Generally speaking, Congress has given EPA the authority to regulate emissions from newly manufactured heavy-duty diesel engines, and has left regulation of emissions from existing, or in-use, engines to the states and local government authorities.

Federal Regulation

In recent years, EPA has adopted a "systems approach" to the regulation of new diesel engines, regulating the emissions and the fuel together. This makes sense because the removal of sulfur from diesel fuel is necessary to allow the efficient use of highly effective after-treatment emission control devices.

Recent EPA regulations will:

• Reduce nitrogen oxides (NOx) and fine particulate matter PM_2.5 from new heavy-duty highway diesels (e.g., trucks and buses) by about 90%, effective in 2007 for PM , and 2007-2010 for NOx.
• Reduce the sulfur content in highway diesel fuel to 15 ppm ("ultra-low sulfur diesel" fuel, or "ULSD" fuel) beginning in late 2006.
• Reduce nitrogen oxides (NOx) and fine particulate matter PM_2.5 from new heavy-duty nonroad diesels (e.g., construction, farming and logging equipment) by about 90%, effective in the 2011-2014 time frame depending on the pollutant and the size of engine.
• Reduce the sulfur content in land-based nonroad diesel fuel in two steps, to 500 ppm in 2007 and 15 ppm beginning in 2010.
• Reduce the sulfur content in diesel fuel used in new locomotive and many marine engines in two steps, to 500 ppm in 2007 and 15 ppm beginning in 2012.

EPA has also recently announced its intention to commence a rulemaking designed to substantially reduce emissions from new locomotives and many marine engines, taking advantage of the highly effective after-treatment control devices made possible by the availability of ULSD fuel.

CATF has participated in many of the proceedings that led to these rules, urging EPA to adopt the most stringent limits achievable with reasonable, cost-effective approaches. CATF will continue to stay involved in this important regulatory arena.

State and Local Regulation

Although recent EPA rulemaking will eventually reduce diesel emissions substantially, that will take some time. The federal rules affect only new engines, and due to the long lives of heavy-duty diesels – 30 years or more in many cases – substantial turnover of the existing diesel fleet will not likely occur until for 2 or 3 decades after the rules become effective in the 2007-2014 time period. Most of these existing diesels are quite dirty, and it is a public health imperative to clean them up before another generation or two is exposed to this pollution.

The task of regulating diesel emissions from the existing fleet of heavy-duty highway, nonroad, marine and locomotive diesels largely falls on state and local authorities. California has clear authority to regulate emissions from all types of heavy-duty diesel engines, subject to EPA approval, the limits of California's jurisdictional reach and so long as it does not unduly interfere with or discriminate against interstate commerce. Other states also have clear authority to regulate emissions from in-use highway diesels. However, their authority to regulate existing nonroad, marine and locomotive emissions is presently limited – in general, they are free to copy California emission standards for nonroad engines, but it is not clear that they may adopt separate and different standards.

Therefore, California action on reducing diesel emissions is important, not only for California but also for other states and localities. Fortunately, California is leading the way in regulating in-use diesel emissions, based on its finding that diesel exhaust is a potent air toxic, and on its resulting Diesel Risk Reduction Plan. The Plan recommends that the state adopt retrofit requirements for most types of existing highway and nonroad mobile diesels and stationary diesel engines, in conjunction with ultra-low sulfur diesel fuel. Thus far, the California Air Resources Board has enacted regulations requiring many in-use urban buses and waste collection trucks, as well as stationary and portable diesel engines (used as generators, etc.) and transport refrigeration units, to substantially reduce NOX and PM emissions. Regulations reducing emissions from other California in-use highway, non-road, marine and locomotive diesels are planned.

There is limited activity in other states presently, but CATF is actively working with its partners in a dozen states to advocate the reduction of emissions from heavy-duty diesel engines through both voluntary and regulatory measures.

For more information, see CATF's Fact Sheet, Diesel Engines: Emissions Sources and Regulations, and the CATF report Diesel and Health in America and interactive companion web.

CATF Contact: David Marshall