Mercury exposure in British Columbia: Do we have a problem?

Human exposure to mercury, a naturally occurring element, is universal. Similar to the case with lead, recent research indicates that subtle effects of mercury toxicity may occur at much lower levels than previously believed. There are substantial uncertainties in the interpretation of the public health implications of current exposure levels. It has been alleged that over 60 000 children are born each year in the United States at risk of adverse neurodevelopmental effects due to in utero exposure to methylmercury.[1] However, a recent review points out the basis for this estimate has not been presented and does not appear to be supported by the existing literature.[2]

Mercury exposure to the general population comes primarily from diet, with fish being the main contributor. Thimerosal, a mercury-containing preservative in some vaccines, may also contribute to exposure although it has now been eliminated from most vaccines. Dental amalgam and indoor spills of elemental mercury are potential sources of exposure to mercury vapor.

Eliminating or reducing consumption of fish to curb mercury intake may seem like an effective way to reduce exposure. However, health benefits of fish consumption have been demonstrated in a number of studies; reducing fish intake can do more harm than good.

Mercury exposure through fish is influenced by the concentration of mercury in the fish consumed as well as the frequency and amount consumed. Because the concentration of mercury varies substantially between species, it is possible to choose low-mercury species in preference to those with a higher mercury content. In this manner the benefits of frequent fish consumption can be obtained while keeping mercury intake at a minimum. Mean mercury concentrations found in surveys of US commercial fish range from 0.99 ppm in shark and 0.97 ppm in swordfish to 0.12 ppm in canned light tuna, 0.03 ppm in freshwater trout and 0.01 ppm in fresh/frozen salmon. Species with intermediate mean concentrations include fresh/frozen tuna at 0.38 ppm, canned albacore tuna at 0.35 ppm, and halibut at 0.22 ppm.[3]

The United States conducts periodic surveys that measure exposure to a variety of environmental contaminants in a sample of the general population. The most recent data indicate a mean blood mercury of 0.3 ppb (1.5 nmol/L) in children aged 1 to 5 and a mean of 1.2 ppb (6 nmol/L) in women aged 16 to 49. The 90th percentiles were 1.4 ppb (7 nmol/L) and 6.2 ppb (31 nmol/L) respectively.[4] We lack comparable population-based survey data for Canada.

However, the recent finding of substantially elevated blood mercury in two Lower Mainland children by the Trace Elements Laboratory at Children’s and Women’s Health Centre of BC raises questions about the prevalence of excessive mercury exposure. In one case the initial blood mercury was 1000 nmol/L, in the other 400 nmol/L. Blood mercury tested by the same lab for a recent survey at a dental clinic yielded a 95th percentile of 36 nmol/L and a maximum of 68 nmol/L. In both cases the children were of preschool age and had a history of unusually high fish consumption. In one case the fish contained mercury at twice the Canadian guideline of 0.5 ppm. Investigation of exposure in the second case is still underway. In both cases, the children were of Asian ethnicity.

Additional cases of excessive mercury exposure may be present in the BC population. Physicians providing care for young children and women of childbearing age can identify additional individuals at risk of elevated mercury exposure through questions on the frequency and species of fish consumed. Where there is a history of unusually frequent consumption of fish species that may have high mercury content, blood mercury testing can provide information on the extent of exposure and serve as a guide to the need for any dietary changes or other therapy. Salmon (farmed and wild) and commercially sold freshwater trout can be recommended as species that are low in mercury.

—Ray Copes, MD
British Columbia Centre for Disease Control 
—Jan Palaty, PhD
Children’s and Women’s Health Centre of BC
—Gillian Lockitch, MD, FRCPC
Children’s and Women’s Health Centre of BC

References

1. National Research Council. Toxicological Effects of Methylmercury. Washington, DC: National Academy Press; 2000. Full Text 
2. Davidson PW, Myers GJ, Weiss B. Mercury exposure and child development outcomes. Pediatrics. 2004;113:1023-1029. PubMed Abstract Full Text 
3. Mercury levels in commercial fish and shellfish. US Food and Drug Administration. Center for Food Safety and Applied Nutrition. www.cfsan.fda.gov/ (accessed 23 August 2004). 
4. Center for Food Safety and Applied Nutrition. Blood and hair mercury levels in young children and women of childbearing age—United States, 1999. MMWR Morb Mortal Wkly Rep. 2001;50:140-143. PubMed Abstract Full Text   Site ©, 2003 | Contact Us