Hazardous Air Quality (AQI 301–500)
Health emergency — the entire population is likely to be affected. This is a rare, extreme pollution event.
0–50
51–100
101–150
151–200
201–300
301–500
What Does This Mean?
PM2.5 exceeds 250.5 µg/m³ — over 17× the WHO guideline and 7× the EPA standard. This is a public health crisis level. Common only during the worst episodes: Delhi's Diwali season, extreme crop burning in Chiang Mai/Chiang Rai, or intense wildfire smoke. Visibility may drop below 1 km.
Who Is Affected?
- The entire population — healthy adults will experience respiratory symptoms
- Emergency room visits for respiratory and cardiac events spike significantly
- There is no safe outdoor exposure duration at this level
What Should You Do?
- Stay indoors as much as possible
- Keep all windows and doors sealed
- Run HEPA air purifiers on maximum — in rooms where you spend the most time
- If you must go outside, wear a properly fitted N95 mask (not KN95 — fit matters at these levels)
- Avoid all physical exertion outdoors
- Sensitive groups should strongly consider relocating temporarily if levels persist
- Schools should close or move to remote learning
- Seek medical attention immediately for chest pain, difficulty breathing, or severe wheezing
Key Pollutants at This Level
PM2.5 is the sole concern. At AQI 301+, PM2.5 concentrations are catastrophic. Other pollutants are secondary.