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Infection Control

CM/Spotlight: Infection control — Science-based steps of the cleaning process

September 19, 2010
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Published with permission by the Cleaning Industry Research Institute © 2008.

This is part one of a four-part series on the "Science-based steps of the cleaning process."

Each step contains scientific concepts and principles explaining cleaning''s effectiveness in putting unwanted matter in its proper place.

What is unwanted matter?
Unwanted matter is brought indoors by foot traffic, air deposits, impact, filtration or other outdoor sources.

Basic physics suggests all matter transfers in and out of compartments, such as carpet, hard floors, fabrics and, to some degree, HVAC.

It is carried between compartments by physical displacement, gravitational settling, air current impact or filtration, electro-attraction or direct contact, like tracking.

Substances circulating indoors have different shapes, sizes, chemical compositions, densities, absorption, adsorption and adhesion characteristics.

Dusts are the heterogeneous mixture of these solid particles.

Substances are measured as a micron or micrometer unit. A micrometer (µm) = 1/1,000,000 of a meter.

A micron or micrometer-size particle is about 100 times smaller than a grain of salt.

Particles between 30 and 40 microns are nearly invisible.

Their mass is calculated in grams.

Substances measured as air concentrations are calculated as micrograms (millionth of a gram) per cubic meter of air (µg/m3).

How is matter measured and transferred?
Particle counts measure matter in an environment''s compartment.

Generally, this is irrelevant since our interest is the mass of substance, not the number of microscopic particles.

Consequently, virtually all environmental health information relates to substance mass not particle count.

How is matter transferred?

Large, dense substances, such as clay, sand, oils, grass, leaves and some biomass, usually are tracked inside at ground level.

Irregular or porous surfaces tend to collect and hold fast to foreign matter.

Carpet has a large, irregular surface area and strong holding characteristics.

Temporary containment — also known as the trap effect — helps carpet control indoor pollutants.

Carpet dusts commonly are 85 to 95 percent dry soil and are tracked in from outside.

Typically, these dusts are greater than 20 µm.

These larger, denser particles are trapped by carpet close to entrances.

Because of their size and density, once they inhabit the carpet it is unlikely they will return to air.

Matter transfers from air to surfaces through gradual settling, impaction, filtration and electronic attraction.

Generally, large dense particles transfer faster than less dense matter.

Matter transferring from surfaces to air primarily is a function of attractive forces between the matter and its surface.

Matter with multiple contact points is more strongly attracted to the surface and more force is required for displacement.

Denser matter is less buoyant and has greater difficulty re-entering the air.

Without human activity most indoor air matter originates outdoors.

Gravitational settling causes larger, denser airborne substances to sink to the ground or floor faster.

Outdoor sources generally are bigger and fall to the ground rapidly.

Once indoors, they are deposited in the building envelope''s outer regions, like window sills, more than inside.

Large-size substances, such as hair, skin scales and fibers generated by indoor activities, usually settle out rapidly in carpet close to the activity area.

Smaller atmospheric particles (<15 µm) most influence indoor air quality (IAQ).

These particles usually are transported indoors on air currents.

Particles <1 µm are a major component of atmospheric dust.

If they become part of indoor air it takes time for them to settle out, if at all.

Know your surface
Particles settling out or becoming trapped, such as in carpet, are difficult to remove and re-entrain or to become airborne.

Depending on their shape and the number of contact points, particles settle and attach themselves to ceilings, walls, shelves, air ducts and floors with various attraction forces.

Typically, an indoor room environment has four walls, a ceiling and floor.

The flooring constitutes roughly 16 percent of the surface area.

Elevated surfaces also influence IAQ but not to the same extent as flooring.

Flooring, especially carpet, holds more matter.

Hard, elevated surfaces, such as tabletops and shelves, are not as strong a compartmental sink as fabrics.

Vertical surfaces tend to hold matter by electrostatic forces.

This matter often is smaller in size and density.

It takes 10 times more energy to re-suspend matter from fabric than a hard surface.

Less matter is transferred from an elevated surface when regular cleaning is conducted.

In the first order matter transfer process, the amount of matter in a compartment determines how much matter transfers to another compartment, such as air.

Flooring and elevated surfaces are loaded with matter.

As matter is transferred from sources to air, concentrations increase based on the transfer rate.

Factors influencing IAQ
A building''s IAQ is influenced by outdoor air, effective ventilation and compartment (source) loading.

Source loading first occurs through air deposition followed by human tracking.

No activity or ventilation results in positive feedback.

Without ventilation particle mass accumulates on interior surfaces, transfers to air and degrades IAQ.

This occurs in unoccupied, unventilated and ill-maintained buildings.

Ventilation contributes to the possibility of achieving a steady state of air quality when faced with continuous compartment loading and infrequent cleaning.

Ventilation also reduces the particle mass loading of compartments and surfaces.

It also tends to keep IAQ concentrations near background levels.

Removing minimal matter through cleaning brings source loading to a steady state condition in unoccupied buildings.

Even removing 5 to 20 percent of particle mass produces the same condition observed in several research studies.

Carpet is stronger at trapping matter than transmitting it.

When there is human tracking activity in a building, carpet source loading occurs more than with other compartments.

Depending on the amount of loading, air quality degrades primarily in the carpet compartment.

The most significant or likely air contaminants that can become airborne into the indoor air are particulate matter ranging from 1 to 10 µm.

Field data suggest that IAQ over carpet is adversely affected by a high loading (>2000 mg/m2) of dusts.

This is consistent with the principle that matter transfers from one compartment, such as carpet, to another, such as air, in proportion to the amount of matter in the originating compartment.

Excessively loaded carpet degrades IAQ well beyond background levels.

Cleaning reduces source strength.

As mitigation remedies are introduced, such as managing activity, source strength is managed by steady state conditions.

The IAQ also is brought to an acceptable control level.

Field demonstration research indicates that cost effective cleaning technologies easily maintain the source strength of carpet and other surfaces at levels not adversely affecting IAQ.


Recent Articles by Michael A. Berry, Ph.D.

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