Air quality and pollution

What AQI means

The Air Quality Index combines several pollutants into a single 0–500 scale. PM2.5 (particulate matter under 2.5 microns) is usually the binding constraint in Chinese cities. Bands:

• 0–50 — good. No precautions.
• 51–100 — moderate. No precautions for most people.
• 101–150 — unhealthy for sensitive groups (asthmatics, children, elderly).
• 151–200 — unhealthy. Mask if outdoors for extended periods.
• 201–300 — very unhealthy. Limit time outdoors. Mask essential.
• 301+ — hazardous. Stay indoors with air purifier.

Apps

• AQICN (aqicn.org and app)
• AirVisual (now IQAir)
• The native iOS and Android weather apps include AQI in mainland Chinese cities.

Masks

If AQI exceeds 150 and you have respiratory sensitivity, an N95 / FFP2-rated mask helps. KN95 is the Chinese-standard equivalent and widely sold (¥3–¥10 per mask). Cloth masks don't filter PM2.5.

Worst months by region

• Beijing, Tianjin, Hebei: November–February (heating season, more coal). Spring dust storms in March–April.
• Shanghai, Yangtze Delta: November–January (winter inversions).
• Chengdu, Sichuan basin: persistent low-grade haze due to topography. Winter worse.
• Pearl River Delta, Hong Kong: generally cleaner; January–February occasional spikes.
• Xinjiang, Lanzhou, Gansu: dust storms in spring; otherwise dry-clean.
• Yunnan, Hainan, Tibet: typically very clean throughout the year.

What's improved

Beijing's air has improved substantially since 2013 — annual mean PM2.5 fell roughly 50%. Major coal-to-gas conversion in the residential sector and factory relocations away from the city are the main reasons. The improvement is real, but bad days still happen and 'good' Beijing air is not as good as a Western capital's average.