Permissible Daily Airborne Particle Mass Levels Encompass Brief Excursions to the “London Fog” Range, Which May Contribute to Daily Mortality and Morbidity in Communities

Abstract

PM₁₀ levels within the Environmental Protection Agency's (EPA's) 24-hour, 150-μg/M³ standard have been associated with mortality and morbidity, which remain causally unexplained. EPA's standard, refocused on PM_2.5, has been adjusted downward, assuming that effects observed daily are caused by daily exposures. Real-time particulate matter (PM) monitoring was used to inquire whether 24-hour average PM levels might encompass brief, health-significant excursions. Data from Birmingham, Alabama; Penticton, British Columbia; and Zeebrugge, Belgium, revealed many excursions, one reaching 2000 μg/M³ or twice the level in the lethal 1952 London fog. Harm from brief exposure to PM in the range of observed excursions was demonstrated experimentally in people, dogs, and rats. In rats, the area of lung surface developing lesions was elevated when the 4-hour average aerosol dose rate included a short-term burst. Elevations were observed with each of two aerosols, and were more pronounced with four excursions than with one excursion. In another study, short-term PM exposure killed rats. In dogs, short-term PM exposure caused serious electrocardiogram changes. In humans, asthma symptoms were associated with PM levels, but the strength of the association increased as PM averaging time decreased from 24 hours to 8 hours to 1 hour. Short-term human response to PM also was evident when 300 people were overcome after wind lofted particles into the air during a 4-day outdoor music festival in Queen Creek, Arizona, in April 1997. These findings support four conclusions: (1) EPA's decision to change the PM standard may be justified by residual health risks associated with the old standard. (2) The time required for PM to cause adverse effects may be briefer than the time interval over which the effects are observed. (3) Greater risk reduction would appear to be achievable via control of 1-hour PM averages than 24-hour averages. (4) Additional study of PM excursions is needed.