Particulate matter (PM)
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.
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, Sao 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.
Other studies that looked at PM indicators other than PM10 also suggest 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.
In particular, more recent reanalysis 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. 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.
A. Particle Epidemiology Reanalysis Project
The Harvard Six City Study and the American Cancer Society study examined the long-term effects of exposure to particulate air pollution on mortality.
The Harvard Six-Cities Study by Dr. Douglas Dockery of the Harvard School of Public Health, and others, was published in December 1993 in the New England Journal of Medicine. Researchers followed the health of more than 8,000 people in six small cities that fell along a gradient of air pollution concentrations for a period of 14 to 16 years.
As particle concentrations increased, there was an almost directly proportional increase in the death rate in the residents studied. Residents of the most polluted city in the study, Steubenville, Ohio, had a 26 percent increased risk of premature mortality, compared to the residents of the cleanest city studied, Portage, Wisconsin. According to study authors, this translates into a shortened life expectancy of one to two years for residents of Steubenville compared to residents of Portage.
The March 1995 American Cancer Society study, by Dr. Arden Pope of Brigham Young University, and others, found an association between chronic exposure to fine particle air pollution and premature death in a study group of over half a million people in 151 cities. Sulfate pollution was also associated with early death. The study reported strong associations between sulfates and fine particles and death by cardio-pulmonary causes.
These original studies used statistical techniques to adjust for age, and to control for the effects of smoking, diet, and occupational exposure.
Dr. Daniel Krewski of the University of Ottawa and his associates conducted the newly released reanalysis of these two studies for the Health Effects Institute. First, the HEI - funded researchers undertook a reanalysis of the original studies and a quality audit of the underlying data. Second, researchers performed an extensive sensitivity analysis using alternative statistical methods, and considering the role of 20 potential confounders such as other pollutants, climate, and socio-economic factors on study results.
The reanalysis by independent investigators validates the original studies.
B. The National Morbidity, Mortality and Air Pollution Study (NMMAPS)
The Health Effects Institute also commissioned an original nationwide study of the short-term effects of air pollution on human health in the 90 largest American cities.
A team of investigators led by Dr. Jonathan Samet and Dr. Scott Zeger of the Johns Hopkins University School of Hygiene and Public Health examined short-term increases in mortality rates caused by short-term elevations in particulate air pollution. Harvard School of Public Health researchers Dr. Douglas Dockery and Dr. Joel Schwartz studied effects on hospitalization in a subset of these cities. NMMAPS developed a new standardized methodology for examining pollution effects across many cities. Investigators developed state-of-the-art statistical techniques to examine the effects of multiple pollutants and the extent of life shortening.
Some critics have argued that short-term increases in the death rate are unimportant because the individuals affected are very frail and near death, even in the absence of air pollution. NMMAPS dispels this "harvesting" notion. NMMAPS investigators report that life is not shortened by a matter of days, but that life shortening is on the order of months or more.
Critics have also argued that other pollutants may be responsible for observed health effects. NMMAPS found strong evidence linking daily increases in particulate pollution to increases in death, in the twenty largest U.S. cities. The association between particulate matter and mortality persisted even when other pollutants were included in the analysis.
In addition, NMMAPS found stable and robust associations between particulate pollution and increased hospital admissions for cardiovascular disease, pneumonia, and chronic obstructive pulmonary disease.
In an abstract prepared for the HEI Annual Conference, investigators concluded "these complementary analyses of mortality and morbidity provide new and strong evidence linking particulate air pollution at current levels to adverse health effects."
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