Air
PM2.5
Also known as fine particulate matter, particulate matter 2.5
Airborne particles 2.5 micrometers in diameter or smaller. The size fraction is small enough to stay suspended for hours and to reach deep into the lungs, which is why it is tracked separately from larger dust.
In more detail
PM2.5 is a size class, not a substance. The same measurement covers cooking aerosols, candle and fireplace smoke, wildfire smoke, road and brake dust, combustion byproducts, and particles formed by reactions in the atmosphere. Composition varies. Size is what determines where a particle deposits in the respiratory tract, and size is what the standard instruments measure.
Indoors, concentration is the sum of what infiltrates from outside and what is generated inside. Cooking is usually the largest indoor source in a home — searing, frying, and high-heat oven use can push a kitchen well above outdoor levels within minutes, and a recirculating range hood does nothing about it. Outdoor infiltration depends on the enclosure and on the ventilation system's filtration, which is the mechanism by which a tight, well-filtered house holds steady through a wildfire event while a leaky one tracks the outdoor curve.
Low-cost optical sensors have made continuous monitoring cheap and widespread. They estimate mass by scattering light off particles, which makes them good at showing trends and events in a home and unreliable for absolute accuracy, since response varies with particle composition and humidity.
Why it matters
Of the pollutants a homeowner can influence, fine particulate has the strongest health evidence behind it and the most controllable indoor sources. It also produces the most legible feedback loop: a monitor in a kitchen shows the spike from a seared steak and the decay curve after the hood runs, which turns an abstract argument about ducted range hoods into an observation. Two decisions carry most of the outcome — exhausting cooking to the outside, and filtering the air the house brings in.
The common misunderstanding
That a HEPA purifier in the living room covers it. Particle removal only competes with particle generation, and a purifier sized for a bedroom cannot keep up with an unvented range at full output. Capturing at the source with a ducted hood removes the particles before they mix into the house, and no amount of downstream filtration matches that.
Related terms
On Compound
Where this term does real work — and where the decisions get made.
Sources
Last reviewed July 2026 by Compound Editorial. Definitions are educational and not project-specific engineering or medical advice.