exposed.  Research efforts of the Agency in cooperation with the Alternative Fluorocarbons Environmental Acceptability Study (AFEAS) are underway to define the potential ecological risks associated with the formation of TFA in the environment.

      Although there are significant uncertainties about the potential risks associated with TFA, and research is continuing to address these risks, a number of preliminary statements can be made based on the research completed to date:

      	Research indicates that virtually 100% of the HCFC-123 and HCFC-124 released to the environment will degrade to TFA, and suggests that approximately 25% to 40% of HFC-134a will also degrade to TFA.

      	TFA is a highly soluble acid that will exist primarily in aqueous environments (rain, rivers, lakes, ocean, and groundwater).  Tests have shown that it is very resistant to decomposition and will therefore reside in the environment for long periods of time.

      	Given conservative estimates of global HCFC and HFC use and emissions, the environmental concentrations of TFA will be low over the next few decades.

      	Experimental results show at high concentrations TFA has low to moderate acute toxicity to animals and initial tests indicate that relatively high concentrations of TFA cause toxic effects on some plants and algae.  Research to evaluate these effects is continuing.

      	The expected environmental concentrations of TFA over the next few decades are unlikely to pose a significant hazard to animals or plants.

      	There has been concern that TFA might degrade to MFA (monofluoroacetic acid) and that MFA could exist at high enough concentrations to pose health and environmental risks.  Research to date has identified no environmental process that would convert TFA to MFA.  Research to elucidate all the degradation pathways for TFA, including potential pathways to MFA is continuing.

      EPA is closely monitoring the research efforts underway with respect to TFA, and will assess the long term ecological risks associated with this compound when the research is completed.  As noted above, research completed to date has not uncovered any risks that warrant the implementation of measures to control atmospheric concentrations of TFA.  If such risks are identified in the future, the Agency will re-assess its position on TFA and evaluate measures to mitigate such risks.

	2.  OVERVIEW OF SUBSTITUTES AND END USES

2.1	END USES

      The refrigeration industry was the first to make widespread use of CFCs after this class of chemical compounds was discovered in the 1930's.  In 1990, refrigeration and air conditioning accounted for almost 22 percent of the total use of Class I substances in the United States.  There are over 500 million pieces of refrigeration and air conditioning equipment that use these chemicals as the working fluids in a vapor compression cycle.

      Many Class I substances exhibit desirable thermophysical properties for use in refrigeration cycles.  They are relatively nontoxic, nonflammable, and inexpensive to produce  characteristics that have contributed to their appeal as refrigerants.  CFC-12 is the most widely used refrigerant, with applications in mobile air conditioners (MACs), household refrigerators and freezers, various appliances, chillers, retail food refrigeration equipment, cold storage warehouses, refrigerated transport systems, and industrial equipment.  CFC-11 is most commonly used to provide cooling for large buildings, while CFC-115, as a component in the refrigerant blend R-502, is used for low temperature applications in a variety of sectors.  CFC-113 and CFC-114 are used in special applications chillers.

      Of the Class II controlled substances, HCFC-22 is the most common refrigerant in small to medium size air conditioning systems, and some types of retail food and industrial process refrigeration systems.

      EPA has divided the refrigeration and air conditioning sector into the following general end uses:

      	Retail Food Systems:  These include stand alone refrigeration cases found in small markets, convenience stores, restaurants and other food establishments, large parallel systems found in supermarkets, and HCFC-22 systems found in a wide variety of retail and service establishments;

      	Cold Storage Systems:  Public and private facilities used to store meat, produce, dairy products, frozen foods, and other perishable goods.  These systems refrigerate warehouses of varying sizes and include high pressure and low pressure systems;

      	Chillers:  These are large commercial air conditioning systems used to cool office buildings, shopping malls, and other large indoor spaces.  End uses modeled are CFC-11 (low pressure) chillers, CFC-12 centrifugal chillers, CFC-12 reciprocating chillers, CFC-114 centrifugal chillers, and CFC-500 centrifugal chillers;

      	Industrial Process Refrigeration:  Refrigeration systems used in the chemical, pharmaceutical, petrochemical, and other manufacturing and food processing industries.  These include both packaged and built-up systems used in manufacturing processes as well as extra-large ice makers;

      	Ice Skating Rinks:  There are several different types of ice rink systems, but all are grouped into one end use in this analysis;

      	Ice Makers:  These include small, medium, and large ice makers used by a number of entities, including restaurants and hotels;

      	Household and Light Commercial Air Conditioning:  These include window units, packaged terminal air conditioners, central air conditioners, direct expansion commercial air conditioners, and heat pumps used for space cooling and heating in homes and businesses.  All systems in this end use currently use HCFC-22;

      	Refrigerated Appliances:  These include household refrigerators and freezers, water coolers, vending machines, and dehumidifiers.  Most currently use CFC-12 as a refrigerant, but there are some CFC-502 units in existence;

      	Refrigerated Transport Systems:  These include systems designed to refrigerate materials in transit.  They are found in trucks, trains, and ships; and

      	Mobile Air Conditioning:  This end use includes systems installed in motor vehicles for the comfort of occupants.

      Industry has invested heavily in the search for suitable alternative refrigerants that exhibit the favorable characteristics of the controlled substances, but that do not contribute to stratospheric ozone depletion or global warming.  The hydrochlorofluorocarbons (HCFCs) and hydrofluorocarbons (HFCs) have received the most attention, along with expanded use of traditional refrigerants such as ammonia and hydrocarbons.  In many cases the most promising solution appears to be a blend of refrigerants.  Despite great progress, the suitability of many replacements is still uncertain.  Toxicity concerns, high ozone depletion potentials, or flammability may limit the ultimate attractiveness of potential substitutes in many applications when testing is completed.  The 1991 report by UNEP's Refrigeration, Air Conditioning, and Heat Pumps Technical Options Committee contains detailed information about the status of alternative refrigerants in various applications.

      EPA believes that HCFCs have an important role to play as transitional refrigerants, both in retrofit applications and in new equipment.  HCFCs have the disadvantage that they do contribute to the destruction of stratospheric ozone, although to a much lesser extent than CFCs.  HFCs have ozone depletion potentials (ODPs) of zero, but their practical application is years away in many cases.  The interim use of HCFCs, until such time as safer alternatives are available, will allow industry to move away from CFC refrigerants more rapidly.  EPA believes that this approach will have greater environmental benefits than permitting the continued use of CFCs until equipment that uses HFCs becomes available.

2.2	ALTERNATIVE REFRIGERANTS

2.2.1	HCFCs

      HCFCs are chemically similar to CFCs except that they contain hydrogen in addition to chlorine and fluorine.  HCFC-22 has been used as a refrigerant for many years.  It is the primary refrigerant used in small to medium sized air conditioners and chillers, and has found increasing application in medium temperature retail food refrigeration systems.  HCFC-123 holds promise as the primary retrofit option for CFC-11 in low pressure centrifugal chillers.  HCFC-124 has potential applications in blends for many types of refrigeration equipment and, as a pure refrigerant, has been suggested as a replacement for CFC-114 in high pressure centrifugal chillers.

      Because they contain hydrogen, the HCFCs break down more easily in the atmosphere, and therefore have lower ODPs.  Their global warming potentials are lower than either the CFCs or the HFCs.  Production of HCFCs is controlled by the Clean Air Act Amendments of 1990.  Beginning in 2015, it will be unlawful to produce HCFCs for uses other than in equipment manufactured before 2020.  Beginning in 2030, it will be unlawful to produce HCFCs, except for certain purposes that are exempted.

      Because their thermophysical properties are, in many cases, similar to CFCs, equipment designed to use CFCs can sometimes be retrofitted to operate with HCFCs.  As noted above, EPA believes that HCFCs will play an important role as transitional refrigerants.  There are clear environmental benefits to be gained by allowing their use until better substitutes are developed.

2.2.2	HFCs

      HFCs do not contain chlorine and do not contribute to destruction of stratospheric ozone.  Many, however, have relatively high greenhouse gas potentials and contribute to global warming.  Although a few HFCs have been in use for some time (HFC-152a is a component in the azeotropic blend R-500, used in smaller tonnage reciprocating equipment and large tonnage centrifugal equipment), the potential for HFCs as a replacement for CFCs has only recently been investigated.  HFC-134a and HFC-152a hold the most promise as single-substance replacements for Class I and Class II refrigerants.  HFC-143a and HFC-125, as parts of blends, have been considered as substitutes for R-502 in low temperature refrigeration applications.  HFC-227ea has possible applications as a refrigerant in mixtures used in high pressure centrifugal chillers.  HFC-23 and HFC-32, when used in blends with other chemicals, have been proposed as a potential substitute for certain HCFC-22, CFC-13, and R-503 end-uses.

