Mercury Factoids

As noted in Effect Measure and in this blog, that august scientific group, being the leading Republican members of the Resources Committee of the House of Representatives has prepared a report concluding that the risks of mercury pollution have been overstated. Not a surprising conclusion when you consider the source, which might actually be the right-wing, funded by Exxon-Mobil think tank, Frontiers of Freedom.

While the report isn’t terribly useful for informing the debate about the mercury emissions from power plants or mercury risks (c’mon, did you really think it was there for any reason other than an excuse for a press release), I’ve put it to use as a platform for deconstructing some of the right’s talking points about mercury.

I’ve been wanting to write about the EPA’s Reference Dose for methyl mercury for sometime now, and this provides the perfect opportunity. But that post is taking awhile to prepare; because unlike some people, I’m trying to frame the issues transparently and in a manner that readers might find useful. So, in the meantime, let’s look at one of the factoids, instead.

The Resources Committee report (which I’ll call the RC report herein) poses the question, “others wonder whether the overall increase in global mercury emissions and deposition mean that Americans will be exposed to higher concentrations of methylmercury in fish.” In answering that question, it offers “recently published and ongoing scientific research can provide some insight to this question”. One of those pieces of research is this study published in Environmental Health Perspectives (referred to here as the EHP paper), which the RC report characterizes as follows, with a quote from the study:

Concentrations of mercury, lead and persistent organic pollutants in the umbilical cord blood of Inuit infants born in Nunavik, Quebec decreased between 1994 and 2001.

“Inuit inhabitants of Nunavik consume great quantities of marine food and are therefore exposed to high doses of food chain contaminants. …We analyzed 251 cord blood samples collected from 1994 through 2001 for polychlorinated biphenyls (PCBs), dichlorodiphenyl trichloroethane (DDT), dichlorodiphenyl dichloroethylene (DDE), hexachlorobenzene (HCB), chlordanes, lead and mercury. Using an exponential model, we found strongly significant decreasing trend for PCBs (7.9% per year, p <0.001), DDE (9.1% per year, p < 0.001), DDT (8.2% per year, p< 0.001), and HCB (6.6% per year, p < 0.01). No significant trends were detected for chlordanes. A significant reduction of lead and mercury concentrations was found [i.e., by more than 8% decrease per year], ...”

The quote appears to have been lifted from the abstract of the EHP paper. Those who bother to read further into the paper will find the authors’s opinions about why the trends in mercury, lead and chlorinated hydrocarbon compounds looked like this.

The chlorinated hydrocarbon trends (i.e. PCBs and the pesticides) were explained by the restrictions in the use of the substances since the 1970s, though the authors suggest that some foodchain exposures could still occur from global cycling and long-range transport of these substances. They also note the changes in the Inuit diet as a possible factor in exposure reduction to chlorinated hydrocarbon compounds:

The eating habits of Inuit populations have changed enormously during the last 50 years. Since market-bought food has been introduced in their diet, added carbohydrates, junk food, pork, chicken, milk products, and other "foreign" food items have become increasingly popular, especially among adolescents and young adults. . . . Market food usually has a lower trophic level than does traditional Inuit food and is consequently less contaminated by POPs. This is reflected by the much lower mean concentrations of PCBs and chlorinated pesticides detected in cord blood samples from populations whose diet is almost exclusively composed of market-bought food, such as those in southern Québec. . . . However, although a gradual switch from traditional food to market food would result in a decrease in blood concentrations of food chain contaminants, it seems unlikely that dietary modifications would be of such magnitude that they alone would cause an annual decrease of 5-10% in the body burden of contaminants.

Ellipses have been added only to remove the citations in the original EHP paper. The authors also comment that:

The generalized downward tendency of OC concentrations observed in wildlife and human tissues throughout the world strongly suggests that the environmental contaminant burden is steadily declining and that this tendency can be observed in all levels of the food chain. We believe that most of the decrease of OC concentrations observed in this study can be attributed to descending concentrations in the traditional food items of the Inuit diet.

This seems to offer some explanations for the declining trend in chlorinated hydrocarbon compounds (PCBs, DDT, DDE, chlordane and HCB). What’s the story with lead?

We’re told that biomagnification does not play an important role in lead exposure. Lead shot used in hunting is considered to be a more important source of exposure. A recent ban in lead shot, coupled with an information campaign, are considered to be the reasons for the decline in lead exposures.

The RC report authors don’t explain what any of this has to do with mercury exposure, possibly because they may not have read through the entire EHP paper.

What do the EHP authors really say about mercury exposures?

Because no data on mercury temporal trends in arctic wildlife since 1994 are available, it is hazardous to speculate on the cause of the variations observed in this study. In contrast to OCs*, our results for mercury do not support an exponential decrease. The concentrations were constant across the years, except for markedly lower levels in 1998 and 2000. When we used n-3 fatty acids in cord blood as a surrogate of maternal fish consumption, we observed a slight decrease that was not statistically significant and was not related to the mercury concentration. We also searched for relation between mercury concentration and the numbers of beluga caught in each village between 1996 and 2001, but no relation was found (Department of Fisheries and Oceans 2002). A thorough dietary survey would be necessary in order to elucidate the cause of the variation observed. Other studies with longer follow-up would clarify whether the lower levels observed in 1998 and 2002 were due to chance or were signs of a new temporal trend of mercury.

*OC – organochlorine compounds – PCBs, DDT, DDE, chlordane and HCB; synonym for chlorinated hydrocarbon compounds.

This was a very interesting study that provided some deeper insights on persistent organic pollutant (POP) exposures. It also seems to provide some indication of the effectiveness of precautionary measures, such as restricting PCB and chlorinated pesticide use, reducing use of lead shot and lead health education measures. However, it appears that with the exception of a couple of years, there is no declining trend in mercury concentrations; and at this time, the most that can be said is that the trends in mercury exposures and their underlying explanations, are inconclusive. It is scarcely the definitive scientific statement to help the readers of the RC report judge “whether the overall increase in global mercury emissions and deposition mean that Americans will be exposed to higher concentrations of methylmercury in fish.”

So, what’s the deal here? Is this simply an example of sloppy scholarship? A lack of awareness (or concern) that anyone was going to do any fact checking? In the end, it doesn’t really matter that the report is really a 33 page, turgidly written op-ed piece, with footnotes. On the heels of this report are stories in a hundred different newspapers titled “Mercury Risks Overstated, House Panel Says”. Mission accomplished. But it does a deep disservice to anyone really trying to understand mercury issues.

Edited slightly on February 20, 2005.