Particulate Matter

Overview. Particulate matter (PM) refers broadly to particles suspended in the air, emitted from numerous sources that vary widely in size and chemical composition. As such, PM is best understood as a complex class of air pollutants that differs in many ways from other ubiquitous gaseous air pollutants. Sources of PM are both primary and secondary in origin. Primary processes include sources that directly emit PM into the atmosphere, such as mobile sources, forest fires, and metallurgical operations. Secondary PM processes are those involving the formation of particles through atmospheric chemical reactions and include gases emitted from power plants, several biogenic sources as well as mobile sources.

The scientific community typically differentiates PM into several size-based categories: coarse (particle with diameters between 2.5 and 10 microns), fine (particles with diameters between 0.1 and 2.5 microns), and ultrafine (particles with diameters less than 0.1 microns) PM. As a reference of comparison, the average diameter of a human hair is approximately 70 microns 1. Classifying PM by size is useful from an environmental health standpoint, since the fate and transport of these particles, both in the atmosphere as well as in human respiratory tract, are largely governed by its size 2. Much recent attention has been focused on both fine and ultrafine particles, whose small size allows these particles to penetrate deep into the alveoli, or gas exchange regions of the lung. While much of fine particulate matter (PM2.5) originates from the combustion of fossil fuels, numerous other sources exist. Due to the diverse nature of these sources, such cars, trucks, industrial processes and fire, outdoor PM2.5 is comprised of a variety of chemical species, including elemental and organic carbon, sulfates, nitrates and metals. Indoor sources of particulate matter can also constitute a major component of exposure for some individuals. PM from indoor cooking, for example, is a growing concern for millions of people living in developing countries that use wood and other biomass resources for their primary cooking fuel (WHO, 2002).

Health Effects. Associations between outdoor PM concentrations and a variety of acute and chronic adverse health outcomes have been well documented in epidemiological and toxicological studies 3-6, with results indicating that PM2.5 is the particulate size fraction most strongly associated with these observed effects6-8. In response to these findings, the U.S. Environmental Protection Agency, the World Health Organization and other environmental regulatory agencies have adopted new standards regulating outdoor concentrations of PM2.5 as well as the previously-regulated larger class of particulate matter, PM10. Notably, these standards are based on total mass concentrations and do not regulate either emissions or ambient concentrations of specific PM components or sources. To date, it is unclear whether the biological relationships between PM and health are due to specific chemical components of PM, total mass or PM size 5.

Numerous adverse health outcomes have been reported to be associated with PM of various sizes including increased hospital admissions and emergency room visits for respiratory and cardiovascular conditions, exacerbation of asthma symptoms, decreased lung function, increased lung cancer and premature deaths3, 4, 6, 9, 10. Several large, population-based studies of chronic health effects have estimated that increases 10 g/m3 of PM2.5 are associated with increased all-cause mortality rates ranging from 6 to 16% 4, 11, with higher observed rates for cardiopulmonary deaths specifically. Importantly, as PM2.5 is still a broad means of characterizing particulate air pollution, current research has shifted towards identifying the specific chemical components and sources of PM responsible for the observed health effects 5. Indoor air pollution, especially in developing countries, has also been implicated as a major contributor to global morbidity and mortality rates. In its 2002 report characterizing the burden of global diseases, the World Health Organization estimated about 35.7% of lower respiratory infections, 22.0% of chronic obstructive pulmonary disease and 1.5% of trachea, bronchus and lung cancer is due to pollution from indoor cooking sources, which are typically dominated by PM12. Studies examining susceptibility to PM health effects indicate that specific sub-populations may be at greatest risk, including individuals with preexisting heart or lung disease, diabetics, older adults, and children5.

Until recently, little was known concerning the biologic basis for the observed health effects, especially for those results indicating associations between PM and non-respiratory health effects. Although the exact mechanisms and pathways responsible for PM-mediated health effects remain uncertain, several plausible hypotheses have emerged from both controlled and observational studies13-17. There is evidence, for example, that exposures to PM2.5, or a component of PM2.5, are associated with oxidative stress in the lung leading to acute and chronic elevation of pulmonary and systemic blood inflammation and coagulability markers14, 18. The resulting inflammation may, in turn, progress to impaired lung and vascular function, atherosclerosis and heart attack. Likewise, findings have linked PM2.5 with altered autonomic function affecting both heart rate and heart rate variability (HRV)19-23. It is hypothesized that, for some individuals, altered autonomic function may lead to arrhythmias and heart attacks.

Finally, several studies have shown that controlled PM2.5 exposures are related to endothelial dysfunction and/or acute arterial vasoconstriction24-27. Researchers have speculated that sudden vasoconstriction may trigger plaque instability in individuals with preexisting atherosclerosis 25.

Trends. In many parts of the U.S. and developed world, the evidence linking outdoor and indoor PM with adverse cardiopulmonary health has led to efforts aimed at reducing PM exposures. Scrubbers, electrostatic precipitators and baghouses are technologies have been used to reduce PM emissions at pollution point sources, such as power plants and metal refineries. In addition, limits on the emissions of PM gaseous precursors, including sulfur dioxide from coal fired power plants, has also contributed to reductions. Despite these encouraging trends, there is convincing evidence to suggest that excess morbidity and mortality is still occurring at and below the current international standards 3, 28, 29. It is also worth noting that in many parts of the developing world, opposite trends are occurring for PM levels. Recent annual averages of PM10 in Delhi and Beijing, for example, were shown to be around 150 ug/m3, or three times EPA’s national standard for annual PM10 levels30.

Reducing PM, Protecting Health: The Harvard Six Cities as a Case Study.

Among the most influential studies examining the link between long-term exposure to PM and human health has been the Harvard Six Cities (HSC) study6. The HSC study measured air pollution levels in six northern and Midwestern US cities between 1974 and 1989 and examined corresponding health effects for a cohort of over 8,000 individuals. Among the findings from the initial HSC analyses was that differences in PM2.5 concentrations was associated with a 26% higher mortality rate in the city with the highest PM2.5 levels (Steubenville, OH) as compared to the city with the lowest PM2.5 concentrations (Portage, WI). Subsequent analyses of the HSC data has provided insight on the specific sources of PM2.5 responsible for the observed health effects, suggesting that PM2.5 from mobile and coal combustion sources were most harmful31. In the eight years following the initial HSC analyses, PM levels decreased in the Six Cities consistent with trends occurring in much of the U.S. In Steubenville, for example, the average annual PM2.5 concentration dropped from 29 to 22 ug/m3 between the initial study period and the eight-year follow-up period. This decline afforded researchers with the rare opportunity to assess whether improvements in outdoor air quality were associated with corresponding decreases in adverse health. A recent analysis comparing the HSC results during the initial and follow-up periods found that decreases in PM2.5 concentrations of 10 ug/m3 were associated with reduced risks of 27% for mortality across the cities11, providing strong support for policies aimed at reducing emissions and exposure to PM pollution.