Smoke is a mixture of lightweight particles and gases created through burning or combustion. The exact types of particles and gases depend on the fuels burned.
Smoke is a complex substance that contains many different chemicals. Wood smoke is high in carbon monoxide, methane and volatile organic compounds. It also typically contains air that didn't react during combustion, water vapor from moisture in the wood and mold spores that are released during burning. Some smoke is transparent because it is composed of chemicals that are completely invisible. Invisible smokes are sensed by their odor. Many smokes contain toxic chemicals that are harmful to the environment. Filters installed in home and industrial chimneys can reduce some of the pollutants that are released into the environment.