The Mad Hatter + Fish: Mercury Levels & Your Health

by Brenton Bauer, MD 2011 |

You are probably wondering why the title of this article references a character from Lewis Carrol’s famous childhood story, Alice and Wonderland, when we are supposed to be discussing the effects of mercury on human health. Well, interestingly enough, it has everything to do with mercury. To take you back on a quick journey through history, 19th century hat makers (aka “hatters”) used to utilize a mercury-based solution in the process of curing animal pelts for their hats. Furthermore, they typically worked in very poorly ventilated workshops and, during the curing process, they would inhale large quantities of vaporized mercury. Over time, these hatters began developing neurological symptoms, which Carrol used as the framework for the idiosyncrasies seen in the character of the Mad Hatter. As Lewis Carrol has shown us, mercury and its impact on human health is not a novel idea. There is, however, a lot of hype regarding this heavy metal in contemporary news. Below, we strive to break down some of the key issues regarding mercury’s interaction with human health in today’s day and age.

What is Mercury?

Mercury is a chemical element (known in the scientific community by the symbol, Hg) and is the only metal that is naturally in the liquid state at standard temperature and pressure (a.k.a. “room temperature”). Interestingly, the nickname for elemental mercury is “quicksilver” because of these physical properties (no relation to the surf company).

How am I exposed to Mercury?

Mercury is present in various forms in nature but the primary forms that affect humans beings are methylmercury, elemental mercury, and to a minor degree a variety of inorganic mercury-based salts. Methylmercury exposure most commonly occurs when you consume fish and shellfish that have been contaminated with methylmercury formed by various marine microorganisms (these little critters convert inorganic mercury substances into methylmercury). On the other hand, elemental mercury exposure typically occurs when a device that containing mercury (eg. thermometers, barometers, thermostat switches, dental fillings, etc.) breaks and spills some of its contents. The spilled mercury subsequently evaporates into a toxic vapor that is both odorless and colorless. This poisonous vapor is inhaled, particularly in poorly ventilated spaces, and the elemental mercury then builds up in your body and can potentially cause a variety of symptoms.

How does Mercury affect the body?

To make a generalization, you can think of mercury intoxication as a problem that affects the nervous system (a.k.a. your brain, spinal cord, and nerves). Furthermore, the type of mercury compound causing the intoxication makes a big difference as it will affect the severity of symptoms and whether any any other parts of your body will be affected.

Let’s start with Methylmercury

Methylmercury, which we previously mentioned as being a possible contaminant in fish and shellfish, can cause health effects in adults if accumulated in large enough quantities. In the US, this is exceedingly rare as the EPA and FDA set forth high standards and regulations regarding this very issue. Historically, however, this has definitely been a problem. Most notably, there was a large-scale methylmercury-poisoning epidemic that occurred over a thirty-six year period in Minamata City, Japan in the 1930’s. A Japanese chemical company had been allowing large quantities of methylmercury-containing hazardous waste to spew into the bay which served as the major hub for the region’s fishing industry. Subsequently, people in the region began experiencing hand and foot numbness, ataxia (a.k.a. stumbling gait), hearing and vision abnormalities, generalized weakness, coma, and even death. After an extensive investigation, the government Identified severe methylmercury intoxication as the source of these symptoms. Since then, this spectrum of symptoms related to severe methylmercury intoxication has been referred to as Minamata disease.

Should I stop eating fish?

If you have a seafood allergy, then yes (as we hope you already have). However, if you’re giving up seafood because of a paranoia about methylmercury poisoning then you can put your mind at ease. As we mentioned with regard to the heavy environmental regulations in place in the US, it is extremely rare to see mercury toxicity in adults after exposure to regularly caught seafood that you purchase at your neighborhood market.