2.2.3	Hydrocarbons

      Propane, ethane, propylene, and, to some extent, butane are used as refrigerants in specialized industrial applications, primarily oil refineries and chemical plants, where they are frequently available as part of the process stream and where their use contributes only slightly to the incremental risk of fire or explosion.  These industrial systems are specially designed to meet rigid requirements for reliability, durability, and safety.  ASHRAE Standard 15, Safety Code for Mechanical Refrigeration, and Standard 34, Refrigerants, are incorporated into building codes in most of the U.S.  These standards limit the use of flammable refrigerants in many applications.  Perfluoropropane and other perfluorocarbons (PFCs) have potential applications in the refrigeration sector as well.

2.2.4	Ammonia

      Ammonia has been used as a refrigerant in vapor compression cycles for more than 100 years.  It is by far the refrigerant of choice in many food processing and industrial applications, such as the meat packing, chicken processing, dairy, frozen juice, brewery, and cold storage industries.  It is also widely used to refrigerate holds in fishing vessels.  Industrial process refrigeration equipment uses rotary screw or reciprocating compressors.  Ammonia's main application is when moderate to low temperatures are required.  Ammonia has a characteristic pungent odor, excellent refrigerant properties, is low in cost, and has no long-term environmental drawbacks.  It is moderately flammable and toxic, however, but it is not a cumulative poison.  Occupational Safety and Health Administration (OSHA) standards specify that the maximum allowable concentration of ammonia for an eight-hour working exposure is 50 ppm.

2.2.5	Chlorine

      Chlorine has been proposed as a Class I substitute refrigerant for use in chlorine liquification, a processing step in the manufacture of the chemical.  When chilled below its boiling point, chlorine can be stored as a liquid at atmospheric pressure, a method that for safety reasons is much preferred to storing the chemical as a pressured gas at ambient temperatures.  Compatibility of the refrigerant with liquid chlorine is critical because of chlorine's high reactivity; CFC-12 has been widely used because it is nonreactive with chlorine.

      Chlorine compressors would be specialized units made to resist chemical attack by liquid and gaseous chlorine.  Because a chlorine refrigeration system would use part of the process stream as the refrigerant, the proposed use of chlorine as a refrigerant is analogous to that of hydrocarbon refrigerants in the oil and gas industry.  EPA has determined that, if the refrigeration system is placed so that any leakage or losses of chlorine would be contained and neutralized by the process safety mechanisms, chlorine can be used safely in these specialized applications.

2.2.6	Potential Substitutes that Manufacturers have Submitted as Confidential Business Information

      A few substitutes that may be used in certain refrigeration and air conditioning end uses have been submitted by their manufacturers as confidential business information.  As a result, the identity of these substitutes are not revealed in this analysis.  The results of the risk analyses for these compounds are presented under the following identities:  R200b through R200i and HCFC/HFC/Fluoroalkane Blend A.

2.3	ALTERNATIVE TECHNOLOGIES

2.3.1	  Absorption Refrigeration Systems

      Absorption refrigeration systems are the only major existing alternative to systems based on vapor compression cycles.  Ammonia is also used in absorption refrigeration and air conditioning systems.  Small ammonia refrigeration units are popular in recreational vehicles and in some household applications as they need no electrically driven mechanical compressor, relying instead on a propane flame as an energy source.  Small refrigerators using absorption technology are produced for use in hotel rooms, where the focus is on their silent operation rather than the lack of a suitable supply of electricity.  Small absorption type systems use hydrogen to maintain a system pressure high enough to allow the ammonia refrigerant to evaporate at low pressure and temperature (and condense at room temperature), and are constructed to withstand high internal operating pressures.  The absorption mechanism itself is a sealed unit, which usually needs no servicing over its operating life.

      Commercial ammonia absorption systems are used for air conditioning comfort cooling, particularly in instances where waste heat is available.  As with all chillers, these produce chilled water, which is circulated to the space being cooled.  Lithium bromide is also used in commercial absorption systems, where it serves as an absorbent.  Such systems operate at very low pressure to allow water to act as a refrigerant.  Lithium bromide is a relatively nontoxic, nonflammable, nonexplosive, chemically stable compound.  Both types of absorption chiller systems have traditionally been competitors to electrically driven CFC chillers.

2.3.2   Evaporative Cooling

     Evaporative cooling is another technology that could be an alternative to CFC- and HCFC-based vapor compression systems in some applications.  Three types of EC systems exist: direct, indirect, and desiccant-assisted.  In direct EC, the airstream temperature is decreased by bringing the air into direct contact with liquid water.  Because it humidifies when it cools, direct evaporative cooling can give customer comfort only where the ambient air is hot and dry, such as in the southwestern parts of the United States.  Indirect systems use a heat exchanger between the water and the airstream, and desiccant-assisted systems use a desiccant to dry the air before it is exposed to water.  These new developments have greatly expanded the applicability of EC to include the entire U.S.

      Contrary to vapor compression systems, evaporative coolers do not require compressors and consequently can offer considerable energy savings.  Evaporative coolers have been used for many years to provide cooling for comfort and for a variety of industrial and agricultural applications.  They are energy-efficient, inexpensive to purchase, and have low operating costs.

	3.  TOXICITY REFERENCE VALUES FOR SUBSTITUTES

      To assess potential health risks from exposure to substitutes for ozone-depleting substances (ODSs) in the refrigerants sector, EPA identified or developed toxicity reference values for each substitute.  These values include occupational exposure limits, reference concentrations, and cancer slope factors.  The toxicity reference values were compared to or combined with predicted exposure concentrations to estimate risks to workers, consumers, and the general population, as described in subsequent chapters of this report.

      This chapter first discusses EPA's general approach for identifying or developing the toxicity reference values for substitutes examined in the SNAP background documents.  Section 3.1 discusses occupational exposure limits, and Section 3.2 discusses general population toxicity reference values.  Both of these sections pertain to the inhalation route of exposure.

      This chapter concludes by presenting the most recent toxicity reference values available for the substitutes covered in the refrigerants sector at the time of publication of this background document (Exhibit 3-1).  The list of substitutes presented in Exhibit 3-1 is not all inclusive; however, the Agency believes that the substitutes selected for this review are representative of the types of agents that could replace ODSs in this sector.  The Agency has also included in this screen replacement chemicals that may have high toxicity potential to ensure that the assumptions used in the subsequent hazard assessments are valid and conservative.

3.1	OCCUPATIONAL (INHALATION) EXPOSURE LIMITS

      Allowable occupational exposure limits (OELs) for continuous and short-term exposure, denoted as the workplace guidance level (WGL) and emergency guidance level (EGL), respectively, are used to examine potential risks to workers.  Whenever available, the WGLs and EGLs are based on OELs developed by the Occupational Safety and Health Administration (OSHA), the National Institute for Occupational Safety and Health (NIOSH), the American Conference of Governmental Industrial Hygienists (ACGIH), or the American Industrial Hygiene Association (AIHA).

      The Occupational Safety and Health Administration (OSHA) is a governmental agency within the Department of Labor that sets enforceable occupational standards.  Limits set by OSHA include permissible exposure limits (PELs) and short-term exposure limits (STELs).  The PEL is an 8-hour time-weighted average (TWA) airborne exposure limit that is designed, to the extent feasible, to reduce a significant risk of material impairment of health or functional capacity associated with exposure to a hazardous substance.  The STEL is the employee's 15-minute TWA exposure that should not be exceeded at any time during a work day.  It is designed to protect workers from experiencing adverse acute health effects such as respiratory irritation, eye irritation, and narcosis.  It must be noted that because OSHA's exposure limits are legally enforceable, OSHA must consider a variety of factors including technologic and economic feasibility in establishing them.  Thus, the limits set by OSHA do not solely reflect the toxicological properties of the material.

Exhibit 3-1

Exhibit 3-1 (cont.)

