Effect of Droplet Size on Respiratory Responses to Inhaled Sulfuric Acid in Normal and Asthmatic Volunteers

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Abstract
We exposed groups of healthy and asthmatic volunteers to sulfuric acid aerosols with volume median droplet diameters of approximately 20, 10, and 1 .mu.m, at nominal concentrations of 2,000 .mu.g/m3, and exposed them similarly to aerosols of purified water as a control. Exposures lasted 1 h each, and included three 10-min periods of exercise (ventilation rate typically 40 to 45 L/min). Exposures occurred in randomized order 7 days apart. Temperature was 10.degree.C, relative humidity was .apprx. 100% in 20- and 10-.mu.m (fog) exposures, and .apprx. 75 to 80% in 1-.mu.m aerosol exposures. Healthy subjects showed no statistically significant changes in lung function or in bronchial reactivity to methacholine attributable to acid exposures. They showed significant increases in lower and upper respiratory irritant symptoms when exposed to 20- or 10-.mu.m acid fog, but not when exposed to 1-.mu.m acid aerosol. Asthmatics showed significant excess decreases in forced expiratory performance, increases in airway resistance, and increases in irritant symptoms during acid exposures, relative to control conditions. Lung function changes in asthmatics tended to increase with time during exposure; they did not vary significantly with acid droplet size. Symptoms in asthmatics were slightly worse with 10- or 20-.mu.m fog as compared with 1-.mu.m aerosol. In a few instances, symptoms and lung function decrements necessitated stopping exercise or terminating the exposure early. Thus, asthma is a risk factor for unfavorable physiologic response to sulfuric acid at occupational exposure concentrations. Large droplet size (i.e., fog) tends to exacerbate short-term symptomatic response, but we have not been able to demonstrate a consistent effect of droplet size on physiologic response.