London just put out a high air pollution alert today, even though the AQI was 76.
After a weekend of thick fog and still air, tiny pollutant particles built up to five times above safe limits.
The warning came on expert advice, with Mayor Sadiq Khan asking people at risk to be extra careful.
London City Hall issues health advice
Smoke from Bonfire Night, Diwali fireworks, and wood burning made things worse.
City Hall sent alerts to residents and schools, and signs went up across town.
Londoners are being asked to help out by taking public transport, avoiding busy roads, and not burning waste.
Deputy Mayor for Environment and Energy Mete Coban pointed out that toxic air is a serious health issue behind thousands of early deaths in the city each year.