Press Releases

2000 Releases

Diesel or Natural Gas? New Harvard Study Finds Environmental Pros and Cons with Both

For immediate release: January 10, 2000

Boston, MA--Which fuel is the right choice for heavy trucks and buses? It's a decision facing policymakers in California, at the EPA, and at government agencies around the world, as well as executives at automakers and corporations that operate fleets of buses or trucks. Phase 1 of a study comparing the two fuels, by the Harvard Center for Risk Analysis (HCRA) at Harvard School of Public Health, finds that there are advantages and disadvantages to each. Environmentally, natural gas is better at reducing particulate and NOx pollution. Diesel is better for reducing greenhouse gasses.

Diesel is the fuel of choice now, but concerns about particulate pollution in diesel exhaust have prompted a move toward alternatives. The HCRA analysis finds that natural gas reduces emissions of fine particulates, those smaller than 2.5 microns. But natural gas may generate more ultra fine particles than diesel. Those are less than .1 micron. Several studies indicate that ultrafine particles may have an even more dramatic impact on health than those in the fine category.

The study finds that because natural gas is primarily methane, a relatively simple molecule, it combusts more completely than many fuels, producing fewer emissions of several types, particularly NOx, an important contributor to ground level ozone and the formation of fine particulates.

The advantages of diesel come from its efficiency. Diesel engines convert a large fraction of the available energy into useable work. As a result, diesel engines consume less fuel overall than if they were converted to natural gas. The HCRA study suggests that converting heavy trucks and buses from diesel to natural gas would increase emissions of C02, a significant greenhouse gas. In addition, the study finds that more widespread use of natural gas would likely increase the escape of methane into the atmosphere. Methane is approximately 20 times more potent as a greenhouse gas than CO2.

The study finds that European regulators seem to be favoring diesel fuel as part of their effort to comply with the Kyoto agreements to stabilize CO2 and other greenhouse gas emissions. They are using tax incentives and emissions standards to encourage the use of new cleaner-burning diesel fuels. European vehicle manufacturers appear to be increasing their application of "green" diesel technology that captures significant amounts of particulates.

The study finds that diesel has safety advantages over natural gas, which is a more flammable and explosive fuel to handle and store. It finds that diesel has a short-term cost advantage, but that natural gas might end up with roughly the same costs if engines and refueling infrastructure become common.

For a complete copy of the report, please view the December 1999 issue of Risk in Perspective (PDF, 205 KB, PDF information).

The Harvard Center for Risk Analysis promotes a more reasoned response to health, safety, and environmental risks.

For further information, please contact:

Edmond Toy, lead author, 617-432-1566,

David Ropeik, Director of Risk Communications, 617-432-6011,