About AirCheck
Track air quality (AQI) in real-time across major cities worldwide. Every breath. Everywhere.
What this is
Air quality describes how clean or polluted the air is, and what that means for your health. The single number you usually see is the Air Quality Index (AQI) — a 0–500 scale that converts the measured concentration of several pollutants into one easy-to-read value with a color, from green “Good” to maroon “Hazardous”.
AirCheck pulls live readings from thousands of ground sensor stations worldwide, normalizes each pollutant onto the AQI scale, and shows you the worst (highest) pollutant for every place — because on any given day a city’s air is only as healthy as its dominant pollutant.
Learn the basics
All articles →- 🎚️ How the Air Quality Index worksWhy one pollutant’s concentration becomes a 0–500 number, the six categories, and how the reported AQI is the worst pollutant, not an average.
- 🧪 The pollutants that drive the indexPM2.5, PM10, ozone, nitrogen dioxide, sulfur dioxide and carbon monoxide — what each is, where it comes from, and why it matters.
- 🫁 What polluted air does to the bodyShort- and long-term effects, and why children, older adults, pregnant people and those with heart or lung conditions are most at risk.
- 😷 Protecting yourself on bad-air daysPractical steps by AQI category — timing outdoor activity, filtering indoor air, masks that actually work, and when smoke changes the rules.
- 📡 Where the numbers come fromReference-grade monitors vs. low-cost sensors, why two nearby readings can differ, and how AirCheck normalizes them.
Health guidance by AQI category
General-public guidance below; sensitive groups should act one category earlier. This is informational, not medical advice — follow your doctor’s plan for any condition.
Good (0–50) · Moderate (51–100)
- Air quality is acceptable; most people can be active outdoors.
- Unusually sensitive individuals may notice symptoms at the upper Moderate range and can ease intense, prolonged exertion.
Unhealthy for Sensitive Groups (101–150)
- Children, older adults and people with heart or lung disease should reduce prolonged or heavy outdoor exertion.
- Watch for coughing or shortness of breath; take more breaks and lower the intensity.
Unhealthy (151–200) · Very Unhealthy (201–300)
- Everyone should reduce or reschedule outdoor exertion; sensitive groups should avoid it.
- Keep windows closed, run a HEPA purifier, and wear a fitted N95/FFP2 if you must go out.
Hazardous (301–500)
- Health-warning conditions — everyone should stay indoors and keep activity light.
- Create a clean-air room, avoid adding indoor smoke or fumes, and follow local emergency guidance.
Data sources
- Open-Meteo Air Quality (CAMS) — Current PM2.5 and US AQI from the Copernicus Atmosphere Monitoring Service (CAMS) model, served key-free by Open-Meteo.
- US EPA AirNow — Official US AQI and the breakpoint methodology used to convert concentrations to the index.
- WHO Air Quality Guidelines (2021) — Global health-based guideline limits for PM2.5, PM10, ozone, NO2 and SO2.
Frequently asked
- Is a lower or higher AQI better?
- Lower is better. 0–50 (green) is Good; the scale climbs through yellow, orange, red and purple to maroon (301–500) Hazardous. Each step up means more people should reduce outdoor exertion.
- Why does the AQI here differ from another app?
- Apps use different stations, different source networks (reference-grade vs. low-cost sensors), and sometimes different national AQI formulas. We show the station and source for each reading so you can see where a number comes from.
- What counts as a dangerous AQI?
- From 101 (orange) sensitive groups should take care; from 151 (red) everyone should reduce prolonged outdoor exertion; 201+ (purple/maroon) is very unhealthy to hazardous for all — move activity indoors and keep it light.
- Do masks help with air pollution?
- A well-fitted N95/FFP2 respirator filters fine particles like PM2.5 and wildfire smoke. Cloth and surgical masks do little for particulate pollution, and no mask helps much with gaseous ozone.
- Does “Good” air mean there’s no pollution?
- No — it means pollution is low enough to pose little risk to most people. There is no concentration of fine particles proven completely safe, so the WHO guideline limits are stricter than many national standards.
⚠️ Important
AirCheck is an informational service, not an official health authority. Readings can be delayed, estimated or missing, and low-cost sensors are less precise than regulatory monitors. For health decisions follow your doctor and your national air-quality agency.