Prenatal Mercury and Autism

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 in: Neuroscience/Mental Health, Public Health

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31 thoughts on “Prenatal Mercury and Autism

  1. Søren Furbo Skov says:

    I thought the reason for the apparant difference between Faroe Islanders and Seychellians was explained by including different persistent organic pollutants (POPs) in the analysis.

    The northen atlantic is filled with POPs, while the Indian Ocean is not. POPs and mercury would show up in the same foods (they accumulate in the food chain), so maternal levels of them would correlate in both populations, but not between populations. If POPs were (some of) the reason for the apparant mental effects of mercury in the Faros Islands, we would expect exactly the pattern we are seeing.

    Have this hypothesis been discarded?

  2. WilliamLawrenceUtridge says:

    Why does the Arctic (and Antarctic?) seem to gather and retain pollutants in this manner? Are they just not broken down due to the cold?

    1. angorarabbit says:

      It’s geography. Heavier industrial pollution north of the equator and especially North America and northern Europe. There just aren’t the comparable pollution sources near Antarctic waters.

      1. Calli Arcale says:

        Angorarabbit — the Antarctic retains pollutants too, despite being less industrialized, most famously the CFCs which contribute to the ozone hole. There is a circumpolar vortex in the atmosphere over each pole, a feature which appears to be very common among worlds with atmospheres; it’s particularly obvious on cloudy worlds like Venus and Saturn. This has a tendency to trap atmospheric pollutants. The whole process of ozone depletion is heavily driven by this, as well as the midnight sun and noontime darkness that you get in these regions.

      2. WilliamLawrenceUtridge says:

        That still doesn’t quite make sense to me, because then people would discuss the heavy pollution of the North Atlantic as well as the Arctic – though I suppose north-emptying rivers could contribute to this.

        I’m wondering who I would ask about this – an oceanographer?

      3. stuastro says:

        10% of the world’s population lives in the southern hemisphere So I suppose that it isthe lack of people down here that leads to lower pollution levels.

    2. Søren Furbo Skov says:

      One effect that does this is distillation.

      Volatile compounds will evaporate from hot surfaces and condense on cold ones. When a farmer in India spreads DDT, some of it will evaporate and move with the atmosphere. When it comes to the arctic, it will condense out. Given enough time, this can move vast amounts of persistent volatile compounds to the polar regions.

      This is not the effect for CFCs, as they will not condense. I think they are equally distributed in the entire atmosphere, they just have more chance of doing damage in polar regions (I think this has to do with a special type of clouds present here, and with ozone primarily being brought to the upper atmosphere around the equator, so the circumpolar winds keep new ozone out).

      1. Calli Arcale says:

        It’s not exactly the type of clouds. The two main factors are the polar vortex and the long polar winter, which removes sunlight from the equation for a couple of months.

        1. WilliamLawrenceUtridge says:

          Thanks for the clarifications, it makes more sense now.

  3. angorarabbit says:

    Soren, as I recall (I am a neurotoxicologist) you are correct; the northern waters have higher levels of polyaromatics (TCDDs, PAHs, etc) as compared with the Indian Ocean. For this reason there are also significant concerns with First Native peoples and Inuits, as well as others with similar diet practices. I wonder if there is also a contribution of longevity – whales (and seals?) live longer and have more opportunities to concentrate and accumulate toxicants, as compared with shorter lived fish species.

    I agree with Steven’s final conclusion, with the caveat that regional differences must be considered. For example, here we recommend limited consumption of US Great Lakes prey fish by pregnant / child-bearing age women. Which is sad because coho and northern pike are yummy!

  4. Soren – that’s a good point. There is probably something different between whale meat and fish, and POPs may be the answer. This is still a viable theory, but I could not find any confirming studies. If you have a reference, please post it. Thanks.

  5. anthro49 says:

    I welcome the reassurance this study brings to the question, but I wonder if we should talk about alternative sources of the nutrients in question? Not only do many people dislike seafood in general–other than deep fried prawns maybe–but there is the problem of overfishing.

  6. daedalus2u says:

    The levels of mercury found in the Faroe Island cohort was gigantic compared to what is found in “normal” individuals in the US population. As I recall 25% of the ~1000 member cohort had cord-blood mercury levels above 200 nM/L. There was autism screening in the Faroe Islands that covered that cohort and the levels were not different from other populations.

