Mercury: A Potent Toxin With Widespread Health Impacts
Mercury is a potent neurotoxin—in its liquid form it has even been called ‘liquid death’. It has been designated by the World Health Organization as one of the ten most dangerous chemicals to public health.1 Exposure to as little as 1.3 grams of dimethylmercury causes irreversible neurological problems and death.2
The toxicity of mercury is unsettling because the metal is all around us. Volcanoes, mines, and oceans release mercury into air, soil, and groundwater; however, nearly 80% of environmental mercury is a consequence of human activity such as fossil fuel combustion, agriculture, and mining.3 Developing countries produce and release as much as 200 metric tons of mercury every year.2 The US is no less culpable—we create about 10% of the world’s mercury pollution each year.2
Before we can understand mercury’s damaging health effects, we need to know about the different forms. Mercury can exist as elemental (or metallic) mercury, inorganic mercury salts, and organic mercury. Elemental mercury is the silver liquid found in old thermometers and amalgam dental fillings.4Elemental mercury enters the body primarily through inhalation of mercury vapors. Most of the mercury in water, soil, plants, and animals exists as inorganicmercury salts.5 Organic mercury includes substances such as methylmercury, dimethylmercury, ethylmercury, and phenylmercuric compounds. Organic mercury is freely absorbed in the gut. The main source of organic mercury exposure, in the form of methylmercury, is from contaminated fish. Vaccines that contain the preservative, thimerosal, contain ethylmercury.6 Methylmercury and dimethylmercury are highly toxic.
Elemental mercury and most forms of organic mercury (such as methylmercury) are lipophilic, or fat-loving. They can readily cross the blood-brain barrier and the placenta. Once these forms of mercury are absorbed into the bloodstream they can damage the brain, heart, kidneys and numerous tissues and organs throughout the body.7,8 Inhaled elemental mercury is acutely toxic to the lungs, mucous membranes of the mouth, the gastrointestinal tract, the eyes, and the skin. Chronic exposure to elemental mercury vapors has a particularly potent and long-lasting effects on the central nervous system.9
Mercury and the Immune System
Mercury creates a highly destructive, pro-oxidant state that damages immune function. Many immune cells are ultimately damaged, including B cells, total white cell counts, neutrophils, and more.10,11,12 One major target is the T lymphocyte, or the T cell. Mercury blocks the T cell's ability to secrete chemicals, called cytokines, that help coordinate an immune response.2 It also shifts the balance between two types of T cells (helper Th1 and suppressor Th2 cells) to create an environment in which autoimmune disease development is more likely. Essentially, the immune system is shifted toward an excessive helper Th1 response, and this, in conjunction with other factors of risk, leads the immune system cells to recognize normal human proteins as foreign invaders and mount an autoimmune response.13 This relationship is hinted at in epidemiological studies which show that people with autoimmune diseases have higher levels of mercury on average than people without autoimmune diseases.14
Mercury and the Body’s Antioxidant Systems
Mercury impairs the function of our most potent endogenous antioxidant, glutathione. Mercury (particularly methylmercury) binds to glutathione, reducing its availability in the body, as well as creating a glutathione-mercury complex that penetrates virtually all membranes in the body and is highly toxic to cells and tissues.15
Mercury also damages the body through its interference with the important trace mineral selenium. Mercury can actually bind to selenium, indirectly causing a selenium-deficiency state in the body.16 This is significant because mercury then displaces selenium from a family of important enzymes called selenoenzymes, which includes glutathione peroxidase.17 These enzymes have powerful antioxidant ability.18
Mercury and the Nervous, Endocrine and Reproductive Systems
The classic symptoms of acute mercury poisoning include tremor, gingivitis, and bizarre behavior (e.g., excessive shyness or aggression). Inhaled mercury vapor is associated with neuropsychological problems such as emotional lability at high exposures or more subtle motor and memory deficits at low exposures.19
Children are particularly vulnerable to mercury. Numerous studies have shown a correlation between mercury exposure and autism.20,21 Childhood obesity has also been linked to thimerosal-containing vaccines.22
Mercury is a potent disruptor of our adrenal, pituitary, thyroid glands.23 These glands tend to preferentially absorb and store mercury.24 Chronic exposure to mercury has been shown to be associated with autoimmune thyroiditis and subsequent hypothyroidism.25 Mercury in the adrenal gland lowers plasma levels of corticosterone, which can lead to adrenal hyperplasia.26 This effect is believed to contribute to the development of Addison's disease.27
In men, mercury exposure can decrease sperm count, and the sperm cells that are produced have structural problems especially in the tails, which decreases sperm motility.28 Women who are exposed to mercury before or during pregnancy may suffer from menstrual cycle disruption and are at increased risk of miscarriage, preterm delivery, or a newborn with low birth weight.29
Mercury and the Pregnant Woman
Pregnant women have reason to be concerned about mercury exposure. The fetus, nestled in the protective womb, is vulnerable to the effects of elemental and organic mercury, because they easily cross from the maternal bloodstream through the placenta.30,31 Heavy metal interference with fetal growth and development can cause long-term physical and cognitive problems. For example, toddlers that were exposed to even low levels of methylmercury during gestation have diminished fine motor skills, correlated with the amount of methylmercury found in the umbilical cord blood.32
Children with in utero exposure to mercury have impaired language and communication skills when measured at the age of five. In this study, it was also found that the amount of seafood consumed during pregnancy correlated with language and communication deficits in the children.33 The neurological effects of mercury on fetuses may extend beyond simple cognitive, motor, and communication deficits. Mercury exposure in prenatal and neonatal periods may increase the risk for autism spectrum disorders.34
Protecting Yourself From Mercury Exposure
Even though mercury is nearly universal in our environment, there are measures we can take to reduce exposure and to mitigate its impact. Perhaps the biggest risk to mercury exposure in the general population is the consumption of mercury contaminated-fish. But not all fish are equal. The amount of mercury contained in seafood varies depending on the species and the source. Fish harvested from polluted freshwater including bass, pike, and walleye may contain high levels of methylmercury. Likewise, fish on the higher end of the food chain such as shark, swordfish, and tuna have particularly high levels of methylmercury because they concentrate the compound from the numerous mercury-contaminated fish that they eat themselves.
Regulatory authorities have established maximum safe levels for mercury in fish, suggesting that only a certain number of fish should be consumed in a given month. These recommendations vary depending on the species of fish and the source; however, without testing the fish directly or monitoring one's blood or urine for mercury it is impossible to determine exactly how much mercury one is consuming. A diet rich in fish can convey a number of health benefits; however, this must be balanced with the risk of mercury exposure.
Certain nutrients may be able to further protect against the effects of mercury. We know that mercury irreversibly inhibits selenoenzymes thioredoxin reductase and glutathione peroxidase.35 These enzymes are important for restoring glutathione, as well as vitamin E, to their reduced forms, so they can function as antioxidants.36 Selenium supplementation to support adequate selenium levels may be protective. A good food source of selenium is the brazil nut. Since mercury exposure decreases the body’s level of glutathione, maintaining adequate glutathione levels is of particular importance. Glutathione supplementation can help to restore levels depleted by mercury exposure. Vitamin E supplementation also may further protect against the pro-oxidative state caused by mercury exposure.37 Of particular importance is binding and removing the toxic mercury from the body, which can be safely and effectively done via oral supplementation with a thiol-functionalized silica, which tightly binds the mercury in the gut, removing it from the body.
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