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6. Health Effects and Exposures to Particulate Matter and Associated Air Pollutants

Air pollution in the United States is regulated under the authority of the Clean Air Act to protect public health and welfare. Recently, EPA's Clean Air Scientific Advisory Committee reviewed and reached consensus that there is increasing scientific confidence, based on numerous epidemiological studies, that particulate matter (PM) is associated with increased morbidity and mortality and these effects occur at exposure levels below the current standards. Significant uncertainties remain, however, about the biological mechanisms that could cause increased mortality or morbidity from PM exposures and about the nature of human exposures. The question of biological plausibility, .e., "How could PM be causing these effects?" has received much less study. Animal toxicology studies have reproduced at higher concentrations the effects reported in humans: mortality, asthma-like effects, and increased infection-related morbidity. While several hypotheses regarding possible mechanisms underlying recently reported PM effects have been proposed, little research has been conducted to evaluate these hypotheses and to explore issues of dose-response and exposure scenarios.

The lack of understanding about biological mechanisms that could explain (a) the observed effects; (b) the reported independence of effects from particle composition; and © the lack of an obvious threshold for effects (i.e., the effects observed at very low exposures) underscores the critical need for research on mechanisms of PM toxicity. In addition the lack of research as to whether, and to what extent, the effects attributed to PM exposures are modified by other commonly occurring pollutants such as SO₂ and ozone, leads to uncertainties in interpretation of epidemiological studies.

In addition, there are important scientific uncertainties regarding PM exposures. Uncertainties regarding exposure assessment (e.g., particle concentration, size, chemical speciation, spatial and temporal variability, and copollutants) for important subpopulations (e.g., children, the elderly, individuals with pre-existing disease) are critically important since they affect interpretation of the epidemiological studies on which PM risk estimates are based. Understanding regional and temporal variability in particle characteristics (e.g., Western versus Eastern U.S.) and toxicity (e.g., coarse natural fugitive dust particles versus fine combustion-derived particles) may also lead to more effective risk management.

Research is needed in the following areas:

Investigation of causal mechanisms of PM toxicity. New clinical, epidemiological, toxicological, and in vitro research is needed for pulmonary, cardiovascular, and immunological effects (or other effects) in normal and sensitive subpopulations to better understand causal mechanisms by which PM, alone and/or in combination with other air pollutants, may cause health effects at levels below the current standard.
Studies using intermediate biological endpoints (i.e., which might relate to morbidity) hypothesized to be important to a causal mechanism(s) are needed to simultaneously test mechanism hypotheses and be indicative of dose-response relationships for PM toxicity. Research is needed on coarse, fine, and ultrafine particles.
Research to reduce uncertainties in exposure assessment for PM and associated copollutants. Research is needed to improve the characterization of individual and population exposures to PM (concentration, size, composition, fine mode versus coarse mode, etc.) and copollutants, including relationships between personal exposure to ambient PM, indoor PM, and total PM, and to allow new epidemiological studies to better define relationships between exposure to PM and other atmospheric constituents and adverse health effects.
Research is needed on the composition of little understood components of PM such as organic compounds (non-volatile and semi-volatile), primary biological materials, and species dissolved in liquid particles. Characterization of the spatial, temporal, and indoor/outdoor patterns of species such as NH₄NO₃ and parameters such as particle number is included. New or improved instruments or techniques may be required for these studies also.

Funding:

Approximately $2 million is expected to be available in FY 97 for awards in this program. Proposals in the $50,000 to $200,000/year range are encouraged. Duration of awards may be up to three years.