road-paving material in the late 19th and early 20th centuries, but ultimately fell out of favor because they did not generally hold up well enough over time. two later methods for creosoting wood were introduced after the turn of the century, referred to as empty-cell processes, because they involve compressing the air inside the wood so that the preservative can only coat the inner cell walls rather than saturating the interior cell voids. this is a less effective, though usually satisfactory, method of treating the wood, but is used because it requires less of the creosoting material. the first method, the "rüping process" was patented in 1902, and the second, the "lowry process" was patented in 1906. later in 1906, the "allardyce process" and "card process" were patented to treat wood with a combination of both creosote and zinc chloride. in 1912, it was estimated that a total of 150,000,000 gallons were produced in the us per year. medical coal-tar creosote, despite its toxicity, was used as a stimulant and escharotic, as a caustic agent used to treat ulcers and malignancies and cauterize wounds and prevent infection and decay. it was particularly used in dentistry to destroy tissues and arrest necrosis. industrial coal-tar creosote is the most widely used wood treatment today; both industrially, processed into wood using pressure methods such as "full-cell process" or "empty-cell process", and more commonly applied to wood through brushing. in addition to toxicity to fungi, insects, and marine borers, it serves as a natural water repellent. it is commonly used to preserve and waterproof cross ties, pilings, telephone poles, power line poles, marine pilings, and fence posts. although suitable for use in preserving the structural timbers of buildings, it is not generally used that way because it is difficult to apply. there are also concerns about the environmental impact of the leaching of creosote into aquatic ecosystems. due to its carcinogenic character, the european union has regulated the quality of creosote for the eu market and requires that the sale of creosote be limited to professional users. the united states environmental protection agency regulates the use of coal-tar creosote as a wood preservative under the provisions of the federal insecticide, fungicide, and rodenticide act. creosote is considered a restricted-use pesticide and is only available to licensed pesticide applicators. oil-tar creosote derivation and general composition of water-gas-tar creosote oil-tar creosote is derived from the tar that forms when using petroleum or shale oil in the manufacturing of gas. the distillation of the tar from the oil occurs at very high temperatures; around 980 °c. the tar forms at the same time as the gas, and once processed for creosotes contains a high percentage of cyclic hydrocarbons, a very low amount of tar acids and tar bases, and no true anthracenes have been identified. historically, this has mainly been produced in the united states on the pacific coast, where petroleum has been more abundant than coal. limited quantities have been used industrially, either alone, mixed with coal-tar creosote, or fortified with pentachlorophenol. water-gas-tar creosote water-gas-tar creosote is also derived from petroleum oil or shale oil, but by a different process; it is distilled during the production of water gas. the tar is a by-product resulting from enrichment of water gas with gases produced by thermal decomposition of petroleum. of the creosotes derived from oil, it is practically the only one used for wood preservation. it has the same degree of solubility as coal-tar creosote and is easy to infuse into wood. like standard oil-tar creosote, it has a low amount of tar acids and tar bases, and has less antiseptic qualities. petri dish tests have shown that water-gas-tar creosote is one-sixth as anti-septically effective as that of coal-tar. lignite-tar creosote lignite-tar creosote is produced from lignite rather than bituminous coal, and varies considerably from coal-tar creosote. also called "lignite oil", it has a very high content of tar acids, and has been used to increase the tar acids in normal creosote when necessary. when it has been produced, it has generally been applied in mixtures with coal-tar creosote or petroleum. its effectiveness when used alone has not been established. in an experiment with southern yellow pine fence posts in mississippi, straight lignite-tar creosote was giving good results after about 27 years exposure, although not as good as the standard coal-tar creosote used in the same situation. peat-tar creosote there have also been attempts to distill creosote from peat-tar, although mostly unsuccessful due to the problems with winning and drying peat on an industrial scale. peat tar by itself has in the past been used as a wood preservative. health effects according to the agency for toxic substances and disease registry (atsdr), eating food or drinking water contaminated with high levels of coal-tar creosote may cause a burning in the mouth and throat, and stomach pains. atsdr also states that brief direct contact with large amounts of coal-tar creosote may result in a rash or severe irritation of the skin, chemical burns of the surfaces of the eyes, convulsions and mental confusion, kidney or liver problems, unconsciousness, and even death. longer direct skin contact with low levels of creosote mixtures or their vapours can result in increased light sensitivity, damage to the cornea, and skin damage. longer exposure to creosote vapours can cause irritation of the respiratory tract. the international agency for research on cancer (iarc) has determined that coal-tar creosote is probably carcinogenic to humans, based on adequate animal evidence and limited human evidence. the animal testing relied upon by iarc involved the continuous application of creosote to the shaved skin of rodents. after weeks of creosote application, the animals developed cancerous skin lesions and in one test, lesions of the lung. the united states environmental protection agency has stated that coal-tar creosote is a probable human carcinogen based on both human and animal studies. as a result, the federal occupational safety and health administration (osha) has set a permissible exposure limit of 0.2 milligrams of coal-tar creosote per cubic meter of air (0.2 mg/m3) in the workplace during an 8-hour day, and the environmental protection agency (epa) requires that spills or accidental releases into the environment of one pound (0.454 kg) or more of creosote be reported to them. there is no unique exposure pathway of children to creosote. children exposed to creosote will probably experience the same health effects seen in adults exposed to creosote. it is unknown whether children differ from adults in their susceptibility to health effects from creosote. a 2005 mortality study of creosote workers found no evidence supporting an increased risk of cancer death, as a result of exposure to creosote. based on the findings of the largest mortality study to date of workers employed in creosote wood treating plants, there is no evidence that employment at creosote wood-treating plants or exposure to creosote-based preservatives was associated with any significant mortality increase from either site-specific cancers or non-malignant diseases. the study consisted of 2,179 employees at eleven plants in the united states where wood was treated with creosote preservatives. some workers began work in the 1940s to 1950s. the observation period of the study covered 1979–2001. the average length of employment was 12.5 years. one third of the study subjects were employed for over 15 years. the largest health effect of creosote is deaths caused by residential chimney fires due to chimney tar (creosote) build-up. this is entirely unconnected with its industrial production or use. build-up in chimneys burning wood and fossil fuels in the absence of adequate airflow (such as in an enclosed furnace or stove), causes incomplete combustion of the oils in the wood, which are off-gassed as volatiles in the smoke. as the smoke rises through the chimney it cools, causing water, carbon, and volatiles to condense on the interior surfaces of the chimney flue. the black oily residue that builds up is referred to as creosote, which is similar in composition to the commercial products by the same name, but with a higher content of carbon black. over the course of a season creosote deposits can become several inches thick. this creates a compounding problem, because the creosote deposits reduce the draft (airflow through the chimney) which increases the probability that the wood fire is not getting enough air for complete combustion. since creosote is highly combustible, a thick accumulation creates a fire hazard. if a hot fire is built in the stove or fireplace, and the air control left wide open, this may allow hot oxygen into the chimney where it comes in contact with the creosote which then ignites—causing a chimney fire. chimney fires often spread to the main building because the chimney gets so hot that it ignites any combustible material in direct contact with it, such as wood. the fire can also spread to the main building from sparks emitting from the chimney and landing on combustible roof surfaces. in order to properly maintain chimneys and heaters that burn wood or carbon-based fuels, the creosote buildup must be removed. chimney sweeps perform this service for a fee. release into environment broken creosote piling exposed by weathering even though creosote is pressurized into the wood, the release of the chemical – and resulting marine pollution – occurs due to many different events: during the lifetime of the marine piling, weathering occurs from tides and water flow which slowly opens the oily outer coating and exposes the smaller internal pores to more water flow. frequent weathering occurs daily, but more severe weather, such as hurricanes, can cause damage or loosening of the wooden pilings. many pilings are either broken into pieces from debris, or are completely washed away during these storms. when the pilings are washed away, they come to settle on the bottom of the body of water where they reside, and then they leach chemicals into the water slowly over a long period of time. this long-term secretion is not normally noticed because the piling is submerged beneath the surface, hidden from sight. the creosote is mostly insoluble in water, but the lower-molecular-weight compounds will become soluble the longer the broken wood is exposed to the water. in this case, some of the chemicals become water-soluble and further leach into the aquatic sediment while the rest of the insoluble chemicals remain together in a tar-like substance. another source of damage comes from wood-boring fauna, such as shipworms and limnoria. though creosote is used as a pesticide preservative, studies have shown that limnoria is resistant to wood preservative pesticides and can cause small holes in the wood, through which creosote can then be released. chemical reactions with sediment and organisms once the soluble compounds from the creosote preservative leach into the water, the compounds begin reacting with the external environment or are consumed by organisms. the reactions vary depending on the concentration of each compound that is released from the creosote, but major reactions are outlined below: alkylation alkylation occurs when a molecule replaces a hydrogen atom with an alkyl group that generally comes from an organic molecule. alkyl groups that are found naturally occurring in the environment are organometallic compounds. organometallic compounds generally contain a methyl, ethyl, or butyl derivative which is the alkyl group that replaces the hydrogen. other organic compounds, such as methanol, can provide alkyl groups for alkylation. methanol is found naturally in the environment in small concentrations, and has been linked to the release from biological decomposition of waste and even a byproduct of vegetation. the following reactions are alkylations of soluble compounds found in creosote preservatives with methanol. m-cresol the diagram above depicts a reaction between m-cresol and methanol where a c-alkylation product is produced. the c-alkylation reaction means that instead of replacing the hydrogen atom on the -oh group, the methyl group (from the methanol) replaces the hydrogen on a carbon in the benzene ring. the products of this c-alkylation can be in either a para- or ortho- orientation on the molecule, as seen in the diagram, and water, which is not shown. isomers of the dimethylphenol (dmp) compound are the products of the para- and ortho-c-alkylation. dimethylphenol (dmp) compound is listed as an aquatic hazard by characteristic, and is toxic with long lasting effects. phenol this diagram shows an o-alkylation between phenol and