Mercury’s Health Effects

by Alice Alpert, Ellen Czaika, and Amanda Giang

Pathways to exposure

Although these negotiations are explicitly focused on creating an environmental treaty, mercury’s major significance is its toxicity to humans. When you think about mercury, you probably picture a mercury thermometer. In a thermometer, you can literally see the silvery mercury in its bulb – this is liquid, elemental mercury. If you are absent minded and accidentally drop that mercury thermometer on the bathroom floor, the mercury will spills and form into beads. Although it’s not a good idea to touch this mercury, it is also not easily absorbed by the digestive system in this form.

The more pernicious way for this mercury to enter your body is if it vaporizes, which happens to a small amount of the liquid mercury at room temperature. If you inhale the vapor it can easily pass from your lungs into your blood stream and damage tissues. In fact, vacuuming up the spilled mercury can increase its vaporization and therefore the danger.

In truth, most people will not be exposed to mercury in this form. Instead, people working in chlor-alkali production, mercury mining and refining, thermometer production, dentistry, and in the production of mercury-based chemicals are at increased risk. Although measures have been taken to limit occupational exposure to mercury, many workers may continue to be at risk. Similarly, artisanal or small-scale gold miners are routinely exposed to mercury vapor at very high levels, in the process of burning the mercury-gold amalgam used to extract gold from ore. Indeed, miners and their communities often exhibit clear signs of mercury poisoning.

Another important pathway for mercury exposure is through eating seafood. In fact, according to the World Health Organization (WHO) (Section 2.4, paragraph 128), for many people this is the main pathway for human exposure to methylmercury. Exposure happens through the process of bioaccumulation and biomagnification. In brief, mercury is methylated to methylmercury (CH3HgX) by bacteria in the ocean and then accumulates in fish and marine mammals. Long-lived predatory fish at the top of the food-chain, such as swordfish, tilefish, shark, and tuna, can accumulate dangerously high concentrations of mercury. The US EPA lists guidelines for safe consumption of fish. Women who are pregnant or who could become pregnant should be especially careful about eating mercury contaminated fish because the mercury can be harmful to the developing fetus.

In addition, exposure could happen through dental amalgams. Elemental mercury is used in dental amalgam, and it can be ingested or its vapors can be inhaled. This is a contentious issue in the negotiations. The American Dental Association and the US Environmental Protection Agency state that mercury in dental amalgam is safe, while a report by the WHO (p.11) states that dental amalgam is a significant source of mercury exposure in those who have mercury fillings. We encourage you to look into the reports if you are concerned about this issue. For a solid overview of all pathways, see the WHO report on mercury exposure.

How and why does mercury make us so sick?

The most serious effects of elemental mercury vapor concern the nervous system, including tremors, erethism (a neurological disorder characterized by irritability and shyness), insomnia, muscle weakness, and memory loss. At especially high concentrations, the kidneys, thyroid, and pulmonary system can be affected. Similarly to elemental mercury, mercury in its organic form, methylmercury, has serious neurological effects, including neurobehavioral deficits, neuronal loss, loss of muscle movement, hearing loss, paralysis, and death.

Why is mercury so toxic for the nervous system? There are two specific processes: first, elemental mercury and methylmercury can easily cross the blood-brain barrier and once in the brain, can be oxidized to the mercuric ion (Hg2+), which cannot cross back across the barrier. Instead, mercury is trapped in the brain, where the second process begins, neurotoxin by excitotoxicity. What? Okay, we’ll slow down and explain these multi-syllabic words: in studies of rats, Hg2+ inhibits glutamine and glutamate transport, causing receptors for these molecules to become overexcited. This causes a large influx of the calcium ion into the cell, which activates enzymes that can lead to the neuron’s death, and thus the serious neurological effects. This second process is the reason why mercury is so toxic.

Mercury crucially effects developing fetuses. In the same way that methylmercury can cross the blood-brain barrier, it can also pass through the placenta from a mother to her fetus and then to the developing fetus’ brain. As a neurotoxin, methylmercury can also damage its nervous system, and in fact mercury has lasting negative effects when fetuses are exposed to concentrations at levels that are only 10%-20% of toxic levels for adults.

Babies born to women who consumed significant amounts of methylmercury while pregnant display symptoms similar to cerebral palsy, including delayed walking and talking, altered muscle tone and reflexes. Tragically, these impairments are permanent and affected will suffer from these impairments for their entire life. In fact, recently published research estimates that IQ reductions due to chronic, low-level fetal mercury neurotoxicity costs the European Union alone € 8-9 billion euros per year. Clearly, there are significant social and economic impacts from mercury exposure, particularly for the young.

5 thoughts on “Mercury’s Health Effects

  1. William Clark

    Are the mercury loadings in arctic wildlife high relative to those we find alarming in people? If so, what are the consequences? What are the chances that the last polar bear die from mercury-induced dementia?

    1. Julie van der Hoop

      While I don’t expect that polar bears will have diminished IQs, it is true that Arctic marine mammals have high mercury levels due to biomagnification. A lot of research is focusing on whether marine mammals have specific mechanisms to cope with high mercury burdens, such as binding to Selenium or forming other compounds to reduce toxic effects.

      One of the major problems with marine mammals is that they do make up a significant portion of some specific diets. Populations in Arctic Canada, Siberia, Sweden, the Faroe Islands, Japan, and others, consume marine mammal meat and are thus exposed to high levels of mercury. There have been recent efforts to identify lower-mercury organs that may be safer for consumption than others; still, if mercury levels in the environment and in these animals keep rising, these populations will be even more at risk.

