HUMAN EXPOSURES TO PARTICLE STRONG ACIDITY

This article reviews the findings of recent research programs and atte mpts to present an update of our current understanding about human exp osures to particle strong acidity IPSA). This review tells the key ste ps in our recent progress, such as aerosol chemistry and physics, impr ovements in samplers and measurement techniques, increases in the data bases, and exposure assessments. Remedying an earlier paucity in direc t measurements of PSA, there has been a great increase in the database from several recent field studies. These studies have shown that wher e appreciable PSA exists, virtually all exposures occur in the warmer months, and the highest PSA levels are specifically associated with su mmertime, regional stagnation periods. Frequently, PSA episodes are co incident with photochemical smog and high ozone levels, although the c onverse is not always the case. A number of new studies have shown tha t the effect of the indoors on human exposures to PSA is entirely prot ective. That is, there are rarely important sources indoors, and most factors affecting the indoor air quality lead to attenuation or PSA le vels. 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 f ound to fall between measured outdoor and indoor values. Because of lo w indoor PSA concentrations and the higher activity individuals sustai n while outdoors, even for relatively a minor part of the day, time sp ent outdoors may be the chief determinant of personal dose. Time/activ ity-weighted models of indoor and outdoor concentrations give better e stimates of personal exposures, compared to outdoor concentrations alo ne. However, they currently fall short of providing accurate predictio ns of personal exposure. There remains problems in determining the mos t appropriate exposure metric for epidemiological investigations.