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Smoke is a suspension of airborne particulates and emitted when a material undergoes combustion or pyrolysis, together with the quantity of air that is entrained or otherwise mixed into the mass. It is commonly an unwanted by-product of fires (including , , internal combustion engines, , and ), but may also be used for pest control (fumigation), communication (), defensive and offensive capabilities in the military (smoke screen), cooking, or smoking (tobacco smoking, cannabis, etc.). It is used in rituals where incense, sage, or resin is burned to produce a smell for Spirituality or magical purposes. It can also be a flavoring agent and preservative.
Smoke inhalation is the primary cause of death in victims of indoor . The smoke kills by a combination of thermal damage, and lung irritation caused by carbon monoxide, hydrogen cyanide and other combustion products.
Smoke is an aerosol (or mist) of solid particles and liquid droplets that are close to the ideal range of sizes for Mie theory of visible light.
Pyrolysis of burning material, especially incomplete combustion or smoldering without adequate oxygen supply, also results in production of a large amount of hydrocarbons, both aliphatic (methane, ethane, ethylene, acetylene) and aromatic (benzene and its derivates, polycyclic aromatic hydrocarbons; e.g. [benzopyrene|benzo[apyrene]], studied as a carcinogen, or retene), . It also results in the emission of a range of smaller oxygenated volatile organic compounds (methanol, acetic acid, Hydroxyacetone, methyl acetate and ethyl formate) which are formed as combustion by products as well as less volatile oxygenated organic species such as phenolics, and . Heterocyclic compounds may be also present. Heavier hydrocarbons may condense as tar; smoke with significant tar content is yellow to brown. Combustion of solid fuels can result in the emission of many hundreds to thousands of lower volatility organic compounds in the aerosol phase. Presence of such smoke, soot, and/or brown oily deposits during a fire indicates a possible hazardous situation, as the atmosphere may be saturated with combustible pyrolysis products with concentration above the upper flammability limit, and sudden inrush of air can cause flashover or backdraft.
Presence of sulfur can lead to formation of gases like hydrogen sulfide, carbonyl sulfide, sulfur dioxide, carbon disulfide, and ; especially thiols tend to get adsorbed on surfaces and produce a lingering odor even long after the fire. Partial oxidation of the released hydrocarbons yields in a wide palette of other compounds: (e.g. formaldehyde, acrolein, and furfural), ketones, alcohols (often aromatic, e.g. phenol, guaiacol, syringol, catechol, and ), (formic acid, acetic acid, etc.).
The visible particulate matter in such smokes is most commonly composed of carbon (soot). Other particulates may be composed of drops of condensed tar, or solid particles of ash. The presence of metals in the fuel yields particles of metal . Particles of inorganic salts may also be formed, e.g. ammonium sulfate, ammonium nitrate, or sodium chloride. Inorganic salts present on the surface of the soot particles may make them hydrophilic. Many organic compounds, typically the aromatic hydrocarbons, may be also on the surface of the solid particles. Metal oxides can be present when metal-containing fuels are burned, e.g. solid rocket fuels containing aluminium. Depleted uranium projectiles after impacting the target ignite, producing particles of . Magnetic particles, spherules of magnetite-like ferrous ferric oxide, are present in coal smoke; their increase in deposits after 1860 marks the beginning of the Industrial Revolution. (Magnetic iron oxide can be also produced in the smoke from burning in the atmosphere.) Magnetic remanence, paleomagnetism in the iron oxide particles, indicates the strength of Earth's magnetic field when they were cooled beyond their Curie temperature; this can be used to distinguish magnetic particles of terrestrial and meteoric origin. Fly ash is composed mainly of silica and calcium oxide. are present in smoke from liquid hydrocarbon fuels. Minute metal particles produced by abrasion can be present in engine smokes. Amorphous silica particles are present in smokes from burning ; small proportion of silicon nitride particles can be formed in fires with insufficient oxygen. The silica particles have about 10 nm size, clumped to 70–100 nm aggregates and further agglomerated to chains. Radioactive particles may be present due to traces of uranium, thorium, or other in the fuel; can be present in case of fires during (e.g. Chernobyl disaster) or nuclear war.
Smoke particulates, like other aerosols, are categorized into three modes based on particle size:
Aerosol of particles beyond visible size is an early indicator of materials in a preignition stage of a fire.
Burning of hydrogen-rich fuel produces water vapor; this results in smoke containing droplets of water. In absence of other color sources (nitrogen oxides, particulates...), such smoke is white and cloud-like.
