PM2.5 and PM10: the difference, and which one to worry about
Updated 2 min read
PM stands for particulate matter, the particles suspended in air. The number after it is the maximum diameter in micrometres. PM10 means particles 10 micrometres and smaller; PM2.5 means 2.5 micrometres and smaller.
For scale, a human hair is about 70 micrometres across. PM2.5 is roughly 30 times finer than that.
Why size decides the harm
Larger particles are mostly caught in the nose and throat. PM2.5 is fine enough to travel deep into the lungs, reaching the alveoli, and some of it crosses into the bloodstream.
That is why PM2.5 is linked to more serious effects: not just coughing and breathlessness, but heart and blood vessel effects when exposure goes on.
Which one is driving your reading
During haze it is almost always PM2.5. Smoke from biomass burning produces very fine particles. Road dust, construction sites and dry ground tend to raise PM10 instead.
SemakAPI shows the main pollutant on every city card, so you can see which one set the number that hour.
From concentration to index
Concentration is measured in micrograms per cubic metre (µg/m³). The Department of Environment converts it to a sub-index using official breakpoint tables, and the API is the highest sub-index.
| PM2.5 (24-hour average) | Sub-index |
|---|---|
| 0 – 12.0 µg/m³ | 0 – 50 |
| 12.1 – 50.4 µg/m³ | 51 – 100 |
| 50.5 – 55.4 µg/m³ | 101 – 150 |
| 55.5 – 150.4 µg/m³ | 151 – 200 |
| 150.5 – 250.4 µg/m³ | 201 – 300 |
Note the 24-hour average. The PM2.5 and PM10 indices are defined on a 24-hour mean, not an instantaneous reading. That is why the API does not spike the moment smoke arrives, and does not drop the moment it passes.
The Department of Environment publishes the index but not the concentrations. The µg/m³ figures shown on this site are worked back from the official index using the same tables, for the main pollutant only.