An analysis of more than 40 studies on transportation fuels has found strong evidence that substituting natural gas for diesel in heavy-duty trucks and buses significantly reduces the health risks from diesel exhaust.

The analysis of government, private industry and university research on emissions from heavy-duty diesel and natural gas vehicles and their impact on air quality and public health was sponsored by Chicago-based GRI and conducted by Engine, Fuel, and Emissions Engineering Inc., Sacramento, CA.
More than 40 studies were examined, including research by the U.S. Environmental Protection Agency, National Renewable Energy Laboratory, Society of Automotive Engineers, California Air Resources Board, and the South Coast Air Quality Management District. Also reviewed were studies by the mechanical engineering departments at the University of Minnesota, Colorado School of Mines, and West Virginia University, as well as related congressional testimony and court records.
"The analysis refutes contentions by some that natural gas vehicles are unnecessary because a new generation of 'clean diesel' engines emit fewer pollutants," said Rajeana Gable, GRI program manager. "Study after study documents that diesels in actual use still emit large amounts of pollutants. While gasoline engines for passenger cars have achieved impressive emission reductions during the past 20 years, diesel engines in new heavy-duty vehicles continue to produce high levels of pollution. In fact, research shows that new diesel transit buses still emit 80 times the nitrogen oxides and 60 times the particulate matter emitted by today's gasoline-powered vehicles even though they easily pass applicable standards."
Newly available data also show that particulate emissions from diesel engines in heavy-duty vehicles in-use consistently exceed certification standards. Data from a series of tests by the NREL and the U.S. Department of Energy show that diesel trucks with engines certified to the current PM standard of 0.1 gram per brake horsepower-hour (g/BHP-hr) had average in-use PM emissions of 0.23 g/BHP-hr -- more than twice the standard. Many trucks were emitting 0.4-0.6 g/BHP-hr by the time they had accumulated 200,000 miles. The NREL data show that actual average natural gas engine emissions of vehicles in-use were well below the standard at 0.016 g/BHP-hr for both buses and trucks.
One of the criticisms of natural gas vehicles is cost, which the analysis indicates is usually higher than the price of a comparable diesel vehicle. However, the analysis also shows that costs have declined in recent years, and that the cost differential is likely to be reduced further in the next few years as economies of mass production reduce natural gas engine and cylinder prices. Compliance with the 2004 emission standards is also expected to increase diesel engine and fuel costs.
The GRI report is available at no charge and can be ordered directly from the GRI Document Fulfillment Center, 1510 Hubbard Drive, Batavia, IL 60510, by fax at (630) 406-5995.