Diesel-fueled trucks and buses have served the nation’s freight and transportation needs for over half a century because of their reliability, durability and relative efficiency. This source category includes a wide variety of vehicles from 18-wheel tractor-trailer combinations, to school and transit buses to dump trucks and refuse haulers. These vehicles can be found in large numbers on major highways, urban streets and rural roads.
The exhaust from these heavy-duty diesel trucks and buses is a complex mix of thousands of gases and fine particles (commonly referred to as soot) that contains over 40 toxic air contaminants, including many known or suspected cancer-causing substances such as benzene, formaldehyde and arsenic. It also contains other harmful pollutants, including nitrogen oxides (NOx), sulfur oxides (SOx), particulate matter (PM), other toxic air pollutants, volatile organic compounds (VOCs), carbon monoxide (CO) and black carbon. These pollutants contribute to ground-level ozone (smog), particle pollution, haze, global warming, acid rain and eutrophication of water bodies.
Diesel exhaust and the components that comprise it are responsible for a range of serious adverse health effects. The fine particles emitted by diesel vehicles can contribute to mutation in cells that can result in cancer. Exposure to diesel exhaust can also contribute to premature death, particularly in people with heart and lung disease, and cause, among other things, reduced lung function, making normal breathing difficult; irritated airways, resulting in coughing, a scratchy or sore throat and shortness of breath; increased frequency of asthma attacks and aggravation of asthma and other chronic lung diseases such as emphysema and bronchitis; and increased susceptibility to respiratory infection. In addition, it can lead to more frequent doctor and emergency room visits, increased hospital admissions and lost work or school days. People with lung or respiratory diseases, children, older adults and people who are active outdoors may be especially susceptible to the ill effects of smog.
Historically, emission standards for diesel engines and vehicles have lagged behind those for gasoline engines and vehicles and were not truly a viable option until 2006 when ultra-low-sulfur diesel fuel became widely available. In 2000, EPA adopted a comprehensive national program to both regulate onroad heavy-duty diesel vehicles and their fuel, beginning with model year 2007. This rule reduced emissions from onroad heavy-duty diesel vehicles by 90 percent. In 2011, EPA and the U.S. Department of Transportation (DOT) collaborated on the first-ever regulations to reduce greenhouse gas emissions from and increase the fuel economy of medium- and heavy-duty trucks and buses. This federal program applies to tractor-trailer combination trucks, heavy-duty pickup trucks and vans, buses and refuse and utility trucks beginning with the 2014 model year. EPA estimates it will reduce carbon dioxide by about 270 million metric tons and save about 530 million barrels of oil over the life of model year 2014 through 2018 vehicles. In February 2014, President Obama directed EPA and DOT “to explore further opportunities for fuel consumption and emissions reductions beyond the model year 2018 timeframe” and to develop and issue, by March 2016, the next phase of fuel efficiency and GHG emission standards for medium- and heavy-duty vehicles. EPA and DOT's National Highway Traffic Safety Administration (NHTSA) issued a joint proposal, Greenhouse Gas and Fuel Efficiency Standards for Medium- and Heavy-Duty Engines and Vehicles - Phase 2, on July 13, 2015. Public hearings on this proposal were held on August 5, 11 and 18, 2015; public comments were accepted through September 2015. EPA and NHTSA also issued a Notice of Data Availability (NODA) related to the Phase 2 rule on March 2, 2016 and accepted public comments until April 1, 2016.
It is significant to note that, with good maintenance and engine rebuilding, many diesel engines last up to one million miles allowing older, dirtier engines to remain on the road for 15 to 30 more years. Therefore, various policies and practices have been put in place to ensure that diesel engines currently in use are maintained at the lowest emission levels possible and do not emit excessive smoke. Such programs include ones to retrofit diesel engines with emission control devices (e.g., diesel oxidation catalysts, diesel particulate filters and selective catalytic reduction), reduce diesel engine idling and require compliance with opacity limits.