PM2.5: fine particle pollution
Learn what PM2.5 measures, why fine particles can travel beyond their source and how to read a concentration or forecast.
What PM2.5 measures
PM2.5 is the mass of airborne particles with an aerodynamic diameter of about 2.5 micrometres or smaller. It describes a size class that includes many different substances. Smoke from combustion contains fine particles, and gases emitted by activities such as transport, industry and agriculture can react in the air to form additional particles.
Because these particles are small enough to reach deep into the lungs, they are an important measure in outdoor air quality monitoring. A PM2.5 value reports particle mass per volume of air. Its value alone cannot identify which sources produced the particles.
Why a city forecast can change
Fine particles can be transported from outside the immediate area. Local combustion, regional pollution and weather that limits dispersion can all affect a reading. When checking a change on a city page, compare the same pollutant across hours and note whether the chart shows a station or model.
PM2.5 is included in PM10. Adding the two concentrations would count the fine fraction twice. Compare each value with its own index band and time basis.
Worked example: reading PM2.5 alongside PM10
Suppose one station reports 12 µg/m³ of PM2.5 and 28 µg/m³ of PM10 for the same hour. The difference is 16 µg/m³. This gives an approximate concentration for the coarser fraction between the two size cutoffs. Compare the same station, averaging period and units before subtracting.
Both readings fall in the Fair band for their respective pollutants. Another pollutant could still set a worse overall index. The difference between the particle readings measures mass. Identifying the source would require more evidence, such as particle composition or local monitoring information.