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This article is about the substance. For other uses, see Smoke .

Smoke is the collection of airborne solid and liquid particulates and gases 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 stoves, candles, oil lamps, and fireplaces),but may also be used for pest control (cf. fumigation), communication (smoke signals), defense (smoke-screen) or smoking (tobacco, marijuana, etc.). Smoke is used in rituals, when incense, sage, or resin are burned to produce a smell for spiritual purposes. Smoke is sometimes used as a flavoring agent and preservative for various foodstuffs. Smoke is also sometimes a component of internal combustion engine exhaust gas, particularly diesel exhaust.

Smoke inhalation is the primary cause of death in victims of indoor fires. The smoke kills by a combination of thermal damage, poisoning and pulmonary irritation caused by carbon monoxide, hydrogen cyanide and other combustion products.

Smoke particles are an aerosol (or mist) of solid particles and liquid droplets that are close to the ideal range of sizes for Mie scattering of visible light. This effect has been likened to three-dimensional textured privacy glass — a smoke cloud does not obstruct an image, but thoroughly scrambles it.

Chemical composition

The composition of smoke depends on the nature of the burning fuel and the conditions of combustion.

Fires with high availability of oxygen burn at high temperature and with small amount of smoke produced; the particles are mostly composed of ash, or with large temperature differences, of condensed aerosol of water. High temperature also leads to production of nitrogen oxides. Sulfur content yields sulfur dioxide. Carbon and hydrogen are almost completely oxidized to carbon dioxide and water. Fires burning with lack of oxygen produce a significantly wider palette of compounds, many of them toxic. Partial oxidation of carbon produces carbon monoxide, nitrogen-containing materials can yield hydrogen cyanide, ammonia, and nitrogen oxides. Content of halogens such as chlorine (e.g. in polyvinyl chloride) or other halogens may lead to production of e.g. hydrogen chloride, phosgene, dioxin, and chloromethane, bromomethane and other halocarbons.

Pyrolysis of burning material 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. benzo[a]pyrene, studied as a carcinogen, or retene), terpenes. Heterocyclic compounds may be also present. Heavier hydrocarbons may condense as tar.
Presence of sulfur can lead to formation of e.g. hydrogen sulfide, carbonyl sulfide, sulfur dioxide, carbon disulfide, and thiols; 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: aldehydes (e.g. formaldehyde, acrolein, and furfural), ketones, alcohols (often aromatic, e.g. phenol, guaiacol, syringol, catechol, and cresols), carboxylic acids (formic acid, acetic acid, etc.).

The visible particles in such smokes are 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 oxides. Particles of inorganic salts may also be formed, e.g. ammonium sulfate, ammonium nitrate. Many organic compounds, typically the aromatic hydrocarbons, may be also adsorbed on the surface of the solid particles.

Smoke emissions may contain characteristic trace elements. Vanadium is present in emissions from oil fired power plants and refineries; oil plants also emit some nickel. Coal combustion produces emissions containing aluminium, arsenic, chromium, cobalt, copper, mercury, selenium, and uranium.

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 phenols. Retene, a product of pyrolysis of conifer trees, is an indicator of forest fires. 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, hopanes, steranes, 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. [4456]

Dangers of smoke

Smoke from oxygen-deprived fires contains a significant concentration of compounds that are flammable. A cloud of smoke, in contact with atmospheric oxygen, therefore has the potential of being ignited – either by another open flame in the area, or by its own temperature. This leads to effects like backdraft and flashover. Smoke inhalation is also a danger of smoke that can cause serious injury and death.

Many compounds of smoke from fires are highly toxic and/or irritating. The most dangerous is carbon monoxide leading to carbon monoxide poisoning, sometimes with the addative effects of hydrogen cyanide and phosgene. Smoke inhalation can therefore quickly lead to incapacitation and loss of consciousness.

Cigarette smoke is a major modifiable risk factor for lung disease, heart disease, and many cancers.

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 firemarker in Worcester, Massachusettsmarker was the exact reason why the trapped rescue firefighters couldn't evacuate the building in time. Due to the striking similarity that each floor shared, the dense smoke caused the firefighters to become disoriented.

Visible and invisible particles of combustion

Depending on particle size, smoke can be visible or invisible to the naked eye. This is best illustrated when toasting bread in a toaster. As the bread heats up, the products of combustion increase in size. The particles produced initially are invisible but become visible if the toast is burned.

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.

Measurement of Smoke

As early as the 15th Century Leonardo Da Vinci commented at length on the difficulty of assessing smoke, and distinguished between black smoke (carbonised particles) and white 'smoke' which is not a smoke at all but merely a suspension of harmless water droplets.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.

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 recognised 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, ie, from bituminous coal.

Ringelmann scale. A measure of smoke colour. Invented by Professor Maximilian Ringelmann in Paris in 1888, it is essentially a card with squares of black, white and shades of grey which is held up and the comparative greyness of the smoke judged. Highly dependent on light conditions and the skill of the observer it allocates a greyness 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 90º, 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.

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.

Medicinal smoke

Throughout recorded history, humans have used the smoke of medicinal plants to cure illness. A sculpture from Persepolis shows Darius the Great (522–486 B.C.), the king of Persia, with two censers 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. However, the hazards of inhaling a particulate are unacceptable to some people. Although the use of cannabis for medicinal purposes has been recorded for centuries, it has only recently become a subject of intense public scrutiny. So far, only a few examples of medicinal smoke have been studied in detail (e.g. cannabis). Smoke-based medicinal substances represent multiple opportunities for studies on the chemical constituents, applications, and introduction and preparation of new drugs and dosage forms.


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