PM10 is airborne particulate matter up to 10 micrometers across; PM2.5 is the fraction up to 2.5 micrometers. PM2.5 is the more important health number because those particles penetrate deep into the lungs and can enter the bloodstream. PM10 includes coarser dust, pollen and road grit that mostly stops in the nose and throat. Both are measured in micrograms per cubic meter (µg/m³).
Size in perspective
A human hair is about 70 µm wide. PM10 particles are a seventh of that; PM2.5 particles are a thirtieth. At 2.5 µm and below, particles stay airborne for hours to days and follow airflow into the smallest branches of the lungs. That is why health guidance focuses on PM2.5.
Where they come from
| Fraction | Typical outdoor sources | Typical indoor sources |
|---|---|---|
| PM2.5 (fine) | Vehicle exhaust, wood and coal smoke, wildfire smoke, industrial combustion, secondary particles formed from gases | Cooking (especially frying and searing), candles and incense, tobacco, wood stoves, laser printers |
| PM10-2.5 (coarse) | Road dust, construction, pollen, sea salt, soil | Dust disturbed by cleaning, pet dander, textile fibers |
Guideline values
The World Health Organization’s 2021 air quality guidelines set PM2.5 at 5 µg/m³ annual and 15 µg/m³ for a 24-hour mean, and PM10 at 15 µg/m³ annual and 45 µg/m³ for 24 hours. National standards are typically looser. For indoor monitoring a practical target is to keep the daily PM2.5 average under about 12 to 15 µg/m³ and to notice spikes above 35 µg/m³, which is where short-term effects on sensitive people become likely.
How monitors measure particles
Consumer monitors use a laser scattering sensor: a small fan pulls air past a laser, and a photodiode measures the light scattered by each particle. Particle count and scatter intensity are converted to mass concentration using an assumed particle density. The method is good at detecting change and reasonable for PM2.5 in typical indoor mixtures. It is rough for PM10, because large particles are few and heavy, and it can read high in humid air when particles absorb water. Reference-grade instruments weigh filters or use beta attenuation and cost thousands; a consumer sensor within ±10 to 15 µg/m³ of them is doing well.
What to do with the readings
- Cooking spikes: run the hood on high and keep it running ten minutes after; the reading should fall back within half an hour.
- Persistent elevation: look for candles, an old printer, a wood stove or outdoor infiltration; a HEPA purifier sized for the room brings PM2.5 down quickly.
- Outdoor smoke events: close windows, run filtration, and use the monitor to see whether the house is holding.
Which to track
If a monitor reports only one, PM2.5 is the one to have. Monitors that add PM10 give a rough view of coarse dust, useful for spotting cleaning and pollen episodes. The office air quality guide covers monitors that combine PM2.5 with CO₂ and VOCs.
Frequently Asked Questions
Is PM2.5 included in PM10?
Yes. PM10 means all particles up to 10 micrometers, which includes the PM2.5 fraction. The coarse fraction between 2.5 and 10 µm is sometimes written PM10-2.5.
What is a safe PM2.5 level?
The World Health Organization's 2021 guideline is an annual mean of 5 µg/m³ and a 24-hour mean of 15 µg/m³. Many national standards are higher. Indoors, keeping the daily average under 12 to 15 µg/m³ is a reasonable target.
Can a consumer monitor measure PM10 accurately?
Consumer laser sensors count small particles and estimate PM10 from the size distribution. They are reasonable for PM2.5 and rough for PM10, so treat PM10 readings as indicative.