      The National Institute for Occupational Safety and Health (NIOSH), a governmental institute under the jurisdiction of the Department of Health and Human Services, develops recommended exposure limits (RELs) that are non-enforceable.  Unlike OSHA, NIOSH is not constrained by issues of technical or economic feasibility or significance of risk when deriving RELs.  NIOSH also estimates concentrations of chemicals that are immediately dangerous to life and health (IDLH), which represents "the maximum concentration from which. . . one could escape within 30 minutes . . . without experiencing any escape-impairing (e.g., severe eye irritation) or irreversible health effects" (NIOSH 1990).

      The American Conference of Governmental Industrial Hygienists (ACGIH) is a non-governmental limit setting body.  ACGIH's non-enforceable TWA threshold limit values (TLV-TWAs) are similar to OSHA's PELs.  ACGIH does not formally analyze for technical feasibility, but there appears to be some implicit recognition of feasibility since TLVs are set to be protective for most workers but not so low as to be "unduly restrictive."  ACGIH TLVs provided much of the basis for OSHA's 1989 air contaminant standards.

      The American Industrial Hygiene Association (AIHA) is also non-governmental.   The AIHA workplace environmental exposure levels (WEELs) are like the TLVs in that the justification of a particular WEEL is based predominantly on toxicological information rather than feasibility.  However, it is likely that feasibility is considered implicitly since the WEELs are limits that are protective and practical.

      The long-term occupational exposure limits (WGLs) used for the SNAP risk screening assessments are based on OSHA PELs, NIOSH RELs, ACGIH TLV-TWAs, or AIHA WEELs, if available.  The short-term occupational exposure limits (EGLs) are based on OSHA STELs or NIOSH IDLHs, if available.  EPA recognizes that in an industrial setting OSHA STELs are typically much lower than IDLHs and other emergency guidance levels, and that exposure at the OSHA STEL is not a situation requiring emergency corrective action.  Therefore, using OSHA STELs as EGLs is a conservative way to screen risks from short-term exposure to chemicals.

      If OSHA, ACGIH, NIOSH, or AIHA OELs were not available for a substitute, and WGLs or EGLs were considered important for the risk screening assessment, they were estimated by EPA.  Although the estimated WGLs are intended to be analogous to OSHA PELs, they are different in that PELs are developed by a regulatory process and incorporate complex decision criteria such as technological feasibility.  In estimating WGLs, EPA took into consideration the characteristics of workers who are healthier and younger than the general population, and experience intermittent rather than continuous exposure.  Where appropriate, WGLs were estimated to be ten times greater than the suggested reference concentration (see Section 3.2.1).  If the resulting estimate was greater than the 1,000 ppm TLV-TWA set by ACGIH (1986) ". . . as a guide for good hygiene practice for vapors of low toxicity" (i.e., the "good housekeeping" limit), then the latter was used for the risk screening analysis.

      The EGL estimate is similar to the NIOSH IDLH (i.e., the maximum concentration from which one could escape within 30 minutes without experiencing escape-impairing or irreversible health effects).  It also resembles the emergency exposure guidance level (EEGL) and the short-term public emergency guidance level (SPEGL) suggested by NRC (1986) and the emergency response planning guideline suggested by AIHA (1989).  In instances when a substitute has an OEL but the available toxicological data were not sufficient to develop a STEL, the ACGIH excursion limit recommendation as stated below was used to estimate the EGL:

      "Excursions in worker exposure levels may exceed 3 times the TLV-TWA for no more         than a total of 30 minutes during a work-day, and under no circumstances should they        exceed 5 times the TLV-TWA, provided that the TLV-TWA is not exceeded" (ACGIH,     1992).

Thus, EGLs can range from three to five times the OEL.

In the event of a fire, explosion, or catastrophic emission, the EGL estimates for most of the HCFCs and HFCs and several other chemicals in this sector are based on the lowest-observed-adverse-effect level (LOAEL) or on the no-observed-adverse-effect level (NOAEL) for cardiotoxicity in epinephrine-sensitized dogs because the primary concern for acute high-level exposure to these compounds is cardiotoxicity.  Cardiac sensitization is of particular interest in these acute, episodic exposures because human heart arrhythmias and sudden death resulting from overexposure to halons, CFCs, and other halogenated hydrocarbons have been documented in workplace settings and in volatile substance abuse (e.g., glue sniffing).  As defined by the Agency, cardiotoxicity is the ability of a compound to cause serious and sometimes fatal cardiac arrhythmia, and is best evaluated by exposing the appropriate species, usually the dog, to an agent by inhalation in the presence of epinephrine (Rubenstein and Bellin 1993).  The Agency is using these values in the SNAP risk screen to ensure protection of the worker population.

      The protocols used to determine the cardiotoxic LOAEL and NOAEL concentrations for each agent are conservative; they entail measurement of cardiotoxic effects in animals made more sensitive to these effects by the administration of epinephrine.  The administered doses of epinephrine are just below the concentrations at which epinephrine alone would cause cardiotoxicity in the experimental animal, and are approximately ten times greater than the concentration a human being would likely secrete under stress.  Thus, the estimated LOAELs and NOAELs would be conservative for humans even in high-stress situations.

      EPA is adopting OSHA standard (29 CFR 1910, subpart L) section 1910.162 arising from the cardiac sensitization induced by Halon 1301, and is basing its risk assessment decisions for the substitutes that induce cardiotoxicity upon this standard.  Under safe use conditions for Halon 1301 (29 CFR 1910.162), the exposure limits, based upon the length of time it takes to evacuate, can be summarized as follows:

      	Where egress takes longer than 30 seconds but less than one minute, the employer shall not use the agent in a concentration greater than its LOAEL for cardiotoxicity (e.g., 100,000 ppm for Halon 1301).

      	Agent concentrations greater than the LOAEL for cardiotoxicity are only permitted in areas not normally occupied by employees provided that any employee in the area can escape within 30 seconds.  The employer shall assure that no unprotected employees enter the area during agent discharge.

      	Where egress from an area cannot be accomplished within one minute, the employer shall not use this agent in concentrations exceeding its NOAEL for cardiotoxicity (e.g., 75,000 ppm for Halon 1301).

      In addition, EPA recognizes that agents should not be used at a concentration that would significantly displace oxygen in the lungs.  Since most of the refrigerant substitutes are gaseous, heavier-than-air compounds, following a leak or catastrophic emission, they may tend to pool near the ground, (i.e. in the breathing zone).  These agents are, in the main, colorless with minimal odor and little toxicity or irritant effect, therefore, they can lead to asphyxiation by oxygen displacement, if the unwary person inadvertently walked into an area of oxygen depletion.

      Under the SNAP risk screen, EPA is only intending to set conditions for the safe use of refrigerant substitutes in the workplace until OSHA incorporates specific language addressing gaseous agents into the OSHA regulation.  These general conditions will no longer apply once OSHA establishes applicable work place requirements.

      Where appropriate, the Agency also based its analysis of short-term exposures to consumers and the general population on these cardiotoxic levels.

3.2	GENERAL POPULATION/CONSUMER (INHALATION) TOXICITY REFERENCE VALUES

      For the refrigerants sector, risks to the general population and consumers were screened for short-term and/or long-term exposure scenarios.  Risks from short-term exposure were screened using the EGLs and cardiotoxic LOAELs and NOAELs discussed in the previous section.  This section discusses EPA's general approach for identifying and developing toxicity reference values applicable to chronic (i.e., lifetime) exposure:  verified or interim reference concentrations (RfCs) for noncarcinogens and cancer slope factors (SFs) for carcinogens.

3.2.1	Reference Concentrations (RfCs)

      When possible, the RfC verified by the Agency's RfD/RfC Work Group, whether or not it has been listed on the Agency's Integrated Risk Information System (IRIS) database, was used to assess the risk from long-term exposure to ODS substitutes in the SNAP background assessments.  The RfC is designed to protect the general population against adverse systemic (i.e., noncancer) effects, and is defined as:

      "An estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without appreciable risk of deleterious effects during a lifetime.  The inhalation reference concentration is for continuous inhalation exposures and is appropriately expressed in units of mg/m[3]" (U.S. EPA 1990).

      However, when verified RfCs were not available, interim RfCs were used to evaluate risks associated with chronic exposure to ODS (listed in decreasing order of preference):

      	Currently undergoing verification by the Work Group;

      	Not yet undergoing verification but contained in the Agency's Health Effects Assessment Summary Tables (HEAST; U.S. EPA 1992);

      	Estimated from other data for the substitute; or

      	Estimated based on surrogate limits.

The remainder of this section discusses the interim RfCs derived from other data for the substitute or estimated based on surrogate limits.

