Each year, more than 4 million people die early because of outdoor air pollution, according to the World Health Organization (WHO). The main culprits are fine particles with diameters of 2.5 micrometres or less (PM 2.5). These can penetrate deep into the lungs, heart and bloodstream, where they cause diseases and cancers.

But global average estimates such as this assume that these particles are the same the world over. They are not: PM 2.5 is a cocktail of chemicals (hydrocarbons, salts and other compounds given off by vehicles, cooking stoves and industry) and other, natural components such as dust and microorganisms. The mix — and its toxicity — varies from place to place and over time, in ways that are not tracked, understood or managed.