Mercury

Mercury is a naturally occurring element that has garnered significant attention due to its potential health effects on humans. Found in various forms in the environment, mercury can enter the body through different pathways, leading to concerns about its impact on overall health. 

Mercury is a heavy metal element with the symbol Hg and atomic number 80. It exists in several forms, including elemental (or metallic) mercury, inorganic mercury compounds, and organic mercury compounds.

Mercury has no known beneficial or physiological function in the human body.  Exposure to mercury has detrimental effects on human health, particularly on neurological development and function.

Understanding the role of mercury, its dietary sources, recommended intake, and interpretation of lab test results is essential for maintaining optimal health.

What is Mercury?  [8.] 

Mercury (Hg) is a highly toxic metal found in air, water, and soil that exists in three main forms: elemental or metallic mercury (Hg0), inorganic mercury (Hg+, Hg2+), and organic mercury (such as methyl or ethyl mercury). 

Elemental mercury, liquid at room temperature, releases hazardous vapor when evaporated. 

It is shiny silver-white and odorless in its liquid form, but becomes colorless and odorless gas when heated. Major sources of mercury pollution include agricultural runoff, industrial discharges, and mining activities.  

Mercury compounds have been used not only in industrial processes but also in medicinal uses and others. For instance, mercury has been used in mining, fluorescent light bulb production, and as fungicides. 

In skin care products, mercury chloride (HgCl2) is used for its skin brightening properties, inhibiting melanin formation. 

Thimerosal, a mercury-containing compound, has been used as a preservative in vaccines.

Exposure to mercury leads to adverse neurological and behavioral changes in animals, and it poses a serious risk to human health as well. The brain is the primary target organ for mercury toxicity, but it can also affect the kidneys, muscles, and nerves. 

Mercury binds to thiols with high stability, disrupting cellular function and causing oxidative stress.

Mercury is absorbed differently depending on its form.  

Elemental mercury vapor is easily absorbed through mucous membranes and the lungs, where it is oxidized and deposited, primarily in the brain and also in the placenta, with serious implications.  [4.]

Organic or methylmercury, found in fish, is  the most toxic form of mercury.  It is absorbed through the gut and distributed throughout the body, including the central nervous system and kidneys. While mercury salts are relatively stable and less absorbed, methylmercury is almost entirely absorbed and eliminated mainly through hair, urine, and feces.  [4.] 

Mercury can be methylated or demethylated in the gastrointestinal tract by some gut microbiota.  [9.] 

Organic Mercury

Organic mercury compounds, like methyl mercury (Me-Hg) and ethyl mercury (Et-Hg), are more toxic than inorganic forms.  [2.]  The most toxic form of mercury is dimethylmercury, and even minimal skin exposure can lead to rapidly progressive cerebellar deficits, coma, and subsequent death despite appropriate management.  [3., 10.] 

Organic mercury, primarily methylmercury (MeHg), is a significant environmental pollutant, notably found in aquatic ecosystems due to its low water solubility and high lipid solubility. It bioaccumulates in fish, making them a primary source of human exposure.

Once ingested, MeHg is absorbed through the gastrointestinal tract with high affinity, binding to proteins like glutathione and cysteine, and is distributed to red blood cells and the brain.  

Urinary excretion is minimal; instead, MeHg is eliminated primarily in an inorganic form through the biliary system. Sources of organic mercury include fossil fuel emissions, medical waste incineration, dental amalgam, and various consumer products like skin creams, medications, and vaccines.  [10.]

Mercury poisoning can lead to severe health effects, including death, neurological deficits, developmental defects, and impaired muscle function.

 Practitioners can use mnemonic aids to recognize potential MeHg toxicity.

Function/What Does Mercury Do in the Body?

The toxicity of mercury is well-documented, with methylmercury being particularly neurotoxic. It can cause microtubule destruction, mitochondrial damage, and lipid peroxidation, leading to neurological disorders. 

Mercury emissions into the environment are estimated to be around 2,200 metric tons annually, with significant impacts on both aquatic and terrestrial ecosystems.  [8.] 

Neurological Effects  [4.]

Methylmercury, a potent neurotoxin, is implicated in significant neurological damage in both humans and animals. Its toxic effects are primarily attributed to increased production of reactive oxygen species (ROS), leading to oxidative stress. Studies show that antioxidants can mitigate methylmercury-induced cytotoxicity, suggesting a potential therapeutic approach.

Prenatal exposure to methylmercury is associated with reduced neuronal numbers and altered cytoarchitecture, leading to impaired motor and cognitive function in both humans and animal models. Methylmercury inhibits cell division and migration, disrupts microtubule organization crucial for CNS development, and interferes with intracellular signaling and ion channel function.

