Human Exposures to Particle Strong Acidity

Abstract

This article reviews the findings of recent research programs and attempts to present an update of our current understanding about human exposures to particle strong acidity (PSA). This review tells the key steps in our recent progress, such as aerosol chemistry and physics, improvements in samplers and measurement techniques, increases in the databases, and exposure assessments. Remedying an earlier paucity in direct measurements of PSA, there has been a great increase in the database from several recent field studies. These studies have shown that where appreciable PSA exists, virtually all exposures occur in the warmer months, and the highest PSA levels are specifically associated with summertime, regional stagnation periods. Frequently, PSA episodes are coincident with photochemical smog and high ozone levels, although the converse is not always the case. A number of new studies have shown that the effect of the indoors on human exposures to PSA is entirely protective. That is, there are rarely important sources indoors, and most factors affecting the indoor air quality lead to attenuation of PSA levels. While sulfate aerosol effectively infiltrates from outdoors, the strong acid portion is largely neutralized by ammonia present indoors. The chemical, physical, and phenomenological vagaries of PSA in the human environment are such that absolute exposures cannot be predicted by outdoor data alone. Personal (i.e., breathing zone) exposures are found to fall between measured outdoor and indoor values. Because of low indoor PSA concentrations and the higher activity individuals sustain while outdoors, even for relatively a minor part of the day, time spent outdoors may be the chief determinant of personal dose. TimeJactivity-weighted models of indoor and outdoor concentrations give better estimates of personal exposures, compared to outdoor concentrations alone. However, they currently fall short of providing accurate predictions of personal exposure. There remains problems in determining the most appropriate exposure metric for epidemiological investigations.