RfCs Estimated From Other Data

      In the absence of a verified RfC or an RfC pending verification by the Agency's RfD/RfC Work Group, EPA calculated an interim RfC using the methods specified in EPA's Interim Methods for Development of Inhalation Reference Concentrations (U.S. EPA 1990).  Since adequate human data were not available to calculate RfCs for the proposed substitutes that did not already have RfCs, data obtained from animal inhalation toxicity experiments served as the basis for estimating the RfCs.  This approach was adopted in developing an interim RfC for HFC-143a.  For the purposes of the SNAP risk screen, the interim RfC that has been estimated for HFC-143a was based on limited animal data.  The Agency's RfD/RfC Work Group reviewed the quality and quantity of the animal data and determined that the available information was inadequate to derive an RfC.  The compound's interim RfC is, therefore, currently listed as "not verifiable".  In general, the verification process requires the existence of a sizeable body of information.  Ideally, this should include two well-conducted chronic inhalation studies, one reproductive study, and two developmental toxicity studies in different species.  The term "not verifiable" does not imply that if only limited data are available that these data are technically deficient.  The Agency will develop the verified RfC for HFC-143a when these data become available; use of the interim value should be restricted solely to the SNAP risk screen.

      If adequate animal inhalation toxicity data were unavailable for a substitute, the interim general population RfC was estimated by extrapolation from oral references doses (RfDs) or oral NOAELs or LOAELs.  According to the Interim Methods for Development of Reference Concentrations, such route-to-route extrapolation can be performed if pharmacokinetic data, including measurements of absorption efficiency and comparative excretion data (when the associated metabolic pathways are equivalent) are available for both routes of interest, and comparative systemic toxicity data indicate equivalent effects by each route of interest.  However, the Interim Methods stipulate that oral data should not be used to estimate inhalation exposure limits when the following conditions exist:

      (1)	When groups of chemicals are expected to have different toxicity by the two routes of exposure (e.g., metals, irritants, and sensitizers);

      (2)	When a first-pass effect is expected by the liver, or when the pulmonary system was not adequately studied in the oral studies;

      (3)	When a pulmonary effect is established, but dosimetry comparison cannot be clearly established between the two routes; and

      (4)	When short-term inhalation studies or in vitro studies indicate potential portal-of-entry effects at the lung, but studies themselves are not adequate for RfC development.

The appropriate data to assess these conditions were not available for substitutes requiring a route-to-route extrapolation.  Therefore, the interim RfCs that were estimated by extrapolation from oral data would most likely be considered "not verifiable" by the Agency's RfD/RfC Work Group.  However, in the absence of adequate data and in need of a chronic reference toxicity value for the purpose of the SNAP risk screens, extrapolation from oral data was nevertheless performed according to the following procedures:

      (1)	Oral RfDs (expressed in mg/kg/day) were converted to inhalation RfCs (expressed in mg/m[3]) by multiplying the RfD by the default assumption for average human body weight (70 kg) and dividing by the default assumption for average human breathing rate (20 m[3]/day) (EPA 1988).

      (2)	Alternatively, in the absence of adequate experimental inhalation toxicity data, RfCs for some substitutes were estimated by converting the OEL for that substitute (or a suitable surrogate, as discussed below) to a general population exposure limit.  For example, the aliphatic hydrocarbon isobutane is best represented by pentane.  The OSHA PEL for pentane is 1,800 mg/m[3].  This value is based on an 8-hour TWA worker exposure over a 40-hour work week.  To extrapolate to a threshold for long-term continuous general population exposure, the PEL was multiplied by a factor of 10 m[3]/20 m[3] (assuming that 10 m[3] are inhaled in an 8-hour day by a healthy worker, extrapolated to 20 m[3] in a 24-hour day by the general population) (Jarabek and Hasselblad 1991), and then divided by an uncertainty factor of 1,000 to account for sensitive subpopulations and other uncertainties.

Interim RfCs Based on Surrogate Limits

      For some of the substitutes, there are neither RfCs nor adequate experimental inhalation toxicity data, oral RfDs, oral data, or OELs from which to estimate an RfC.  In these cases, exposure limits for a surrogate such as HFC-134a, the surrogate for several HFCs in this sector, was used.  Surrogates were chosen by structure-activity relationship (SAR) analysis to provide the worst-case/most conservative estimate for the exposure limit.  SAR is used to predict the biological activity of an untested compound based on an examination of analogous chemicals with well-documented health or environmental effects data.

      SAR has been an important tool in guiding the development of new chemicals and drugs for many years.  EPA has used SAR in assessing new chemicals prior to their manufacture or importation under the Premanufacture Notice Program.  Also, the pharmaceutical industry has used SAR to assist in the design and synthesis of new drugs.

3.2.2	Cancer Slope Factors

      The cancer slope factor (SF) is used to estimate the upper-bound risk for cancer.  Because chemicals that produce cancer are assumed not to show a threshold for their effects, the observed threshold level (i.e., the NOAEL) is not used in the calculation of the cancer SF.  Rather, the slope of the dose-response for the effect is used to extrapolate levels at which the risk of the effect is small.  It has been argued that the dose-response function for carcinogenicity could be linear and that it is unlikely to exceed linearity in the low-dose region.  Thus the model chosen to extrapolate low-dose effects from the much higher doses at which effects are observed assumes low-dose linearity for cancer production.  EPA guidelines (U.S. EPA 1986) recommend that, in absence of adequate information to the contrary, the linearized multistage procedure should be used for the extrapolation.  This model expresses upper confidence limits on cancer risk as a linear function of dose in the low-dose range.  Cancer SFs are estimated by fitting the model to experimental animal carcinogenicity data using the maximum likelihood method.  In addition, an upper bound on the dose-response curve is calculated, reflecting uncertainty of extrapolating the curve to low doses at which human exposures are anticipated to occur.  This upper bound can be considered to represent the largest reasonable linear extrapolation to low doses consistent with the data.  The 95 percent statistical upper limit on the linear term, q1, is referred to as the q1*, or the SF.

      The SNAP cancer risk estimates were based on cancer SFs that have been verified by EPA's Cancer Risk Assessment Verification Endeavor (CRAVE) Work Group and are contained in IRIS.  If these were unavailable, a second choice was SFs that have not yet been verified but are contained in HEAST (U.S. EPA 1992).  The interim SFs for HCFC-22 and HCFC-123 were estimated using raw data from recently completed chronic inhalation cancer bioassays, according to the methods recommended in the EPA Risk Assessment Guidelines (U.S. EPA 1986).

                                  REFERENCES

ACGIH.  1986.  Documentation of the Threshold Limit Values and Biological Exposure Indices.  Fifth Edition.

ACGIH.  1992.  1992-1993 Threshold Limit Values for Chemical Substances and Physical Agents and Biological Exposure Indices.

AIHA.  1989.  Concepts and Procedures for the Development of Emergency Response Planning Guidelines (ERPGs).  ERPG Committee.

Jarabek, A.M. and V. Hasselblad.  1991.  Inhalation Reference Concentration Methodology:  Impact of Dosimetric Adjustments and Future Directions Using the Confidence Profile Method.  Presented at 84th Annual Meeting and Exhibition, Vancouver, B.C.  June 16-21, 1991.

NIOSH.  1990.  NIOSH Pocket Guide to Chemical Hazards.  NIOSH Pub. No. 90-117.  US DHHS.

NRC.  1986.  Criteria and Methods for Preparing Emergency Exposure Guidance Level (EEGL), Short-term Public Emergency Guidance Level (SPEGL), and Continuous Exposure Guidance Level (CEGL) Documents.  Committee on Toxicology.  National Academy Press, Washington, DC.

Rubenstein, R. and J.S. Bellin.  1993.  Short Duration High-Level Exposure to Halon Substitutes:
      Potential Cardiovascular Effects.  Presented at Halon Alternatives Technical Working Conference, Albuquerque, NM, May 1113, 1993.

U.S. EPA.  1992.  Health Effects Assessment Summary Tables.  Annual Update.  Environmental Criteria and Assessment Office, Office of Health and Environmental Assessment.  Cincinnati, OH.  OHEA ECAO-CIN-821.

U.S. EPA.  1990.  Interim Methods for Development of Inhalation Reference Concentrations.  Review Draft.  August 1990.  Environmental Criteria and Assessment Office, Office of Health and Environmental Assessment, Research Triangle Park, NC.  EPA 600/8-90/066A.

