A threshold of effect is a level below which a substance has no effect at all although it is present
Recent research has shown that, at a population level, no thresholds of effect can be identified for several of the common air pollutants.
Until about 1990 it was believed by many that it should be possible to define concentrations of air pollutants that had no effects on health. Such levels could then be used as a basis for setting standards. Recent research has shown that, at a population level, no thresholds of effect can be identified for the common air pollutants.
Though epidemiological studies relate effects to measured concentrations of pollutants it is likely that there is in fact a distribution of exposure across the population.
We also need to take into account the likely wide range of sensitivity of people to air pollutants. Epidemiological data suggest that for ozone, nitrogen dioxide and sulphur dioxide, some individuals may be much more sensitive than others, responding to much lower levels of pollutants. Their individual thresholds may differ. Furthermore, a wide range of individual thresholds can be identified in studies of human volunteers exposed to ozone or sulphur dioxide. Some people – for example those with asthma – fall into a sensitive sub-population with regard to the effects of sulphur dioxide on the airways.
The combination of a likely distribution of sensitivity and a likely distribution of exposure makes it difficult to be sure whether a threshold does or does not exist at a population level.
When considering the Air Quality Index (AQI) COMEAP recognised the possibility that there may be no threshold for the health effects of air pollutants. Despite this, the committee agreed that the AQI provides useful information on the possible effects on health at different pollution levels in the short-term, and identifies individuals likely to be most susceptible.
For ozone, the 2012 report 'The Health effects of Climate Change in the UK' published calculations for short-term exposure to ozone and the effects on health. These were quantified using no threshold and thresholds of 70 µg/m3 and 100 µg/m3 for health effects.
COMEAP Long-Term Exposure to Air Pollution: Effect on Mortality (2009) PDF,1.51 MB [specifically the working paper ' Thresholds and Non-Linearities']
HPA The Health effects of Climate Change in the UK 2012 [external link]