This paper estimates the causal effect of increased ambient air pollution on the frequency of road traffic accidents in the United Kingdom between 2009 and 2014. An instrumental variable approach is applied, exploiting atmospheric temperature inversions as a source of plausibly exogenous variation in daily air pollution levels. The paper estimates the local average treatment effect for a geographic grid cell (1° X 1º) as an increase of 0:3 accidents per day for each additional 1μg=m³ in the daily concentration of NO2. The effect is equivalent to an increase of 2% relative to the average number of daily accidents. The results appear robust to multiple sources of potential confounding, measurement error and co-emission bias.

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