Beijing Sets New Rules to Automatically Trigger 'Red' Smog Warnings

(Beijing) — Beijing has revised its air pollution alert system to allow environmental officials to issue "red" smog alerts in the city when air-quality readings exceed 500 — a move that appears meant to assuage public criticism over the city's perceived inadequate response to dangerous smog levels.

The air quality index (AQI) measures the level of pollutants in the air, especially the concentration of PM2.5 — fine cancer-causing particles smaller than 2.5 micrometers in diameter — as well as levels of PM10 pollutants and poisons such as sulfur dioxide.