On the Air

The ARIES Study

Questions About the Health Effects of Fine Particulates

The U.S. Environmental Protection Agency (EPA) sets the National Ambient Air Quality Standards (NAAQS) for particulate matter (PM). Total suspended particles (TSP) of 100 micrometers (µm) or less in diameter was the PM standard for the 1970s. Beginning in 1987, however, the standard was revised to include, as part of its basis, PM₁₀ (PM of 10 µm or less in diameter). Over the past quarter century, both TSP and PM₁₀ concentrations in the air have been reduced dramatically throughout the United States. The NAAQS for TSP was eventually cancelled, leaving PM₁₀ as the PM standard. Currently, the entire Tennessee Valley meets the PM₁₀ standard. In 1997, EPA revised the NAAQS for PM to include fine particles, defined as being less than or equal to 2.5 µm in diameter, and usually referred to as PM_2.5.

As measurement capabilities and the quality of health studies improved in the 1990s, concerns were raised about the potential health impacts of ambient PM_2.5. PM_2.5 are particles small enough to be inhaled and retained deep in the lungs. Also, a number of environmental health studies have indicated an association between ambient PM_2.5 levels and various measures of human health. One study that showed such an association—the Harvard Six Cities Study—has been widely distributed and discussed. This study included measurements from the Kingston-Harriman, Tennessee, area of the Tennessee Valley. Data from this study and others were used to make projections of “premature” mortality that was assumed to result from levels of PM_2.5 in ambient air. But these “premature” mortality projections were based on questionable assumptions. Further scientific research was needed to determine the extent to which these assumptions were valid and to reduce the uncertainty of health impact projections. Some of the more questionable of these assumptions are listed below.

Common Questionable Assumptions Made about Fine Particles:

1. PM_2.5 Mass Is the Primary “Cause” of Adverse Health Impacts. In most studies that found a significant statistical association between PM_2.5 levels and health effects, only total ambient PM_2.5 mass was related to “premature” mortality. This was true, in great part, because only PM_2.5 mass data were available. However, elevated PM_2.5 levels often are associated with hot, humid, and hazy weather conditions. Under such “stagnant” weather conditions, the levels of many other air pollutants increase along with PM_2.5 levels. If only PM_2.5 levels are measured, any adverse impacts associated with the other elevated air pollutants may be unjustifiably attributed to PM_2.5. This circumstance makes it impossible to accurately separate the individual health effects of a variety of pollutants. Also, several of these studies used a statistical model—Generalized Additive Model (GAM)—under conditions in which the risk factors for PM_2.5 were calculated incorrectly. Subsequent reanalysis of the data determined that many of the associations between PM_2.5 levels and health had been over-predicted by approximately a factor of two, and many previously significant associations between PM₁₀ or PM_2.5 and health endpoints were, in fact, no longer significant.
2. All PM_2.5 Components Are Equally Toxic. EPA’s adoption of a mass-based standard implies that all fractions of PM_2.5 mass are equally toxic. However, this is not the case. Ambient PM_2.5 is a complex mixture of many chemical compounds. Toxicological studies of various particulate constituents confirm that differing components of PM_2.5 mass have vastly different biological impacts. For example, fine particulate ammonium sulfate (often a secondary product of coal combustion, and the largest constituent of fine particles in the Eastern United States) appears to be relatively benign. However, urban particle mixes that contain high levels of carbon particles are much more likely to result in a harmful response.
3. Scientific Explanations Exist for How PM_2.5 Causes Adverse Human Health Effects That Are Widely Accepted. At present, no simple biological explanations exist for how and why various PM_2.5 constituents might cause observed harmful health effects, including pulmonary and cardiovascular diseases and premature mortality. Statistical associations, although suggestive, are of limited usefulness without a science-based understanding of how the effects happen, especially when the risk factors are small.

