Air Quality FAQ — AQI, PM2.5 & Pollution Explained

What is AQI (Air Quality Index)?

The Air Quality Index is a standardized scale from 0 to 500 that measures air pollution levels. Lower values mean cleaner air. CERNS.IO uses the US EPA AQI standard, which is the most widely adopted globally.

AQI Range	Level	Health Advice
0–50	Good	Air quality is satisfactory. Enjoy outdoor activities.
51–100	Moderate	Unusually sensitive people should reduce prolonged outdoor exertion.
101–150	Unhealthy for Sensitive Groups	Sensitive groups should reduce prolonged outdoor exertion.
151–200	Unhealthy	Everyone should reduce prolonged outdoor exertion.
201–300	Very Unhealthy	Everyone should avoid outdoor exertion.
301–500	Hazardous	Everyone should stay indoors and keep windows closed.

What is PM2.5?

PM2.5 refers to fine particulate matter smaller than 2.5 micrometers in diameter — about 30 times smaller than a human hair. These tiny particles can penetrate deep into the lungs and enter the bloodstream, causing respiratory and cardiovascular problems. The WHO guideline for 24-hour average PM2.5 is 15 µg/m³.

What is PM10?

PM10 refers to particulate matter smaller than 10 micrometers. These coarser particles include dust, pollen, and mold spores. While they don't penetrate as deeply as PM2.5, they can still irritate the respiratory tract. The WHO guideline for 24-hour average PM10 is 45 µg/m³.

What is the UV Index?

The UV Index measures the intensity of ultraviolet radiation from the sun. Values range from 0 (nighttime) to 11+ (extreme). At UV Index 3 or above, sun protection (sunscreen, hat, shade) is recommended. At 8+, avoid prolonged sun exposure.

Where does CERNS.IO get its data?

CERNS.IO aggregates data from multiple sources:

• Air quality stations — ground-level monitoring stations via AQICN (thousands of stations worldwide)
• Weather forecasts — ECMWF IFS (Integrated Forecasting System), one of the world's most accurate weather models
• AQI forecasts — ECMWF CAMS (Copernicus Atmosphere Monitoring Service) for PM2.5 and PM10 predictions

How often is the data updated?

Station readings are updated every 1–2 hours depending on the monitoring network. Weather forecasts are updated every 6 hours from ECMWF model runs. City AQI values combine station readings with model forecasts, using bias correction when both are available.

Why might the AQI differ from other websites?

Different websites may use different AQI standards (US EPA, China MEP, India NAQI), different averaging periods, or different data sources. CERNS.IO uses the US EPA AQI standard with station-primary data when available, falling back to bias-corrected model estimates.

How many cities does CERNS.IO cover?

CERNS.IO provides air quality and weather data for over 33,000 cities worldwide, sourced from the GeoNames cities15000 dataset (all cities with population above 15,000).

When should I wear a mask outdoors?

Consider wearing an N95 or KN95 mask when PM2.5 exceeds 35.5 µg/m³ (AQI above 100). At AQI 150+, masks are strongly recommended for everyone. Standard surgical masks do not effectively filter PM2.5.

Should I use an air purifier indoors?

When outdoor AQI exceeds 100, running a HEPA air purifier indoors can significantly reduce indoor PM2.5 levels. Keep windows and doors closed during high pollution episodes.