Mercury in unequivocally a neurotoxin. It is especially damaging to the developing brain. But it’s the dose that makes the toxin, and so a low enough exposure even to something known to be potentially harmful may not be. Further, the body has mechanisms for dealing with toxins, and toxins in the body may not be reaching the cells they can potentially damage in significant amounts. Therefore if we want to know if a potential toxin is actually causing any harm to people we need to do some type of epidemiological study – correlating exposure to possible adverse outcomes. All the studies in petri dishes and with cell cultures just won’t answer the question of harm.

The question of whether or not mercury in vaccines has caused neurological harm, specifically autism, has been largely answered. Numerous studies have shown no association between the amount of mercury exposure from vaccines and the risk of developing autism. A separate mercury-related question, however, is whether or not there is any risk of harm from mercury exposure from seafood. Mercury is methylated by bacteria into methymercury, and through them gets into the food chain in the oceans. Fish that eat other fish then concentrate the mercury in their tissues, and so predatory fish and sea mammals tend to have high concentrations of methymercury.

This has led to some precautionary recommendations, including that pregnant women should not eat tuna or other fish with high mercury levels. This makes sense, but what is the actual risk? The precautionary principle can also cut both ways. Fish contain many high-quality nutrients important for a developing brain, such as polyunsaturated fatty acids. Removing this food source from the diet of pregnant women may have unintended negative consequences.

The best way to address this issue is to look at net clinical outcomes associated with the consumption of seafood. There are a number of studies that do this, and a recently published follow up study adds to the growing evidence-base addressing this question.

Wingaarden et al. recently published the study “Autism Spectrum Disorder Phenotypes and Prenatal Exposure to Methylmercury”. In this study:

We administered the Social Communication Questionnaire to parents of a cohort of 1784 children, adolescents, and young adults. The Social Responsiveness Scale was administered to teachers of 537 cohort subjects at about 10 years of age. Prenatal exposure to methylmercury was measured in maternal hair samples collected at or near the time of birth. Multivariable regression models evaluated the relationship between prenatal methylmercury exposure and ASD phenotypic scores, adjusting for relevant covariates.

The conclusion:

No consistent association between prenatal methylmercury exposure and ASD screening instrument was found, using linear and nonlinear regression analyses.

This is a large and well-conducted study looking specifically at features of autism. Of course, no study can every demonstrate a risk of zero, but the evidence from this and other studies suggests that any such risk must be minimal – too small to show up in the data. The other possibility is that any negative effects of mercury are more than compensated for by the nutritional benefits of eating seafood.

Davidson et al. published a previous study in 2010 in Neurotoxicology, “Fish consumption, mercury exposure, and their associations with scholastic achievement in the Seychelles Child Development Study”. The Seychelles are the “canary in the coal mine” for mercury exposure from seafood. The inhabitants of these islands have a fishing economy and consume about ten times the amount of seafood as Americans and Europeans.

In this study they measured mercury exposure through hair analysis, with an average prenatal exposure level of 6.8 ppm (compared to an average of 1 ppm in the US). They compared prenatal mercury exposure and postnatal exposure (hair mercury levels in infants) with various scales of neurological development and academic performance (the Southern and Eastern African Consortium for Monitoring Educational Quality, the Family Resource Scale, the Henderson Environmental Learning Profile Scale, and standard IQ tests).

In this study they found:

For the subgroup of 215 subjects who participated in the SACMEQ test, there were significant adverse associations between examination scores and postnatal exposure, but only for males. The average postnatal exposure level in child hair for this subgroup was significantly higher than for the overall cohort. These results are consistent with our earlier studies and support the interpretation that prenatal MeHg exposure at dosages achieved by mothers consuming a diet high in fish are not associated with adverse educational measures of scholastic achievement. The adverse association of educational measures with postnatal exposure in males is intriguing, but will need to be confirmed by further studies examining factors that influence scholastic achievement.

In other words, they found no consistent pattern of negative outcomes. They found the typical scatter of results indicative of the random results seen when looking at multiple outcomes. The outlier of postnatal exposure (not prenatal) in males only would have to be independently replicated before it should be considered anything but a random result.

The Seychelles cohort is a resource of information that can be examined in multiple ways. Essentially you have a large cohort of children followed for years with standard measures of IQ and academic performance. Another study, for example, looked at prenatal mercury exposure from dental amalgam fillings and found no association with a negative outcome.

At odds with the evidence from the Seychelles are previous studies of prenatal mercury exposure and IQ from the Faroe Islands. This research has found a relationship between mercury exposure and decreased IQ.

How do we reconcile these differing results? As others have pointed out – the Faroe Islanders get their mercury from whale meat, while inhabitants of the Seychelles get it from fish. It’s not clear why this would cause a difference, but it is a possible factor. The Seychelles have a genetically diverse population, while the Faroe Islands are homogeneously Scandinavian. Overall, the Seychelles data is more robust, especially when the latest study is taken into consideration.


While the data is not unanimous (it rarely is), the bulk of the evidence indicates that prenatal exposure to methylmercury from seafood is not a significant risk for reduced IQ or the development of autism. The data is more compelling for fish than for sea mammals such as whales. This is true even at mercury exposure levels many times that in the US and Europe.

What is the bottom line for pregnant women? The most common recommendation is still to balance the possible risk from mercury with the nutritional benefits of seafood. It is counterproductive to avoid seafood completely out of fears of mercury. It is reasonable to seek out fish that have generally lower mercury levels but still contain the polyunsaturated fatty acids that are beneficial.

A quick guide from the Mayo Clinic advises:

Eat a variety of seafood that’s low in mercury and high in omega-3 fatty acids, such as:

  • Salmon
  • Anchovies
  • Herring
  • Sardines
  • Trout
  • Atlantic and Pacific mackerel

Other safe choices include shrimp, pollock, catfish and canned light tuna. However, limit albacore tuna and tuna steak to no more than 6 ounces (170 grams) a week.

The Seychelles data is very reassuring, however. Any significant risk would have likely been identified in this data.

Posted by Steven Novella

Founder and currently Executive Editor of Science-Based Medicine Steven Novella, MD is an academic clinical neurologist at the Yale University School of Medicine. He is also the president and co-founder of the New England Skeptical Society, the host and producer of the popular weekly science podcast, The Skeptics’ Guide to the Universe, and the author of the NeuroLogicaBlog, a daily blog that covers news and issues in neuroscience, but also general science, scientific skepticism, philosophy of science, critical thinking, and the intersection of science with the media and society. Dr. Novella also contributes every Sunday to The Rogues Gallery, the official blog of the SGU.