PM2.5

PM stands for particulate matter, the mixture of solid particles and liquid droplets suspended in air. The number is a size. PM2.5 means particles with a diameter of generally 2.5 micrometres and smaller. The United States Environmental Protection Agency puts that in scale by comparing it with hair: an average human hair is about 70 micrometres across, roughly 30 times wider than the largest particle that counts as PM2.5.

Size is what makes these particles dangerous. Particles less than 10 micrometres in diameter can get deep into the lungs, and some may even get into the bloodstream. They are not one substance. They come in many shapes and can be made up of hundreds of different chemicals. Some are emitted directly from a source, such as construction sites, unpaved roads, fields, smokestacks or fires, but most form in the atmosphere through reactions involving chemicals such as sulphur dioxide and nitrogen oxides, which are released by power plants, industry and vehicles.

Exposure is measured as a concentration, so PM2.5 levels are quoted in micrograms per cubic metre of air. The World Health Organization's 2021 global air quality guidelines set the recommended annual level at 5 micrograms per cubic metre, and European health agencies report the deaths attributable to the exposure people experience above that line.