EPA says in a new assessment that the widely used solvent trichloroethylene (TCE) is carcinogenic to humans by all routes of exposure. But the EPA has simply pulled the same trick on TCE as it did on environmental tobacco smoke (ETS) — a trick that got the EPA’s ETS cancer risk assessment trashed by a federal court.
The EPA says in its TCE assessment that:
Following EPA Guidelines for Carcinogen Risk Assessment, TCE is characterized as “carcinogenic to humans” by all routes of exposure. This conclusion is based on convincing evidence of a causal association between TCE exposure in humans and kidney cancer. The human evidence of carcinogenicity from epidemiologic studies of TCE exposure is strong for non-Hodgkin’s lymphoma )NHL)], but less convincing than for kidney cancer, and more limited for liver and biliary tract cancer. Less human evidence is found for an association between TCE exposure and other types of cancer, including bladder, esophageal, prostate, cervical, breast, and childhood leukemia. [Footnotes omitted]
So what is the evidence for TCE causing kidney cancer in humans? Below is the EPA’s summary chart of the studies it views as bolstering its case:
Of the 15 studies listed in the table, only 2 report a statistically significant association (correlation) between TCE exposure and kidney cancer.
What about the evidence for TCE causing NHL? Once again, check out the EPA’s own summary:
Of the 13 studies listed in the table, only 2 report a statistically significant association (correlation) between TCE exposure and NHL.
Albert Einstein famously said that to defeat the theory of relativity one did not need the word of 100 scientists, just one fact. With respect to kidney cancer and NHL, there are at least 24 facts against EPA’s (at best) 4.
So how did the EPA overcome a combined score of 24-4 against TCE being associated with kidney cancer and NHL? It combined all the studies together in the highly inappropriate technique of a meta-analysis — the same stunt pulled on environmental tobacco smoke (ETS) in its 1993 risk assessment of that substance.
While a meta-analysis can be an appropriate statistical technique for combining similar studies (like similarly-designed and tightly-controlled clinical trials) into a single study with more statistical power, meta-analysis is highly inappropriate for epidemiologic studies of varying protocols and uneven quality.
Even accepting for the sake of argument the EPA’s meta-analysis as a valid technique, the results remain weak statistical associations that are best described as reflecting statistical noise.
Keep in mind that none of the epidemiologic studies in question are based on confirmed exposure levels to TCE — just self-reported guesstimates. None of the studies medically link cases of cancer with TCE.
Also consider that that kidney cancer and NHL studies were the most persuasive to EPA — and they don’t implicate TCE as a carcinogen in the least.
When a federal court finally got around to reviewing EPA’s cancer risk assessment for ETS, it vacated the EPA’s conclusion for, among other reasons, the dubiousness of EPA’s meta-analysis.
We’ll see if the chemical industry pushes back on the EPA’s TCE junk science.