Results of recent research show that particulate matter (PM) composition and size vary widely with both space and time. Despite the variability in PM characteristics, which are believed to influence human health risks, the observed relative health risk estimates per unit PM mass falls within a narrow range of values. Furthermore, no single chemical species appears to dominate health effects; rather the effects appear to be due to a combination of species. Non-PM factors such as socioeconomic status and lifestyle are also believed to affect the health risk, although accounting for these confounding factors is challenging. Airborne PM is also responsible for a number of effects aside from human health, such as alterations in visibility and climate. Because the PM problem is associated with a range of societal issues such as energy production and economic development, making progress on reducing the effects of PM will require integrated strategies that bring together scientists and decision makers fromdifferent disciplines to consider tradeoffs holistically.
Particulate matter refers to small particles consisting of solid or liquid droplets suspended in air. EPA currently regulates particles in two size ranges to help protect public health. These include PM10 and PM2.5. PM10 refers to particles less than 10µm in aerodynamic diameter (about 1/10th the diameter of a human hair), while PM2.5 (fine particles) refers to particles less than 2.5 µm in aerodynamic diameter. Because PM10 includes PM2.5, EPA is in the process of promulgating new standards for coarse particles (PMc). This refers to particles with aerodynamic diameters between 2.5 µm and 10 µm. ...
Reducing the health (and welfare) effects associated with ambient PM is not a simple undertaking. It involves understanding not only the effects of PM, but also the linkages between PM (or precursors) emitted from sources and how that PM makes its way through the air and into the human body. Each step along the way between source and health effect is complicated and makes it difficult to link the observed effect back to the specific source or even source type. For example, roughly half the total global emissions of PM2.5 and precursor species (~500 Tg/year) are emitted from anthropogenic sources with the other half from natural sources. The distribution of those emissions in air and resulting PM varies greatly in size and composition, and thus, PM2.5 concentrations vary widely over space and time.
The health effects of PM are thought to be strongly associated with particle size, composition, and concentration, even though relative risk estimates indicate that the risk per unit PM mass falls within a limited range of values for these parameters. As well, a combination of species and daily variations in PM mass and composition are believed to contribute to the toxicity of the particulate matter air pollution. Furthermore, measuring the relevant parameters and quantifying the health effects are extremely challenging, as numerous external factors, including meteorology and socioeconomic aspects of the human lifestyle, strongly affect human morbidity and mortality. Identifying and correcting for the effects of confounding factors remain a difficult task.
People are exposed to PM2.5 from many sources as they go about their daily activities, spending time in their homes, at work, in recreation, and in traveling. This is further complicated by the knowledge that some individuals or segments of the population are more susceptible to PM exposures, due to factors such as respiratory habits (e.g., mouth breathing versus nose breathing), pre-existing diseases, or genetics. Given all these complications, it is interesting to note that fixed monitoring stations at a central urban site seem to provide reasonable estimates of total exposure of an individual to PM2.5 mass and some secondary species like SO2. 4 . However, determining concentrations of other species such as metals and organic compounds may require measurements in each microenvironment. …