Hightower, JM. 2003. Mercury Levels in High-End Consumers of Fish. Environmental Health Perspectives doi:10.1289/ehp.5837

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Physician Jane Hightower reports that patients in her medical practice in the San Francisco Bay Area who consume large predatory fish like tuna and swordfish carry high levels of methylmercury in their blood and/or hair and are likely to report various medical symptoms consistent with low-level mercury poisoning. The good news is that when they stopped eating fish contaminated with mercury, their blood mercury levels went down over a period of 20+ weeks. For the most part these patients were socially and economically advantaged people for whom cost was not an issue in determining what fish to eat.

What did she do? Hightower interviewed patients on patterns of fish consumption and also about other sources of mercury exposure (mecury amalgam fillings, thimerosal in vaccinations). Those whose fish diet indicated a possibility of high mercury exposure were then asked to participate in a study that involved sampling blood or hair mercury levels over time, during which they were also encouraged to lower their predatory fish consumption.

What did she find? Approximately 140 out of 720 patients surveyed had fish consumption patterns suggesting high mercury exposure. Of these 116 allowed blood measurements; another with 7 allowed hair analysis. Mercury averaged 14 µg/L in serum. Hair levels ranged from 1.55 to 14.81 µg/g. EPA and the National Academy of Sciences recommend not allowing mercury levels to rise above 5 µg/L and hair not above 1 µg/g. Four of Hightower's patients had blood levels in excess of 50 µg/g.

Data from eighty-nine patients were put through more detailed statistical analysis to look for relationships among fish consumption patterns and mercury level. Among these patients, mercury ranged from 2 to 89.5 µg/L, with a mean of 14.5 µg/L. 82 subjects had levels above 5 µg/L and sixteen subjects had levels above 20 µg/L. The estimates of fish consumption were based on questionnaire responses about past histories and therefore subject to considerable uncertainty. The only strong pattern revealed by this analysis was for swordfish: the more frequently a patient ate swordfish, the higher their mercury levels.

Hightower tracked 67 patients over time to see if reducing fish consumption reduced mercury levels. As evident in the graph below, all but two of these patients reduced their levels to under 5 µg/L after 41 weeks; the two exceptions continued to eat large predatory fish.

Serum mercury levels in patients after beginning a diet with reduced predatory fish intake. Two patients did not comply fully.     from Hightower 2003

What does it mean? The bad news is that some of the fish currently consumed by Americans is sufficiently contaminated by mercury to elevate levels far beyond thresholds of concern. This is not a new story. The twist that attracted media attention to Hightower's research is that her patients tend to come from tonier economic backgrounds and many are eating fish-rich diets because they thought it was good for them.

Hightower does not expect her findings to be unique to the San Francisco area: "It would be expected that places such as New York, Maine, Florida, Hawaii, Martha’s Vineyard Massechusettes, and Los Angeles, Monterey, Santa Cruz, San Diego, California could demonstrate similar data."

The good news is that not all fish creates this risks, and that by avoiding mercury-laden fish even people with high mercury levels can bring them down (assuming no other sources).

The tragedy is that we have allowed mercury to escape into the environment which now means people must avoid important sources of protein. This mercury comes from multiple sources, the most pervasive of which is generation of electricity from coal contaminated by mercury. The further tragedy is that we have the technology today to be able to use coal without letting that mercury escape, but the political process puts higher value on cheap electricity than on avoiding neurological damage to children.

Like many other state agencies, Washington State's Department of Health provides recommendations for limits to consumption of different type of fish because of mercury contamination. For example, they would limit consumption of canned tuna to 2 oz. per week for a child weighing 50 pounds. That's one third of a can.