To obtain an accurate temperature reading, a glass thermometer must remain in the mouth for three minutes, while a digital thermometer remains in the mouth until it beeps or the temperature reading appears in the window, according to Drugs.com. The tip of the thermometer must be placed under the tongue.
Digital thermometers are preferred for oral measurement of body temperature because they are easy to use and provide a temperature reading within a few seconds. Glass thermometers with alcohol inside are safe for use with anyone over the age of 5. Younger children may bite on glass thermometers and break them. The Environmental Protection Agency and the American Academy of Pediatrics are among the organizations that no longer recommend the use of glass thermometers that contain mercury due to the risk of mercury being inhaled or absorbed through the skin if the thermometer breaks, notes Drugs.com.