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Research Programs: Particulate Matter (PM) Health Effects
Research Areas: Hazardous Components
PM Health Effects
What are the physical and chemical characteristics of PM that are responsible for causing adverse health effects?
The term "particulate matter" refers to a complex mixture of solid and liquid particles that exhibit a diversity of chemical and physical properties and which range in size over several orders of magnitude. In light of evidence that exposure to PM causes adverse health effects, researchers are interested in identifying what specific attributes or components of PM are responsible for the observed effects.
Comparison of PM to a hair. Click for a larger image. (50 KB 600x420)
Scientists commonly differentiate PM based on size, characterizing PM as either ultrafine, fine, or coarse. Studies using insoluble particles as surrogates for ultrafine PM (PM with diameters < 100 nm) have shown that, based on mass, the same amount of ultrafine PM can induce more injury than other size classes of PM. Evidence from epidemiological studies suggests that fine particles (PM with diameters less than 2.5 µm , or PM2.5) are strongly associated with cardiovascular and respiratory effects. Other studies have shown that coarse particles (PM with diameters less than 10 µm, or PM10) do not contribute significantly to an increased risk of adverse health effects. Research is ongoing to determine the extent to which adverse health effects can be attributed to PM belonging to a particular size class.
The chemical composition of PM has received considerable scrutiny in toxicological studies. The major chemical components of fine PM are sulfate, nitrate, ammonium, and carbonaceous material (organic carbon or OC, composed itself of hundreds of organic compounds, and elemental carbon or EC, also referred to as soot, inorganic carbon, or black carbon). Coarse PM (PM10) is composed of primarily crustal material (typically oxides or salts of elements found in dirt; e.g., Fe, Ca, Si, Al).
To date, the most data has been generated about the inorganic constituents of PM. Sulfate and nitrate anions derived from combustion emissions usually combine with water soluble constituents of PM to form "acid aerosols." Although the independent toxicities of many sulfates and nitrates appear to be rather low, it is hypothesized that they may influence the toxicity of other PM components. Little is actually known about the cardiovascular effects of acidic aerosols, but the possibility that they might mediate some of the reported PM effects is currently being investigated.
There is compelling evidence from epidemiological studies that the metal components of PM are particularly toxic and are responsible for lung inflammation and cardiac arrhythmias. A study conducted in the Utah Valley revealed that there was a significant decrease in hospital admissions for respiratory and cardiovascular-related causes in the vicinity of a steel mill while the mill was temporarily closed because of a labor dispute. In corresponding clinical studies, humans, animals, and cell cultures were exposed to extracts of particles collected from the mill during the labor dispute and when the plant was open to compare the effects associated with the particles during these two time periods. Analysis of the extracts revealed higher levels of several transition metals when the mill was operational. More recent studies have shown that extracts stripped of transition metals lose their potency and that solutions of pure transition metals in the ratios and concentrations found in the particles can induce similar effects in animals as the complete extracts. This conformed to a theory of particle toxicity emphasizing the ability of metals on fine particulates to induce toxic oxygen radical species and subsequent inflammatory response in the lung. The implication of this study is that, while particle size does matter, particle composition seems also to be extremely important in the pulmonary inflammatory response to particulate air pollution. These studies showed a pattern of response that was consistent with the reported epidemiologic findings — i.e., metal-containing PM induced measurable adverse effects.
EPA is investigating the toxicity of other chemical attributes of PM. Organic constituents are of particular concern due in large part to the contribution of industrial, diesel, and other mobile sources to the fine PM fraction. While not as overtly toxic as the some of the metal compounds, some organic compounds appear to be able to generate oxidants that might cause delayed or subtle effects that are not readily measured by conventional methods. Studies are also underway to investigate the relationship between health outcomes and PM that can be attributed to a specific source. If health effects can be linked to particular types of air pollution sources, this information will assist in designing targeted control strategies that will yield the greatest reduction in risk at the least cost.
Glossary
Size:
The physical attributes of PM-size, surface area, and number-are interrelated descriptive metrics of PM. These properties influence PM deposition, penetration, and persistence in the lung, as well as the potential for systemic transport and the inherent toxicity of the particle itself. It is not difficult to perceive that a given mass of fine PM would have a greater surface area than a comparable mass of coarse PM; likewise, there would be much more surface associated with ultrafines compared to a similar mass of fine PM. The effect of size on surface area is exponential. Thus there is concern that smaller PM fractions have a potentially enormous surface carrying capacity. As PM is a complex mixture, the potential role of surface-associated chemicals may be of considerable importance. Thus, size may come into play in determining toxicity in a variety of roles.
