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Incomplete page!!! IntroductionChlorinated solvents have been used industrially for degreasing, for over a century, with great success. They have replaced light hydrocarbons, which were the cause of many serious accidents, because of their flammability. Generally speaking, chlorocarbons have little or no flammability. In fact, carbon tetrachloride has been used as a fire extinguisher. Although this list is not exhaustive, the most popular ones, in chronological order of their introduction, are as follows, listed by their chemical name, see the links for their synonyms: 1,1,1-trichloroethane 1,2-trans-dichloroethene Chlorinated solvents, generally, have been progressively given a "bad press", mainly because of anecdotal cases of toxic outcomes and an insidious campaign by some ecopolitical NGOs to denigrate all chloroorganic compounds. This has meant that the dangers of using chlorocarbon solvents have been grossly exaggerated out of all proportion and, consequently, sales have dropped. Worse, manufacturers of other cleaning products and process materials and equipment have not been slow to suggest, often wrongly, that chlorocarbon solvents should not be used. Furthermore, this disinformation has spread to many developing countries. Enterprises seeking substitutes for ozone-depleting solvents have been scared to use chlorocarbon solvents and have been uselessly forced, in many cases, to seek much more expensive and technically less suitable solutions. This has cost the UNEP Multilateral Fund a lot of money for their grants and the situation has been exacerbated by the disinformation propagated by at least one Implementing Agency. I don't know whether this was done innocently because the persons concerned genuinely believed that the chlorocarbon solvents were more dangerous than they are, or whether it was done deliberately to increase the cost of the projects and thus the IA's revenue. It is true that, in a few cases, the cost of replacing an ozone-depleting solvent by a chlorocarbon solvent would have been so low, that the beneficiary enterprise and the IA would have received no revenue from the project. This would nevertheless have cost the IA a considerable sum to put forward, so the IA would have been motivated to suggest a different process where they could, at least, have recovered their costs. The purpose of this page is to attempt to redress the balance by demonstrating that chlorocarbon solvents can be used safely in many applications, at much lower costs than other methods. It is really a paradox that I'm promoting chlorocarbon solvents as, for many decades, I've been very wary of them and have discouraged the use of all halogenated solvents, where other methods may be better for one reason or another. This is still the case. Nevertheless, my mind is not closed to the proven fact that there some applications where chlorocarbon solvents offer the best solution for some jobs.
Synonym: chloroform. Although a good solvent for oils, greases and waxes, its anaesthetic properties and high hepatotoxicity very quickly reduced its use as an industrial solvent.
Synonyms: carbon tetrachloride, carbon tet. This was, by far, the most popular industrial solvent for degreasing and dry cleaning up to about 1960. It was cheap, easily available and very effective. However, long-term exposure proved it to be a bad hepatic toxin and many persons suffered severely reduced liver function. It is also a suspected carcinogen, although this has never been proved. It was withdrawn from general use, as a result, in developed countries although it is still very widely used, often under bad exposure conditions, in some developing countries. It is a bad ozone-depleting substance, some 10% worse than the popular refrigerants and is controlled under the Montreal Protocol with total phase-out for developed countries and a gradual phase-down for the others.
Synonyms: trichloroethylene, ethylene trichloride, trike, tri, TCE. This has been the most-used chlorinated solvent for metal degreasing and some other applications since about 1903, when it was first commercialised. This long experience means that there is little that is not known about it. It is less stable than tetrachloromethane, forming hydrochloric and trichloroacetic acids in contact with moisture, which means that stabilisers are added to the commercial blends to prevent this. The stabiliser level should be maintained to ensure a long working life, especially when used in vapour degreasers. The latter have a tendency to condense atmospheric humidity on the cooling coils, exacerbating the decomposition. ToxicityThe toxicity of TCE has been the subject of many heated debates, often fuelled by misinformation and disinformation propagated by companies offering other types of solvent. Let's look at what is certain: with long continual exposure to the vapour at high concentration, it will cause damage to the liver. For this reason, most countries have maximum time-weighted exposure levels for operators of 25-100 ppm. These are deemed to be the maximum continuous level over 8 hours/day, 5 days/week to which an operator may be subjected with zero risk of compromising his health, with a generous safety margin of between 3:1 to 10:1, depending how the calculations are carried out. The big question is whether TCE is carcinogenic. All the evidence would indicate that it is not, even at exposure levels well above the permitted maximum. Firstly, remember that workers have been exposed to TCE vapour for over a century and there have not been any indications of problems, even in the days when there was no limit or control of exposure levels. Tests with rodents have induced cancerous tumours at high exposure levels; however, it has been shown that this will not happen, in the same way, in humans because the metabolic path of the breakdown of the substance is different. It is an error to extrapolate animal tests to humans, without examining the comparative effects of metabolites and how they are produced. The key factor in determining the potential carcinogenicity is in properly conducted, full scale, epidemiological studies over many years and with large cohort groups and control groups. I have a copy of part of Vol 10 of Occupational Toxicants by the Deutsche Forschungsgemeinschaft (Wiley) devoted to the toxicology of TCE. It is fully referenced and mentions many studies on the carcinogenicity of TCE. The following references showed no statistical increase in the incidence of tumours:
