Air Quality and Health and Welfare

2.3 Particulate Matter

2.3.1 Introduction
Particulate matter (PM) represents a broad class of chemically and physically diverse substances that exist as discrete particles (liquid droplets or solids) over a wide range of sizes. Human-generated sources of particles include a variety of stationary and mobile sources. Particles may be emitted directly to the atmosphere or may be formed by transformations of gaseous emissions such as sulfur dioxide or nitrogen oxides. The major chemical and physical properties of PM vary greatly with time, region, meteorology, and source category, thus complicating the assessment of health and welfare effects as related to various indicators of particulate pollution. At elevated concentrations, particulate matter can adversely affect human health, visibility, and materials. Components of particulate matter (e.g., sulfuric or nitric acid) contribute to acid deposition (“Air Quality Criteria for Particulate Matter,” 1996).

The key health effects categories associated with PM include premature death; aggravation of respiratory and cardiovascular disease, as indicated by increased hospital admissions and emergency room visits, school absences, work loss days, and restricted activity days; changes in lung function and increased respiratory symptoms; changes to lung tissues and structure; and altered respiratory defense mechanisms. Most of these effects have been consistently associated with ambient PM concentrations, which have been used as a measure of population exposure, in a large number of community epidemiological studies. Additional information and insights on these effects are provided by studies of animal toxicology and controlled human exposures to various constituents of PM conducted at higher than ambient concentrations. Although mechanisms by which particles cause effects are not well known, there is general agreement that the cardio-respiratory system is the major target of PM effects.

Individuals with respiratory disease (e.g., chronic obstructive pulmonary disease, acute bronchitis) and cardiovascular disease (e.g., ischemic heart disease) are at greater risk of premature mortality and hospitalization due to exposure to ambient PM.

Individuals with infectious respiratory disease (e.g., pneumonia) are at greater risk of premature mortality and morbidity (e.g., hospitalization, aggravation of respiratory symptoms) due to exposure to ambient PM. Also, exposure to PM may increase individuals’ susceptibility to respiratory infections.

Elderly individuals are also at greater risk of premature mortality and hospitalization for cardiopulmonary problems due to exposure to ambient PM.

Children are at greater risk of increased respiratory symptoms and decreased lung function due to exposure to ambient PM.

Asthmatic individuals are at risk of exacerbation of symptoms associated with asthma, and increased need for medical attention, due to exposure to PM.

There are fundamental physical and chemical differences between fine and coarse fraction particles. The fine fraction contains acid aerosols, sulfates, nitrates, transition metals, diesel exhaust particles, and ultra fine particles and the coarse fraction typically contains high mineral concentrations, silica and resuspended dust. Exposure to coarse fraction particles is primarily associated with the aggravation of respiratory conditions such as asthma. Fine particles are most closely associated with health effects such as premature death or hospital admissions, and for cardiopulmonary diseases.

The strongest evidence for ambient PM exposure health risks is derived from epidemiologic studies. Many epidemiologic studies have shown statistically significant associations of ambient PM levels with a variety of human health endpoints in sensitive populations, including mortality, hospital admissions and emergency room visits, respiratory illness and symptoms, and physiologic changes in mechanical pulmonary function. The epidemiologic science points to fine PM as being more strongly associated with some health effects, such as premature mortality, than coarse fraction PM, which is associated with other health effects.

Time-series analyses strongly suggest a positive effect on daily mortality across the entire range of ambient PM levels. Relative risk (RR) estimates for daily mortality in relation to daily ambient PM concentration are consistently positive, and statistically significant (at P ? 0.05), across a variety of statistical modeling approaches and methods of adjustment for effects of relevant covariates such as season, weather, and co-pollutants.

Within the body of evidence, there is considerable agreement among different studies that the elderly are particularly susceptible to effects from both short-term and long-term exposures to PM, especially if they have underlying respiratory or cardiac disease. These effects include increases in mortality and increases in hospital admissions. Children, especially those with respiratory diseases, may also be susceptible to pulmonary function decrements associated with exposure to PM or acid aerosols. Respiratory symptoms and reduced activity days have also been associated with PM exposures in children.

Numerous time-series analyses published in the late 1980s and early 1990s demonstrate significant positive associations between daily mortality or morbidity and 24-hour concentrations of ambient particles indexed by various measures (black smoke, TSP, PM10, PM2.5, etc.) in numerous U.S. metropolitan areas and in other countries (e.g., Athens, São Paulo, Santiago). These studies collectively suggest that PM alone or in combination with other commonly occurring air pollutants (e.g., SO2) is associated with daily mortality and morbidity, the effect of PM appearing to be most consistent. In both the historic and recent studies, the association of PM exposure with mortality has been strongest in the elderly and for respiratory and cardiovascular causes of death.

Evidence from studies that looked at PM indicators other than PM10 also suggests that fine particles may be important contributors to the observed PM-health effects associations given the increased risks (of mortality, hospitalization, respiratory symptoms, etc.) associated with several different fine particle indicators (e.g., PM2.5, SO, H+). In particular, more recent reanalyses of the Harvard Six-City Study by Schwartz et al. examined the effects on daily mortality of 24-hour concentrations of fine particles (PM2.5), inhalable particles (PM15/10), or coarse fraction particles (PM15/10 minus PM2.5) as exposure indices. Overall, these analyses suggest that, in general, the association between excess mortality and thoracic particles appears to be stronger for the fine than the coarse fraction.

In addition to short-term exposure effects, mortality and morbidity effects associated with long-term exposure to PM air pollution have been assessed in cross-sectional studies and more recently, in prospective cohort studies. A number of older cross-sectional studies provided indications of increased mortality associated with chronic exposures to ambient PM (indexed mainly by TSP or sulfate measurements); however, unresolved questions regarding adequacy of statistical adjustments for other potentially important covariates tended to limit the degree of confidence that could be placed on such studies.

Some more recent studies used improved methods to examine relationships between chronic PM exposures indexed by different particle size indicators (PM15, PM2.5, PM15 to PM2.5). These studies observed associations between increased risk of mortality/morbidity and chronic (annual average) exposures to PM10 or fine particle indicators in contemporary North American urban air sheds.

Recently, the Health Effects Institute (HEI) released the results of two major studies that are central to the debate over the adverse impact of particulate on human health. The first report is a re-analysis of two long-term community health studies: the Harvard Six Cities Study (1993), and the American Cancer Society Study (1995). The second, called the National Morbidity, Mortality, and Air Pollution Study (NMMAPS), is original research on hospitalization and deaths associated with air pollution in major U.S. cities.