Fine particulate matter (PM2.5) is the airborne particle pollution made of particles with diameters generally 2.5 micrometers or smaller. An average human hair is about 30 times wider than the largest of these particles, so they are easily inhaled.
Where it comes from
Some fine particles are emitted directly by sources such as construction sites, unpaved roads, smokestacks and fires. Most form in the air through chemical reactions involving gases such as sulfur dioxide and nitrogen oxides.
Wildfire smoke is a mix of gases and fine particles from burning vegetation, buildings and other material. The AirNow Fire and Smoke Map uses PM2.5 levels to show how smoke is affecting air quality during fires.
Why health agencies watch it
The U.S. Environmental Protection Agency (EPA) says particles smaller than 2.5 micrometers pose the greatest health risk among particle sizes. It also notes that some inhaled particles can get deep into the lungs, and some may even reach the bloodstream.
The Centers for Disease Control and Prevention (CDC) lists people at increased risk from wildfire smoke as those with chronic conditions such as asthma, chronic obstructive pulmonary disease (COPD), diabetes, chronic kidney disease or heart disease, along with pregnant people, children and responders.
How it is measured
Concentrations are reported in micrograms per cubic meter. The AirNow Fire and Smoke Map shows permanent regulatory-grade monitors, temporary monitors that agencies place to track smoke events, and air sensors that meet performance targets.
Without correction, some widely used sensors tend to overestimate PM2.5, and their response becomes nonlinear in very heavy smoke. EPA applies a correction equation to reduce that bias for the map, though it notes sensors still have limitations, including bias.
Forecasting smoke, and its limits
NOAA's High-Resolution Rapid Refresh smoke model (HRRR-Smoke) uses fire radiative power from NASA and NOAA satellites, an estimate of the heat a fire releases, to place smoke sources. It forecasts how much smoke fires produce, how high plumes rise and how smoke spreads, with separate near-surface and vertically integrated smoke products.
Satellites see smoke at any height, so a plume on a map may not change the air at the ground. Near-surface forecasts and monitor readings are the better guide to what people are breathing.