PM10 (coarse particulate matter)

PM10 is the mix of airborne particles 10 micrometers or smaller in diameter, small enough to inhale. It covers everything a PM2.5 sensor sees plus the larger coarse dust that sensor misses: wind-driven soil, sea salt, road and brake wear, and pollen. Because the coarse part is larger, it settles in the nose, throat, and upper airways and tends to show up as a stuffy nose, cough, or asthma flare rather than something felt deep in the chest.

PM10 stands for particulate matter 10 micrometers or smaller in diameter. The PM is particulate matter, the airborne mix of solid and liquid particles, and the 10 is the size cutoff in micrometers (a micrometer is a thousandth of a millimeter). It is a mass measurement of every particle at or below that size, so it includes the fine PM2.5 fraction plus the coarse fraction between 2.5 and 10 micrometers.

By definition, 10 micrometers in diameter or smaller. The label draws a line at that size and counts every particle at or under it. Particles between 2.5 and 10 micrometers are the coarse fraction, mostly dust, sea salt, road wear, and pollen; anything 2.5 micrometers and under is the fine PM2.5 fraction. Both are far too small to see one at a time, but the coarse fraction is what shows up as visible dust settling on surfaces and vents after a windy or dusty stretch.

There is no PM10 level known to be free of effect, so the health target is as low as feasible. For regulatory reference, the federal 24-hour limit is 150 micrograms per cubic meter, and California sets a stricter 24-hour limit of 50. Those are legal compliance limits, not health-based safety lines, and coarse-dust days can push outdoor readings well above them. Indoors, running HEPA continuously is what keeps your own level low.

PM2.5 is the fine fraction, particles up to 2.5 micrometers, which reaches the deep lung and bloodstream. PM10 includes PM2.5 plus the coarse fraction up to 10 micrometers, which is dust, sea salt, road wear, and pollen. The coarse part deposits in the nose and upper airways and irritates them. PM10 is additive to PM2.5, not a substitute for it.

Dry, gusty wind erodes loose soil and carries coarse dust across a region, and severe events have measured well above the federal 24-hour limit. In Southern California these are the Santa Ana winds, where dry desert winds loft soil and carry dust across urban areas. That dust deposits in the nose, throat, and large airways, which is why it shows up as a stuffy nose, cough, sinus flare, or asthma symptoms rather than something you feel deep in the chest.

Yes, very effectively. HEPA filters capture particles down to 0.3 micrometers at 99.97 percent, and PM10 particles are larger, so capture is essentially complete on each pass. Run a correctly sized unit continuously in the rooms where you spend the most time, and keep it on through wind-dust and wildfire events.

The standard sensor measures PM2.5, not PM10, and the audit is upfront about that gap. PM10 is captured through context instead: how close you are to busy roads or construction, recent wind-dust events, pets and flooring, and visible dust on surfaces and vents. The mitigation, HEPA and MERV-13 and the wind-day protocol, handles both fractions regardless.

Homes within a few miles of the coast carry a steady sea salt contribution to their PM10. It is largely benign for health, so it is worth knowing about for context rather than as something to fix. That contribution is part of why a coastal home can read a higher PM10 baseline than an inland one without that meaning more risk.