U.S. EPA.  1988.  Recommendations for and Documentation of Biological Values for Use in Risk Assessment.  February 1988.  Office of Health and Environmental Assessment, Environmental Criteria and Assessment Office, Cincinnati, OH.

U.S. EPA.  1986.  The Risk Assessment Guidelines of 1986.  Office of Health and Environmental Assessment.  Washington, DC.  EPA/600/8-87/045.

	4.  ATMOSPHERICS MODELING

4.1	INTRODUCTION

      EPA has conducted a risk screen of possible alternatives to Class I ozone depleting substances (ODSs) as part of its implementation of Section 612 of the Clean Air Act Amendments of 1990.  Class I substances include chlorofluorocarbons (CFCs), halons, carbon tetrachloride, and methyl chloroform (MCF).  Class I ODSs deplete stratospheric ozone and may contribute to global warming.  These atmospheric changes have potentially serious consequences for human health and the environment.  The depletion of stratospheric ozone increases the amount of biologically active ultraviolet radiation (UV-B) at the earth's surface.  Among the consequences of increased UV-B are the increased incidence of cataracts and skin cancers, damage to plants and animals, and damage to polymers (chemical compounds often used in plastics, paints, and coatings).  This chapter presents estimates of the potential atmospheric and human health effects of the alternatives available to replace Class I compounds in the refrigeration and air conditioning sector.

      The remainder of this chapter is divided into the six following sections:  Section 4.2 presents an overview of the refrigeration and air conditioning sector, including a description of the major end uses; Section 4.3 summarizes the potential replacements for Class I compounds in the refrigeration and air conditioning sector; Section 4.4 describes the baseline for the analysis; Section 4.5 details the modeling framework used to quantify the atmospheric and human health impacts of the alternatives; Section 4.6 presents the results of the analysis; and Section 4.7 presents a qualitative discussion of global warming and global warming potentials.

4.2	DESCRIPTION OF REFRIGERATION AND AIR CONDITIONING END USES

      CFCs are used as refrigerants in numerous refrigeration and air conditioning applications.  For the purposes of this analysis, the refrigeration and air conditioning sector was divided into several end uses to facilitate appropriate comparisons between possible substitutes.  Two general principles define how equipment types were classified into the various end uses:  (1) all equipment types in an end use employ roughly comparable refrigeration technologies (e.g., low and high pressure cold storage warehouse systems are separated ); and (2) all equipment types in an end use serve the same general consumer or industrial purpose (e.g., ice skating rinks and parallel retail food systems form separate end uses).

      The advantage of this end use classification is that substitutes applicable to a given end use will typically be able to be used in all segments of that end use.  A consequence, however, is that some end uses are considerably larger than others.  For example, all high-pressure industrial process refrigeration systems form one end use, while another end use consists only of CFC-12 reciprocating chillers.

      While the principal focus of this analysis is to evaluate alternatives to Class I substances, many end uses in the refrigeration and air conditioning sector primarily use HCFC-22 (a Class II compound).  In order to comprehensively examine the health and environmental effects of substitutes throughout this sector, HCFC end uses were analyzed in the same manner as CFC end uses.

      The end use categories are as follows:

      	Retail Food Systems:  These include stand alone refrigeration cases found in small markets, convenience stores, restaurants and other food establishments, large parallel systems found in supermarkets, and HCFC-22 systems found in a wide variety of retail and service establishments.

      	Cold Storage Systems:  These systems refrigerate warehouses of varying sizes and include high pressure and low pressure systems.

      	Chillers:  These are large commercial air conditioning systems used to cool office buildings, shopping malls, and other large indoor spaces.  End uses modeled are CFC-11 (low pressure) chillers, CFC-12 centrifugal chillers, CFC-12 reciprocating chillers, CFC-114 centrifugal chillers, and CFC-500 centrifugal chillers.

      	Industrial Process Refrigeration:  These include both packaged and built-up systems used in manufacturing processes as well as extra-large ice makers.  High and low pressure systems are modeled as separate end uses, although the overwhelming majority of industrial process systems are high pressure units.

      	Ice Skating Rinks:  There are several different types of ice rink systems, but all are grouped into one end use in this analysis.

      	Ice Makers:  These include small, medium, and large ice makers used by a number of entities, including restaurants and hotels.

      	Household and Light Commercial Air Conditioning:  These include window units, unitary air conditioning systems, and heat pumps.  All systems in this end use currently use HCFC-22.

      	Refrigerated Appliances:  These include household refrigerators and freezers, water coolers, vending machines, and dehumidifiers.  Most currently use CFC-12 as a refrigerant, but there are some CFC-502 units in existence.

      	Refrigerated Transport Systems:  These include systems designed to refrigerate materials in transit.  They are found in trucks, trains, and ships.

      	Mobile Air Conditioning:  This end use includes systems installed in motor vehicles for the comfort of occupants.

      Exhibits 4-1 and 4-2 display the consumption of ODSs in the refrigeration and air conditioning sectors as a fraction of total U.S. ODS consumption.  The consumption amounts presented for ODSs in Exhibit 4-1 are weighted according to the chemicals' ozone depletion potentials (ODPs).

ex 4-1  &  4-2

4.3	CONTROL OPTIONS

      The alternatives to CFC refrigerants with the widest applicability are HCFC and HFC refrigerants, although other alternatives such as ammonia, propane, and butane are possibilities in some end uses.  Whether or not a chemical substitute is feasible in a given end use depends on a number of factors, including the required operating temperatures and pressures that need to be attained in that end use.

      Substitutes can replace CFCs in both new and existing equipment in the refrigeration and air conditioning sector.  In general, existing equipment will have to be modified  or retrofitted  to be capable of using the substitutes.  In this analysis, the effects of substitutes in new equipment were distinguished from the effects of the same substitutes as retrofits.  This allows one to study the implications of substitutes in new equipment and existing equipment independently from one another.

      Additional information on possible substitutes, their availability, their potential end use market penetration, and their impact on energy consumption is presented in Section 4.6 with the results.

4.4	THE BASELINE FOR THE ANALYSIS

      The baseline provides a benchmark against which the effects of possible substitution scenarios can be measured.  An appropriate baseline incorporates assumptions regarding chemical emissions in the particular end use being examined, but at the same time it must account for all other sources of emissions of relevant compounds both in the U.S. and in the rest of the world (ROW).  A global baseline is required because the incremental atmospheric effects of emissions in an end use depend on the quantity emitted from all other end uses worldwide.

      The baseline is described in three steps.  First, the uniform assumptions made throughout the analysis are specified.  Second, the assumptions that are applied to all of the end uses (including end uses in other sectors such as solvents, fire extinguishing, foams, and sterilants) other than the one in which multiple substitutes are being compared are specified.  Finally, the baseline assumptions used within an end use when evaluating multiple substitutes are specified.

4.4.1	Uniform Assumptions Made Throughout the Analysis

      This section describes the uniform phaseout and recycling assumptions made throughout the analysis.  The U.S. phaseout assumptions are listed in Exhibit 4-3.  The exhibit lists the last year in which a given chemical can be produced for use in either new equipment or existing equipment.  Other developed countries (Canada, Japan, Australia, New Zealand, and the nations of Western Europe) are assumed to follow the same schedule.  The phaseout of ODSs in developing countries is assumed to lag that of the developed world by a decade.  In addition, this analysis assumes that CFCs and HCFCs are recycled beginning in 1992 during the servicing and disposal of certain refrigeration equipment and that HFCs are likewise recycled starting in 1995.  These recycling options are implemented in both the U.S. and the rest of the world (with a ten-year lag for developing countries).

                                  EXHIBIT 4-3
             PRODUCTION PHASEOUT ASSUMPTIONS FOR THE UNITED STATES

                               Chemical Compound
                                 Phaseout Date
                                    for New
                                   Equipment
                                 Phaseout Date
                                 for Existing
                                   Equipment
               CFCs, methyl chloroform and carbon tetrachloride

                                     1996

                                     1996
                                    halons
                                     1994
                                     1994
                                methyl bromide
                                     2001
                                     2001
                                   HCFC-141b
                                     2003
                                     2003
                             HCFC-22 and HCFC-142b
                                     2010
                                     2020
                                All other HCFCs
                                     2015
                                     2030

4.4.2	 Assumptions Made for All Other End Uses

      This section summarizes the substances industry is assumed to choose to replace CFCs, halons and MCF in all end uses other than the one in which multiple substitutes are being compared.  Although in a few sectors it remains uncertain which substitutes will be chosen, a "best estimate" substitute or combination of substitutes was assumed for each end use under the 1996 phaseout in order to best approximate total worldwide emissions of Class I and Class II substitutes.  These substitution scenarios were developed based on EPA's best judgment of the likely substitutes to be used in each end use under the 1996 phaseout.

