What is PM2.5?
In one line: PM2.5 is fine airborne particle pollution — particles 2.5 microns across or smaller. They're small enough to reach deep into the lungs, which is why PM2.5 is the pollution most closely linked to health effects, and the main thing a home air purifier is there to remove.
What it is
"PM" stands for particulate matter; the number is the particle size in microns. PM2.5 means particles up to 2.5 microns — about 30 times thinner than a human hair. (You may also see PM10, which includes coarser particles up to 10 microns.) Because PM2.5 is so fine, it stays suspended in the air for a long time and can be inhaled deep into the lungs and even enter the bloodstream.
Where indoor PM2.5 comes from
- Cooking — frying and high-heat cooking are among the biggest indoor sources.
- Outdoor air getting in — traffic, wood smoke and urban pollution seep indoors.
- Smoke — tobacco, candles, wood burners, wildfires and bonfires.
- Dust and allergens — the finer fraction of dust, pollen fragments and pet dander.
How air purifiers remove it
A purifier removes PM2.5 by drawing room air through a filter fine enough to trap it. Both true HEPA and MERV-13 capture the PM2.5 fraction well. What decides how quickly your room's PM2.5 falls is the purifier's clean-air delivery rate and the resulting air changes per hour — a higher clean-air delivery clears a pollution spike (like cooking or outdoor smoke drifting in) far faster.
A useful habit: pair a purifier with a cheap PM2.5 monitor. You'll see levels spike when you cook or when outdoor air is bad, and watch the purifier bring them down — which also tells you it's sized right for the room.
Next step
Choosing a unit to keep PM2.5 low? Compare picks at Best Air Purifier UK. Dealing specifically with smoke or cooking? See the smoke and odour guidance there.
FAQ
What's a safe PM2.5 level? Lower is better; the WHO's air-quality guidance sets stringent targets and there's no threshold below which fine particles are known to be completely harmless — so reducing indoor exposure is worthwhile.
Do purifiers remove gases and smells too? Particle filters (HEPA/MERV-13) capture PM2.5 but not gases or odours — for those you need activated carbon in addition to the particle filter.