What is wildfire smoke?

Smoke generated by forest fires is comprised of water vapor, fine particles, and gases. The exact composition of wildfire smoke varies depending on temperature, wind, and the materials burning, but gases commonly include carbon monoxide, carbon dioxide, nitrogen oxide and volatile organic compounds (VOCs).

The fine particles often referred to as PM 2.5, or particulate matter of 2.5 micrometers in diameter, in smoke are most harmful to health.