Written and maintained by CASRAI Editorial Board
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Mercury is dangerous because even small exposures can harm the nervous system, kidneys and other organs, and the risk depends on which form of mercury is involved. The World Health Organization says exposure to mercury, even in small amounts, may cause serious health problems and is especially harmful to a child in the womb and in early life. WHO lists it among the top ten chemicals of major public health concern. This page compares the three forms of mercury, the ways exposure happens, and the published exposure figures that labs and clinics can look up.
Definition and scope: three forms, three different hazards
“Mercury hazard” is not one hazard. WHO and the US EPA both separate mercury into forms that behave differently in the body, and that is why a sealed thermometer, a contaminated fish and a laboratory salt carry different risks.
- Elemental (metallic) mercury is the silver liquid metal found in older thermometers and gauges. It causes health effects mainly when its vapour is inhaled.
- Inorganic mercury compounds, such as mercuric chloride, are corrosive to the skin, eyes and gut.
- Organic mercury includes methylmercury, which builds up in fish and shellfish, and ethylmercury, used as the vaccine preservative thiomersal. WHO does not consider ethylmercury at those levels a health risk.
How exposure happens
Why is mercury so dangerous when it is a liquid?
The liquid itself is less of a concern than the vapour it gives off. The EPA states that metallic mercury mainly causes health effects when inhaled as a vapour, and that exposure usually follows a spill or the breakage of a mercury-containing product. WHO adds that inhaling elemental vapour is a main route, mostly in industrial processes.
Other routes
Eating fish or shellfish contaminated with methylmercury is the main route for organic mercury. Some inorganic compounds can harm through ingestion or skin contact. The NIOSH Pocket Guide lists four routes for mercury compounds: inhalation, skin absorption, ingestion, and skin or eye contact.
Health effects of mercury
WHO and the EPA describe effects on the nervous, digestive and immune systems, the lungs, kidneys, skin and eyes. Reported effects include tremors, insomnia, memory loss, headaches, mood changes and poorer performance on mental tests. Higher exposure to metallic mercury may cause kidney damage, respiratory failure or death, according to the EPA. WHO describes kidney effects ranging from protein in the urine to kidney failure.
The EPA states that effects range from very severe to subtle or none depending on the form, amount, duration, route of exposure, age and health of the person. The EPA also notes that no human data currently link mercury to cancer, though the data are limited. Unborn infants are the most vulnerable: the EPA lists later problems with thinking, memory, attention, language, fine motor skills and visual-spatial skills.
Comparison: the three forms side by side
The rows below summarise the WHO fact sheet and the EPA health-effects page listed under Sources.
| Form | Main route | Main concern reported |
|---|---|---|
| Elemental (metallic) | Inhaling vapour after a spill or product breakage | Nervous system effects such as tremors, insomnia and headaches; kidney damage at higher exposure |
| Inorganic compounds | Ingestion or skin contact | Corrosive to skin, eyes and gut; high exposure can damage the digestive tract, nervous system and kidneys |
| Organic (methylmercury) | Eating contaminated fish and shellfish | Loss of peripheral vision, tingling in hands, feet and mouth, poor coordination, impaired speech, hearing and walking, muscle weakness |
Where it matters for labs, clinics and supply decisions
Exposure limits a lab can look up
Different organisations publish different numbers because they measure different things. The figures below are quoted as the publishers state them; they are reference points, not a compliance determination.
- The NIOSH Pocket Guide lists a NIOSH recommended exposure limit of 0.05 mg/m3 as a time-weighted average for mercury vapour, with a skin notation, and a ceiling of 0.1 mg/m3 for other mercury compounds.
- The same page lists an immediately-dangerous-to-life-or-health value of 10 mg/m3 (as Hg) and, as shown on that page, an OSHA permissible exposure limit of 0.1 mg/m3 as a time-weighted average.
- WHO reports that workers exposed to airborne elemental mercury at 20 micrograms per cubic metre or more for several years may show mild subclinical signs of central nervous system toxicity. That is an observation about workers, not an exposure limit.
Where mercury is found in workplaces
OSHA lists thermometers, gauges, switches, batteries, high-intensity discharge lamps and dental amalgam among the workplaces and products where mercury occurs. The CASRAI guide on mercury in a thermometer covers how much mercury a thermometer holds, and the guide to medical thermometers covers the alternatives. For waste classification, see RCRA hazardous waste codes and laboratory waste disposal. For spill preparedness, see chemical spill kits. Hazard information for any specific product is on its safety data sheet; see how to read a safety data sheet.
In the US, the poison control number given on the EPA page is 1-800-222-1222.
Common misunderstandings
- “Small amounts are harmless.” WHO states that even small amounts may cause serious health problems. The EPA adds that outcomes vary with form, dose, duration and who is exposed.
- “The liquid is the danger.” For metallic mercury the EPA identifies inhaled vapour as the main cause of health effects.
- “All mercury is the same.” Elemental, inorganic and organic forms differ in route and target, as the table shows.
FAQ
What is the mercury hazard?
It is the harm to the nervous system, kidneys and other organs from inhaling, swallowing or touching mercury, with severity depending on the form and amount.
What are the hazards of mercury in a laboratory?
The NIOSH Pocket Guide lists target organs as the eyes, skin, respiratory system, central nervous system and kidneys, with symptoms such as irritation, cough, tremor, insomnia, headache and proteinuria.
Why is mercury so dangerous to unborn children?
WHO and the EPA identify the developing child as the most vulnerable group, with effects on thinking, memory, attention, language and motor skills reported later in life.
Sources
- World Health Organization, Mercury and health (checked 29 October 2026).
- US Environmental Protection Agency, Health effects of exposures to mercury (checked 29 October 2026).
- NIOSH Pocket Guide to Chemical Hazards, Mercury compounds [except (organo) alkyls] (checked 29 October 2026).
- OSHA, Mercury (checked 29 October 2026).
This page gives general hazard information and is not medical advice.
