Thermodynamics and toxicity of sulfuric acid mists.

The hygroscopic nature of sulfuric acid mist has not been totally appreciated in previous toxicological work. Sulfuric acid mist, by adsorption or desorption of water, equilibrates rapidly with the relative humidity of its environment. The measured particle diameters of a given sulfuric acid mist will increase in size as the particles adsorb water on entering the respiratory tract. For a dry climate of 5% relative humidity, sulfuric acid mist particles will triple in size in the respiratory tract. At 60% relative humidity, particles will double in size, while particles in humid regions will increase very little. In the respiratory tract, the particles will deposit according to their size at 98% relative humidity. In order to compare toxicity data for sulfuric acid mist, relative humidity must be carefully recorded throughout the experimental procedure.