Smoke emissions may contain characteristic trace elements. Vanadium is present in emissions from oil fired power plants and Oil refinery; oil plants also emit some nickel. Coal combustion produces emissions containing aluminium, arsenic, chromium, cobalt, copper, iron, mercury, selenium, and uranium.
Traces of vanadium in high-temperature combustion products form droplets of molten . These attack the passivation layers on metals and cause high temperature corrosion, which is a concern especially for internal combustion engines. Molten sulfate and lead particulates also have such effect.
Some components of smoke are characteristic of the combustion source. Guaiacol and its derivatives are products of pyrolysis of lignin and are characteristic of wood smoke; other markers are syringol and derivates, and other methoxy . Retene, a product of pyrolysis of conifer trees, is an indicator of . Levoglucosan is a pyrolysis product of cellulose. Hardwood vs softwood smokes differ in the ratio of guaiacols/syringols. Markers for vehicle exhaust include polycyclic aromatic hydrocarbons, , , and specific nitroarenes (e.g. 1-nitropyrene). The ratio of hopanes and steranes to elemental carbon can be used to distinguish between emissions of gasoline and diesel engines.
Many compounds can be associated with particulates; whether by being adsorption on their surfaces, or by being dissolved in liquid droplets. Hydrogen chloride is well absorbed in the soot particles.
Inert particulate matter can be disturbed and entrained into the smoke. Of particular concern are particles of asbestos.
Deposited of radioactive fallout and bioaccumulated radioisotopes can be reintroduced into the atmosphere by and ; this is a concern in e.g. the Zone of alienation containing contaminants from the Chernobyl disaster.
Polymers are a significant source of smoke. Aromatic , e.g. in polystyrene, enhance generation of smoke. Aromatic groups integrated in the polymer backbone produce less smoke, likely due to significant charring. Aliphatic polymers tend to generate the least smoke, and are non-self-extinguishing. However presence of additives can significantly increase smoke formation. Phosphorus-based and halogen-based decrease production of smoke. Higher degree of between the polymer chains has such effect too.
An ionization chamber type smoke detector is technically a product of combustion detector, not a smoke detector. Ionization chamber type smoke detectors detect particles of combustion that are invisible to the naked eye. This explains why they may frequently false alarm from the fumes emitted from the red-hot heating elements of a toaster, before the presence of visible smoke, yet they may fail to activate in the early, low-heat smoldering stage of a fire.
Smoke from a typical house fire contains hundreds of different chemicals and fumes. As a result, the damage caused by the smoke can often exceed that caused by the actual heat of the fire. In addition to the physical damage caused by the smoke of a fire – which manifests itself in the form of stains – is the often even harder to eliminate problem of a smoky odor. Just as there are contractors that specialize in rebuilding/repairing homes that have been damaged by fire and smoke, fabric restoration companies specialize in restoring fabrics that have been damaged in a fire.
Smoke can obscure visibility, impeding occupant exiting from fire areas. In fact, the poor visibility due to the smoke that was in the Worcester Cold Storage Warehouse fire in Worcester, Massachusetts was the reason why the trapped rescue firefighters could not evacuate the building in time. Because of the striking similarity that each floor shared, the dense smoke caused the firefighters to become disoriented.
Corrosivity of smoke produced by materials is characterized by the corrosion index (CI), defined as material loss rate (angstrom/minute) per amount of material gasified products (grams) per volume of air (m3). It is measured by exposing strips of metal to flow of combustion products in a test tunnel. Polymers containing halogen and hydrogen (polyvinyl chloride, with halogenated additives, etc.) have the highest CI as the corrosive acids are formed directly with water produced by the combustion, polymers containing halogen only (e.g. polytetrafluoroethylene) have lower CI as the formation of acid is limited to reactions with airborne humidity, and halogen-free materials (polyolefins, wood) have the lowest CI. However, some halogen-free materials can also release significant amount of corrosive products.
Smoke damage to electronic equipment can be significantly more extensive than the fire itself. Electrical cable fires are of special concern; low smoke zero halogen materials are preferable for cable insulation.
When smoke comes into contact with the surface of any substance or structure, the chemicals contained in it are transferred to it. The corrosive properties of the chemicals cause the substance or structure to decompose at a rapid rate. Certain materials or structures absorb these chemicals, which is why clothing, unsealed surfaces, potable water, piping, wood, etc., are replaced in most cases of structural fires.
