Ozone is a naturally occurring compound that, in the upper atmosphere shields us from harmful ultraviolet light from the sun.

In the lower atmosphere however, ozone is a pollutant that contributes to smog and is a respiratory hazard. Ozone is created naturally by nitrogen oxide and carbon oxide (nitrogen and carbon combined with oxygen) compounds reacting with sunlight.

When forest fires burn, they release large amounts of the compound carbon monoxide into the air (the same compound created by burning gasoline in your car!). The carbon monoxide reacts with sunlight, and creates ozone. The huge fires that are burning in California right now are adding a lot of carbon monoxide to the air, and that's why the air quality is so poor right now.