The Devil’s in the Details on TCE

For some weeks, I’ve been struggling with a post on TCE, trying to answer to my satisfaction several questions. These included: whether or not we’re spending too much time trying to understand a very small risk, or spending too much time arguing over something that’s a clear environmental health problem; how significant of a cancer risk are we facing with a substance that shows relative risks less than two in epidemiological studies:

Analytical studies of individual risk factors today often show relative risks of only 1.5 to 2, and these are difficult to translate into preventive recommendations.

Although epidemiologists have become more introspective with questions on the utility of epidemiologic studies for providing insight on associations of relative risks of 2.0 or lower (Taubes, 1995, Monson, 1980), epidemiologic evidence continues to inform the hazard step in risk assessment and is increasingly examined for dose-response inferences.

and, how did we end up with an 17 nanograms per cubic meter (ng/m3) of TCE in air (a tiny, tiny amount), corresponding to a 1 in 1,000,000 excess lifetime cancer risk, as a regulatory guidance level? For that matter, how did we end up with 1 in 1,000,000 as a target level for evaluating cancer risks to the public? Here’s one perspective – you might not agree with it, but it’s thought-provoking.

These questions have proven to be difficult and time-consuming to answer for TCE. However, the stakes are large – more and more, TCE is showing up in air samples in peoples’ basements, and the costs for remediation of TCE-contaminated soil and groundwater potentially run to several billions of dollars. A substantial portion of those costs are borne directly by us taxpayers, for contamination at Superfund sites with no responsible parties (i.e. sites where the individuals who contaminated the environment can’t possibly pay for the cleanup) and contamination at Federal facilities.

Some would argue that TCE is a classic case that argues for imposition of the precautionary principle:

When an activity raises threats of harm to human health or the environment, precautionary measures should be taken even if some cause and effect relationships are not fully established scientifically.

However, the devil’s in the details in just how you accomplish that feat. There’s also merit in the perspective of risk management:

“What should we do when the safe and the dangerous are inextricably intertwined?” It is foolish to seek maximum benefit without considering the risks involved, but it is just as foolish to pursue minimum risk without regards for the cost. It is madness to ask for zero risk. Risk management is a balancing act.

This issue has been thrown in to the lap of the National Academy of Sciences. Some will hope, for the sake of decision makers, homeowners with TCE in their basements, and workers exposed to TCE everywhere, that the NAS will pass some kind of miracle and show us the path between neglecting a potentially significant threat to human health and alarming people with possibly needless precaution. I’m not that optimistic. A successful resolution to the TCE problem requires all of us to do some of the heavy lifting.

This wasn’t the TCE post I’ve been trying to write, but then again, cancer epidemiology and pharmacokinetic modeling aren’t terribly relevant without this kind of framework.