Furthermore, methylmercury alters neurotransmitter release and nitrergic activity, contributing to neurological dysfunction. Correlations have been observed between maternal methylmercury exposure during pregnancy and psychomotor retardation in children. Exposure to methylmercury during pregnancy or early childhood leads to developmental deficits, including motor and cognitive impairments.

Mercury toxicity is sometimes referred to as "Mad Hatter's disease" due to historical associations with hat-making in the 18th and 19th centuries. During this period, mercury compounds, particularly mercurous nitrate, were commonly used in the production of felt hats. 

Hatters, also known as milliners, were exposed to mercury vapors while working with these compounds, leading to chronic mercury poisoning.

Effects on the Cardiovascular System  [4.] 

Mercury exposure, once primarily associated with neurological effects, is now recognized for its profound cardiotoxicity. Studies have shown elevated mercury levels in populations, particularly in regions like the Amazon basin, where fish consumption is common. 

This exposure is strongly correlated with increased arterial blood pressure and heightened risk of cardiovascular diseases, including hypertension, myocardial infarction, and atherosclerosis.

Mercury's adverse effects on the cardiovascular system are attributed to oxidative stress, where exposure leads to increased production of free radicals and decreased activity of antioxidant enzymes. This imbalance promotes the oxidation of low-density lipoprotein (LDL) and inactivation of enzymes like paraoxonase, crucial for preventing atherosclerosis.

Animal studies have further elucidated mercury's impact on the cardiovascular system, showing effects such as cardiac diastolic failure, pulmonary hypertension, and increased blood pressure. 

Chronic exposure to low doses of mercury induces endothelial dysfunction, reducing nitric oxide (NO) bioavailability and increasing oxidative stress. These changes impair vascular function and contribute to cardiovascular risk, comparable to traditional risk factors like hypertension and diabetes.

Effects on the Kidneys  [6.] 

The kidney is highly susceptible to mercury toxicity due to the metal's strong affinity for renal tissue. Chronic exposure to mercury leads to kidney damage, presenting as proteinuria and toxic encephalopathy.

Clinical manifestations of mercury-induced kidney disease include proteinuria, edema, and urinary changes, often accompanied by nervous system symptoms such as tremors and insomnia. Pathological examinations reveal various renal conditions, with membranous nephropathy and minimal-change disease being the most common findings.

Chelation therapy is employed to remove mercury from the body, resulting in a reduction in urinary mercury concentrations and an improvement in renal function. However, transient elevation in urinary protein levels may occur during treatment.

Overall, chronic mercury poisoning can cause significant renal damage, leading to nephrotic syndrome and associated complications. Early diagnosis and appropriate treatment are crucial in managing mercury-induced kidney disease and preventing long-term complications.

Respiratory System Effects

Inhalation of mercury vapors can result in respiratory problems such as bronchitis and asthma. Moreover, mercury interferes with protein structure, cellular function, and immune response, leading to the formation of free radicals and cellular damage, to which the respiratory system is particularly sensitive. 

Chelation therapy is used to mitigate mercury toxicity by binding it to surrounding ligands and promoting its excretion from the body.  [8.] 

Effects of Mercury on Developing Brains  [1.] 

Mercury, particularly methylmercury, is especially neurotoxic to developing embryonic nervous systems and newborns due to its heightened neurotoxicity compared to mature neurons.

Normal brain development relies on a delicate balance of cellular processes like proliferation and differentiation, which mercury disrupts. Methylmercury, in particular, affects cell signaling, leading to impaired proliferation rather than increased cell death. 

It inhibits DNA synthesis, alters gene expression, increases oxidative stress, and disrupts intracellular calcium ion homeostasis, ultimately interfering with neurogenesis. Moreover, mercury exposure during neurogenesis can result in long-lasting effects, altering the proliferation capacity of future generations of cells and leading to epigenetic changes. 

Dietary Sources of Mercury  [13.] 

Overwhelmingly, fish is the largest source of dietary mercury.  

Fish: Some types of fish are known to contain higher levels of mercury due to environmental contamination. These include predatory fish such as shark, swordfish, king mackerel, and tilefish. Other types of fish, such as tuna (especially albacore or white tuna), marlin, orange roughy, and bigeye tuna, may also contain elevated levels of mercury.

Shellfish: Certain species of shellfish, including lobster, crab, and shrimp may contain mercury, although typically in lower levels compared to fish.

‍Seafood: Other types of seafood, such as squid, octopus, and sea bass, may also contain mercury, albeit in varying concentrations depending on factors like location and environmental conditions.

Rice: In some regions, rice may contain mercury due to contamination of soil and water sources.

Wine and liquor: wine and liquor may contain mercury.  

Certain fruits and vegetables: While fruits and vegetables are not typically significant sources of mercury, they may contain trace amounts if grown in soil or water contaminated with mercury.

