Results may have been confounded by viral infections and faulty equipment

doi:10.1136/bmj.312.7046.1607b

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Graham Bentham,
Peter Brimblecombe

Senior lecturer Professor School of Environmental Sciences University of East Anglia Norwich NR4 7TJ

EDITOR,—Roger Buchdahl and colleagues report that hospital attendances for acute childhood wheezy episodes were significantly elevated on days when ozone concentrations were very high and very low.1 They suggest that this U shaped relation may indicate an optimal protective concentration of atmospheric ozone, with adverse health effects being associated with higher or lower values. An alternative explanation is that the observed association is the result of confounding by exposure to viral infections that are known to be …