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Regulating Fine Particles

Developing a new paradigm for managing fine particulate air pollution.

Fortnightly Magazine - February 2012

Fine particulate matter—particles less than 2.5 micrometers in diameter, or PM2.5—has been linked with a number of adverse health effects, including premature mortality, cardiovascular disease, and respiratory diseases such as asthma. PM2.5 is a complex mixture comprised of hundreds of different components, both inorganic and organic. Inorganic components include sulfate and nitrate—formed from gaseous sulfur dioxide (SO 2) and nitrogen oxides (NOx)—as well as elemental species such as selenium, mercury, arsenic and chromium. Coal-fired power plants are the most significant source of SO 2, while traffic is responsible for most NOx. Organic PM components include polycyclic aromatic hydrocarbons and a host of other compounds. These organic materials are derived from a variety of sources, including traffic and wood smoke, and from reactions between emissions of volatile organic compounds (VOC) from anthropogenic and natural vegetative sources and other atmospheric constituents. Black carbon (BC)—sometimes referred to as soot—is found primarily in tailpipe emissions, especially those from diesel vehicles, and wood smoke. BC is a significant contributor to ambient PM in urban areas.

The composition of PM2.5 varies geographically. In the eastern United States, organic compounds and sulfate dominate the mixture, whereas in the western U.S., organic compounds, dust particles, and nitrate dominate the mix. There’s also temporal variability on both short-term and longer term scales; sulfate and organic compounds are more important in the warmer summer months, while nitrate levels increase in the colder winter months.

Much of what we currently know about PM2.5 and health comes from the field of epidemiology, which evaluates statistical associations between population exposure to PM and resultant health effects. There are many studies that demonstrate a statistically significant association between PM2.5 and health; however, there is a scientific consensus that all components within the PM mixture aren’t equally harmful to health. Indeed, accumulating evidence from epidemiology as well as toxicology—the use of controlled exposures in either animals or humans—indicates that different PM components impact health differently. Authoritative studies on such evidence could help inform efforts to regulate PM emissions in the ways that will best protect public health.

Component-Based Standards

Epidemiological and toxicological research by EPRI and others indicates that PM health effects differ significantly depending on PM’s composition. EPRI’s Aerosol Research and Inhalation Epidemiology Study (ARIES) is one of the most comprehensive air pollution epidemiology studies ever conducted, in terms of the breadth of air quality monitoring and health endpoints studied. The study, which began in 1998 and is still ongoing, measures more than 100 air pollution variables each day and links changes in these variables to changes in mortality, emergency department visits, hospital admissions, unscheduled physician visits, and arrhythmias in patients with implanted defibrillators.

Results from ARIES have been published in the scientific literature and point to the importance of carbon-containing compounds and some