After a bit of a break, I’m getting back to my series of posts related to mercury. This time I’ll focus on methylmercury, which is generally considered to be one of the “nastier” forms, since it is relatively toxic (primarily neurotoxicity, but also believed to cause cardiovascular and reproductive toxicity at high doses) and also bioaccumulates in animals. Since the main source of methylmercury exposure is food, and in particular fish, I’ll look at how the amount of mercury in fish relates to potential effects on humans.
Methylmercury is an organic mercury compound that exists naturally in the environment, particularly in water where some organisms can convert inorganic mercury to methylmercury. It normally isn’t present in very high concentrations in the environment; however, it is readily accumulated in aquatic organisms such as fish. Concentrations in fish increase as the animal gets older; in addition, methylmercury biomagnifies, which means that predators higher in the food chain have higher mercury concentrations than their prey. As a result, the methylmercury is found in relatively high concentrations in fish compared to their surrounding environment, particularly in top predators, and mercury exposure from eating fish is primarily in the form of methylmercury.
Of course, fish are also generally considered to be a healthy food source, with high quality proteins and relatively large amounts of omega-3 fatty acids. So eating fish is considered to have a positive effect on human health if you don’t consider the methylmercury. In order to determine whether eating fish is a good idea, and if so how much you should eat, we need to look at how much methylmercury is in fish, and how much you can be exposed to without having adverse effects.
The Canadian Food Inspection Agency measured mercury concentrations in several types of fish that are commercially available for consumption. Most fish had mercury concentrations less than 0.2 micrograms per gram of fish (ug/g). However, several species had higher concentrations; the highest values were reported for swordfish, with an average concentration of 1.82 ug/g and a maximum concentration of 3.85 ug/g. Other fish with relatively high mercury concentrations include barracudea, escolar, marlin, sea bass, shark, bigeye tuna and fresh tuna, followed by grouper, orange roughy and walleye. While these values are total mercury, they are generally assumed to be 100% methylmercury when determining whether their is a health risk; the majority of mercury in most of these fish would be methylmercury.
The average portion size is 145 grams per meal for fish, 99 g/meal for shellfish, and 136 g/meal for seafood. Using these values, we can determine that for most fish the total mercury exposure would be (0.2 ug/g x 145 g/meal) 29 ug methylmercury per meal. For swordfish, the average exposure would be 264 ug/meal.
The World Health Organization and Health Canada both historically considered a methylmercury exposure of 0.47 micrograms per kilogram body weight per day (ug/kg-bw/d) to be “safe” for the general population. However, methylmercury is known to affect the development of the brain in embryos, fetuses, and possibly young children. Studies undertaken in the Faroe Islands and Seychelles looked at these effects and related them to mercury concentrations in their mother’s hair. They found that a mercury concentration in hair of 14 ug/g did not appear to have any adverse effect on the children; this in turn was found to correspond to a concentration in blood of 0.056 ug/L and a methylmercury intake of 1.5 ug/kg-bw/d. WHO decided that, based on the available data, they should divide this number by a factor of 2 to account for variation in the relationship between mercury in hair and blood in different individuals, and by 3.2 to account for differences in sensitivity to mercury in different people, and as a result they considered 1.6 ug/kg-bw per week or 0.23 ug/kg-bw/d to be a “safe” amount of mercury for pregnant women and young children. They considered a dose at least twice this high to be safe for other members of the population, however, which is essentially the same as the original exposure limit of 0.47 ug/kg-bw/d.
Since we know the “safe” amount of methylmercury someone can be exposed to, and we know how much mercury someone might be exposed to by eating fish, it is then possible to determine how often it is safe to eat fish. In the case of the majority of fish with methylmercury concentrations below 0.2 ug/g, where we established that a typical meal would result in exposure to 29 ug of methylmercury, and assuming that a typical adult has a mass of about 70 kg, we find that person can eat fish (1.6 ug/kg-bw/week x 70 kg ÷29 ug) 3.8 times per week using the numbers for a pregnant women, or 3 times per week to err on the side of safety. For other adults, 7 times per week or once per day would be safe.
When we look at swordfish, however, a 70 kg pregnant woman could only eat (1.6 ug/kg-bw/week x 70 kg ÷ 264 ug) 0.4 meals per week based on the average concentration, and even less when you consider that some swordfish will have higher than the average methylmercury concentration. In other words, pregnant women shouldn’t be eating swordfish. Even for the rest of the population, consumption once per week would exceed the recommended exposure limit.
Based on similar calculations, Health Canada concluded that regular consumption of barracuda, escolar, marlin sea bass, shark, swordfish, big eye tuna and fresh tuna could result in methylmercury exposure above the “safe” limit for pregnant women; for adults, only swordfish was likely to exceed this limit. For young children, several other fish were also on the list potentially posing a risk, though it was determined that of these only albacore tuna was likely to be consumed by children on a regular basis. Other commonly consumed fish and seafood, including cod, rainbow trout, shrimp and light tuna, were not predicted to result in risks from methylmercury exposure.