Press Releases

2003 Releases

CNG Yields More Health Improvement Than 'Clean Diesel' for Urban Buses

For immediate release:  April 03, 2003

Boston, MA - An analysis of the two most popular alternative propulsion technologies for reducing air pollution from urban transit buses finds that compressed natural gas (CNG) affords a third more health benefits than so-called “clean diesel”, but that the cost per unit of health improvement is six to nine times higher for CNG than for emission controlled diesel (ECD). The analysis was done by the Harvard Center for Risk Analysis, part of the Harvard School of Public Health. It appears in the current issue of the journal Environmental Science and Technology.


The study, led by Senior Research Associate Joshua Cohen, measures the public health damages of air pollution from urban transit buses in units of Quality Adjusted Life Years (QALYs). It finds that new ECD buses reduce health damages by 40%, and that new CNG buses cuts health damages by 55%, compared with new conventional diesel buses. Both CNG and ECD reduce emissions of fine particles by about 75%. CNG has a further health advantage because it also reduces emissions of NOx, a gas that contributes to ground level ozone, or smog, and to the formation of fine particles.


But the cost per QALY saved using CNG would be 6 to 9 times greater than for ECD because of the higher cost of acquiring and maintaining CNG vehicles, installing and maintaining infrastructure to fuel them, and paying more for fuel to run them.


The study finds that both ECD and CNG buses generate more greenhouse warming gas emissions than conventional diesel buses. However, the incremental impact is small compared to the baseline emissions generated by conventional diesel buses. Furthermore, the possibility that diesel exhaust causes cancer has little effect on the results because the estimated total mortality from fine particles dwarfs any effect from cancer.


Cohen and co-authors James Hammitt and Jonathan Levy note that the study does not account for the safety risk of CNG, which must be stored at high pressure and is readily ignitable. Nor does it consider some of the drawbacks of diesel technology, including a strong odor and noisier operation. In addition, the authors point out that their analysis requires assumptions about many uncertain factors, and that their modeling of population exposure and health effects also reflects a number of simplifying assumptions.


But the authors note that their analysis is “…the first to compute and compare aggregate incremental costs and health benefits for bus propulsion technologies.” “These first order ball-park estimates of the costs and benefits of these alternative propulsion systems provide an important way to think about the pros and cons of different ways to address this important environmental issue,” said Cohen.


The study was funded by the International Truck and Engine Corporation. The work was informed by an advisory panel of 18 academic, industry, and government experts, including five senior managers of public transit authorities from across the United States.

For further information, please contact:

Joshua Cohen, Ph.D.

617 432-0394

David Ropeik

Director of Risk Communication

Harvard Center for Risk analysis

617 432-6011