Controlled Exposures Of Volunteers To Respirable Carbon And Sulfuric Acid Aerosols

Abstract

Respirable carbon or fly ash particles are suspected to increase the respiratory toxicity of coexisting acidic air pollutants, by concentrating acid on their surfaces and so delivering it efficiently to the lower respiratory tract. To investigate this issue, we exposed 15 healthy and 15 asthmatic volunteers in a controlled-environment chamber (21 °C, 50 percent relative humidity) to four test atmospheres: (i) clean air; (ii) 0.5-μm H2SO4 aerosol at ≈100 μg/m3, generated from water solution; (iii) 0,5-fim carbon aerosol at ≈250 μg/m3, generated from highly pure carbon black with specific surface area comparable to ambient pollution particles; and (iv) carbon as in (iii) plus ≈100 fig/m3 of ultrafine H2SO4 aerosol generated from fuming sulf uric acid. Electron microscopy showed that nearly all acid in (iv) became attached to carbon particle surfaces, and that most particles remained in the sub-jim size range. Exposures were performed double-blind, 1 week apart. They lasted 1 hr each, with alternate 10-min periods of heavy exercise (ventilation ≈50 L/min) and rest. Subjects gargled citrus juice before exposure to suppress airway ammonia. Lung function and symptoms were measured pre-exposure, after initial exercise, and at endexposure. Bronchial reactivity to methacholine was measured after exposure. Statistical analyses tested for effects of H2SO4 or carbon, separate or interactive, on health measures. Group data showed no more than small equivocal effects of any exposure on any health measure. One individual's responses were consistent with a clinically significant excess airway constriction from H2SO4 plus carbon, and 2-3 others showed slight excess responses to the combined pollutants, but all these observa-Implications Epidemiologic evidence suggests substantial health risk from acidic ambient particulate pollution, but is not strongly supported by human laboratory evidence. Thus, specific regulation of acidic aerosols remains controversial. This study aimed to resolve the discrepancy through more realistic laboratory exposures, employing acidcoated insoluble respirable particles instead of aqueous aerosol. Exposed volunteers were not affected meaningfully, except for an equivocal response by one individual. Thus, the discrepancy between epidemiologic and laboratory evidence remains to be resolved. tions might have reflected chance variations. We conclude that coexisting carbon aerosol did not increase respiratory irritancy of H2SO4, in most healthy and asthmatic subjects exposed for 1 hr under simulated “worst-case” ambient conditions. © 1992 Air & Waste Management.