    This study is based on hair mercury, which is not that great a measure compared to blood. There is a lot of scatter, hair color has effects as does light exposure and exposure to environmental mercury. It is also indirect, comparing maternal hair mercury with infant neurodevelopment.

    The largest mercury exposure was likely due to teething powders, which were once very commonly given to infants and which had ~1 grain of calomel, about 65,000 micrograms of HgCl. We know that infant exposures were very high because tens of millions of doses of these teething powders were sold per year. There used to be a disease of childhood, called “pink disease”, which we now know was mercury poisoning due to teething powder containing mercury. It was once a major cause of death in childhood, with over a thousand infants dying from it before the cause was discovered and mercury removed from teething powders. Where were the cases of autism when infants were dying from mercury poisoning due to teething powders?

  7. Publicjoy says:

    @anthro49. There are many healthy, low mercury fish that are NOT in danger of being overfished and should be consumed (at least) two meals per week for a variety of positive health benefits. Wild Alaska salmon comes immediately to mind as it is safe for unlimited consumption by all population groups and is one of the world’s most sustainably managed fisheries.

    1. anthro49 says:

      Thank you–and I DO eat Wild Alaskan salmon twice a week, but as a native Nothwesterner, I am well aware that salmon are stressed. I buy Alaskan salmon, but not everyone can, so I still think we need to address alternative sources of fatty acids. I live in the Midwest now, and am wondering about Great Lakes salmon which my spouse and kids catch in good quantity–fills the freezer! Trout as well.

  8. Kim Pomares says:

    Sensory Enrichment has been found to enhance functionality in rodents even after neurological challenges including exposure to toxins. The caveat is that the toxins considered did not include Mercury. They included pre-natal alcohol and lead.

    I copy here an extract from a study by the University of California, Irvine on the effectiveness of Sensory Enrichment as a therapy for autism. (

    “Enhanced environmental stimulation ameliorates the effects of a wide range of neurological challenges that are experimentally produced in animals. These challenges include: brain lesions, toxin exposure, exposure to addictive drugs, brain trauma, stroke, seizures, aging, and hypoxia (Laviola, Hannan, Macri, Solinas, & Jaber, 2008; Nithianantharajah & Hannan, 2006; Pang & Hannan, 2013). In animal models, enriched sensorimotor experience also ameliorates genetically based neurological disorders such as Down’s syndrome, Alzheimer’s disease, Huntington’s disease, Parkinson’s disease, schizophrenia, and autism (Laviola et al., 2008; Nithianantharajah & Hannan, 2006; Pang & Hannan, 2013).”

  9. Stopthequacks says:

    The Japanese prize tuna, but it may be their diet priorarises quality over quantity. Are there any studies of their diet to reveal if they have high intake of fish with potentially high mercury levels, and of any potential consequences?

    I remember Minamata Disease was a highly topical story in my undergraduate days but that was an extreme case.

  10. pmoran2013 says:

    “The precautionary principle can also cut both ways”

    Love this sentence and the thought behind it, despite the unneeded “also”.

  11. DavidRLogan says:

    I’m probably an idiot (as usual) or misunderstood the post. But do some of these measures seem problematic? (academic “performance”, “achievement” questionairres, IQ tests)

    I am speculating wildly and way outside my level of education…but I don’t understand why a cell culture model is a problem (where a cell assay would seem at least somewhat objective, at least to me) but multiple regressions on, say, IQ testing are better evidence? I guess I am skeptical an IQ test measures what it says, and more skeptical it reliably measures something across different people in the same way (and maybe this is true of the other measures…I don’t know.)

    Just curious…I know absolutely nothing about this topic except what I just read.

    Thanks for the thoughtful post, Dr. Novella.

    1. WilliamLawrenceUtridge says:

      The use of a large samples is a way of offsetting this – it is thought to average out most of the confounds so long as there’s nothing systematic in the confounds themselves. In addition, this group has the advantage of being more culturally, linguistically and genetically homogeneous than most other groups studied in population-level research. The tests may be imperfect, but you can’t argue that they are measuring something that is within squinting distance of some sort of cleverness.

      But no study is perfect, not even close, which is why multiple different types of studies and evidence converging on a coherent point is such an important signal to rise out of noise.

