Cancer, Animals and Man – The Case of TCE, Part 1

This was getting too long, so I’m going to have to roll it out in multiple posts. Previously, I introduced a recently published report that highlights the role of carcinogenic chemicals in the environment as possible causes for the incidence of cancer in the U.S. Revere addresses the issue of why we need to be able to identify environmental carcinogens, and why we need to rely on cancer bioassays using laboratory animals for carcinogen identification. Well-conducted epidemiological studies that show an association between exposure to an agent and occurrence of disease are the most convincing evidence of human health risk. But epidemiological studies, even when they aren’t confounded, often don’t point clearly to an exposure-disease relationship. Few studies are sufficiently powerful to detect anything but highly potent carcinogens. Finally, epidemiological studies don’t produce findings for many years after exposure occurs, and not until a sizeable number of people are ill or dying. Epidemiology has a crucial role in evaluating the human carcinogenicity of chemical substances, but if you’re trying to “get out in front” of carcinogenic exposures and address them promptly, epidemiological studies aren’t the tool you want to put a lot of reliance on.

However, carcinogenicity is not a “yes/no” property in a chemical. Even with animal bioassays, identification and evaluation of carcinogenicity in a chemical can be murky. If it’s a chemical where substantial costs might be involved in mitigation or substitution, the deliberations of its carcinogenicity can become torturous and prolonged.

Take trichloroethylene (TCE), for example. TCE is used as a intermediate in chemical manufacturing, and as a solvent for metal cleaning. Historic waste management and disposal practices associated with the latter use in particular have resulted in soil and groundwater contamination at thousands of sites in the U.S. Use of TCE as a cleaning solvent has decreased in favor of less toxic, water soluble alternatives, due to occupational health concerns and more stringent waste management regulations. However, there remains an enormous mortgage for the cleanup of the contamination remaining from past practices.

IARC judges TCE to be probably carcinogenic in humans, based on limited evidence in humans and sufficient evidence in laboratory animals. The NTP states that:

Trichloroethylene (TCE) is reasonably anticipated to be a human carcinogen based on limited evidence of carcinogenicity from studies in humans, sufficient evidence of carcinogenicity from studies in experimental animals, which indicates there is an increased incidence of malignant and/or a combination of malignant and benign tumors at multiple tissue sites in multiple species of experimental animals and information suggesting TCE acts through mechanisms that indicate it would likely cause cancer in humans.

Even the solvents industry acknowledges that TCE is an animal carcinogen, but they do question the relevance of the tumors to human health.

For many people, that summation of TCE’s carcinogenicity is enough to demand that we get it out of our environment – stop using it and use safer alternatives, and clean it up from contaminated soil and groundwater. As always, the devil’s in the details, in terms of figuring out step-by-step how those objectives are accomplished; particularly in the cleanup department – how low do you have to go? As you will see in the follow up post, science has its limits in helping answer these questions. Some have argued (as Ellen Silbergeld did several years ago) that risk assessment was a waste of time and an impediment to regulation. However, it also may be one of the tools to help all of us get involved with answering the questions of what to do about chemicals and our health.

I'm wading further into TCE science, and I hope to write more about this again soon.