Fine particulate matter (PM2.5) is the pollutant that most often drives unhealthy AQI. These particles are 2.5 micrometers across or smaller — about 1/30th the width of a human hair — small enough to travel deep into the lungs and pass into the bloodstream. Sources include combustion: vehicle exhaust, power plants, wood burning and wildfire smoke.
Coarse particulate matter (PM10) includes dust, pollen, mold and construction debris up to 10 micrometers. It irritates the airways and is the main pollutant during dust storms and dry, windy spells.
Ground-level ozone (O3) is not emitted directly — it forms when sunlight cooks nitrogen oxides and volatile organic compounds from traffic and industry. That photochemistry makes ozone a hot-afternoon, sunny-day pollutant that peaks in summer and downwind of cities, even in otherwise clean rural areas.
Nitrogen dioxide (NO2) comes mainly from vehicles and combustion and is a marker of fresh traffic pollution; it both irritates airways and feeds ozone formation. Sulfur dioxide (SO2) comes from burning sulfur-rich fuels and smelting. Carbon monoxide (CO), an odorless product of incomplete combustion, is dangerous in enclosed spaces and elevated near heavy traffic.