AirCheck — glossary

Name: AirCheck live feed
Creator: AirCheck

Plain-language definitions of the terms used across AirCheck (Open-Meteo (CAMS)).

The index

Air Quality Index (AQI): A 0–500 scale that converts pollutant concentrations into one health-oriented number with six color-coded categories. The reported value is the highest (worst) of the individual pollutant sub-indices, not an average.
Breakpoints: The concentration values that anchor each AQI category for a pollutant. The index is interpolated linearly between them — e.g. PM2.5 of 12.0 µg/m³ = AQI 50, 35.4 µg/m³ = AQI 100.
Responsible (dominant) pollutant: The pollutant with the highest sub-index at a location — the one that sets the reported AQI and the one whose guidance you should follow that day.
Sensitive groups: People who feel effects at lower pollution levels: children, adults over 65, pregnant people, and anyone with asthma, heart disease or lung disease. The orange category (101–150) is named for them.

PM2.5: Fine particulate matter ≤2.5 µm across — small enough to reach deep lung tissue and the bloodstream. The pollutant that most often drives unhealthy air; comes from combustion and wildfire smoke.
PM10: Coarse particulate matter ≤10 µm — dust, pollen, mold and construction debris. The main pollutant during dust storms and dry, windy weather.
Ozone (O3): Ground-level ozone, formed when sunlight reacts with nitrogen oxides and VOCs from traffic and industry. A hot-afternoon, sunny-season pollutant that can be high downwind of cities.
Nitrogen dioxide (NO2): A reddish-brown gas from vehicles and combustion; a marker of fresh traffic pollution that irritates airways and helps form ozone.
Sulfur dioxide (SO2): A sharp-smelling gas from burning sulfur-rich fuels and metal smelting; irritates the respiratory system and forms fine sulfate particles.
Carbon monoxide (CO): A colorless, odorless gas from incomplete combustion. Outdoors it is elevated near heavy traffic; indoors it can be acutely dangerous.

µg/m³: Micrograms of a pollutant per cubic meter of air — the concentration unit for particulate matter, converted to AQI via breakpoints.
Reference-grade monitor: A calibrated regulatory instrument that meets official accuracy standards. Accurate but sparse; the authoritative source where available.
Low-cost sensor: An inexpensive particle sensor that enables dense networks. Greatly improves coverage but is less precise, and tends to over-read in high humidity unless corrected.
Averaging period: The time window a pollutant is averaged over before conversion (e.g. ozone over 8 hours, PM over a 24-hour basis). Why the AQI reflects recent air, not an instantaneous spike.