4.4.3	 Assumptions Made Within the End Use of Concern

      This section describes the baseline assumptions used within the end use in which multiple substitutes are to be compared.  In this analysis, two different baselines are used to present the results of the substitution scenarios.  Before detailing these baselines, it is important to note that the choice of the baseline affects the presentation, and not the substance, of the results.  Under either of the baselines described below, the relative differences between the substitutes with respect to ozone depletion will be the same.  The absolute results presented for each substitute will, however, differ between the two baselines.

      Each of the baselines used with the end use of concern are discussed below.  One is a "no-risk baseline"; the other is a "no-substitution" baseline.  These two baselines bound the range of health effects, since the first examines impacts from a risk-free scenario and the second presents effects assuming continued use of all CFCs.

<IP_ADDRESS>	Ideal Substitute Baseline

      The "ideal" substitute is a hypothetical alternative that has an ozone depletion potential (ODP) of zero.  The concept of the "ideal" substitute was developed to provide a "no-risk" point of reference from which actual alternatives could be compared against one another.

      In this case, the difference between substitute and baseline scenarios isolates the effects of implementing the substitute in the U.S. relative to an ideal or "no-risk" substitute.  As presented schematically in Exhibit 4-4, the reported effects of a substitute are the difference between the estimated effects of the substitution scenario and the "ideal" substitute baseline.  The "ideal" substitute baseline consists of the "ideal" substitute in the end use of concern in the U.S. only and the "best estimate" substitute in all other end uses  both in the U.S. and in the rest of the world.  The substitution scenario consists of the substitute in the end use of concern in the U.S., and the "best estimate" scenario in all other end uses.

      In the refrigeration and air conditioning sector, special care also has to be taken to distinguish between new and existing equipment, especially regarding baseline issues.  As mentioned previously, this analysis examines separately the effects of substitutes in new equipment and in retrofits (existing equipment).  As a result, for new equipment scenarios, the analysis measures the impact of replacing the ideal substitute with the substitution scenario.  The retrofit scenarios assess the impact of replacing the ideal substitute in existing equipment with the substitution scenario.

                                  EXHIBIT 4-4
               SUMMARY OF METHODOLOGY USED IN ATMOSPHERICS RUNS
               TO CALCULATE IMPACTS RELATIVE TO "IDEAL" BASELINE

	United States

	Rest-of-World

	End Use
	of Concern
	All Other
	End Uses

	End Use
	of Concern
	All Other
	End Uses

BASELINE SCENARIO
	"Ideal"
	Substitute
	Best
	Estimate
	Substitute

	Best
	Estimate
	Substitute
	Best
	Estimate
	Substitute

SUBSTITUTION SCENARIO

	Substitute

	Best
	Estimate
	Substitute

	Best
	Estimate
	Substitute
	Best
	Estimate
	Substitute

      The model includes consideration of speed of market penetration.  In all analyses, the "ideal" substitute fully penetrates the new equipment market in 1992 and begins to penetrate the existing equipment market in 1992, reaching a maximum market penetration in 2000.  The slower rate of penetration and the smaller final market share for retrofits reflects the behavior that would be expected in practice for most possible retrofit scenarios.  As a result, a substitute that has low ODP but only penetrates that market slowly shows adverse health effects that are an artifact of the modeling, since until the time the substitute becomes available, Class I compounds must continue to be used.

      The advantage of the "ideal" substitute baseline is that it allows for a "no-risk" point of reference for presentation purposes but accounts for changes in atmospheric conditions caused by activity in other end uses.  It must be emphasized that the "ideal" substitute is only used as a reference point.  In all end uses, the relevant comparisons are those between one substitute and another, not between the substitute and the ideal.

<IP_ADDRESS>  No Substitution Baseline

      Under this baseline, the incremental performance of each substitute within an end-use is judged against a no substitution baseline (i.e., continued use of Class I substances within the end use).  This option clearly indicates the reduction in ODP risks associated with replacing the use of Class I compounds with each of the potential substitutes and allows for direct comparisons in risk reduction achievable through each substitute.

      In this case, the difference between substitute and baseline scenarios isolates the effects of implementing the substitute in the U.S. relative to a scenario that assumes the continued use of the Class I compound.  As presented schematically in Exhibit 4-5, the reported effects of a substitute under this baseline are the difference between the estimated effects of the substitution scenario and the no substitution baseline.  The no substitution baseline consists of the continued use of the Class I compound in the end use of concern in the U.S. only and the "best estimate" substitute in all other end uses  both in the U.S. and in the rest of the world.  The substitution scenario consists of the substitute in the end use of concern in the U.S., and the "best estimate" scenario in all other end uses.

                                  EXHIBIT 4-5
               SUMMARY OF METHODOLOGY USED IN ATMOSPHERICS RUNS
          TO CALCULATE IMPACTS RELATIVE TO "NO SUBSTITUTION" BASELINE

	United States

	Rest-of-World

	End Use
	of Concern
	All Other
	End Uses

	End Use
	of Concern
	All Other
	End Uses

BASELINE SCENARIO

	Continued
	CFC Use
	Best
	Estimate
	Substitute

	Best
	Estimate
	Substitute
	Best
	Estimate
	Substitute

SUBSTITUTION SCENARIO

	Substitute

	Best
	Estimate
	Substitute

	Best
	Estimate
	Substitute
	Best
	Estimate
	Substitute

      As discussed above, this analysis examines separately the effects of substitutes in new equipment and in retrofits (existing equipment).  As a result, for new equipment scenarios, the analysis measures the impact of phasing out the use of the Class I compound in new equipment in a given end use and replacing it with the substitution scenario.  The retrofit scenarios assess the impact of replacing the use of the Class I compound in existing equipment in a given end use and replacing it with the substitution scenario.

      The intent of the Agency in presenting the resulting of the tradeoff analysis relative to the no substitution baseline is to indicate that all of the substitutes analyzed are substantially less harmful to the ozone layer than continued use of CFCs.  In presenting the results in this fashion, however, EPA is cognizant of the fact that the reduction in ODP risks due to the phaseout of Class I compounds are attributable to the Class I Phaseout Rule and not the Safe Alternatives Program.

4.5	OVERVIEW OF THE MODELING APPROACH

      This section provides a brief description of the Atmospheric and Health Effects Framework (AHEF), the modeling framework used to assess the global environmental impacts of alternatives to Class I substances in the United States.

4.5.1	Chemical Use

      The starting point of an AHEF analysis is to forecast the consumption of the relevant chemical compounds in each of the end uses in each sector.  For U.S. estimates, this task is usually performed by the Vintaging Model, which simulates the chemical requirements of the various equipment types in each end use over their lifetimes.  The Vintaging Model has been used to support several significant Agency rulemakings including the phaseout of Class I and II compounds under Section 604 of the Clean Air Act.  It is documented in Appendix A of EPA's Regulatory Impact Analysis: Compliance with Section 604 of the Clean Air Act for the Phaseout of Ozone Depleting Substances (EPA 1992), which was included in the Phaseout rulemaking docket and was subject to public review and comment.

      For other regions of the world, chemical use estimates are typically derived by applying the London Amendment reduction schedule to estimates of what demand would be in the rest of the world had this phaseout never occurred.

4.5.2	Chemical Emissions

      Emissions of a compound into the atmosphere are not necessarily simultaneous with use.  The delay between use and emission of a compound depends on the kind of equipment in which the compound is employed.  For example, hermetically sealed refrigerators emit CFCs over a very long period of time, whereas packaging foams emit similar compounds in less than one year.  AHEF simulates compound emissions by means of release functions which specify the time delay between use and emissions for each particular end use.

4.5.3	Atmospheric Effects

      The output from the emissions module are used in an atmospheric model, the Atmospheric Stabilization Framework (ASF), that predicts changes in stratospheric ozone associated with changes in atmospheric composition.  ASF computes the globally and annually averaged concentrations of climatically important atmospheric constituents, taking into account various feedbacks between climate parameters and the constituents themselves.  Examples of feedbacks accounted for in ASF are:

      	The dependence of a compound's atmospheric lifetime on column (stratospheric) ozone and temperature;
      	Radiative and chemical feedbacks due to water vapor;
      	Ocean absorption;
      	Atmospheric circulation effects; and
      	Chemical interactions between compounds.

