Improving air quality in subway systems: An overview.

This article comments on the current reality of particulate matter (PM) concentrations breathed by commuters on subway train platforms and considers what can be done to improve air quality underground. We propose the introduction of a targeted, color-coded approach to the problem, based on the methodology of the World Health Organisation and designed to encourage transport authorities to aim for progressive PM reductions. The method defines thresholds that cascade down through bands of decreasing PM concentrations towards the ideal WHO Air Quality Guideline of PM2.5 annual mean level of 10 μg m-3, where negative health effects of long term particle inhalation are minimal.