Soot // is impure carbon particles resulting from the incomplete combustion of hydrocarbons. It is more properly restricted to the product of the gas-phase combustion process but is commonly extended to include the residual pyrolysed fuel particles such as coal, cenospheres, charred wood, petroleum coke, and so on, that may become airborne during pyrolysis and that are more properly identified as cokes or chars.
Soot, as an airborne contaminant in the environment has many different sources but they are all the result of some form of pyrolysis. They include soot from coal burning, internal combustion engines, power plant boilers, hog-fuel boilers, ship boilers, central steam heat boilers, waste incineration, local field burning, house fires, forest fires, fireplaces, furnaces, etc. These exterior sources also contribute to the indoor environment sources such as smoking of plant matter, cooking, oil lamps, candles, quartz/halogen bulbs with settled dust, fireplaces, defective furnaces, etc. Soot in very low concentrations is capable of darkening surfaces or making particle agglomerates, such as those from ventilation systems, appear black. Soot is the primary cause of "ghosting", the discoloration of walls and ceilings or walls and flooring where they meet. It is generally responsible for the discoloration of the walls above baseboard electric heating units and can be known as a gas. The formation of soot depends strongly on the fuel composition. The rank ordering of sooting tendency of fuel components is: naphthalenes → benzenes → aliphatics. However, the order of sooting tendencies of the aliphatics (alkanes, alkenes, alkynes) varies dramatically depending on the flame type. The difference between the sooting tendencies of aliphatics and aromatics is thought to result mainly from the different routes of formation. Aliphatics appear to first form acetylene and polyacetylenes, which is a slow process; aromatics can form soot both by this route and also by a more direct pathway involving ring condensation or polymerization reactions building on the existing aromatic structure.
Soot is a powder-like form of amorphous carbon. The gas-phase soots contain polycyclic aromatic hydrocarbons (PAHs). The PAHs in soot are known mutagens and are classified as a "known human carcinogen" by the International Agency for Research on Cancer (IARC).. Soot can be classified as soot nanoparticles. . Soot forms during incomplete combustion from precursor molecules such as acetylene. It consists of agglomerated nanoparticles with diameters between 5 to 30 nm. The soot particles can be mixed with metal oxides and with minerals and can be coated with sulfuric acid. 
Long-term exposure to urban air pollution containing soot increases the risk of coronary heart disease, according to a major study published in New England Journal of Medicine in 2007. Diesel exhaust (DE) gas is a major contributor to combustion derived particulate matter air pollution. In several human experimental studies using a well-validated exposure chamber setup DE has been linked to acute vascular dysfunction and increased thrombus formation. This serves as a plausible mechanistic link between the previously described association between particulate matter air pollution and increased cardiovascular morbidity and mortality.
Soot also tends to form in chimneys in domestic houses possessing one or more fireplaces. If a large deposit collects there, it can ignite and create a chimney fire. Regular cleaning by a chimney sweep should eliminate the problem.
- Activated carbon
- Atmospheric particulate matter
- Black carbon
- Carbon black
- Chimney sweep
- Diesel particulate matter
- Indian ink
- Soot blower
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