      1. William Clark

        Why don’t you expect them to have diminished IQs (or other chronic neurological damages)? I see from Noelle’s post that the data on tissue loadings are below the ‘safe’ level. But ‘safe’ is defined by a paper that is 17 years old, and seems to refer to acute illness. Is there no more recent data on acute and chronic impacts on non-human animals? Do we care about such loadings only because we might eat them and suffer ourselves? Should the world of WWF and Co. be more aware of the chronic health impacts of pollutants such as Hg on their species of interest? How might one communicate this to them (they are silent on the matter), and get them and their members behind the effort to regulate emissions?

  2. Noelle Selin

    Our current understanding is that levels in Arctic biota are at or above concentrations where health impacts would be expected. I posted a neat figure from the latest Arctic Monitoring and Assessment Programme report for
    polar bears and
    other marine mammals , showing levels relative to health thresholds. The full AMAP report is available here pdf and the figures are from pages 117 and 119.

    Julie van der Hoop is one of the students here in Geneva — her research focuses on marine mammals and she may have some information to add as well.

  3. Randall

    Alice, Ellen and Amanda
    I’ve sent a similar email to Leah thanking her for her efforts covering the happenings at INC5… and that deep appreciation is extended to all of you as well. It’s so great to see others interested in this topic and I appreciate you giving a day by day reporting on INC5.

    Your recent article above had some really great information in it. I really like that you broke down why mercury is so toxic for the nervous system in an easy to understand way for readers. When one looks at literature reviews by the EPA and ATSDR, it is truly staggering the number of various adverse health effects mercury can cause.

    I appreciated that you mentioned dental amalgam in your article, although, there are some specifics that were overlooked that I feel are quite important to understand when discussing exposure and risk from amalgam.

    when discussing mercury vapor, your article states: “In truth, most people will not be exposed to mercury in this form.”

    Mercury Vapor is one of the main forms of mercury anyone with dental amalgam is exposed to daily, which in our country represents over 120 million people (Richardson 2011). Richardson 2011 is the only amalgam risk assessment performed on the US population from the CDC’s NHANES data-set and was done with input from the FDA. This study found that 67.2 million Americans would exceed the Hg dose associated with the reference exposure level (REL) of 0.3 μg/m3 established by the US Environmental Protection Agency; and 122.3 million Americans would exceed the dose associated with the REL of 0.03μg/m3 established by the California Environmental Protection Agency. You can watch a video of Mark discussing his study on

    Later in your article you refer readers to the WHO report on mercury exposure which references UNEP 2002 for exposure to amalgam (perhaps taken from WHO 1991) at 3 – 17 ug/day. In 2003 WHO released a report increasing those levels to 1 – 27 ug/day from dental amalgam. Using either of the estimated exposures from UNEP or WHO is quite significant when one considers not only the higher exposure to mercury from amalgam compared to all other sources, but that the EPA’s Reference Concentration Dose (RfD) for mercury vapor (0.048 Hg/kg-day) is much lower than that of methylmercury ( 0.1 μg Hg/kg-day). Unfortunately it is almost universally promoted that methylmercury is the “toxic” or “dangerous kind of mercury, while most often the toxicity of mercury vapor from amalgam is downplayed.

    It is interesting to note that the EPA’s mercury safe levels are close to 20 years old and that when CAL-EPA issued updated levels for mercury vapor in 2008, they were ten times lower than national EPA. I’ve sent you a recent article from Mark Richardson (previously of Health Canada) that outlines the various exposures and risk comparisons between the recommended amount of tuna intake to that of amalgams for further thought.

    Additionally, studies from the Children’s Amalgam Trials (CAT) (long term study examining what if any health effects might arise from placing mercury fillings in children) show increased mercury body burden, kidney dysfunction and neurological problems (all the things one would expect to find when exposing kids to mercury). These studies showing harm have been largely ignored by our regulatory agencies. I’ve sent the studies for you and you can watch a video of one of the published researchers talking about their findings (from the CAT dataset)

    From my perspective, It has been disheartening to see both researchers and journalists outline symptoms from mercury exposure and occasionally acknowledge the largest exposure is from amalgams, but then quote the ADA’s position of no harm…which is largely based upon very limited literature reviews and the research of Rodney Mackert (ADA spokesperson) who has stated it would take over 400 amalgams to become toxic. The context is that his findings are absurdly low when compared to other risk assessments done on amalgams – see the details of various risk assessment comparisons are outlined in Richardson et al 2011.

    The position taken by the EPA and ADA do not line up with the science (or common sense) to say that on one hand amalgam represents that largest exposure to mercury in people who have them, yet on the other hand they are safe. This unfortunate position leaves newcomers to the subject of amalgam (especially those from the environmental side of the mercury issue) to grapple with understanding the disparity of these two positions outlined above.

    Through the much needed collaborative efforts at INC5 we are seeing mountains being moved to limit mercury being released into the environment in order to protect human health, and I greatly applaud those efforts. But it is frustrating to see the largest contributor of mercury to a significant portion of the population largely ignored or downplayed at these negotiations and in the press, especially considering those levels during placement, polishing and removal are thousands of times higher than safety levels and violate occupational safety regulations.

    I look forward to more of your continued blogging on INC5 happenings. Keep up the great work !


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