Justification of an Air Quality Standard for Ozone

17. The Panel accept the clear evidence from the medical literature that ozone has irritant effects on the lungs, resulting in certain circumstances in cough or discomfort on deep breathing. In experimental and epidemiological studies there is evidence that inflammation of the inner surface lining of the lung's airways and impairment of ventilatory capacity may occur on exposure to concentrations of ozone higher than about 100 ppb, usually for several hours. Smaller effects can be detected in individuals undertaking intermittent exercise at concentrations of about 80 ppb over periods of 6.6 hours. There is, however, no evidence to date that such effects cause any long term damage to the airways or contribute to chronic lung disease at the levels of ozone likely to occur in the United Kingdom.
18. The Panel recognise that the main factors determining rises in tropospheric ozone concentrations are the coincidence of sunlight and traffic pollution, and that the highest levels of ozone are likely to be encountered some distance downwind of the source of precursor pollutants. This means that excursions of ozone concentrations above a Standard may occur in circumstances outside the complete control of any one nation. Moreover, such excursions may occur purely as a consequence of the natural phenomenon of stratospheric air penetrating to the troposphere, and during the months of October to March such natural increases in ozone concentrations may occur. Thus any health-based Standard set may be expected to be exceeded from time to time, and will be exceeded more frequently in the south of the UK than in the north and in rural areas rather than in towns.
19. The Panel have decided to recommend an Air Quality Standard for ozone in terms of a concentration with a specified averaging time. We have chosen a level of 50 ppb - appreciably lower than the concentration at which effects have been detected experimentally, in order to include a margin of safety. The Panel recommend that the 50 ppb level be measured as a running 8-hour average, since this most closely represents the exposures likely to be harmful to human health. This recommendation should not be taken to imply that ozone concentrations vary only slowly, indeed the opposite is often the case, with variations between consecutive hourly means of up to a factor of two being recorded. For each day there will be a continuous eight hour period for which the mean concentration is higher than for any other similarly continuous period. The World Health Organisation (WHO) chose a fixed set of 8-hour periods, 00.00 -7.59, 08.00 -15.59 and 16.00 -23.59, so clearly peak concentrations recorded on this basis will be slightly lower than those on the running average basis for a given ozone exposure. The 50 ppb maximum 8-hour running average employed here Is equivalent to 37- 48 ppb on the rigid and arbitrary definition of averaging periods used by WHO. The WHO equivalent value appears as a range because of the variability of ozone concentrations both between sites and at the same site on different days.
20. Our examination of levels of ozone recorded in the United Kingdom leads us to recognise that the Standard may be exceeded (Figure 7) and that when this occurs 8-hour ozone concentrations may be considerably greater than 50 ppb. The highest 8-hour concentration recorded in a year has been shown (Figure 8) to increase with the number of times that the recommended Standard (50 ppb, running 8-hour average concentration) is exceeded. We note that if the recommended Standard is exceeded on less than 10 days per year, at any one site, one would not expect, in most years, the highest 8-hour concentration to exceed 100 ppb (Figure 8).
21. Present ozone concentrations show exceedences of a running 8-hour average of 50 ppb at some sites in the order of 80 days per year (Figure 7), and, in recommending such a Standard, the Panel recognise that its achievement will require continued determined action to reduce the major sources of primary pollutants in the United Kingdom and the rest of Europe.