Mercury is a heavy metal with well-documented toxic effects on human health and the environment. It exists in three main forms, each with distinct toxicological profiles and mechanisms of action. Understanding the differences between these species is crucial for assessing risks, diagnosing exposure, and implementing effective treatment strategies.
Mercury Species and Their Toxicity
1. Elemental Mercury (Hg⁰):
Physical state: Liquid at room temperature but easily vaporizes into a toxic gas.
Exposure routes: Inhalation of vapors (e.g., from broken thermometers, artisanal gold mining).
Toxicity: Absorbed through the lungs, elemental mercury crosses the blood-brain barrier, oxidizing to Hg²⁺ and causing neurotoxic effects. Symptoms include tremors, mood changes, memory loss, and, in severe cases, respiratory failure.
Oral ingestion: Surprisingly, liquid mercury has low gastrointestinal absorption, so swallowing it is less toxic than inhalation.
2. Inorganic Mercury (Hg²⁺ and Hg⁺):
Physical state: Often found in salts like mercury chloride (HgCl₂) or mercury sulfide (HgS).
Exposure routes: Industrial processes, some skin-lightening creams, and certain traditional medicines.
Toxicity: Primarily affects the gastrointestinal tract and kidneys. Hg²⁺ binds to sulfhydryl groups in proteins, disrupting enzyme function and leading to nephrotoxicity.
Symptoms: Oral exposure causes abdominal pain, vomiting, and diarrhea; chronic exposure leads to kidney damage.
3. Organic Mercury (Methylmercury and Ethylmercury):
Methylmercury (CH₃Hg⁺): The most hazardous form due to bioaccumulation in fish and seafood.
Ethylmercury: Found in some vaccines as thimerosal, but rapidly cleared from the body and less toxic than methylmercury.
Exposure routes: Predominantly through consumption of contaminated fish.
Toxicity: Methylmercury crosses the blood-brain barrier and placenta, binding to thiol groups in the central nervous system. It leads to neurodevelopmental issues, cognitive deficits, and motor dysfunction. Minamata disease is a well-known example of severe methylmercury poisoning.
Mechanisms of Toxicity
Mercury’s toxicity largely stems from its strong affinity for thiol (-SH) groups in proteins, disrupting enzymatic activity and cellular function.
Elemental mercury vapor oxidizes to Hg²⁺, accumulating in the brain and causing neuronal damage.
Bioaccumulation: Methylmercury magnifies through food chains, affecting top predators like large fish.
Global transport: Mercury vapors travel long distances, depositing in water bodies and converting to toxic methylmercury.
Conclusion
Mercury toxicity remains a critical public health and environmental issue. Proper understanding of its speciation, mechanisms of action, and clinical effects is essential for effective prevention, diagnosis, and treatment. Continued research and regulation are crucial to mitigate mercury exposure and its associated risks.
Michael Sperling
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This
comprehensive review discusses the various forms of mercury, their
sources, mechanisms of toxicity, clinical presentations, and treatment
options.
Yuan-Seng Wu, Ahmed I. Osman, Mohamed Hosny, Ahmed M. Elgarahy, Abdelazeem S. Eltaweil, David W. Rooney, Zhonghao Chen, Nur Syafiqah Rahim, Mahendran Sekar, Subash C. B. Gopinath, Nur Najihah Izzati Mat Rani, Kalaivani Batumalaie, Pow-Seng Yap, The Toxicity of Mercury and Its Chemical Compounds: Molecular Mechanisms and Environmental and Human Health Implications: A Comprehensive Review, ACS Omega, 9 (2024) 5100−5126. 
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