PM1 is particulate matter smaller than 1 micrometer in diameter. It is a subset of PM2.5 (everything under 2.5 µm), which is itself a subset of PM10. PM1 is dominated by combustion products: soot from vehicles, wood smoke, candles, cooking flames and cigarettes, plus secondary particles formed from gases. These are the particles that reach the deepest parts of the lungs and can cross into the bloodstream, which is why some monitors now report PM1 separately.
The nesting
| Fraction | Size | Contains | Typical sources |
|---|---|---|---|
| PM10 | Under 10 µm | PM2.5 and PM1 | Dust, pollen, road grit, plus everything below |
| PM2.5 | Under 2.5 µm | PM1 | Combustion, secondary aerosol, fine dust |
| PM1 | Under 1 µm | Ultrafine (under 0.1 µm) | Combustion soot, vehicle exhaust, secondary aerosol |
Because each is contained in the next, a monitor always shows PM1 ≤ PM2.5 ≤ PM10. The gaps between them say something about the source: a large PM10 to PM2.5 gap means coarse dust; PM2.5 nearly equal to PM1 means combustion.
Why PM1 matters
Particle deposition in the lungs depends on size. Above about 2.5 µm most particles are caught in the nose and upper airways. Between 1 and 2.5 µm they reach the bronchi. Below 1 µm they reach the alveoli, and the smallest fraction can cross into blood. Health studies mostly use PM2.5 because that is what monitoring networks measure, but the harm is concentrated in the PM1 fraction, which typically makes up 60 to 90 percent of PM2.5 mass in urban and smoky air.
How sensors report it
Consumer laser sensors count particles by the light they scatter and sort them into size bins. Converting counts to mass in each bin uses an assumed density and shape. The PM1 estimate depends on the smallest bins, where the sensor is least sensitive, so it is the least reliable of the three numbers. Treat PM1 on a consumer monitor as a trend indicator: when it rises with PM2.5, a combustion source is active. See PM2.5 vs PM10 for the sensor method.
Reading the pattern
- PM1 ≈ PM2.5, PM10 slightly higher: combustion (traffic, smoke, candles, gas cooking).
- PM1 low, PM2.5 moderate, PM10 high: coarse dust from cleaning, construction, pollen or wind.
- All three rising together while frying: cooking oil aerosol spans the sizes.
- All three rising in humid air: particles absorbing water and growing; readings can overstate mass.
Do you need a PM1 reading?
For deciding whether to run a purifier or open a window, PM2.5 alone is enough. PM1 adds source diagnosis: it tells you whether a rise is smoke or dust. Monitors that report all three, such as the IQAir AirVisual and the Temtop and Qingping ranges, cost little more than PM2.5-only units. Combination monitors are compared in the office air quality guide.
Recommended Tools
Products shown because their documented specifications fit this guide. Price and availability are on Amazon and can change.
Frequently Asked Questions
Is PM1 more dangerous than PM2.5?
Particles under 1 µm penetrate deepest and can pass into the bloodstream, so PM1 is the fraction of most health concern. There is no separate PM1 guideline yet; PM2.5 guidelines are used because most PM2.5 mass in polluted air is PM1 anyway.
Why does my monitor show PM1 lower than PM2.5?
Because PM1 is contained within PM2.5. PM2.5 includes everything up to 2.5 µm, so it is always equal to or greater than PM1.
Do consumer sensors measure PM1 accurately?
They estimate it from scattered light and an assumed size distribution. The PM1 number is the least reliable of the three on consumer sensors, but the trend is informative.
Affiliate disclosure: Akermin earns a commission from qualifying Amazon purchases made through links on this page. Our editorial picks are based on documented specifications and owner feedback, not commissions.