Justification of an Air Quality Standard for Sulphur Dioxide

24. The Panel has considered two matters with respect to its recommendation for a Standard: the concentration at which the Standard should be set and the averaging time over which measurements of sulphur dioxide should be monitored and reported. With respect to the first of these, we accept the evidence from human experimental studies that people suffering from asthma are generally more sensitive to sulphur dioxide than are fully fit individuals and, since such people comprise a substantial and increasing minority of the population, we have concluded that any Standard should be based on likely effects on patients with asthma rather than on the healthy. Furthermore, while we are aware of the possibility that sulphur dioxide may have long-term effects on human health, we believe that the evidence for this is weak and insufficient to contribute to the process of standard-setting. The Panel has therefore concluded that the Standard should be based upon the relatively well-characterised shortterm effects.
25. In coming to a conclusion as to the concentration at which the Standard should be set, the Panel recognises that the response of airways in people suffering from asthma occurs rapidly after the commencement of exposure and that any averaging time will conceal short-term excursions to concentrations higher than the average. While most studies have shown no effect below about 250 ppb, occasional subjects have responded to lower concentrations with transient changes in measurements of lung function, insufficient to be associated with symptoms. The artificial nature of such studies, which have involved asthmatic subjects breathing through a mouthpiece while exercising on a bicycle, are such that the Panel feel it would be unwise to generalise from them. Rather, we have concluded that it is unlikely that clinically significant effects will occur in the majority of people with asthma exposed to concentrations below 200 ppb in the ambient air.
26. The Panel has considered the time periods over which concentrations of suphur dioxide should be monitored and reported in the ambient atmosphere. Since the effects of the gas on the lung's airways may occur very rapidly, a short averaging period is clearly desirable. Very short periods, say of 1 minute, while technically possible, are impracticable, and we have concluded that a 15-minute averaging period is a sensible compromise between desirability and practicality. Such a measurement will include brief periods of higher concentrations, which could be as much as double the average, and could therefore have an effect on susceptible individuals when the average appears safe. Taking this into consideration, as well as the need to ensure an adequate margin of safety for those individuals more severely affected with asthma, the Panel has concluded that the Standard should be set at 100 ppb, measured over a 15-minute averaging period. Because there are still networks which cannot record sulphur dioxide concentrations as 15-minute averages we have included an Annex which suggests how the data from these could be interpreted in terms of this Standard.