SULFUR-CONTAINING AIR POLLUTANTS (PARTICULATES)

Sulfur oxides are an important class of air pollutants. They include sulfur dioxide, sulfuric acid, and various forms of sulfate. The major determinant of total sulfur oxide production is the sulfur content of fossil fuels, which tends to be highest in coal and lowest in natural gas. Sulfur dioxide, a gas, was found to be associated with mortality and morbidity in London during periods of heavy smog and during other air pollution episodes. While in retrospect the problem had been present for decades and perhaps centuries, it was not demonstrated epidemiologically until the 1950s. This led in the 1960s to effective control measures, including a switch to fuels with lower sulfur content and the banning of many local point sources, such as coal used for heating homes. However, toxicological studies, including controlled human exposures, could not substantiate the epidemiological association of low levels of sulfur dioxide with adverse health effects. It soon became apparent that the causal relation was primarily with sulfuric acid and sulfates, and that atmospheric sulfur dioxide was both a precursor of these particulate forms of sulfur oxides, and a surrogate measure for their air concentrations.

Sulfur dioxide can be oxidized in the atmosphere to particulate sulfates, the oxidation being abetted by oxidant smog conditions, which also lead to ozone formation and to particulate forms of nitric oxides. This oxidation can occur many kilometers downwind from the emission source, which accounts for the occurrence of acid rain in relatively pristine rural areas. The toxicity of particulate sulfates depends upon the physical and chemical attributes of the inhaled particles. Size is particularly important, with only smaller particles being inhaled deeply into the lung, a recognition that has led to a change in the U.S. particulate standard, which originally was based solely on the total weight of all airborne particles. This was changed to a standard that only measured particles with a median diameter of less than 10 microns; a new proposed standard will only measure particles with a median diameter less than 2.5 microns.

Also important to toxicity is the chemical form of the sulfur oxide; that is, particles that are more acidic or more soluble in the lung tend to be more toxic. Recent epidemiological studies have shown an association of morbidity and mortality with atmospheric levels of particles much lower than previously reported. The adverse health effects include increases in mortality of the elderly and those with preexisting heart and lung conditions, as well as increases in the incidence of respiratory disease (including asthma attacks), in children. These new findings are the basis for the more stringent particulate standard recently proposed by the U.S. Environmental Protection Agency, which will require additional, costly control measures. This controversial proposed standard is complicated by the lack of sufficient clarity in the epidemiological and toxicological data. It has been difficult to separate out which chemical type or source of particles is most important, a situation that greatly complicates devising control strategies. Evidence suggests that adverse health effects are due to a gas-aerosol complex consisting of many air pollutant components, among which sulfur oxides play a significant role.

Bernard D. Goldstein

(see also: Acid Rain; Airborne Particles; Air Quality Index; Ambient Air Quality [Air Pollution]; Atmosphere; Inhalable Particles [Sulfates]; Pollution; Smog [Air Pollution]; Total Suspended Particles [TSP] )

Bibliography

Bates, D. V., and Sizto, R. (1987). "Air Pollution and Hospital Admissions in Southern Ontario: The Summer Haze Effects." Environmental Research 43:317–331.

Dockery, D. W.; Pope, C. A.; Xu, X. et al. (1993). "An Association between Air Pollution and Mortality in Six U.S. Cities." The New England Journal of Medicine 329(24):1753–1759.