      ASF relies on simple empirical relations based either on observations or on the results of more complex models in order to quantify physical feedbacks.  In particular, the dependence of model ozone on stratospheric chlorine is adjusted so that it predicts a historical ozone depletion equal to that observed for the current atmosphere.  In addition, ASF input parameters are adjusted to give column ozone changes that are consistent with consensus ODP estimates.

      The ASF is a "consensus" model developed by a committee of prominent atmospheric scientists in government, academia, and private consulting firms.  It was designed to approximate the behavior of more sophisticated two and three-dimensional physical models without requiring the computing power needed for physical simulations.  Additional information on the ASF can be found in National Aeronautics and Space Administration Conference Publication 3023, An Assessment Model for Atmospheric Composition (NASA 1988).

4.5.4	Human Health Effects

      The ozone column changes predicted by the ASF are inputs to a health effects model that estimates the number of skin cancer cases and fatalities that result from the scenario modeled.  Estimates of skin cancers include both melanoma and non-melanoma cancers and represent all cases experienced by individuals born in the United States before 2030.

      In order to quantify the human health impacts of ozone depletion in the U.S., the globally averaged ozone depletion predicted by ASF is converted to estimates of depletion by latitude.  This latitudinal variation is estimated using simple regression techniques from a prior two-dimensional modeling study. (Isaksen, 1986).  Results from a separate UV model are in turn used to estimate changes in UV-B intensity associated with the latitudinally-varying ozone depletion.

      Human health effects are computed by AHEF as a function of UV-B exposure.  The effective exposure of various population cohorts, representing different sexes and age groups, is estimated using appropriate factors that weight exposure at various stages during a person's lifetime.  Empirical "dose-response" relationships for each cohort are then used to predict changes in skin cancer and cataract incidence over time.

      The human health effects model was reviewed in 1987 as part of an EPA Science Advisory Board review of Assessing the Risks of Trace Gases that Can Modify the Stratosphere (EPA, 1987).

4.6	MODEL RESULTS

      This section presents the results of the analysis and lists several considerations that are helpful in interpreting them.

4.6.1	Description of Results

      The results are presented in Exhibits 4-6 through 4-45, one for each end use sector.  Each exhibit contains two tables and two graphs.  Parts A and B of each exhibit present the atmospheric results relative to the "ideal" baseline.  Parts C and D of each exhibit present the results relative to the no substitution baseline.  The table lists the assumptions regarding the availability and replacement potential of each alternative and estimated atmospheric and health effects resulting from those assumptions.  Note that numbers in parenthesis represent decreases from the baseline (e.g., negative numbers).  The graph illustrates the change in cumulative Clx in 2075 for each of the alternatives.

      More specifically, the following information is provided:

Start Date:  The year in which the alternative option is expected to be commercially available (i.e., after all research and development is complete).  A start date of 1992 indicates that the alternative is currently available.

Years to Maximum Market Penetration:  The number of years it would take for an alternative, once it became commercially available, to reach its maximum possible use in the end use market.

Maximum Market Penetration:  The maximum, technically feasible percentage of total ODS consumption in an end use that can be replaced by the alternative.

Cumulative Clx:  This is a measure of the change in the total quantity of chlorine in the stratosphere over time.  More technically, it is the integral of the Clx curve above two parts per billion.

Skin Cancer Cases:  The estimated change in the number of cases of melanoma and non-melanoma skin cancers experienced by individuals in the U.S. population born before 2030.  The estimate of skin cancers includes fatalities that may result from these cancers.

Skin Cancer Fatalities:  The estimated change in the number of fatalities due to melanoma and non-melanoma skin cancers of individuals in the U.S. population born before 2030.  Skin cancer fatalities are a small portion of the total number of skin cancer cases.

4.6.2	Interpreting the Results

      When interpreting the results, one should be aware of the following:

      	For the results presented relative to the ideal baseline, the baseline for the substitution scenarios for new and existing equipment assumes that the "ideal" substitute is fully implemented in new equipment in 1992 and is retrofit in 50% of existing equipment between 1992 and 2000.  As a result, the numbers presented for each alternative in new equipment scenarios account for:  (1) the effects of that alternative; (2) the effects of any continued CFC use occurring in the period after the implementation of the ideal substitute in the baseline and before the implementation of the alternative; and (3) the continued CFC use attributable to the lack of a retrofit.  Accordingly, the later the availability of the alternative, the greater the quantity of continued CFC use that is attributed to that alternative.  Even an ozone-safe alternative can have cancers indirectly attributed to it if it penetrates the market after the "ideal" substitute.

      	Under the ideal baseline, a "no retrofit" scenario (i.e. "ideal" in new equipment and continued CFC use in existing equipment) is included with the retrofit results for each end use to indicate the effects of continued CFC use in existing equipment that are potentially avoidable through retrofitting.  The results of this scenario are helpful in interpreting both retrofit and non-retrofit graphs because they demonstrate the benefits of retrofitting and show how much of the effects in new equipment scenarios can be attributed to the lack of a retrofit.

      	Since different HCFCs have different phaseout dates, the total quantity of substitute chemical ultimately released into the atmosphere may differ depending on the alternative.  For example, the results of scenarios using HCFCs with early phaseout dates reflect the fact that less of the chemical is ultimately used.

      	Blends containing HCFCs are phased out as soon as their earliest constituents are phased out.

      	Modeling reflects the market penetration potential for each substitute.  This means that health effects are dependent on potential market size, so that a low ozone-depleting substitute with large market potential could look worse than a high ozone-depleting substitute with only small uses.

Exhibits 4-6 through 4-45.
80 pages

4.7.	Global Warming

      There is no doubt that human activity is increasing the concentration of greenhouse gases in the atmosphere.  The sources and industrial sectors contributing these gases are extremely varied.  No single economic sector can be held entirely responsible for the greenhouse effect, nor can a single source, because it contributes only a small amount to the problem, be exempt from accountability.  Additional research is needed to better characterize the effects of greenhouse gas emissions on the environment and to evaluate the benefit of various reduction strategies.  However, as in the case of ozone depletion, there is growing international consensus among scientists and policymakers that immediate efforts are necessary to curb the rate of growth of atmospheric concentrations of these gases.

      Based on total emissions, the major greenhouse gases are carbon dioxide (CO2) and methane (CH4), and it is on these major contributors that EPA has focused much of the effort of its nascent voluntary programs such as Green Lights.  However, a substantial contribution is made in the aggregate by other gases including CFCs, halons, and other halocarbons.  These minor or trace gases are important for two reasons.  First, although these gases are currently emitted in smaller quantities than CO2 and CH4, their effectiveness in influencing climate tends to be greater.  Second, while individual use of these gases may add little to the overall burden, the rising concentration of greenhouse gases in the atmosphere cannot be controlled without addressing the aggregate effect of the trace gases.

      As the transition away from the ozone-depleting compounds proceeds, a key policy goal of the SNAP program in reducing overall risk is to be sure that this shift does not also result in a significant rise in contributions to global warming due to an increased reliance on greenhouse gas substitutes.  Alternatives reviewed under SNAP to date have varying potential to contribute to global warming.  Their contribution depends on many factors evaluated under SNAP, including radiative properties and persistence in the atmosphere.  It is difficult to make relative judgements among those which do contribute to global warming, however, because their contributions, when viewed through the SNAP lens of individual end-uses, are uniformly small.

      Recognizing this, SNAP avoids evaluating the viability of greenhouse gases as substitutes based solely on a single measure, such as an end-use by end-use contribution to actual warming.  Instead, a broader evaluation of a substitute's inherent characteristics, such as its absorption spectra and atmospheric lifetime, is used to help clarify whether a substitute could contribute significantly to the problem relative to other substitutes available.  For example, a compound which might be acceptable in its contribution to global warming in limited uses might become a problem in widespread emissive uses.  Thus, a careful review of the volume likely to be used in a particular end-use, and how much is likely to be emitted from that end-use, also helps.  Finally, because SNAP's primary goal is to facilitate the transition away from the ozone-depleting chemicals, a substitute which is also a significant global warming gas might be allowed with controls if there exist few other alternatives in a particular end-use.

