An index is a translation layer
US EPA Air Quality Index categories
An air-quality index turns pollutant data into categories meant to communicate health concern. The US AQI covers five regulated pollutants, including particle pollution and ozone. The highest relevant pollutant sub-index generally drives the headline category, so a favourable PM2.5 result alone does not establish that every pollutant is favourable.
AQI systems are local
The US AQI is EPA’s system; it is not a universal scale. Europe also has an EEA index, and national or regional services may use their own categories, breakpoints and reporting rules. Compare source, index name and pollutant before comparing colours or numbers across services.
Averaging changes the question
AirNow explains that the US daily AQI uses an 8-hour basis for ozone and a 24-hour basis for particle pollution, while its current NowCast draws on multiple recent hours. A current display, a daily index and a forecast can all be valid while showing different values. Read the label before acting on it.
A good index does not establish year-round safety
An index category boundary, a legal limit and a WHO health guideline are different things. No threshold has been identified for particulate matter below which health damage is not observed. A favourable current index or a concentration below a limit therefore does not guarantee that breathing that air every day for years is harmless. An hourly reading cannot replace a daily average, and a daily average does not describe a whole year. Match pollutant, unit and averaging period when comparing values; long-term exposure needs data over a longer period.
Sources
- AirNow: AQI Basics ↗
- AirNow: Using the AQI ↗
- EEA: European Air Quality Index ↗
- WHO: Global Air Quality Guidelines (2021) ↗
- WHO: Particulate matter and risk at low concentrations ↗
Educational information; personal health questions belong with your clinician.
Explore next
- Air quality forecast in the Atmiq app: how to read it
- Why air-quality apps disagree
- PM2.5: what it means and how to use the reading
- PM10: what it is and how it differs from PM2.5
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