On the other hand, there was a much disputed study (Henschler et al. 1995) where 4 out of 169 persons exposed in an E. German cardboard factory were diagnosed with renal tumours (2 others were diagnosed later. The average observation period was 30.7 years. The exposure level was very high, estimated at 200 ppm. These studies were conducted after the events, so the real exposure levels could not be monitored. However, there was a severe confounding factor in this study; the TCE appears to have been stabilised with epichlorohydrin, a known carcinogen. This would tend to render the study of less value, especially as it contradicts the findings of other studies with better controls. Of course, the document cites numbers of other anecdotal cases "proving" that TCE is or is not carcinogenic. A section is devoted to animal experiments, often with massive exposure levels of up to 2000 ppm and administered doses of up to 3,000 mg/kg (equivalent to drinking a 0.25 litre glass of the stuff for an adult human), On the whole, pre-natal concentrations up to 300-500 ppm did not prove toxic for the dams or offspring, nor on sperm count and motility of the sires, although there were contradictions in the findings between some of the studies. Maltoni et al. 1988 subjected two races of mice and one of rats to 100, 300 and 600 ppm. The results with the highest concentration did produce "borderline evidence" of renal tubular adenocarcinomas in male rats after exposure for 78 weeks and observation for 154 weeks, One of the mouse races (males) showed increased lung tumours at the two highest concentrations. There was also an increase in females of all types at all concentrations. Of course, the last paragraphs are only a brief summary of 44 pages and interested readers should consult the full document. Another, even more relevant, document is Technical Report No. 60, Trichloroethylene: Assessment of Human Carcinogenic Hazard, May 1994, published by the European Centre for Ecotoxicology. Brussels. This 77 page report was written by a committee of seven and, more importantly, was peer reviewed by 15 experts from industry and academia. This cites many of the same epidemiological studies as the foregoing document, but in much greater detail, which would be irrelevant here, for each kind of tumour. The metabolic processes in animals and man are also explained. I consider this report as the current, and probably definitive, one on the subject, unless some new discoveries are made. I should like to cite the last sentence of the Conclusion: "Taking all of this information into account, it is concluded that exposure to TCE does not present a carcinogenic hazard to man at levels of current occupational exposure standards." Notwithstanding, some countries have regulations regarding the use, packaging and labelling of TCE that is totally out-of-proportion to the reality of long and deep scientific research into its health and safety issues. This is unfortunate because it is a solvent that is almost universally suitable for most degreasing applications, it is cheap and easy to obtain and is safe to use in modern equipment, yet has become unpopular because of what amounts to unjustified prejudice.
Synonyms: tetrachloroethylene, perchloroethene, perchloroethylene, ethylene tetrachloride, perc, PCE. This solvent may be considered as similar to TCE, except for a lower volatility and higher boiling point. It is used mainly for dry cleaning and metal cleaning. The higher boiling point makes it more effective in removing heavy greases and waxes in vapour degreasers. Although introduced on the commercial market a few years later than TCE, we still have nearly a century of experience in its safe use. ToxicologyThe acute toxicity of PCE is less than that of TCE. It is a relatively stable compound and inhaled vapours are largely exhaled again within a day or two. Notwithstanding, there are many cases of toxic effects from a single massive exposure, with varying symptoms, including nausea, inebriation, unconsciousness, neuropathic effets, respiratory problems and even death. Vol 3 of Occupational Toxicants by the Deutsche Forschungsgemeinschaft (Wiley) has a section devoted to the toxic effects of PCE. Most of the epidemiological studies have been made on workers of dry cleaning establishments, but it is more difficult to interpret them because exposure levels vary enormously from shop to shop and work methods also vary, such as how much solvents remain in the garments when they are removed from the machines and when they are subsequently pressed. Also, it is more difficult to determine confounding factors, such as siting of the shops with respect to urban pollution, individuals' tobacco and alcohol usage, contact with other solvents (many shops use three or four) etc. In some studies, the SMRs from cancers of all types are slightly over 100 per cent and in some, they are very marginally below 100 per cent. Report no. 37, Tetrachloroethylene: Assessment of Human Carcinogenic Hazard (European Chemical Industry Ecology and Toxicology Centre, Brussels, 1990) offers similar reasoning. It was written by a committee of 9 and peer reviewed by a scientific committee of 14 experts from industry. It states: "It is concluded that, overall, the design and outcome of epidemiological studies failed to demonstrate a relationship between exposure to tetrachloroethylene and the occurrence of cancer in man." Note the careful wording, much less categorical than the above quotation for TCE; I suggest that this because the conditions of use of PCE in many dry-cleaning shops is almost anarchic. A much more comprehensive assessment can be found in Joint Assessment of Commodity Chemicals No. 39: Tetrachloroethylene (European Chemical Industry Ecology and Toxicology Centre, Brussels, 1999). This was written by similar committees and peer reviewers to the preceding document. It is a massive work of 271 printed A4 pages and it would obviously be impossible to summarise it in detail on a web page. It makes similar conclusions to the preceding document, but adds that a Lowest Observed Effect Level over a wide range of species is 100 ppm. With a safety margin based on a LOEL:Operator Exposure Level ratio of 3:1 with a TWA of 8 h/day, 5 days/week, this suggests that the OEL should be at about 33 ppm. Most countries have an OEL of 25 or 50 ppm.
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