Now, there is one subset of adult patients who should be more concerned and monitor their fish consumption: pregnant women. While the extremely low levels of methylmercury found in seafood will not cause any effects on the mother, these low levels can cause permanent damage to the unborn baby’s developing nervous system. The EPA has reported that children who were exposed to levels of methylmercury in utero (specifically of a reference dose >0.1mcg/kg/day) may see lasting neurologic effects including impaired cognitive thinking, attention span, language development, memory, and visual/motor skills. Therefore, during pregnancy, it is important for expecting mothers to limit her consumption of fish and shellfish. Which fish should you avoid? The FDA reported that the fish that consistently contain the highest levels of methymercury (and which should strongly be avoided) are mackerel, kingfish, shark, and tilefish. See the chart below for more information about the mercury content of various fish.

What about the other type of mercury?

Elemental mercury is the other major cause of mercury exposure, although occurrences are pretty rare. However, in the case of an elemental mercury spill (for example, a broken mercury thermometer or barometer in a closed space), the health effects have the potential be more severe given the more pure form of mercury and the route of exposure (i.e. inhalation). Symptoms include but are not limited to: tremors, emotional affect changes (i.e. mood swings, excessive irritability, etc.), muscular weakness and atrophy, headaches, sensory disturbances, and, in high doses, kidney and respiratory failure…and, as always, death. Therefore, if you think you have been exposed to significant amounts of elemental mercury or have been exposed in the past and are currently experiencing any of the above-mentioned symptoms, go see your doctor…now!

I’m woman who desires to be pregnant in the near future. How should I protect my baby and myself?

Here is a non-exhaustive list of some of the ways to keep you and your future baby safe:

  1. Limit your exposure to methylmercury by reducingthe amount of seafood you eat during pregnancy. Particularly, avoid mackerel, kingfish, shark, and tilefish.
  2. Some dental fillings contain mercury. The FDA has concluded that dental fillings containing mercury are safe even in pregnant women. However, if you do need to have a tooth filled, talk to your dentist about the various non-mercury options available to you to be extra safe.
  3. If you are potentially exposed to any form of mercury at your job, talk to your employer about temporary reassignment possibilities to limit your exposure during pregnancy.
  4. If you own an “old school” mercury-based thermometer consider buying a new one. If you ignore our advice and that old and busted thermometer breaks, vacate the room immediately and have another non-pregnant adult take care of the clean up.

Speaking of children and mercury exposure, doesn’t the mercury in some vaccines cause Autism?

No. According to the latest large-scale government study, there is no increased risk of autism following thimerosal exposure (thimerosal is a mercury-based preservative found in some vaccines). The British physician and scientist, Andrew Wakefield, who popularized this hypothetical link between the thimerosal-containing vaccines (most notably the MMR vaccine) and autism in children has been discredited worldwide. The Lancet (the original journal to publish his research) issued a formal retraction because the evidence supporting the claims he made came from falsified data. Whoops…ok, really big whoops on their part. Therefore, it is very important to have your children see their pediatrician and have their vaccines kept up to date because the infections that these immunizations prevent can be very serious, if not potentially life-threatening to your child.

Your Take Home Message

Returning to Carrol’s Alice and Wonderland, realize that your next piece of Mahi Mahi is not going to turn you into a crazy elderly gentleman with a penchant for tea that Johnny Depp so brilliantly portrayed to the world. Rather, it is important for you to realize some of the potential hazards of the different types of mercury exposures as well as the symptoms to look out for if you have been exposed. For the particularly vulnerable crowd (that is babies and women with a baby on the way), make a mental note of the added precautions that are necessary to maintain good developmental and overall health and save the shark meat for a more non-pregnant time of your life.

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1. Price et al. Prenatal and Infant Exposure to Thimerosal from Vaccines and Immunoglobulins and Risk of Autism. Pediatrics. Published online September 13 2010. Accessed 12/9/2010. Available at:
2. “Mercury in Pregnancy”. March of Dimes. Updated Aug 2009. Accessed 12/5/10. Available at:
3. “Health Exposure/Effects”. Mercury. Published by United States Environment Protection Agency. Available at:


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