PM2.5: what it means and how to use the reading

PM2.5 describes particle size

PM2.5 is particulate matter with a diameter generally 2.5 micrometres or smaller. It is a size category, not one chemical and not an all-purpose verdict on the day. EPA notes that particle pollution can include solids and liquid droplets from sources such as fires, roads, industry and chemical reactions in the air.

A concentration is not an AQI

A PM2.5 reading is usually a concentration, commonly expressed in micrograms per cubic metre. An AQI is a separate communication scale that translates one or more pollutant readings using local rules. Check the unit, the averaging period and the source before comparing a concentration with an index number or with a result from another country.

Below a limit does not mean harmless

No concentration threshold has been identified for particulate matter below which health damage is not observed. A legal limit, a WHO guideline and an index category boundary serve different purposes; none acts as a switch between harmful and harmless air. If a particular limit were 25 µg/m³, a reading of 24 µg/m³ would be below it, but breathing that air every day for months or years would not become harmless. This is a hypothetical example, not the current WHO recommendation for PM2.5.

Check the averaging period. An hourly reading describes one hour, a daily average covers a whole day, and an annual average helps assess long-term exposure. One reading of 24 µg/m³ does not establish the daily or annual average. WHO’s 2021 PM2.5 guidelines recommend an annual mean of 5 µg/m³ and a 24-hour value of 15 µg/m³, assessed as the annual 99th percentile, allowing about 3–4 exceedance days. A constant concentration of 24 µg/m³ would exceed both values. WHO guidelines are health recommendations, not legally binding standards.

Use the local context

Look at the monitor location and its timestamp before a long outdoor activity. A nearby reading can inform a smaller decision, such as choosing a shorter route or checking again later; it does not describe every street or diagnose an individual health effect. WHO identifies particulate matter as a common proxy for air pollution and links exposure with adverse health effects.