All of us have seen the thick black smoke that belches from the exhaust pipes of large diesel-fueled buses and trucks on our streets and highways. The smoke stains the sky and spreads a cloud of fine black dust. Our instinctive reaction is to avoid inhaling these emissions -- which is a good instinct. Evidence from scientific studies attests to the probable adverse health and environmental effects that result from this type of air pollution.

Heavy-duty diesel exhaust is a significant contributor to air pollution in Vermont and throughout the Northeast. These emissions account for 33 percent of all highway vehicle nitrogen oxides and 78 percent of all transportation-related fine particulate matter emissions in the northeastern United States. (See Figures 1 and 2) Heavy-duty diesel emissions of fine particulate matter, nitrogen oxides, and volatile organic compounds contribute to exceedances of health-based air quality standards for ground-level ozone across the region and may also contribute significantly to violations of new health-based particulate matter standards in urban areas of the Northeast.

The specific composition of heavy-duty diesel engine emissions is particularly worrisome. The volatile organic compounds in these emissions include a number of cancer-causing gaseous compounds such as formaldehyde, benzene, and 1,3-butadiene, all of which have been shown by monitoring in our urban centers to exceed Vermont's health-based exposure standards. The ultra-small size of the carbon soot particles also emitted in diesel exhaust makes it easy for these compounds to enter human lungs through normal breathing. A July 2000 health assessment of diesel exhaust released in draft form by the U.S. Environmental Protection Agency concludes that diesel exhaust "is likely to be carcinogenic to humans by inhalation at any exposure condition."

The most recent assessments cited by EPA indicate that exposure conditions in urban areas of our nation can be up to four times greater than in rural areas. The Vermont Air Pollution Control Division has analyzed fine particulate matter monitoring data to compare exposure to diesel exhaust emissions in downtown Burlington against exposure in rural locations. (See Figure 3) Data obtained over a full sampling year clearly show higher average annual exposure to both organic matter (organic carbon) and carbon soot (elemental carbon) at the downtown location. Analysis of the composition of these samples provides strong evidence that the urban excess is likely due to the higher levels of diesel exhaust emitted in the downtown.

The EPA recently proposed a comprehensive national control program which would regulate heavy-duty diesel vehicles and diesel fuel itself. The proposed standards for exhaust emissions, which would not take effect until 2007, are premised on the use of high-efficiency catalytic exhaust emission control devices or comparable technologies. EPA has also proposed to reduce the level of sulfur in highway diesel fuel by 2006 because it is known that sulfur damages catalytic converters. If enacted, these regulations would reduce emissions from new heavy-duty diesel engines by 90 to 95 percent from current levels.

The Vermont Air Pollution Control Division Mobile Sources Section, in cooperation with the Vermont Department of Motor Vehicles, has been collecting data on heavy-duty diesel smoke for several years. The testing program to date has operated as a voluntary pilot program intended to raise awareness of cost benefits of vehicle engine maintenance to the trucking industry and to collect data. Correctly characterizing these emissions is important because although future heavy-duty diesel vehicles will be manufactured to meet much more stringent emissions standards, there is no federal provision requiring maintenance of vehicles already on the road. The smoke-testing program is one way for Vermont to enforce compliance with these new standards. The Vermont pilot program has established that this approach is not only feasible, but that it could be highly effective if mandatory repairs and maintenance were a part of an ongoing program.

In the period from October 1996 to October 1998, 247 trucks and buses were tested in Vermont. Data from this project indicate that there is a significant number of excessively smoking trucks traveling on Vermont's roads. The research found that Vermont-registered heavy-duty diesel vehicles generally have higher average opacity and higher failure rates for the smoke test than their non-Vermont registered counterparts. (See Figure 4) One contributing factor may be that Vermont is one of the few states in the Northeast that does not require maintenance and repair of smoking heavy-duty diesel vehicles.

The Agency of Natural Resources and the Department of Motor Vehicles in 2001 will jointly advocate for legislation to establish a mandatory program for heavy-duty diesel smoke testing in Vermont. There are several options for implementing such a program:

• Routine annual or biennial smoke testing performed by state-certified inspection facilities;
• Random roadside testing of in-use vehicles associated with DMV safety checks; or
• Routine smoke testing at inspection facilities and random roadside testing for in-use compliance.

The decision as to which option to pursue will involve more analysis and additional involvement by the stakeholders.