The toxic element found in thermometers is mercury. Mercury is a heavy metal that is liquid at room temperature and has been historically used in thermometers due to its ability to expand and contract with temperature changes. However, due to its toxicity and environmental concerns, many countries have phased out the use of mercury in thermometers in favor of safer alternatives such as alcohol or digital thermometers. Exposure to mercury can cause serious health problems, including neurological damage, which is why proper disposal of older mercury thermometers is essential.