Hypothesis: Ill health associated with low concentrations of nitrogen dioxide--an effect of ultrafine particles?

The epidemiological associations between illness and nitrogen dioxide may be the consequence of confounding by particle numbers In 1996 the Expert Panel on Air Quality Standards (EPAQS) recommended an ambient air standard for nitrogen dioxide (NO2) in the UK of 150 ppb measured hourly.1 This recommendation, like those for carbon monoxide (CO) and sulphur dioxide (SO2) that had preceded it, was based on human toxicology rather than on epidemiology. The EPAQS was unable to find evidence that these gases were likely to be toxic to humans at the recommended concentrations. However, at the time of the NO2 recommendation there was already epidemiological evidence that effects on populations rather than individuals might be associated with much lower concentrations and the EPAQS recommended that steps be taken to reduce annual average concentrations, although without proposing a long term standard. The UK government has subsequently adopted, as targets to be achieved by 2005, World Health Organization NO2 guideline standards of 105 ppb (200 μg/m3) over 1 hour and 21 ppb (40 μg/m3) as an annual average, the latter having been based on possible relationships between exposure to the gas and respiratory illness in children.2 Achievement of a long term standard does, of course, have the desirable consequence of reducing peaks and therefore short term exccedences. However, compliance with a very low average concentration of NO2 implies a substantial reduction in the concentration of the primary pollutant released from vehicle exhausts—that is, nitric oxide (NO). Since NO reacts with ozone to form NO2, lower concentrations will result in raised urban ozone concentrations, a gas that also has known toxic effects on the lungs and that, until now, has been seen primarily as a rural pollutant in the UK. Progressive reductions in pollution are …