in ambient settings are not likely to have effects in healthy individuals but can advance the time of angina (chest pain) in people with coronary artery disease and may cause increased incidence of cardiac effects. Some recent epidemiologic studies have found relationships between increased CO levels and increases in mortality and morbidity.

  • Ozone is a gas formed in the atmosphere from combinations of nitrogen oxides and volatile organic compounds (both emitted from vehicles) in certain meteorological conditions normally found in the summer time. It is known to reduce the lung function of some individuals, and epidemiologic studies have found evidence of increased asthma attacks and hospitalization related to increased ambient levels. It may also increase the lung’s reaction to allergens and other pollutants. Although recent studies have found associations of daily increases in ozone with increased mortality, there is not comprehensive evidence that long-term exposure causes chronic health effects, and some evidence suggests that the lung may develop a form of tolerance after repeated short-term exposures.

  • Particulate matter (PM) in the form of PM10 and PM2.5 is material than can be inhaled that is emitted directly from motor vehicles and other sources, and that also formed in the atmosphere from atmospheric reactions with gaseous emissions (e.g., nitrogen oxides become nitrates). Although PM has been of concern for many decades, new short term and long-term epidemiologic studies published in the United States and Europe in the 1990s found associations of PM with increased mortality and morbidity at ambient levels below then-established national air quality limit values. It is these studies that have been the basis for recent action in both the European Union and the United States to establish more stringent standards for PM. These studies, some of which find the highest mortality effects for any air pollutant, also provide the basis for the estimates of population risk in Europe discussed above. In the past several years, several new epidemiologic studies have begun to strengthen the understanding of the relation between exposure to PM and mortality and morbidity. Diesel exhaust PM has been cited as a probable human carcinogen by several national and international bodies. Ultrafine particles (less than 0.1 microns), particles containing metals (e.g., iron) may be the most toxic components of the mixture. To date these studies have not identified one component or characteristic that is significantly more toxic than others.

  • Air toxins have a variety of characteristics and effects. Most of those emitted from motor vehicles are animal carcinogens. Benzene is a known human carcinogen. Butadiene, for which vehicles are the dominant ambient source, was recently designated as a probable human carcinogen by the International Agency for Research on Cancer, and as a known human carcinogen by the U.S. National Institutes of Health. Several aldehydes (including formaldehyde and acetaldehyde) have also been designated as probable human carcinogens. In addition, several of the mobile source air toxins, especially the aldehydes, have exhibited evidence of acute respiratory effects. Recently, the U.S. Environmental Protection Agency identified a total of 21 air toxins emitted from motor vehicle exhaust.

Trends and the Future. The U.S. Environmental Protection Agency took action in 1999 to further improve fuel formulation and reduce emissions of light duty vehicles, and has currently proposed stringent new fuel and emissions standards for heavy-duty vehicles. The European Union (EU) is on a similar path, which is expected to substantially reduce emissions over the coming 20 years. However, continued growth in travel is expected to offset a portion of these reductions. As a result, continued attention to reducing emissions is likely in the future.

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