Particulate matter
Particulate matter is a mixture of tiny solid particles and liquid droplets in the air, including dust, soot, smoke, and fine particles that can affect health.
What particulate matter is
Particulate matter is a mix of tiny solids and liquid droplets floating in the air. Some particles, such as dust, smoke, and soot, may be visible. Others are so small that they require specialized instruments or microscopes to detect. The mixture can change by place, season, weather, and pollution source.
PM10 and PM2.5
Two common size categories are PM10 and PM2.5. PM10 refers to inhalable particles generally 10 micrometers wide or smaller. PM2.5 refers to fine inhalable particles generally 2.5 micrometers wide or smaller. Fine particles are especially important because they can travel deeper into the respiratory system.
Where particles come from
Some particles are emitted directly from sources such as fires, smokestacks, diesel engines, construction sites, unpaved roads, fields, and dust storms. Others form in the atmosphere when gases from vehicles, power plants, industry, and other sources react chemically and create new particles.
Health effects
Particle pollution can irritate the eyes, nose, throat, and lungs. Fine particles can worsen asthma and other lung diseases, contribute to breathing trouble, and affect heart health. Risk is higher for children, older adults, people with lung or heart disease, people with diabetes, pregnant people, outdoor workers, and people exercising hard outdoors.
Wildfire smoke and dust
Wildfire smoke is a major source of fine particles and can travel far from the fire. Dust from dry landscapes, roads, construction, farms, and deserts can raise coarse and fine particle levels. During smoke or dust events, conditions can change quickly, so current air quality information may be more useful than a single daily forecast.
Visibility and haze
Particles scatter and absorb light, which is why particle pollution can make the air look hazy and reduce visibility. Fine particles are a major cause of haze in many places, including parks and wilderness areas. Clear-looking air is not always particle-free, but visible haze is a useful clue that pollution may be elevated.
Monitoring and AQI
Air monitors measure particle concentrations, often reported in micrograms per cubic meter. The Air Quality Index translates particle pollution and other pollutant measurements into color-coded health guidance. For rapidly changing smoke conditions, the NowCast AQI can reflect recent monitor readings more quickly than a full-day average.
Why it matters
Particulate matter matters because small particles can be easy to inhale and hard to see. Understanding PM helps explain wildfire smoke alerts, urban haze, dust warnings, air filtration advice, school activity decisions, and why air quality can affect health even when the sky does not look dramatically polluted.