Exceeding the methyl mercury reference dose: how dangerous is it?

The methyl mercury exposure data presented by Hightower and Moore (2003) with regard to San Francisco fish consumers illustrates how regular consumption of certain species of fish can lead to an exposure that exceeds the current U.S. Environmental Protection Agency (EPA) reference dose (RfD). However, in spite of the impression given by the cover headline "High Health Cost of Eating Expensive Fish," the study sheds little light on the question of whether the health of the authors' patients was affected by their methyl mercury exposure. Hightower and Moore investigated the relationship between methyl mercury exposure, fish consumption, and the U.S. EPA RfD--not the relationship between methyl mercury and health effects.

Not all readers of EHP will appreciate the difference between documenting exposure levels and providing evidence for health effects, and not every reader will have followed the ongoing international controversy over the health effects of methyl mercury for fish consumers. The Science Selections published in EHP can help to bridge these gaps by providing clarification of important issues and context.

Unfortunately, the Science Selection on the Hightower and Moore article (Hood 2003) simply reinforced the impression left by the authors that the methyl mercury exposures in their patients were extraordinarily and dangerously high and that such exposure levels can result in symptoms of fatigue, headache, decreased memory, and joint pain in adults. In fact, methyl mercury exposures many times higher than the U.S. EPA RfD are common in fishing populations around the world, and there has been little epidemiologic investigation into the relationship between exposure levels in fishing populations and these symptoms [Agency for Toxic Substances and Disease Registry (ATSDR) 1999; National Research Council 2000].

The U.S. EPA RfD of 0.1 [micro]g/kg body weight/day is derived on the basis of the health effects of prenatal exposure (U.S. EPA 2001), as historically scientists believed the developing fetus to be most sensitive to methyl mercury. Two recent large prospective epidemiologic studies in fishing populations in the Seychelles (Myers et al. 2003) and Faroe Islands (Grandjean et al. 1997) have given mixed results, however, such that the question of neurologic impairment in children of mothers who consume large amounts of fish during pregnancy remains open. With regard to adults, the epidemiologic evidence for adverse effects at the exposure levels documented by Hightower and Moore (2003) is even more uncertain.

Because of this uncertainty, the determination of the appropriate level for the methyl mercury RfD represents a subjective policy decision, as well as a calculation based on scientific data. The policy element is most apparent in the choice of an uncertainty factor, which sets the RfD many times lower than the exposures that have been associated with prenatal effects. Policy is also reflected in the decision to apply one guideline to all members of the population. Many scientists, physicians, and regulators consider that, given current evidence, a guideline of 0.1 [micro]g/kg body weight/day is too low, particularly for men and older women.

Regardless of whether one agrees with the specific level set by the U.S. EPA for its methyl mercury RfD (U.S. EPA 2001) in the context of the agency's mandate for environmental protection, it is important not to forget how this dose was determined and its potential shortcomings as a dietary guideline. The current RfD may be appropriate for Hightower and Moore's patients, who have a wide range of high-quality food available to them. However, for many individuals, in the United States and around the world, fishing remains an important economic and cultural activity, and the dietary alternatives to the fish they catch are of much poorer nutritional value. For these individuals, adhering to the U.S. EPA RfD may not be the best advice.

The author is an independent consultant currently working under contract for the Cree Board of Health and Social Services of the James Bay (CBHSSJB); this work includes a literature review of the health effects of methyl mercury. Her views in this letter are her own and do not represent the CBHSSJB.

REFERENCES

ATSDR. 1999. Toxicological Profile for Mercury (Update). Atlanta, GA: Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services.

Grandjean P, Weihe P, White RF, Debes F, Araki S, Yokoyama K, et al. 1997. Cognitive deficit in 7-year-old children with prenatal exposure to methylmercury. Neurotoxicol Teratol 19:417-128.

Hightower JM, Moore D. 2003. Mercury levels in high-end consumers of fish. Environ Health Perspect 111:604-608. Hood E. 2003. A diet rich in fish. Environ Health Perspect 111:A233.

Myers G, Davidson PW, Cox C, Shamlaye CF, Palumbo D, Cernichiari E, et al. 2003. Prenatal methylmercury exposure from ocean fish consumption in the Seychelles child development study. Lancet 361:1686-1692.

National Research Council. 2000. Toxicological Effects of Methylmercury. Washington, DC: National Academy Press.

U.S. EPA. 2001. Methylmercury (MeHg) (CASRN 22967-92-6). Washington, DC: Integrated Risk Information System, U.S. Environmental Protection Agency. Available: http:// www.epa.gov/iris/subst/0073.htm [accessed 1 April 2004].

Deborah Schoen

Consultant/Science Writer

Longueuil, Quebec, Canada

E-mail: [e-mail]