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When a substance undergoes combustion or pyrolysis, smoke is formed from a mixture of airborne particles and gases, as well as the amount of air entrained or otherwise incorporated into the mass. It’s a frequent by-product of flames (such as stoves, candles, internal combustion engines, oil lamps, and fireplaces), but it may also be utilized for pest management (fumigation), communication (smoke signals), military defensive and offensive capabilities (smoke screen), cooking, or smoking (tobacco, cannabis, etc.). It’s utilized in spiritual or magical ceremonies when incense, sage, or resin is burnt to create a scent. It can also be used as a preservative and flavoring ingredient.

Smoke rising from a grassland fire in northern Mexico during a heat wave that coincided with Mexico’s forest fire season.
The most common cause of death among victims of indoor fires is smoke inhalation. Carbon monoxide, hydrogen cyanide, and other combustion products produce thermal damage, poisoning, and lung irritation, which all contribute to death.

Smoke is an aerosol (or mist) comprising solid particles and liquid droplets with a size distribution that falls within the optimal range for Mie scattering of visible light.

The nature of the burning fuel and the conditions of combustion determine the composition of smoke. Fires with a lot of oxygen burn at a high temperature and create very little smoke; the particles are largely ash or, if the temperature difference is significant enough, condensed water aerosol. Nitrogen oxides are also produced at high temperatures. Sulfur dioxide is produced when sulfur is present, or hydrogen sulfide is produced when sulfur is not present. Carbon and hydrogen are converted to carbon dioxide and water almost entirely. Fires that aren’t getting enough oxygen create a far broader range of chemicals, many of which are poisonous. Carbon monoxide is produced when carbon is partially oxidized, whereas nitrogen-containing compounds can create hydrogen cyanide, ammonia, and nitrogen oxides. Instead of water, hydrogen gas can be created. Hydrogen chloride, phosgene, dioxin, chloromethane, bromomethane, and other halocarbons can be produced when halogens like chlorine are present (for example, in polyvinyl chloride or brominated flame retardants).

Fluorocarbons, whether fluoropolymers exposed to fire or halocarbon fire control chemicals, can produce hydrogen fluoride. Some fire retardant chemicals can produce phosphorus and antimony oxides, as well as their reaction products, which increases smoke toxicity and corrosivity. Pyrolysis of polychlorinated biphenyls (PCB) can create 2,3,7,8-tetrachlorodibenzodioxin, a strong carcinogen, and other polychlorinated dibenzodioxins, e.g. by burning older transformer oil, and to a lesser extent other chlorine-containing compounds. Carbonyl fluoride (which hydrolyzes rapidly to HF and CO2) is generated when fluoropolymers, such as teflon, are pyrolyzed in the presence of oxygen; additional compounds, such as carbon tetrafluoride, hexafluoropropylene, and the exceedingly hazardous perfluoroisobutene, may also be created (PFIB).

Pyrolysis of burning material, particularly incomplete combustion or smoldering without adequate oxygen supply, produces a large amount of hydrocarbons, both aliphatic (methane, ethane, ethylene, acetylene) and aromatic (benzene and its derivates, polycyclic aromatic hydrocarbons; e.g. benzopyrene, which has been studied as a carcinogen, or retene), terpenes. It also results in the emission of a variety of smaller oxygenated volatile organic compounds (methanol, acetic acid, hydroxy acetone, methyl acetate, and ethyl formate), as well as less volatile oxygenated organic species like phenolics, furans, and furanones, which are formed as combustion byproducts.

Heterocyclic compounds might be present as well. Tar can form when heavier hydrocarbons condense; smoke with a high tar concentration is yellow to brown. In the aerosol phase, solid fuel combustion can result in the release of hundreds to thousands of reduced volatility organic molecules. During a fire, the presence of smoke, soot, and/or brown oily deposits indicates a potentially dangerous situation, as the atmosphere may be saturated with combustible pyrolysis products with concentrations above the upper flammability limit, and a sudden inrush of air can cause flashover or backdraft.

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