Of the suite of pollutants that taint urban air, fine suspended particulate matter, sulfur dioxide (SO2), and ozone pose the most widespread and acute risks; however, airborne lead pollution is a critical concern in many cities as well. Recent studies on the effects of chronic exposure to air pollution have singled out particulate matter as the pollutant most responsible for the life-shortening effect of dirty air, although other pollutants may also play an important role.
Suspended particulate matter is a nearly ubiquitous urban pollutant. Although particulate levels in North America and Western Europe rarely exceed 50 micrograms of particulate matter per cubic meter (µg/m3) of air, levels in many Central and Eastern European cities and in many developing nations are much higher, often exceeding 100 µg/m3 (1).
Particulate air pollution is a complex mixture of small and large particles of varying origin and chemical composition. Larger particles, ranging from about 2.5 microns to 100 microns in diameter, usually comprise smoke and dust from industrial processes, agriculture, construction, and road traffic, as well as plant pollen and other natural sources. Smaller particles those less than 2.5 microns in diameter generally come from combustion of fossil fuels. These particles include soot from vehicle exhaust, which is often coated with various chemical contaminants or metals, and fine sulfate and nitrate aerosols that form when SO2 and nitrogen oxides condense in the atmosphere. The largest source of fine particles is coal-fired power plants, but auto and diesel exhaust are also prime contributors, especially along busy transportation corridors.
The health effects of particulates are strongly linked to particle size. Small particles, such as those from fossil fuel combustion, are likely to be most dangerous, because they can be inhaled deeply into the lungs, settling in areas where the body’s natural clearance mechanisms can’t remove them. The constituents in small particulates also tend to be more chemically active and may be acidic as well and therefore more damaging (2).
Numerous studies associate particulate pollution with acute changes in lung function and respiratory illness (3)(4), resulting in increased hospital admissions for respiratory disease and heart disease, school and job absences from respiratory infections, or aggravation of chronic conditions such as asthma and bronchitis (5)(6). But the more demonstrative and sometimes controversial evidence comes from a number of recent epidemiological studies. Many of these studies have linked short-term increases in particulate levels, such as the ones that occur during pollution episodes, with immediate (within 24 hours) increases in mortality. This pollution-induced spike in the death rate ranges from 2 to 8 percent for every 50-µg/m3 increase in particulate levels. These basic findings have been replicated on several continents, in cities as widely divergent as Athens, São Paulo, Beijing, and Philadelphia (7)(8)(9). During major pollution events, such as those involving a 200-µg increase in particulate levels, an expert panel at the World Health Organization (WHO) estimated that daily mortality rates could increase as much as 20 percent (10). These estimates should be viewed with caution, however, because some of those who die during a pollution episode were already sick,and the pollution may have hastened the death by only a few days.
In the aggregate, pollution-related effects like these can have a significant impact on community health. WHO has identified particulate pollution as one of the most important contributors to ill health within Europe.In those cities where data on particulates were available, WHO estimated that short-term pollution episodes accounted for 7 to 10 percent of all lower respiratory illnesses in children, with the number rising to 21 percent in the most polluted cities. Furthermore, 0.6 to 1.6 percent of deaths were attributable to short-term pollution events, climbing to 3.4 percent in the cities with the dirtiest air (11).
Nor are health effects restricted to occasional episodes when pollutant levels are particularly high. Numerous studies suggest that health effects can occur at particulate levels that are at or below the levels permitted under national and international air quality standards. In fact, according to the WHO and other organizations, no evidence so far shows there is a threshold below which particle pollution does not induce some adverse health effects, especially for the more susceptible populations (12)(13). This situation has prompted a vigorous debate about whether current air quality standards are sufficient to protect public health.
SO2 is emitted largely from burning coal, high-sulfur oil, and diesel fuel. Because this gas is usually found in association with particulate pollution as SO2 is the precursor for fine sulfate particles separating the health effects of these two pollutants is difficult. Together, SO2 and particulates make up a major portion of the pollutant load in many cities, acting both separately and in concert to damage health.
Although ambient concentrations of SO2 have declined in many cities in Western Europe and North America, they remain higher often by a factor of 5 to 10 in a number of cities in Eastern Europe, Asia, and South America, where residential or industrial coal use is still prevalent and diesel traffic is heavy (14).