    2. windriven says:

      What do you think IQ measures? How do you explain the correlation between IQ and academic success? Are your questions medical, philosophical or political?

    3. Young CC Prof says:

      The problem with relying on cell cultures alone is that it doesn’t tell you, for example, how much of the mercury in your food actually gets to your brain, or to the brain of an unborn child. The blood-brain barrier and the placenta are both, well, barriers, and some substances cross them better than others. It also doesn’t tell you whether the damage accumulates and prevents normal development, or whether the damage is transitory at low levels of exposure.

      The test-tube studies clearly say, “Mercury is bad stuff.” Nobody debates that! But you need a population study to be sure of how bad, what the harm is, and what levels of exposure are worth worrying about.

  12. Stephen H says:

    Wait, I thought I was supposed to be worried about my fillings leaking – and you say it’s FISH that gave me Asperger’s?

    Or I could just lay the blame at parents who clearly earned their own places on the autism spectrum.

  13. DavidRLogan says:

    Thanks for the comments, everyone. Windriven thanks for helping me think more about what I was saying. I guess I think IQ tests are claiming to measure *intelligence* (it’s in the name…) I’m skeptical about that claim but doubt I can make a good case on a message board. I don’t know why that correlation you mention exists…it’s interesting. It seems to me (getting nervous because I know nothing) the correlation you mention strengthens the overall case of these studies.

    That said…from what I could tell there was also the more general claim in the post that the precautionary principle should err on the side of eating fish, or at least the precautionary principle should NOT err on the side of not eating fish (whew!…So I take it this would be relevant to any risk not just Autism which looks debunked) If that was a good reading of the post I guess I wonder how the questionnaires/IQ tests would support that more general claim.

    So I guess that is a philosophical worry until I know something about medicine. Every political opinion I’ve ever had I regret :(

    What do you think?

    1. WilliamLawrenceUtridge says:

      I think you should be congratulated for educating yourself, changing your mind, and admitting to it!

    2. Egstra says:

      ” I guess I think IQ tests are claiming to measure *intelligence* (it’s in the name…) I’m skeptical about that claim but doubt I can make a good case on a message board.”

      I would modify that to say that they measure academic intelligence. There are certainly other kinds that the standard tests don’t measure. There is also a strong cultural component, which needs to be taken into account when looking at the results. They are open to misuse (read, The Mismeasure of Man).

      They don’t measure some very important things, like determination and persistence and creativity.

      1. NSC says:

        Is it likely that the cultural component includes socioeconomic status (SES)? That would seem to correlate with academic success. If SES has a moderate impact on scores, could a smaller effect due to minor changes in actual intelligence be obscured in such a study?

        1. Egstra says:

          “Is it likely that the cultural component includes socioeconomic status (SES)? That would seem to correlate with academic success. If SES has a moderate impact on scores, could a smaller effect due to minor changes in actual intelligence be obscured in such a study?”

          I am, by no means, an expert on the nuances of intelligence testing. My impression, however, is that the verbal section of the WAIS (for example) is strongly influenced by SES. There is good evidence, for example, that vocabulary (one of the verbal subtests) is tied to higher SES. The performance section may be less influenced.

          To check on my impressions, I did a quick search and found several studies suggesting that there is highly significant positive correlation between full scale IQ and high SES.

          There are tests which claim to be culture free (such as the Raven Progressive Matrices), but some experts assert that the very notion of culture-free intelligence is a contradiction in terms.

          So, in answer to your question… yes, I believe that an individual with a lower SES might do worse on the standard IQ tests than someone from a higher SES despite being brighter.

          1. NSC says:


            It appears the Seychelles study attempted to control for SES, but it’s not clear to me whether the other studies did.

  14. Egstra says:

    This is going way back to graduate school ( a looong time ago!), but I seem to remember learning that, while the severe forms of mental retardation were spread evenly along the SES levels, mild retardation was significantly more common among those in the lower SES statii (statuses?). The assumption? hypothesis? was that it was the result of poor maternal nutrition, greater levels of pollution in poor areas, and so forth.

    I’m not sure to what extent it’s even possible to tease out the cause/effect relationships here. Perhaps the important thing is to keep the limitations of IQ testing in mind, and try to avoid the abuses of the past.

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