The health warning was sounded by the second report of the Advisory Group on Medical Aspects of Air Pollution Episodes. The report evaluates the effects likely to occur from exposures to SO2, acid aerosols and particulates.
The Group's conclusions on the effects of exposure to SO2 are reassuring for most individuals. The available evidence, it says, indicates that anyone not suffering from respiratory disease will be unaffected by SO2 levels experienced in the UK even during pollution episodes.
However, asthmatic patients are more at risk. Inhalation studies have shown measurable changes in the lung function of asthmatics on exposure to SO2 levels of 400ppb, and in a small number of individuals at levels as low as 200ppb. The report notes that patients with severe asthma or those with highly reactive airways may be still more sensitive, but have generally not participated in such studies.
The report adds that an exposure lasting no more than 10 minutes or so is necessary to produce a maximal response to any particular level of SO2.
The recommendations which the Group drew from these conclusions, however, were influenced by the general lack of short-term data on SO2 levels in the UK.
Its key proposals, which have been accepted by the Government, are that advice on possible health effects should be made available to the public when hourly average SO2 levels are expected to fall within the range 125-400ppb. And a health warning should be issued when hourly average SO2 levels of more than 400ppb are anticipated. The latter recommendation has required the Department of the Environment to reduce the 500ppb threshold at which it begins to describe air quality as "very poor".
Where the Group has left itself exposed to criticism is in relating its recommendations to hourly average SO2 levels. These, as the report acknowledges, may understate by a large margin the SO2 concentrations likely to be experienced over short periods of 10-15 minutes.
In its defence, the Group says that the two thresholds at which it recommended the provision of health advice were lower than they would have been if short-term monitoring data were routinely available. But this merely poses the question why it would have recommended a higher threshold than 400ppb for health warnings when it had itself concluded that asthmatics are likely to suffer effects when exposed to this level of SO2 for only 10 minutes or so.
The number of occasions on which health advice is likely to be given is not known with precision. However, data in the report show that a one-hour average SO2 level of 122ppb was exceeded for 382 hours at 10 monitoring stations in 1990/1. The total was dominated by Belfast, with 269 hours. Exceedances of the 400ppb threshold will be much less common, and are most likely to occur in Belfast.
The Group was unable to come to any definitive conclusions about the health effects of particulates and acid aerosols, primarily because very few suitable monitoring data are available in the UK. The report recommends forcefully that monitoring should be instituted.
The Group also comments that it is "particularly to be regretted" that very little research on the health effects of SO2 is under way in the UK. There is a "clear need for further research", it says, on mechanisms of action of SO2 on the respiratory system, on the health impacts of exposures to SO2 in combination with other pollutants, and on the effects of low levels of pollutants such as SO2 and particulates on the prevalence of respiratory disease in the UK. The Government's response to the report was silent on these recommendations.