What's the Story? The Dry-Cleaning Chemical PERC

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Perc also known as perchloroethylene, PCE, or tetrachloroethylene is a colorless, nonflammable liquid with a sweet odor. It evaporates readily into the air. Some algae produce perc naturally, but the perc used in business and industry is made synthetically. Approximately 300 million pounds of perc were produced in the U.S. in 1993 a decrease of about 50 percent from the amount produced ten years earlier.


Perc can dissolve fatty and oily substances and so has been used extensively for dry-cleaning clothes and for degreasing metals in a variety of industries. In addition, it is used by the chemical industry as a raw material in the production of other compounds, especially new refrigerants developed to replace the ozone-depleting chlorofluorocarbons (CFCs).

The advantages of perc compared to other such compounds include that it is not flammable, has low toxicity to humans, does not accumulate in humans and other living creatures, does not deplete the ozone layer, and is not persistent in the environment. Thus, it can be used beneficially without the negative effects on human health and the environment that are characteristic of some other solvents, such as petroleum products.


Two concerns have been expressed about perc. One relates to the possibility that it can have various non-cancer effects, such as headaches, nausea, dizziness, and liver and kidney problems, and reproductive difficulties in women. These effects are said to occur almost exclusively in workers, especially those in the dry-cleaning and chemical industries. The other relates to the possibility that long-term exposure to perc can cause cancer both in workers and in the general population.

That perc might cause cancer in members of the public who are exposed to it in their air and water has been the source of most of the demands for increased regulation and decreased use of this substance. A particular environmental concern is that perc has been found at about half of the past and current Superfund sites, mainly in the groundwater below the sites. There have been claims that drinking this water, which often is also contaminated with other chemicals, has caused cancer.

Although the claims of cancer in people living near hazardous waste sites contaminated with perc are the most dramatic, the case against perc as a cancer-causing agent is based on experimental studies of mice and rats exposed to high levels of perc daily for a lifetime and on epidemiological studies of workers who were exposed to perc for many years.


Results from observations of people exposed to high levels of perc in air have revealed that nervous system effects such as headache, nausea, and dizziness can occur. Some changes in the blood suggestive of effects on the liver and kidneys have also been observed in persons exposed to such high levels of perc. But even in heavily exposed individuals, these effects appear reversible except where extremely high air concentrations can lead to serious toxic effects and even death. Such cases are very rare. Epidemiological studies do not support the claim that perc can cause reproductive problems.

Based on data from humans exposed to perc, acceptable workplace limits for this compound in air have been established. These acceptable occupational limits are very much higher than perc concentrations that have been measured in indoor or outdoor air. Thus, it is very unlikely that any of these adverse effects would occur in persons exposed in their homes or while carrying out their daily activities.

The question of whether perc can cause cancer in workers or members of the general population has been studied using both experiments on laboratory animals and epidemiological investigations of workers and environmentally exposed individuals. The studies in laboratory animals produced mixed results. After high-dose, daily lifetime exposures to perc in air, male rats had a slightly increased frequency of kidney cancer, while female rats showed no such effect. No liver cancer was found in the rats. Both male and female mice exposed to high air levels of perc daily over a lifetime showed an increase in liver cancer, but not in kidney cancer.

These results were initially interpreted to mean that perc in air could cause cancer in humans since the generally accepted assumption is that anything that can cause cancer in animals can also cause it in humans. But serious doubt about this interpretation arose after a number of careful studies performed to test this assumption indicated that it is not valid for perc.

Some of these studies indicated that the liver cancer in mice was not caused by perc itself, but rather resulted from a byproduct of perc. This byproduct is produced in much greater amounts in mice than in humans. It was also found that it does not occur in humans at all unless perc exposures are very high. This information strongly suggests that the liver cancer that occurred in mice exposed to high levels of perc would not occur in humans, especially those exposed at levels present in the workplace and in the environment.

Additional studies revealed that reactions with perc thought to be necessary for kidney-cancer formation occur much more strongly in rats than in humans. Other experiments showed that male rats can accumulate a special protein that is critical to effects on the kidney and that female rats, mice, and humans cannot accumulate this protein. Taken together, these studies show why perc caused cancer only in male rats and why humans are very unlikely to show the same effects.

Some epidemiological studies have shown a possible connection between perc exposure and cancer in workers in the dry-cleaning and chemical industries, and other people, but other investigations have found no relationship. Evaluation of these studies to determine the sources of this inconsistency revealed a number of serious deficiencies in the way that the investigations were performed. These problems include uncertainties in the perc exposure amounts, lack of consideration of exposures to other chemicals at the same time, and failure to take into account other factors that might also lead to cancer (smoking, for example). Because of such inconsistencies and deficiencies, the epidemiological evidence cannot be used to help decide whether or not perc can cause cancer in humans.


The evidence shows that perc can cause effects on the nervous system, liver, and kidneys only when high levels of exposures occur. Since these levels are much higher than those found in the environment, especially the home, it is very unlikely that the public will suffer such adverse effects from environmental exposures to perc.

Critical analysis of the results of the many studies on perc show that there is no credible animal or epidemiological evidence that perc can cause cancer in humans. On the contrary, careful scientific examination of claims shows that it was the unique characteristics of rodents and their very high exposures to perc that led to their cancers, and that the results of such laboratory-animal studies do not apply to humans. Critical evaluation of the epidemiological studies indicates that, because of study deficiencies, they cannot be used either to establish or to discredit definitively a relationship between perc and adverse human health effects.

Perhaps the best summation of the possible risks of perc to the public was made by a Canadian governmental agency, Health Canada, which concluded that perc "is not entering the environment in quantities or under conditions that may constitute a danger to . . . human life or health."