Cities tackling one major air pollutant risk inadvertently making things worse by fuelling the growth of another, potentially more harmful type of pollution.
Many urban areas around the world are in breach of World Health Organization guidelines on PM2.5, particulate matter with a maximum diameter of 2.5 micrometres. Vehicles are a common source of this kind of pollution.
But simply reducing levels of PM2.5 pollution may not improve the safety of urban air. A Chinese-US team has found that PM2.5 plays a key role in suppressing the formation of another type of pollution in built-up areas – “ultrafine particles”. These have a diameter of under 50 nanometres, and an emerging body of work has linked them to health concerns including birth defects.