AIR QUALITY MONOGRAPHS #75-25 REVIEW OF INHALATION TOXICOLOGY OF SULFURIC ACID AND SULFATES
|Publication Date:||1 January 1975|
Sulfur compounds, largely sulfites and sulfates, comprise an important proportion of atmospheric particulate pollutants, and are responsible for a significant part of old and rent problems of air pollution. Visibility and property damage are recognized to be the better documented and unquestioned effects. As well these compounds have been prominently implicated, rightly or wrongly, in the health effects associated with air pollution episodes. Sulfur compounds, sulfates included, are common components of ambient air. Their sources are many, ranging from natural generation to man made production.
A proposed cycle of sulfur in the environment is outlined in Figure 1.
Natural and anthropogenic sources of sulfates are compared in Table 1. About 1/3 of the total sulfates is produced by modern technology and almost 40% of those in the respirable (<5μ ) are produced by man.
For years sulfur dioxide has been blamed and in fact has carried the major role in the alleged causation of ill effects from air pollution. Yet as early as 1955, sulfates have been implicated in air pollution episodes. Hemeon (1), in reconstructing the composition of the air pollution during the Donora, Pa. episode of 1948, gave special emphasis to the water soluble fraction of particulates. His analysis suggested that acidic salts rather than SO2 were the probable irritant agents. His retrospective analysis, obtained by x-ray diffraction of solids collected on the filter of an electronic air cleaner during the episode, showed that almost 2/3 of the water soluble solids consisted of zinc ammonium sulfate and zinc sulfate. The biological importance of these compounds was emphasized by Amdur and Corn (2), who showed that in guinea pigs, greater irritating capacities are manifested by these salts than by H2SO4 and SO2.
The emphasis on sulfates, naturally, does not entirely detract from the importance of sulfur dioxide. The latter obviously remains a source of sulfates in its role as a precursor. The suggested mechanisms for SO2 conversion to sulfates are multiple, uncertain and schematically summarized by the following hypotheses: