The second edition of the WHO AQG (WHO, 2000) set the guideline value for ozone at 120 µg/m3 for an 8-hour daily average. Since the mid- 1990s there has been no major addition to the evidence from chamber studies or field studies. There has however been a marked increase in health effects evidence from epidemiological time-series studies. Combined evidence from those studies show convincing, though small, positive associations between daily mortality and ozone levels, independent of the effects of particulate matter. Similar associations have been observed in both North America and Europe. These time-series studies have shown effects at ozone concentrations below the previous guideline of 120 µg/m3 without clear evidence of a threshold. Evidence from both chamber and field studies also indicate that there is considerable individual variation in response to ozone. In view of these considerations, there is a good case for reducing the WHO AQG from the existing level of 120 µg/m3.

It is possible that health effects will occur below this level in some sensitive individuals. Based on time-series studies, the number of attributable deaths brought forward can be estimated at 1-2% on days when ozone concentration reaches this guideline level as compared with the background ozone level.

There is some evidence that ozone also represents unmeasured toxic oxidants arising from similar sources. Measures to control ozone are also likely to control the effects of these pollutants.

Hemispheric background concentrations of tropospheric ozone vary in time and space but can reach average levels of around 80 µg/m3. These arise from both anthropogenic and biogenic emissions of ozone precursors and downward intrusion of stratospheric ozone into the troposphere. The proposed guideline value may occasionally be exceeded due to natural causes.

There is some evidence that long-term exposure to ozone may have chronic effects but it is not sufficient to recommend an annual guideline.