CONTROLLED EXPOSURES OF VOLUNTEERS TO RESPIRABLE CARBON AND SULFURIC-ACID AEROSOLS

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-degrees-C, 50 percent relative humidity) to four test atmospheres: (i) clean air; (ii) 0.5-mu-m H2SO4 aerosol at almost-equal-to 100-mu-g/m3, generated from water solution; (iii) 0.5-mu-m carbon aerosol at almost-equal-to 250-mu-g/m3, generated from highly pure carbon black with specific surface area comparable to amblent pollution particles; and (iv) carbon as in (iii) plus almost-equal-to 100-mu-g/m3 of ultrafine H2SO4 aerosol generated from fuming sulfuric acid. Electron microscopy showed that nearly all acid in (iv) became attached to carbon particle surfaces, and that most particles remained in the sub-mu-m size range. Exposures were performed double-blind, 1 week apart. They lasted 1 hr each, with alternate 1 0-min periods of heavy exercise (ventilation almost-equal-to 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 end-exposure. 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 observations 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.