Wildfire smoke is comprised of a mixture of gaseous pollutants (e.g., carbon monoxide), hazardous air pollutants (HAPs) (e.g., polycyclic aromatic hydrocarbons [PAHs]), water
vapor, and particulate matter (PM or soot).
Wildfire smoke is a complex mixture of gases, particles, and water
vapour that contains:
volatile organic compounds
fine particulate matter (PM2.5, roughly 50 times smaller than a grain of sand)
There is no evidence of a safe level of exposure, for most of these pollutants, but PM2.5 may pose the greatest risk.
What exactly is in a wildfire’s smoke*
depends on a few key things: what’s burning – grass
, brush or trees; the temperature – is it flaming or just smoldering; and the distance between the person breathing the smoke and the fire producing it.
Two forms of combustion characterize wildfires: flaming, and smoldering. Flaming is the rapid oxidization of the flame, which occurs rapidly, releasing mainly carbon dioxide and water.
Smoldering is a slow process, where there is a high conversion of fuel
to toxic compounds, such as carbon monoxide, non-methane organic compounds, and aerosols.
The distance affects the ability of smoke to “age,” meaning to be acted upon by the Sun, and other chemicals in the air, as it travels. Aging can make it more toxic.
Importantly, large particles like what most people think of as ash do not typically travel that far from the fire, but small particles, or aerosols, can travel across continents.
Smoke can contain many different chemicals, including aldehydes, acid gases, sulfur dioxide, nitrogen oxides, polycyclic aromatic hydrocarbons (PAHs), benzene, toluene, styrene, metals and dioxins. The type and amount of particles and chemicals in smoke varies, depending on what is burning, how much oxygen is available, and the burn temperature.
Boreal forests tend to produce more carbon monoxide, methane, and fine particulate matter, than other biomes.
Particles can be made up of different components, including acids (e.g., sulfuric acid), inorganic compounds (e.g., ammonium sulfate, ammonium nitrate, and sodium chloride), organic chemicals, soot, metals, soil or dust particles, and biological materials (e.g., pollen and mold
People who are at risk for health
effects due to wildfire smoke should be concerned about particles that are 10 micrometers (µm) in diameter or smaller, because these are the particles that generally pass through the nose and throat, and enter the lungs, with the smallest particles (< 2.5 µm) possibly even translocating into circulation [blood stream].
The air quality index ranges from 0 to 500, though air quality can be indexed beyond 500, when there are higher levels of hazardous air pollution.
Good air quality ranges from 0 to 50 [tho’ any measurement greater than 12.0 μg/m3 (US AQI 50)] can be dangerous to human health
, while measurements over 300 are considered hazardous.
The EPA’s current
24-hour PM2.5 standard was issued in 2006.
o EPA is retaining the existing 24-hour fine particle standard, at 35 μg/m3
[0.035 PPM]. An area meets the 24-hour standard if the 98th percentile of 24-hour PM2.5 concentrations in one year, averaged over three years, is less than or equal to 35 μg/m3.