Air Quality and Health and Welfare

2.6 Sulfur Oxides

2.6.1 WHO Guidelines
Controlled studies with exercising asthmatics indicate that some of them experience changes in pulmonary function and respiratory symptoms after periods of exposure as short as 10 minutes. Based on this evidence, it is recommended that a value of 500 µg/m3 should not be exceeded over averaging periods of 10 minutes. Because exposure to sharp peaks depends on the nature of local sources and meteorological conditions, no single factor can be applied to this value in order to estimate corresponding guideline values over somewhat longer periods, such as an hour.

Day-to-day changes in mortality, morbidity or lung function related to 24-hour average concentrations of sulfur dioxide are necessarily based on epidemiological studies in which people are in general exposed to a mixture of pollutants, with little basis for separating the contributions of each to the effects, which is why guideline values for sulfur dioxide were linked before 1987 with corresponding values for particulate matter. This approach led to a guideline value before 1987 of 125 µg/m3 as a 24-hour average, after applying an uncertainty factor of 2 to the lowest-observed-adverse-effect level. In the 2000 revision, it was noted that recent epidemiological studies showed separate and independent adverse public health effects for particulate matter and sulfur dioxide, and this led to a separate WHO AQG for sulfur dioxide of 125 µg/m3 as a 24-hour average. More recent evidence, beginning with the Hong Kong study of a major reduction in sulfur content in fuels over a very short period of time, shows an associated substantial reduction in health effects (childhood respiratory disease and all age mortality outcomes). In time-series studies on hospital admissions for cardiac disease, there is no evidence of a concentration threshold within the range of 5-40 µg/m3 in both Hong Kong and London. Daily SO2 was significantly associated with daily mortality in 12 Canadian cities with an average concentration of only 5 µg/m3. If there were an SO2 threshold for either the Burnett et al. study of daily mortality, or the annual mortality study of Pope et al., they would have to be very low. For the significant associations in the ACS cohort for 1982-1998 in 126 US metropolitan areas, the mean SO2 was 6.7 µg/m3.

Nevertheless, there is still considerable uncertainty as to whether sulfur dioxide is the pollutant responsible for the observed adverse effects or, rather, a surrogate for ultra-fine particles or some other correlated substance. For example, in Germany (Wichmann et al. 2000) and the Netherlands (Buringh et al. 2000) a strong reduction of SO2 concentrations occurred over a decade. Although mortality also decreased with time, the association of SO2 and mortality was judged to not be causal and was attributed to a similar time trend of a different pollutant (PM). In consideration of: (1) the uncertainty of SO2 in causality; (2) the practical difficulty of reaching levels that are certain to be associated with no effects; and (3) the need to provide greater degrees of protection than those provided by the guidelines published in 2000, and assuming that reduction in exposure to a causal and correlated substance is achieved by reducing sulfur dioxide concentrations, then there is a basis for revising the 24 hour guideline downward for sulfur dioxide, and the above guideline is recommended as a prudent precautionary level.