Among the fruits of the investigations has been an improved understanding of the emissions produced by specific human activities. The combustion of fossil fuels for energy is known to yield substantial amounts of sulfur dioxide (particularly from coal), nitrogen oxides (which form when nitrogen and oxygen in the air are heated) and carbon dioxide. If the burning is incomplete it also yields carbon monoxide (CO), a variety of hydrocarbons (including methane) and soot (carbon particles). Other industrial activities release additional sulfur dioxide (smelting is an example) or in ect such substances as chlorofluorocarbons or toxic metals into the air.