The Health Effects Institute
	PAPA-SAN Public Health and Air Pollution in Asia: Science Access on the Net
Pollutants Commonly Studied Benzene A clear, colorless, volatile, highly flammable liquid with a characteristic odor. It is found in gasoline fumes, vehicle emissions, and tobacco smoke. Benzene is derived from petroleum and is used extensively as a raw material or solvent in many manufacturing processes. Benzene is considered a known human carcinogen by national and international agencies, including the U.S. Environmental Protection Agency and the World Health Organization’s International Agency for Research on Cancer (IARC). Carbon Monoxide (CO) A colorless, odorless gas emitted directly from motor vehicles and other combustion sources. When inhaled, it replaces oxygen in the bloodstream by binding with hemoglobin, thus interfering with normal transport of oxygen to the heart and brain. Exposure to high CO levels may be lethal; low levels found in ambient settings are not likely to affect healthy people but can hasten the onset of angina (chest pain) in people with coronary artery disease and can increase the incidence of cardiac effects. Some epidemiologic studies have found positive relationships between CO level and morbidity, mortality, and adverse pregnancy outcomes. Lead (Pb) A soft, heavy, malleable metal, found naturally in the environment and used in the manufacture of batteries, metal products, paints, and ceramic glazes. Lead is toxic, causing a variety of acute and chronic effects on the central nervous, cardiovascular, gastrointestinal, and urinary systems. Long-term exposure is particularly harmful to cognitive development in children, with reduced growth and other effects also reported. Maternal lead exposure adversely affects birth weight and postnatal neurobehavioral development.  Historically, most ambient lead emissions were produced by motor vehicles and industrial activities (e.g., mining, smelting, and manufacturing). Today, lead in gasoline is banned in many Asian countries and throughout North America and Europe, but its use persists in some parts of Central Asia and Africa. Nitrogen Oxides (NOx) Nitrogen oxides make up a group of highly reactive gases containing nitrogen and oxygen in various proportions, of which nitrogen dioxide (NO2) is the most commonly measured. NO2 is a gaseous by-product of the combustion of fossil fuels in transportation and industrial activities such as waste incineration. In many contemporary urban locations, mobile-source emissions are the chief source of outdoor NO2. NO2 is relatively insoluble and is thus more likely than some other combustion-derived gases, such as SO2, to be deposited in the lower airways. NO2 is also an oxidant. In some experiments in humans, exposure has elicited inflammatory responses at concentrations as low as 1 mg/m3 and has increased responsiveness to ozone (O3, another oxidant gas) and certain allergens, but these responses vary widely among individuals. In epidemiologic studies, NO2 has been associated with increased respiratory morbidity (e.g., exacerbation of asthma and reduced lung function and rate of lung growth in children). Short-term exposure to NO2 is associated with increased daily mortality and hospital admissions due to respiratory and cardiovascular disease. Ozone (O3) A gas formed in the atmosphere from the combination of NOx and volatile organic compounds (both emitted from industrial facilities and motor vehicles) under certain meteorologic conditions, usually during summer. O3 reduces lung function in some individuals. Epidemiologic studies have found evidence that increased asthma attacks and hospitalizations are related to increased ambient O3 levels. O3 might also increase the lung’s reaction to allergens and other pollutants. Although some recent studies have found associations between daily increases in O3 and increased mortality, evidence that long-term exposure to O3 causes chronic health effects is limited. Particulate Matter (PM) Particulate matter (PM), in the form of PM 10 µm or less and 2.5 µm or less in aerodynamic diameter (PM10 and PM2.5, respectively), is inhalable material that is emitted directly from motor vehicles, power plants, and other sources or formed in the atmosphere through reactions with gaseous emissions (e.g., nitrogen and sulfur oxides react to form nitrates and sulfates, respectively). PM is also sometimes measured as total suspended particles (TSP), a mixture that contains larger, non-inhalable particles, as well as the smaller inhalable ones. Although the health effects of PM have been a matter of concern for many decades, short-term and long-term epidemiologic studies published in the United States and Europe in the 1990s found associations of PM with increased morbidity and mortality at ambient levels below the maximal values in national air quality regulations at the time. These results have been the basis of action in both the European Union and the United States to establish more stringent standards for PM. Sulfur Dioxide (SO2) A gaseous by-product of the combustion of fossil fuels that contain sulfur. These include solid fuels (such as certain coals), liquid fuels (such as gasoline, diesel, and fuel oil), and natural gas. Clinical studies have found that exposure to SO2 at levels as low as 0.25 parts per million elicits increased bronchoconstriction in people with asthma, and reductions in lung function have been observed at higher concentrations. SO2 is also associated with increased daily mortality and hospital admissions due to respiratory and cardiovascular disease, even at the low levels now observed in Europe and North America. Long-term exposure to SO2 has been associated with reduced pulmonary function and increased mortality from cardiovascular and respiratory disease. Reductions in ambient SO2 concentrations owing to regulatory action have recently been associated with decreased mortality and improved respiratory health in children. Read more about study designs and abbreviations commonly used in air pollution research.
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