The unofficial air

Methodology

Live PM2.5 readings from Sensor.Community, a global network of self-built citizen sensors. This sits deliberately beside the official LondonAir gauge elsewhere in this atlas: the official network is about a hundred calibrated sites publishing an index the government stands behind, and this is hundreds of uncalibrated ones publishing whatever is in front of them. Sensors marked indoor are dropped, and readings of zero or above 1,000 µg/m³ are treated as a stuck device rather than as air. Nothing here is an official measurement. Colour is banded against the WHO 24-hour guideline of 15 µg/m³.