Smoke is a cloud of particles given off by a material that undergoes combustion. Its chemical composition will vary depending on what material is being burned. Smoke may consist of hydrocarbons, haloalkanes, hydrogen fluoride, hydrogen chloride, and a variety of sulfur-containing compounds, among others. These compounds can vary widely in their toxicity, so the severity of the health hazards associated with the smoke from a fire will depend on what is being burned.