      This section reviews the risk screen for global warming under SNAP.  It includes a discussion of the nature of the global warming problem, general properties of greenhouse gases, and global warming resulting from changes in the energy efficiency of equipment or processes using the alternative chemicals.

4.7.1	The Nature of the Global Warming Problem

      The Earth's stable average temperature is maintained by a radiative balance in which the amount of energy absorbed from the sun is equal to the amount of energy radiated back into space by the Earth and atmosphere.  If a factor is introduced into the atmosphere that changes this balance, the climate must warm or cool until the radiative fluxes are equal and the balance is restored.  A change of this kind is known as a change in radiative forcing (or warming commitment).  Radiative forcing, expressed in watts per square meter (W/m[2]), is a measure of the downward radiation flux received by the climate system.  Concerns about global warming stem from observed increases in the concentrations of certain gases that may be able to alter the climate by causing an increase in radiative forcing.

      A characteristic that these gases share is that they absorb radiation in the infrared (or thermal) part of the electromagnetic spectrum.  Most of the energy that the Earth receives from the sun is in the form of shortwave radiation in the visible and ultraviolet parts of the spectrum.  The energy emitted from the Earth to space is in the form of longwave radiation in the infrared part of the spectrum.  These gases absorb some of the outgoing infrared (or thermal) radiation and re-emit it both up into space and back down to the Earth.  The net result is a decrease in the heat radiation escaping to space and an increase in the heat radiation falling to the Earth (an increase in radiative forcing).  Because this process tends to concentrate heat in the troposphere, it is known as the greenhouse effect.  The gases responsible for the greenhouse effect are known as greenhouse gases.  Greenhouse gases include carbon dioxide, methane, and nitrous oxide, as well as chlorofluorocarbons (CFCs) and some CFC substitutes.

      The greenhouse effect is a natural phenomenon which plays an important role in keeping the Earth warm.  Concerns about the greenhouse effect and global warming arise from indications that human activities have been increasing the concentrations of both naturally occurring and anthropogenic greenhouse gases.  Increasing concentrations of these gases may lead to an enhanced greenhouse effect, which may cause the Earth to warm at a rate much faster than natural rates of climate change.  This accelerated climate change may have a number of adverse consequences including damage to ecosystems, changes in climatic zones, changes in weather patterns, and sea level rise.  Adjusting to these changes quickly may be both difficult and expensive.

4.7.2	Properties of Greenhouse Gases

      All organic compounds that can volatilize into the atmosphere share the property that they absorb energy in the infrared portion of the electromagnetic spectrum.  However, these gases do not all absorb infrared radiation equally well, and thus some have greater impacts than others on global warming.  The following factors determine the global warming impact of emissions of a particular gas:

      	Atmospheric lifetime.  Various physical and chemical processes tend to remove and break down chemicals in the atmosphere.  Atmospheric lifetime is a measure of how long a gas stays in the atmosphere before it is removed by these processes.  The lifetimes of the greenhouse gases are determined by their sources and sinks in the oceans, atmosphere, and biosphere.  In general, the most effective greenhouse gases are those with long lifetimes since the impact of long-lived gases is more persistent than the impact of short-lived gases.  In some cases this persistence may be cause for concern.  With very-long-lived gases it may take thousands of years for concentrations to decline significantly after emissions have stopped.  Thus, the impacts these emissions have are essentially irreversible.

      	Molecular weight.  Emissions are typically measured according to their mass (in kilograms).  However, the increase in atmospheric concentration (by volume) resulting from an emission is proportional to the number of molecules in the emission, not the total mass of the emission.  The number of molecules in an emission of a given mass is inversely proportional to the molecular weight of the gas emitted.  As a result, the molecular weight is needed to determine the increase in concentration resulting from an emission of a given mass.  The lower the molecular weight of the gas, the greater the increase in concentration resulting from a given emission.

      	Radiative forcing per molecule.  This is a measure of the effectiveness of the gas at absorbing infrared radiation emitted by the Earth.  It is proportional to the increase in radiative forcing resulting from a given increase in concentration.

      Of these factors, only molecular weight is an irreducible characteristic of a gas.  Atmospheric lifetime depends on other characteristics such as solubility in water, rates of reaction with other components of the atmosphere, and susceptibility to photolysis.  Radiative forcing per molecule depends on a number of characteristics that are relevant to greenhouse gas behavior.  These include the following:

      	Integrated infrared band strength.  Each kind of molecule absorbs energy at a unique set of wavelengths.  The wavelengths at which a molecule absorbs energy are in patterns of contiguous regions in the electromagnetic spectrum which are referred to as absorption bands.  A molecule's absorption bands together make up its absorption spectrum.  The strength of the gas's infrared absorption spectrum is a measure of how well molecules of the gas absorb radiation at their characteristic wavelengths (i.e., the stronger the absorption band, the more energy that is absorbed by the molecule).

      	Location of infrared absorption bands.  The location of the gas's infrared absorption bands is important for two reasons:  First, the energy radiated from the earth is not evenly distributed across the infrared spectrum.  More energy is radiated at some wavelengths than at others.  Molecules with absorption bands at wavelengths of high infrared radiation will tend to be more effective greenhouse gases than molecules with absorption bands at wavelengths of low infrared radiation.  Second, the impact of a greenhouse gas may be reduced if its absorption bands overlap with those of other gases present in the atmosphere in significant quantities.  For example, if a gas has absorption bands that overlap with those of an abundant greenhouse gas like carbon dioxide, the relative impact of the gas on radiative forcing is reduced since much of the energy at these wavelengths is already absorbed.

      	Initial concentration of the gas.  If the initial atmospheric concentration of a gas is high enough so that a significant portion of the energy at relevant wavelengths is already absorbed, the impact of additional increases in concentration is reduced.  This effect is significant for carbon dioxide.  It is not significant for CFCs and CFC substitutes since they are present in much smaller quantities than carbon dioxide.  Radiative forcing scales linearly with concentration for these chemicals.

      A useful index for comparing emissions that incorporates all of these factors is the Global Warming Potential (GWP).  The GWP depends on the position and strength of the absorption bands of the gas, its lifetime in the atmosphere, its molecular weight, and the time period over which the climate effects are of concern.  Specifically, the GWP is defined as the time-integrated change in radiative forcing resulting from a kilogram of emissions of a given chemical relative to the time-integrated change in radiative forcing resulting from a kilogram of emissions of a reference gas, typically carbon dioxide.

      Because different gases have different lifetimes, the period of integration has an important impact on relative GWPs.  This point is illustrated by the example of two different gases, each with an initial concentration of 1 ppb.  If the lifetime of the first gas is 10 years, its concentration will be 1/e[10] (0.00005) ppb after 100 years (ten lifetimes).  A 100-year GWP captures the entire impact of this gas since the concentration of the gas is effectively zero at the end of the period of integration, and it can no longer influence radiative forcing.  If the lifetime of the second gas is 100 years, its concentration will be 1/e (about 0.37) ppb after 100 years (one lifetime).  A 100-year GWP does not capture the entire impact of this gas since the gas is still present in significant quantities at the end of the period of integration, and it still affects radiative forcing.  If the period of integration were lengthened, the ratio of GWPs between the long-lived and the short-lived gases would increase because the value of the integral in the numerator would increase, while the value of the integral in the denominator would remain the same.

4.7.3	Properties of ODSs and ODS Substitutes

      Exhibit 4-46 presents atmospheric lifetime, molecular weight, integrated infrared band strength and GWPs for a number of ODSs and ODS substitutes.  Although there is overlap between categories, in general the PFCs have the highest GWPs among ODS substitutes.  HFCs are next, followed by HCFCs, which have the lowest GWPs.  PFCs have high GWPs because of their high integrated band strengths and their extremely long atmospheric lifetimes.  HCFCs generally have low GWPs because of their short atmospheric lifetimes (see Exhibit 4-46).  HFCs typically fall in the middle because of strong integrated band strengths and moderate atmospheric lifetimes.  In examining this table the following issues should be considered:

      	Although the GWPs of ODSs and ODS substitutes may be thousands of times the GWP of the reference gas, carbon dioxide, this is offset by the fact that emissions of these compounds are orders of magnitude smaller than emissions of carbon dioxide.  Nevertheless, it is estimated that CFCs accounted for about 23 percent of the direct radiative forcing from US emissions of greenhouse gases in 1990.