Smog is a collection of solid and liquid particles and gases in the air released by burning or pyrolysis of a substance, including a large amount of air entrained or mixed with the substance. It is usually a harmful by-product of fires (including furnaces, candles, oil lamps and fireplaces), but may also be used for pest control (fumigation), communications (smoke signals), military defense and offensive capabilities (smoke) screens), cooking or smoking (tobacco, marijuana, etc.). Smoke is used in rituals in which incense, sage, or resin is burned to produce an odor for spiritual purposes. Smoke is sometimes used as a flavoring agent and as a preservative for various foods. Smoke is also a component of exhaust gas from internal combustion engines, especially diesel engines.

Smoke inhalation is the main cause of death for indoor fire victims. Carbon monoxide, hydrogen cyanide and other combustion products cause heat damage, poisoning and lung irritation combined to kill smoke.

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