COMPARATIVE BIOLOGICAL POTENCY OF ACIDIC SULFATE AEROSOLS - IMPLICATIONS FOR THE INTERPRETATION OF LABORATORY AND FIELD STUDIES

Biological responses to inhaled acid sulfates result from the depositi on of hydrogen ion (H+) on airway surfaces. Thus, effects from sulfuri c acid and ammonium bisulfate, the two major ambient species, have bee n assumed to be the same for a given H+ concentration within the expos ure atmosphere, assuming similar respiratory tract deposition patterns . However, recent inhalation studies have indicated that sulfuric acid is disproportionately potent compared to ammonium bisulfate when the H+ content of the exposure atmosphere is considered, suggesting that s ome factors following inhalation affect the amount of H+ contacting ai rway surfaces. This study assessed a mechanism potentially underlying this phenomenon, namely, the extent of neutralization by respiratory t ract ammonia. This was evaluated using a physical model system designe d to mimic transit of these aerosols in the upper respiratory tract of the animal model used in this laboratory, the rabbit. The results sug gest that for equal exposure quantities of H+, more acid would be depo sited when sulfuric acid is inhaled than when ammonium bisulfate is in haled. Furthermore, results from a series of in vitro exposures of tra cheal epithelial cells to sulfuric acid and ammonium bisulfate aerosol s indicated that the biological response is a function of the total ma ss (ionic) concentration of Ht deliverable to the cells or the total e xtractable Ht per particle. The results of this study have possible im plications for ambient monitoring of particulate associated strong aci dity, suggesting that it may be necessary to speciate such measures in to the relative amounts of H+ as sulfuric acid or ammonium bisulfate i n order to most accurately relate atmospheric acid levels to observed health effects. In addition, since much of the ambient particulate-ass ociated Ht exists as sulfuric acid/ammonium bisulfate mixtures rather than pure compounds, H+-associated health effects from controlled expo sure studies of sulfuric acid may not be transferable to ambient popul ation situations on a 1:1 basis. Since any such errors in exposure ass essment will necessarily bias downward the strength of Ht-related heal th effects associations found via epidemiological studies, failure to address the speciation of H+ may cause such studies to underestimate t he human health effects of strong acids. (C) 1994 Academic Press, Inc.