The Effects of Sulphur Dioxide on Human Health

18. Sulphur dioxide is a gas to which workers in a wide variety of industries may be exposed, and accidental gassings are by no means rare. The Health and Safety Commission has set for industry a Short-Term Exposure Limit (measured as an average over 15 minutes) of 5000 ppb, and a Long-Term Exposure Limit (measured as an average over 8 hours) of 2000 ppb. Exposure to very high concentrations causes painful irritation of the eyes, nose, mouth and throat, and the acute chemical injury to the linings of the airways may cause serious difficulty in breathing, and even death. These effects relate to accidental exposures to concentrations of sulphur dioxide considerably higher than those occurring as a consequence of ambient air pollution.
19. Of greater interest, with respect to outdoor air quality, are the effects on health of the lower concentrations to which we may be exposed in the ambient air. These effects have been studied in a number of different ways, including exposure of volunteers to sulphur dioxide in the air they are breathing and by examination of the effects on members of the population who have been exposed to episodes of atmospheric pollution. A few studies of humans have investigated the effects of prolonged exposure; minor changes in measurements of lung function have been found in healthy adults inhaling up to 8000 ppb continuously in an exposure chamber for five days. These studies are more relevant to industrial than to ambient exposures, although they do indicate the high concentrations necessary to cause significant effects in fit adults over these periods.
20. Sulphur dioxide causes its irritant effects by stimulating nerves in the lining of the nose, throat and the lung's airways. This causes a reflex cough, irritation, and a feeling of chest tightness, and may lead to narrowing of the airways. This latter effect is particularly likely to occur in people suffering from asthma and chronic lung disease, whose airways are often inflamed and easily irritated. It follows that the adverse effects of sulphur dioxide on lung function will be more evident in such people, particularly those whose disease is severe or who are subject to frequent attacks. Moreover, such individuals often already have narrowed airways and any further narrowing will have a disproportionately large effect on the resistance to airflow in their airways and, thus, on the ease with which they can breathe. People with asthma comprise some 4% of the population of the United Kingdom (and a higher proportion among children), and the prevalence of this condition has been rising. Chronic lung disease due to smoking is also common among older people. Clearly, any air quality standard for sulphur dioxide must take account of its possible effects on these sections of the population.
21. Studies of normal healthy volunteers, exposed experimentally to sulphur dioxide in chambers, have shown that measurable narrowing of the airways may occur after breathing the gas for 5 minutes at concentrations of about 4000 to 5000 ppb, but that effects were not detectable at concentrations below 1000 ppb. Tn general, people with asthma are more sensitive to sulphur dioxide, although individuals show considerable variation in sensitivity. Exercise, by increasing the volume of air breathed and by reducing the opportunity for sulphur dioxide to be removed by dissolving in the fluid in the nose, leads to a greater reaction, and studies of exercising asthma patients have shown that some individuals develop airway narrowing at concentrations as low as 200 ppb. Indeed, it was found in one study that transient minor changes in lung function in two particularly sensitive patients could be detected after exposure to 100 ppb during 10 minutes of exercise. However, no such effects have been observed in larger numbers of asthmatic subjects exposed to concentrations of 250 or even 500 ppb in other similar experiments. Nevertheless, there is clear evidence that some people with asthma show reductions in breathing capacity after short-term exposure to concentrations of about 400 ppb. Since the most seriously affected asthmatic subjects do not take part in such experiments, it is not unreasonable to assume that there is a subgroup of such people who would be affected at lower concentrations.
22. It has been suggested that exposure to sulphur dioxide may potentiate sensitisation to inhaled allergens² in animals, and studies have also suggested that people living in polluted areas may be more likely to become sensitised. Whether the latter is due to sulphur dioxide or to other factors is not known. There is no evidence to date that exposure to sulphur dioxide alone increases the likelihood of patients with asthma reacting excessively to inhalation of substances, such as grass pollen, to which they are allergic. However, some data suggest that sulphur dioxide, acting together with nitrogen dioxide, can increase the sensitivity to allergens of some patients with asthma.
23. Studies of populations are complicated because exposure is almost never to one pollutant on its own. Investigations in the 1950s and 1960s showed that excess numbers of deaths occurred in relation to pollution episodes, and it was concluded that such excesses could be detected at concentrations of sulphur dioxide down to about 170 ppb as 24-hour averages in association with comparable rises in the concentrations of particles in the air. At lower concentrations, down to a daily average of 85 ppb (in association with particle concentrations of around 250 µg/m³ as a 24-hour average), there was evidence that an excess of people complained of chest symptoms. It should be noted that a daily average of 85 ppb could conceal the existence of short-term excursions significantly greater than this value. The Panel recognise the difficulty in interpreting the historical data because of the possible interactions associated with the high levels of particles experienced at the time: therefore more weight has been given to the short-term chamber studies. With respect to longer-term effects and, in particular, the question as to whether exposure to sulphur dioxide actually causes lung disease rather than simply provoking attacks of asthma, the view of the Panel is that the evidence remains inconclusive. The question of interactions between sulphur dioxide and particles will be considered in a report from the Department of Health's Committee on the Medical Effects of Air Pollutants.