Toxicologic Appraisal of Particulate Matter, Oxides of Sulfur, and Sulfuric Acid

Abstract

An examination of the available toxicological literature indicates that sulfur dioxide itself would be properly classified as a mild respiratory irritant, the main portion of which is absorbed in the upper respiratory tract. The reported industrial experience of symptoms of mild chronic respiratory irritation from exposures at or above 5 ppm is compatible with what would have been predicted on the basis of available toxicological data. The basic physiological response to inhalation of pure SO₂ appears to be a mild degree of bronchoconstriction reflected in a measurable increase in flow resistance. Although the response is highly variable, most individuals tested have responded to 5 ppm and levels of 5 to 10 ppm have upon occasion produced severe bronchospasm in sensitive individuals. This serves to point up the fact that experience with the industrial Threshold Limit Value (5 ppm) is not applicable as a guide for the general population. Although the majority of individuals tested have shown no detectable response to levels of 1 ppm, there are again sensitive individuals who have responded. It is not known whether these individuals would have responded to concentrations lower than this. The response of these more sensitive individuals to 1 ppm would be classified as detectable response, not as severe bronchospasm. An examination of the available toxicological literature also indicates that sulfuric acid and irritant sulfates, to the extent that the latter have been examined, are more potent irritants than sulfur dioxide. This has been demonstrated in studies using morality and lung pathology as criteria as well as in studies using alterations in pulmonary function in experimental animals and human subjects. The irritant potency of these substances is affected by particle size and by relative humidity, which factors are probably interrelated. It is unfortunate that these substances have not been as yet studied in as great detail as has the less irritant sulfur dioxide. There is evidence which cannot be ignored, even though it is based entirely on animal experiments of one investigator, indicating that the presence of particulate material capable of oxidizing sulfur dioxide to sulfuric acid caused a three to fourfold potentiation of the irritant response. The aerosols causing this potentiation were soluble salts of ferrous iron, manganese and vanadium all of which would become droplets upon inhalation. Insoluble aerosols such as carbon, iron oxide fume, triphenylphosphate or fly ash did not cause a potentiation of the irritant action of SO₂ even when used at higher concentrations. The concentrations of SO₂ used in these various experiments were in some cases as low as 0.16 ppm. The catalytic aerosols were used at concentrations of 0.7 to 1 mg/m³ which is above any reported levels of these metals in urban air. If the SO₂ present as an air pollutant remained unaltered until removed by dilution, there would be no evidence in the toxicological literature suggesting that it would be likely to have any effects on man at prevailing levels. Studies of atmospheric chemistry have shown that SO₂ does not remain unaltered in the atmosphere, especially under onditions of high humidity and in the presence of particulate material, but is converted to H₂SO₄. Such a conversion increases its irritant potency. On this basis the toxicological literature combined with the literature of atmospheric chemistry suggest that sulfur dioxide levels be controlled in terms of the potential formation of irritant particles. This means that control measures as far as feasible should be aimed at both SO₂ and particulate material and not against either alone.