What is PM2.5?

Air Purifier Directory · Plain-English reference · Updated October 2026

What it is

"PM" stands for particulate matter and the number is a size ceiling in micrometres. PM2.5 means everything up to 2.5 µm — roughly thirty times thinner than a human hair. It is a size category, not a substance: wood smoke, cooking particles, brake dust and diesel soot are chemically unalike but all counted as PM2.5 if they are small enough.

That matters when reading any measurement. A monitor reporting PM2.5 is telling you how much fine material is in the air, not what it is or where it came from.

The size scale, and what lives at each size

The right-hand column is the reason purifiers exist. Coarse dust falls out of the air by itself within minutes — sweeping and wiping deal with it. Fine particles are light enough that ordinary air movement in a room keeps them aloft far longer than anyone wants to wait, so something has to actively take them out. That is the job.

Where indoor PM2.5 comes from

• Cooking — frying and high-heat cooking are among the largest indoor sources, and a kitchen spike can easily exceed anything coming in from outside.
• Outdoor air getting in — traffic, wood smoke and urban pollution seep indoors continuously, even with windows shut.
• Smoke — tobacco, candles, wood burners, wildfires and bonfires.
• Dust and allergens — the finer fraction of household dust, pollen fragments and pet dander.

A point worth sitting with: for most people, on most days, the biggest fine-particle exposure of the day happens in their own kitchen rather than on the street.

What the numbers mean

PM2.5 is reported in µg/m³ — microgrammes of material per cubic metre of air. The World Health Organization's 2021 guideline values are 5 µg/m³ as an annual average and 15 µg/m³ averaged over 24 hours. These are guideline levels rather than a safety line: no threshold is known below which fine particles are entirely harmless, which is why the advice is simply that lower is better.

Averages, though, hide the thing a purifier is actually for. Indoor PM2.5 is not a steady background — it is long quiet periods punctuated by sharp spikes when something burns or fries. A room can sit comfortably low all day and then jump many times over during twenty minutes of cooking. Two homes with identical daily averages can have very different peak exposures, and it is the peaks a well-sized unit flattens.

Why a mass reading under-counts the smallest particles

Here is a subtlety that monitor readings never show. µg/m³ is a measure of mass, and mass scales with the cube of diameter. Halve a particle's size and it weighs roughly an eighth as much.

So the ultrafine fraction — the particles small enough to travel furthest into the lungs — contributes almost nothing to the number on the display. A reading of 10 µg/m³ might be a modest number of larger fine particles, or an enormous number of tiny ones, and the monitor cannot tell you which. Frying and candles in particular produce vast quantities of ultrafines while barely moving a mass-based reading.

This is not a reason to distrust monitors, which are genuinely useful. It is a reason not to treat a low number as proof that nothing is happening — and a reason to run the purifier while you cook rather than waiting for the display to justify it.

How air purifiers remove it

A purifier draws room air through a filter fine enough to trap fine particles. Both true HEPA and MERV-13 capture the PM2.5 fraction well — and, counter-intuitively, both capture ultrafines better than they capture PM2.5, because very small particles drift into filter fibres by diffusion rather than having to be intercepted.

What decides how quickly your room's PM2.5 actually falls is not the filter grade but the clean-air delivery rate and the air changes per hour it produces in your particular room. A spike from cooking clears in roughly half an hour at 5 air changes per hour, and takes most of a morning at 2.

Using a monitor sensibly

An inexpensive PM2.5 monitor is the fastest way to understand your own home, and pairing one with a purifier tells you whether the unit is correctly sized — you can watch a spike appear and time how long it takes to come down.

Two caveats worth knowing. Low-cost sensors estimate mass optically, by shining a light through the air and interpreting the scattering, then converting to µg/m³ using assumptions about particle density. Different aerosols break those assumptions differently, so absolute accuracy is limited. And humidity inflates readings, because water condenses onto particles and makes them scatter more light — a damp bathroom can look polluted when it is merely damp.

Treat the readings as a reliable guide to changes in your own home rather than as laboratory-grade absolute values. For deciding whether to run the purifier, that is all you need.

FAQ

What's a safe PM2.5 level? The WHO's 2021 guidelines are 5 µg/m³ annually and 15 µg/m³ over 24 hours. There is no known threshold below which fine particles are completely harmless, so reducing exposure is worthwhile at any level.

Is PM2.5 worse than PM10? They overlap — PM10 includes PM2.5. The fine fraction gets more attention because it stays airborne far longer and penetrates deeper into the lungs.

Do purifiers remove gases and smells too? Particle filters capture PM2.5 but not gases or odours — those need activated carbon alongside the particle filter.

Should I open a window to clear cooking smoke? Usually yes, if outdoor air is reasonable — ventilation and filtration remove particles by separate routes and their effects add. On a high-pollution day, keep it shut and let the purifier work.

Why does my monitor read high in the bathroom? Humidity. Water condenses onto particles and makes them scatter more light, so optical sensors over-read in damp air.