Mercury thermometers do not work below the freezing point of mercury, which is minus 38.83 degrees Celsius or minus 37.89 degrees Fahrenheit. Mercury vapors and physical contact with mercury are very dangerous. Breakage of a mercury thermometer requires proper equipment and procedures to safely clean up and dispose of mercury spills.
Mercury expands and contracts with temperature and, when contained in a marked, narrow diameter glass tube, can accurately measure temperature. Mercury has been used in thermometers since 1714 and is still used by scientists in many parts of the world. For outdoor work, pure mercury thermometers need to be brought indoors in cold temperatures to prevent them from breaking. For very cold applications, a mercury-thallium alloy thermometer is sometimes used, due to its lower freezing point.
Mercury evaporates easily into the air and poses harmful health risks to those that come in contact with it. Due to the inherent health risks that occur when a mercury thermometer breaks, many states and countries have outlawed the manufacturing, sale or distribution of mercury-in-glass thermometers.
For in-home use, alcohol-filled and digital thermometers serve as alternatives to mercury thermometers, especially for fever measurement. However, old mercury thermometers remain in use in some schools, homes and businesses. Lawful disposal of mercury thermometers varies by jurisdiction and should be researched before they are discarded.