The ARIES Study

The Aerosol Research and Inhalation Epidemiology Study (ARIES) was initiated in 1997 in Atlanta, Georgia, by the Electric Power Research Institute (EPRI) in cooperation with the EPA; the U.S. Department of Energy (DOE); several electric utilities, including TVA; and other interested parties. The study was created to provide science-based knowledge that would help lead to improved air quality and more effective protection of public health. Air quality measurements to address concerns about the predictive value of questionable assumptions began in 1998 and continued through 2003.

The design of ARIES overcame many of the limitations that existed in previous PM_2.5 health effect studies by providing data for the full range of relevant air pollutants—gases and PM. As a result, this multi-year, multi-million-dollar project constitutes the most compre-hensive study to date of the effects of air pollution on human health. All known components of PM thought to be related to human health, including the sulfates, nitrates, and carbon-containing compounds and the gas-phase pollutants such as carbon monoxide, nitrogen dioxide, ozone, and sulfur dioxide, were measured simultaneously (Figure 1). Most were measured continuously for an extended period of time. Various adverse human health measures, including unscheduled visits to doctors, emergency room visits, cardiopulmonary disease incidence, and mortalities, were recorded for comparison with air pollution levels.

Study Results

The ARIES investigating team included scientists from Atmospheric Research & Analysis, Desert Research Institute, Emory University, Harvard University, Klemm Analysis Group, Oregon Graduate Institute, Georgia Tech, Kaiser Permanente, and the University of Minnesota. Initial results from the ARIES study have been reported extensively at conferences and published in peer-reviewed scientific journals by the many groups of participating scientists. The ARIES results cast doubt on the soundness of the PM_2.5 mass-based NAAQS standard and on some of the previous claims concerning the magnitude of health effects, including premature mortality attributed to PM_2.5.

ARIES calls into question the following two previous leading assumptions: (1) PM_2.5 mass by itself is associated with harmful health impacts, and (2) all PM_2.5 components are equally toxic. In the ARIES study, the careful measurement of other urban pollutants suggests that carbon monoxide and carbon-based PM_2.5 remain associated with some health problems. However, total PM_2.5 mass, by itself, does not appear to be significantly related to negative health effects. Sulfate, a major component of PM_2.5, when considered separately from other air pollutants, was not significantly associated with any adverse human health effects. Failure to account for co-varying carbon monoxide and carbon-based PM_2.5 may have confounded earlier studies that reported associations between total PM_2.5 mass and health effects. Furthermore, toxicological studies have revealed plausible mechanisms by which exposures to elevated levels of both carbon monoxide and carbon-based PM_2.5 could cause adverse health effects.

In the urban environment, mobile sources, such as on- and off-road vehicles, trains, airplanes, and construction equipment, are major sources of both carbon monoxide and carbon-containing PM_2.5. Furthermore, a recent scientific study in the Netherlands indicated an association between living near a major highway and premature mortality. Other significant sources of carbon-based PM_2.5 include products of incomplete combustion from wood-burning and meat-cooking, volatile organic compounds from vegetation, and the petrochemical industry.

Implications of ARIES

Initial results from ARIES indicate that the current mass-based PM_2.5 standard is of limited value in managing air pollution-based human health effects. Unfortunately, many PM_2.5 management programs are focused primarily on lowering particulate sulfate and nitrate, and do not target the carbon-containing particles that appear to be the more plausible sources of PM_2.5 associated health effects. Certainly, other reasons may exist to continue the reduction of sulfate and nitrate pollution, e.g., reduction of acid rain or visibility improvement. However, it does not appear that sulfate and nitrate particulate compounds are significantly detrimental to human health at current levels. The soundest particulate air pollution management approach appears to be one that is centered on the control of carbon-containing components most likely to cause adverse human health effects, and not on total PM_2.5 mass.

Information Contacts

For more information on this and other air quality issues, please contact:
Thomas A. Burnett, 423-751-3938
William J. Parkhurst, 256-386-2793
Roger L. Tanner, 256-386-2958

If you would like additional information on important air quality topics, please contact Jeanie Ashe by telephone (256-386-2033), e-mail, facsimile (256-386-2499), or TVA mail at P.O. Box 1010, CTR 1K, Muscle Shoals, Alabama 35662-1010.