In the United Kingdom domestic combustion, especially for industrial uses, is the largest single source of PM2.5 annually. In some towns and cities in New South Wales, wood smoke may be responsible for 60% of fine particle air pollution in the winter. A year-long sampling campaign in Athens, Greece found a third (31%) of PAH urban air pollution to be caused by wood-burning, roughly as much as that of diesel and Crude oil (33%) and gasoline (29%). It also found that wood-burning is responsible for nearly half (43%) of annual PAH lung cancer-risk compared to the other sources and that wintertime PAH levels were 7 times higher than in other seasons, presumably due to an increased use of and heaters. The largest exposure events are periods during the winter with reduced atmospheric dispersion to dilute the accumulated pollution, in particular due to the low wind speeds. Research conducted about biomass burning in 2015, estimated that 38% of European total particulate pollution emissions are composed of domestic wood burning.
Wood smoke (for example from or wood ovens) can cause lung damage, artery damage and DNA damage leading to cancer, other respiratory and lung disease and cardiovascular disease. Air pollution, particulate matter and wood smoke may also cause brain damage because of particulates breaching the cardiovascular system and into the brain, which can increase the risk of developmental disorders, neurodegenerative disorders mental disorders, and suicidal behavior, although studies on the link between depression and some air pollutants are not consistent. At least one study has identified "the abundant presence in the human brain of magnetite nanoparticles that match precisely the high-temperature magnetite nanospheres, formed by combustion and/or friction-derived heating, which are prolific in urban, airborne particulate matter (PM)." Air pollution has also been linked to a range of other psychosocial problems.
Smoke from heating appliances is commonly measured in one of the following ways:
In-line capture. A smoke sample is simply sucked through a filter which is weighed before and after the test and the mass of smoke found. This is the simplest and probably the most accurate method, but can only be used where the smoke concentration is slight, as the filter can quickly become blocked.
The ASTM smoke pump is a simple and widely used method of in-line capture where a measured volume of smoke is pulled through a filter paper and the dark spot so formed is compared with a standard.
Filter/dilution tunnel. A smoke sample is drawn through a tube where it is diluted with air, the resulting smoke/air mixture is then pulled through a filter and weighed. This is the internationally recognized method of measuring smoke from combustion.
Electrostatic precipitation. The smoke is passed through an array of metal tubes which contain suspended wires. A (huge) electrical potential is applied across the tubes and wires so that the smoke particles become charged and are attracted to the sides of the tubes. This method can over-read by capturing harmless condensates, or under-read due to the insulating effect of the smoke. However, it is the necessary method for assessing volumes of smoke too great to be forced through a filter, i.e., from bituminous coal.
Ringelmann scale. A measure of smoke color. Invented by Professor Maximilian Ringelmann in Paris in 1888, it is essentially a card with squares of black, white and shades of gray which is held up and the comparative grayness of the smoke judged. Highly dependent on light conditions and the skill of the observer it allocates a grayness number from 0 (white) to 5 (black) which has only a passing relationship to the actual quantity of smoke. Nonetheless, the simplicity of the Ringelmann scale means that it has been adopted as a standard in many countries.
Optical scattering. A light beam is passed through the smoke. A light detector is situated at an angle to the light source, typically at 90°, so that it receives only light reflected from passing particles. A measurement is made of the light received which will be higher as the concentration of smoke particles becomes higher.
Optical obscuration. A light beam is passed through the smoke and a detector opposite measures the light. The more smoke particles are present between the two, the less light will be measured.
Combined optical methods. There are various proprietary optical smoke measurement devices such as the 'nephelometer' or the 'aethalometer' which use several different optical methods, including more than one wavelength of light, inside a single instrument and apply an algorithm to give a good estimate of smoke. It has been claimed that these devices can differentiate types of smoke and so their probable source can be inferred, though this is disputed.
Inference from carbon monoxide. Smoke is incompletely burned fuel, carbon monoxide is incompletely burned carbon, therefore it has long been assumed that measurement of CO in flue gas (a cheap, simple and very accurate procedure) will provide a good indication of the levels of smoke. Indeed, several jurisdictions use CO measurement as the basis of smoke control. However it is far from clear how accurate the correspondence is.
Measurement
As early as the 15th century Leonardo da Vinci commented at length on the difficulty of assessing smoke, and distinguished between black carbon (carbonized particles) and white 'smoke' which is not a smoke at all but merely a suspension of harmless water particulates.
Medicinal smoking
Throughout recorded history, humans have used the smoke of to cure illness. A sculpture from Persepolis shows Darius the Great (522–486 BC), the king of Persia, with two in front of him for burning Peganum harmala and/or sandalwood Santalum album, which was believed to protect the king from evil and disease. More than 300 plant species in 5 continents are used in smoke form for different diseases. As a method of drug administration, smoking is important as it is a simple, inexpensive, but very effective method of extracting particles containing active agents. More importantly, generating smoke reduces the particle size to a microscopic scale thereby increasing the absorption of its active chemical principles.
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