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 reduces health risks from diesel exhaust.
This 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 IL-based GRI and conducted by Engine, Fuel, and Emissions Engineering Inc. (EF&EE), Sacramento CA. Studies included research by the Environmental Protection Agency (EPA), National Renewable Energy Laboratory (NREL), Society of Automotive Engineers, California Air Resources Board (CARB), 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. Results have been compiled into a single report, The Cleaner Choice: Natural Gas as a Substitute for Diesel (GRI-99/0261).
New data also show that particulate matter (PM) 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 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 to 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.
The debate over in-use emissions recently culminated in a lawsuit by the EPA, CARB, and the Department of Justice against seven major United States diesel engine manufacturers. The suit alleged that many of the alternative emission-control devices (AECDs) on the engines were designed to maintain low emissions during official emission tests while later advancing fuel injection timing under on-road driving. The suit also charged that these so-called "defeat devices" helped improve fuel economy and reduce soot contamination of oil during driving, but more than doubled nitrogen oxide emissions. In the largest legal settlement in the history of the Clean Air Act, manufacturers denied their AECDs were defeat devices, but agreed to phase them out and eliminate them from future engines.
Natural Gas Engine Advantages As the analysis indicates, natural gas engines also can be designed to achieve very low nitrogen oxide emissions, and do not appear to incorporate the kinds of AECDs that caused increases in nitrogen oxide emissions in diesel engines.
More than 60,000 natural gas transit buses, pickups, and vans already are in operation in the United States. Heavy-duty natural gas engine manufacturers have or are pursuing certification to CARB or EPA ultra-low emission vehicle (ULEV) standards. These engines will meet the 2004 heavy-duty engine standards when they become effective. In addition, some heavy-duty natural gas engines have been certified to even lower nitrogen oxide levels under California's emission credit program. The most recent models of heavy-duty natural gas pickups and vans have been certified to California's super-ULEV emission standard, with nitrogen oxide emissions less than 0.3 grams per mile and non-methane organic gas (NMOG) less than 0.06 grams per mile. This is less than most light-duty gasoline vehicles.
One of the criticisms of natural gas vehicles is cost, which the analysis indicates is usually higher than that of a comparable diesel vehicle. However, analysis also shows that costs have declined, 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 also is expected to increase diesel engine and fuel costs.
The GRI report summarizes a range of data from the review of studies on the effects of diesel pollutants on air quality and public health. Among the findings:
*Diesel engines are among the main sources in urban areas of particulate matter less than 2.5 micrometers in aerodynamic diameter (PM25). The EPA has found that PM25 significantly increases the risk of a number of diseases and health conditions such as asthma and chronic bronchitis. In 1997, the EPA established new National Ambient Air Quality Standards for PM25. The stricter standards were suspended by a federal court, and were criticized by the diesel industry as too strict. However, had the standards been implemented, they still would have allowed PM25 pollution in urban areas to reach levels that the EPA calculates would increase the death rate by more than 10% during pollution episodes.
*Preliminary results of a study by the South Coast Air Quality Management District of California indicate that 72% of the total cancer risk from toxic air contaminants in the district is attributable to diesel PM. Another 10% is due to 1,3 butadiene, 6% from benzene, and 12% from all other toxic air contaminants combined. Diesel and gasoline engines are primary emitters of 1,3 butadiene and benzene emissions in urban areas, whereas natural gas vehicles-because of their cleaner burning fuel-emit little of either carcinogen.
*CARB has classified diesel PM as a toxic air contaminant. Based on CARB risk factors and exposure data, EF&EE researchers calculate that the risk of lung cancer from the estimated average exposure to diesel PM in California is about 540 cases per million people. However, EPA data show that this cancer threat is only one of a number of risks from exposure to diesel PM. Other risks from PM25 exposure include pneumonia, asthma and other respiratory diseases, and greater risk of death from cardiopulmonary causes. After taking into account these other health risks, the EF&EE study calculates that the lifetime risk of premature death from exposure to diesel PM concentrations in California is about 4,250 cases per million, or one person in 235.
The GRI report is available free of charge and can be ordered from the GRI Document Fulfillment Center, 1510 Hubbard Dr, Batavia IL 60510; or by fax at 630-406-5995.