Comparative potency of inhaled acidic sulfates: Speciation and the role of hydrogen ion

Inhaled acidic sulfate aerosols affect various aspects of lung function, presumably by delivery of hydrogen ion (H +) to target sites. Recent evidence suggests that the relationship between response and H + content of the exposure atmosphere may depend upon the specific sulfate species with which the H + is associated. This study examined comparatively the effects of exposure to the two main ambient acidic sulfates, sulfuric acid (H 2SO 4) and ammonium bisulfate (NH 4HSO 4), using the phagocytic activity of alveolar macrophages as the endpoint. Rabbits were exposed to 250–2000 μg/m 3 H 2SO 4 (as SO 4 −2) and 500–4000 μg/m 3 NH 4HSO 4 (as SO 4 −2) for 1 hr/day for 5 days; bronchopulmonary lavage was then performed for recovery of free lung cells. Phagocytosis, measured by uptake of opsonized latex spheres in vitro, was altered by exposure to H 2SO 4 at concentrations ⩾500 μg/m 3 and to NH 4HSO 4 at ⩾2000 μg/m 3. Assessment of results in terms of the calculated hydrogen ion concentration in the exposure atmosphere showed that identical levels of H + produced different degrees of response depending upon whether exposure was to H 2SO 4 or NH 4HSO 4. On the other hand, macrophages incubated in acidic environments in vitro responded similarly regardless of whether H 2SO 4 or NH 4HSO 4 was used to adjust the pH. Possible reasons for the difference in response observed in vivo and in vitro are discussed. Speciation of ambient acidic sulfate aerosols may be needed in atmospheric monitoring so as to assess the presence of H + posing the greatest biologic hazard following inhalation exposure.