Mercury Rising - NHANES Mercury Exposure Survey

As we discussed previously, the media has been reporting on the risks associated with exposure to mercury, principally from eating fish. This post takes up the question of “what’s the risk that my child could have been affected by mercury because I’ve eaten fish?” Another way of examining it is asking how uniformly is the risk distributed.

The U.S. Environmental Protection Agency (EPA) has concluded that 300,000 newborns may have been overexposed to mercury through their mothers’ consumption of fish. The study that everyone is interested in reports blood organic mercury (i.e., methyl mercury) in 1,709 women who participated in the National Health and Nutrition Examination Survey (NHANES) in 1999 and 2000. Their fish consumption was estimated using 24 hour recall diaries (keeping a diary of everything eaten in the past 24 hours) and interviews of foods consumed in the past 30 days. The mercury concentrations in fish were based on data in published studies. The consumption and concentration data were then combined to estimate daily mercury intake.

Blood methyl mercury concentrations were lowest among Mexican American women. Blood methyl mercury concentrations were highest among participants who designated themselves in the Other racial/ethnic category, which includes Asians, Native Americans, and Pacific Islanders. Blood methyl mercury concentrations were seven times higher among women who reported eating nine or more fish and/or shellfish meals within the past 30 days when compared with women who reported no fish and/or shellfish consumption in the past 30 days.

The blood mercury concentrations in women were compared with the Reference Dose (RfD) for methyl mercury to assess the potential risks of adverse neurobehavioral effects in newborns. The RfD value corresponds to a blood mercury level of 5.8 ug/L in (micrograms of mercury per liter of blood) in umbilical cord blood, which is the closest representation of potential dose to a fetus. The derivation of this value is a topic for its own post, but some of the key points are:

1. The RfD, developed by the EPA, is defined as an estimate (with uncertainty spanning perhaps an order of magnitude) of a daily exposure to the human population (including sensitive subgroups) that is likely to be without an appreciable risk of deleterious effects during a lifetime.
The RfD for methyl mercury was derived from blood mercury level of 58 ug/L (developed by the National Research Council) which is the lower 95% confidence interval (CI) on an estimated dose that doubles the prevalence of children with scores on a test of intellectual development that would fall into the clinically subnormal range. The EPA applied a 10-fold uncertainty factor to NRC’s value to obtain the RfD. See now why this topic requires its own post?

2. In the 1999-2000 NHANES survey, 7.8% of women between 16–49 years of age had blood methyl mercury levels of 5.8 µg/L or higher. EPA applied the 7.8% value to the number of newborns in 2000 (4,058,814, according to The National Vital Statistics Reports) to conclude that > 300,000 newborns per year may have had increased risk of adverse neurodevelopmental effects as a result of in utero methyl mercury exposure.

Another recent study examining the ratio between umbilical cord to whole-body blood methyl mercury concentrations indicates that cord blood is, on average, 70% higher in Hg concentration compared with a whole-body blood mercury level (in other words, the mercury concentration in a blood sample collected from a vein in the arm). When corrected for this ratio of 1.7:1.0, the percentage of women with a blood mercury level that might pose an increased risk of adverse neurodevelopmental effects in newborns increases to 15.7 percent. Does this mean there are really ~600,000 newborns/year at risk? That’s not certain, and EPA did not publish that number in their study. I’m not sure it really matters; either number indicates a potential environmental health crisis.

Another limitation to these findings is that mercury is not the only neurobehavioral toxicant that accumulates in fish as potentially poses a risk to newborns. Persistent organic pollutants including PCBs and dioxins, and polybrominated diphenyl ethers - PBDEs (which aren’t called POPs, but should be) may also pose neurobehavioral risks in the same manner as mercury. As yet, noone has done the cumulative risk assessment addressing mercury and POPs. For the moment, there really isn’t an answer to the question of the combined exposures.

There's more coming on this topic later.