Written and maintained by CASRAI Editorial Board
Last updated
This page describes products and general use; it is not medical advice.
The mercury element in a thermometer is the silver liquid metal sealed in the glass bulb of a traditional fever thermometer. Mercury (symbol Hg, atomic number 80) is one of very few metals that is liquid at room temperature. A typical oral, rectal or baby fever thermometer holds about 0.61 g of it, and the mercury vapors released when one breaks are why digital thermometers are now the usual alternative. Who uses them: clinic staff, households, students and anyone who finds an old thermometer and needs to know what is inside it.
What the mercury in a thermometer is
Mercury is a chemical element, not a compound or an alloy. In a glass thermometer it is the working liquid: it sits in the bulb and its level in the narrow tube is read against a scale. The silver liquid in a bulb may be mercury or a non-toxic look-alike, according to the U.S. Environmental Protection Agency (EPA), so the colour alone does not tell you which one a given thermometer contains.
Element properties of mercury
These values come from the NCBI PubChem periodic table entry for element 80 (checked 2026-10-29).
| Property | Value |
|---|---|
| Symbol and atomic number | Hg, 80 |
| Melting point | 234.32 K (-38.83 C) |
| Boiling point | 629.88 K (356.73 C) |
| Density | 13.5336 g/cm3 |
| State at room temperature | Liquid |
The symbol Hg comes from the Greek hydrargyrum, meaning “liquid silver”. Read side by side, the melting and boiling points show that mercury stays liquid across a wide span of everyday temperatures, from -38.83 C up to 356.73 C.
How much mercury is in a thermometer?
Less than most people expect, but enough to matter. EPA gives two typical amounts (EPA, Mercury Thermometers, checked 2026-10-29):
| Thermometer type | Typical mercury content |
|---|---|
| Oral, rectal or baby fever thermometer | About 0.61 g |
| Basal thermometer | About 2.25 g |
When a mercury thermometer breaks, the mercury releases vapors that EPA describes as harmful to human and ecological health. EPA’s small-spill guidance applies when the amount spilled is less than or similar to the amount in a thermometer.
Safe handling basics
General cautions from EPA’s page on what to do if a mercury thermometer breaks (checked 2026-10-29):
- Never use a vacuum cleaner on mercury; it puts mercury into the air.
- Never use a broom; it breaks the mercury into smaller droplets and spreads them.
- Keep the area ventilated to the outside for at least 24 hours after a successful cleanup.
- Do not let children help, and keep children and pets out of the area.
- For health questions, contact a physician or poison control at 1 (800) 222-1222.
Follow the full EPA cleanup guidance rather than improvising. For broader laboratory context see chemical spill kits, hazardous waste labeling requirements and lab waste disposal.
What replaced mercury thermometers
EPA states that the most similar alternatives to mercury fever thermometers are battery- and solar-powered digital thermometers. A digital thermometer’s button-cell battery may itself contain a little mercury, and EPA advises recycling it through hazardous waste collection. EPA also works to reduce mercury-filled non-fever thermometers in industrial settings where alternatives exist, and the U.S. National Institute of Standards and Technology (NIST) no longer provides calibration services for mercury thermometers. For how current clinical thermometers are built and used, see what is a medical thermometer. Buyers comparing replacements can browse diagnostic and monitoring equipment.
Common mistakes
- Assuming the silver liquid in a thermometer is either mercury or harmless by appearance alone. EPA notes it may be mercury or a non-toxic look-alike.
- Vacuuming or sweeping up a spill.
- Discarding a digital thermometer’s button-cell battery with ordinary trash instead of hazardous waste collection.
FAQ
What element is in a thermometer?
Traditional glass fever thermometers contain mercury (Hg, atomic number 80), a metal that is liquid at room temperature.
Is the mercury element thermometer still used?
EPA describes battery- and solar-powered digital thermometers as the most similar alternatives to mercury fever thermometers, and NIST no longer provides calibration services for mercury thermometers.
How much mercury is in a thermometer?
About 0.61 g in a typical oral, rectal or baby fever thermometer and about 2.25 g in a basal thermometer, per EPA.
Sources
- EPA, Mercury Thermometers, checked 2026-10-29.
- EPA, What to do if a mercury thermometer breaks, checked 2026-10-29.
- NCBI PubChem, Periodic Table, element 80 (mercury), checked 2026-10-29.
