Airborne Particle Excursions Contributing to Daily Average Particle Levels may be Managed via a 1 hr Standard, with Possible Public Health Benefits

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Abstract
Recent studies associate 24 hr PM10 levels within EPA's 150 μg/M3 standard with significantly morbidity and mortality. These findings remain unexplained toxicologically. This study documents the contribution of brief airborne particle excursions to 24 hr average particle levels, and evaluates the possible public health significance of such excursions. Two proven technologies—inertial and beta attenuation—were identified for continuously monitoring particle mass. Ten 24 hr periods distributed among three locations were examined with PM10 < 150 μg/M3. At all three locations, and in six of ten days, excursions exceeded 150 μg/M3. Excursions of 15 min reached 2000 μg/M3, surpassing the 1952 London for (PM10 ≤ 1200 μg/M3) and refuting EPA's assumption that 24 hr averaging protects against excursions. Toxicology literature confirmed the harmfulness of airborne particles in the range of observed excursions. In contrast, EPA has assumed that protracted postexposure morbidity and mortality are explainable by protracted rather than short-term causes. This study suggests that known toxicological mechanisms involving brief but intense particle mass excursions might cause the unexplained effects. EPA has advocated rejecting a 1 hr particle standard, citing uncertainty that “the majority of effects” observe after daily exposure would occur after briefer exposures. However, imposing a 1 hr particle mass limit of 300 μg/M3 might enhance protection against acknowledged short-term particle effects on public health, while avoiding a burdensome change in the 150 μg/M3 24 hr mass limit.

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