Air quality index levels
| US AQI | Level | What to do |
|---|---|---|
| 0–50 | Good | Enjoy the outdoors as usual |
| 51–100 | Moderate | Unusually sensitive people may limit long, hard exercise outside |
| 101–150 | Unhealthy for sensitive groups | People with asthma, heart or lung disease, older adults and children should take it easier |
| 151–200 | Unhealthy | Everyone should cut down hard exercise outdoors |
| 201+ | Very unhealthy to hazardous | Stay indoors with windows closed if you can |
The index is worked out from PM2.5 and PM10 using the US EPA breakpoints revised in 2024, taking whichever is worse. Official indexes average particles over 24 hours, so a short spike, such as smoke from a bonfire, shows higher here than in a daily report.
WHO air quality guidelines
| Pollutant | WHO guideline | Main sources |
|---|---|---|
| PM2.5 | 15 µg/m³ (24-hour) | Traffic, wood burning, industry, wildfire smoke |
| PM10 | 45 µg/m³ (24-hour) | Dust, construction, road and brake wear |
| NO₂ | 25 µg/m³ (24-hour) | Road traffic, especially diesel |
| Ozone | 100 µg/m³ (8-hour) | Forms in sunshine from other pollutants |
The World Health Organization tightened these levels in 2021. Many cities are above them on some days even when the national index says the air is fine, so a reading above the guideline is a nudge to take it easy, not a reason for alarm.
UV index and sun protection
- 0–2, low: little risk for most people.
- 3–5, moderate: wear sunscreen, a hat and sunglasses, and seek shade around midday.
- 6–7, high and 8–10, very high: SPF 30 or higher, cover up, and stay in the shade in the middle of the day.
- 11 and above, extreme: unprotected skin can burn in minutes; avoid the midday sun.