It's essential to note that the mercury content in food can vary depending on various factors, including the species of fish or shellfish, the location where they were caught or harvested, and the individual's dietary habits. 

Pregnant women, nursing mothers, and young children are often advised to avoid or limit consumption of certain types of fish and seafood high in mercury due to potential health risks. 

Additionally, it's recommended to consume a varied diet and to be mindful of potential sources of environmental contaminants like mercury.

Laboratory Testing for Mercury

General Laboratory Testing Information and Sample Types  [5.]

Mercury testing methods vary depending on the form of mercury being assessed. For metallic and elemental mercury, blood samples provide a snapshot of recent exposure, with a half-life of approximately 3 days. 

However, to detect longer-term exposure, such as that from methylmercury, which has a half-life of 40-90 days, hair samples are more suitable.

Urine samples can also be used to detect mercury, particularly for inorganic and elemental mercury, with a detection window of 1 to 3 months post-exposure. 

However, it's important to note that methylmercury is primarily excreted through feces, rendering urine testing less useful for this form of mercury. Therefore, a urine test is not recommended for assessing methylmercury exposure.  Instead, some practitioners will order a fecal test for methylmercury assessment.  [3.]

Preparation for Mercury Testing

Fasting is not required for this test.  However, practitioners may recommend avoiding fish, wine and liquor for 72 hours.  Individuals who have had imaging studies with contrast may also need to wait 72 hours prior to testing.  

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Additionally, ordering practitioners may recommend other preparation steps.  It is important to consult with the ordering provider to confirm preparation steps.  

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Interpreting Test Results

Reference Range for Mercury Testing

It is important to consult with the laboratory company used for testing, as testing across companies and sample types may vary.  

One company reports a reference range for mercury in whole blood of <10 ng/mL for all ages.  [7.]

Clinical Significance of High Levels of Mercury

Elevated levels of mercury indicate the presence of excess mercury in the body.  Individuals may or may not present with symptoms.  

Signs and Symptoms of Mercury Toxicity 

Mercury poisoning typically presents with nonspecific symptoms that can vary depending on the type of mercury that has accumulated.  

Patients with a history of inorganic mercury exposure can present with:  [14.] 

• Angina
• Abdominal pain
• Bloody diarrhea
• Depression
• Edema
• Excessive saliva
• Frothy urine
• High blood pressure
• Hypothyroidism
• Impaired body temperature regulation
• Impaired cognitive ability
• Indigestion
• Memory loss
• Mental disturbances
• Mood swings
• Muscle weakness
• Skin rash or dermatitis
• Sleep disturbances
• Suicidal thoughts
• Tremors
• Thyroid inflammation

Patients with a history of elemental mercury exposure can present with: [14.] 

• Acrodynia (pink skin, peeling rash, irritability, sleeplessness, severe leg cramps, and sweating)
• Anxiety
• Anorexia
• Changes in nerve responses
• Cognitive and mental disfunction
• Dysmenorrhea
• Fatigue
• Forgetfulness
• Headaches
• Insomnia
• Mood swings
• Motor dysfunction
• Muscle atrophy
• Muscle twitching
• Muscle weakness
• Oropharyngeal inflammation
• Polyneuropathy
• Renal impairment
• Sensory disturbances
• Tremors

Patients with a history of organic mercury exposure may present with:  [14.] 

• Primary sensory neuropathy
• Depression
• Difficulty speaking
• Drowsiness
• Extreme irritability
• Fatigue
• Headaches
• Hearing loss
• Hallucinations
• Impaired coordination
• Impaired walking
• Insomnia
• Loss of peripheral vision
• Low body temperature
• Memory loss
• Muscle weakness
• Optic neuropathy
• Paranoia
• Paresthesia of the hands, feet, and perioral region
• Poor concentration
• Reduced sense of smell
• Retinopathy
• Tremors of the hands, head, lips, tongue, jaw, and eyelids
• Weight loss

Natural Ways to Promote Lower Levels of Mercury

Avoiding mercury sources is essential.  Follow the recommendations from the EPA and FDA regarding which fish to choose and which to avoid, to reduce mercury exposure.  [12.]

Sauna therapy may be an effective way to reduce mercury levels, although this should be done under the guidance of a medical professional.  [11.]

Because methylmercury is excreted primarily via the feces, maintaining regular bowel movements is essential.  Plenty of fiber from vegetables, fruits, legumes, and grains supports bowel regularity and may aid in elimination of toxins including methylmercury.  

Treatments to mitigate mercury's toxic effects include compounds like N-acetyl cysteine (NAC), Dendropanax morbifera leaf extract, Lycium barbarum polysaccharides (LBPs), and docosahexaenoic acid (DHA), which have shown promise in protecting developing neurons. [1.] 

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