Thermometers: Understand the options

By Mayo Clinic Staff

A thermometer is a device used to measure body temperature. It helps tell whether someone has a fever or is too hot or too cold. Average temperatures can be different for each person.

On average, temperatures vary slightly by age:

• Adults: 97.9 degrees Fahrenheit (36.6 degrees Celsius), not 98.6 degrees Fahrenheit (37 degrees Celsius) which is commonly cited.
• Children: 98.1 degrees Fahrenheit (36.7 degrees Celsius).
• Infants: 98.9 degrees Fahrenheit (37.2 degrees Celsius).

Body temperature may be higher for people assigned female at birth and temperatures tend to be higher for all age groups in the later afternoon.

But no matter what your temperature is daily, if you think you have a fever, you need a thermometer. Choosing one can be confusing, so here's what you need to know about the most common thermometers.

Types of thermometers

A thermometer measures body temperature by sensing how certain materials change when they get warmer or cooler. (3p3; 8p1) Body temperature can be measured by infrared rays or by how heat changes a liquid, metal, gas or electric signal in a thermometer.

There are two types of thermometers.

• Contact thermometers must touch the body to measure temperature.
• Contactless thermometers can measure body temperature without touching the skin.

Contact thermometers

A common type of contact thermometer is a digital electronic thermometer. It has a sensor that reacts to body heat and sends a signal that's read as a temperature on a digital display.

Contact thermometers may be used in the mouth, armpit or rectum. Or they may be used on the forehead.

Temperatures can be slightly different depending on where you take them on the body. There's no exact way to add or subtract a degree to make them match when temperatures are taken from different parts of the body. For this reason, it's best to compare readings taken the same way each time.

While temperatures from the rectum are the most accurate, rectal thermometers are invasive. They typically are used by healthcare professionals to check the temperature of infants when there are health concerns.

Healthcare professionals also may have parents or caregivers check rectal temperatures for infants if there is concern that they may have a fever. Rectal thermometers can be difficult or uncomfortable for some people, or in some situations. Oral thermometers are used more often, as they provide similar accuracy and are less invasive.

Pros of contact thermometers:

• Most digital contact thermometers can record temperatures very quickly.
• A digital contact thermometer is appropriate for any age, from newborns to adults.

Cons of contact thermometers:

• You need to wait 15 minutes after eating or drinking to take an oral temperature. This is because the temperature of your food or drink might affect the thermometer reading.
• Parents or caregivers may be uncomfortable or worry about a child's discomfort when taking a rectal temperature.
• Contact thermometers have to be cleaned very well after each use. They should be cleaned after each user and each use to limit the spread of germs. If you plan to use a contact thermometer to take both oral and rectal temperatures, don't use the same thermometer in both places. Get two thermometers. Label one for oral use and one for rectal use.

Ear thermometers

Ear thermometers also are called tympanic thermometers. These digital thermometers use an infrared ray to measure the temperature inside the ear canal.

Pros of ear thermometers:

• When positioned properly, ear thermometers are quick and generally comfortable for children and adults.
• Ear thermometers are appropriate for children older than 7 months and adults.

Cons of ear thermometers:

• Ear thermometers aren't recommended for infants under 7 months.
• The shape of the ear canal, infections and earwax could make the ear thermometer less accurate.
• Ear thermometers have to be cleaned very well after each use.
• Air temperatures that are very hot or cold may affect how well an ear thermometer works. So, before using an ear thermometer, make sure to wait about 15 minutes after you or your child has been out in the cold or heat.

Mercury thermometers

Mercury thermometers are made of glass with silver mercury inside. They are no longer recommended because mercury is poisonous. Also, if it breaks the glass shards may cause harm. Mercury thermometers have been banned in many places in the United States.

Before you throw out a mercury thermometer, contact your local pharmacy or recycling collection program to find out how to dispose of it safely.

Contactless thermometers

Contactless thermometers measure temperature without touching the body.

Temperature is found by scanning infrared heat from the forehead or skin, typically at a distance of 1/2 to 2 inches (1.3 to 5 centimeters). Check the manufacturer's instructions, as the distance may be different for each brand. Contactless thermometers used on the forehead are called temporal artery thermometers.

Forehead thermometers

Contactless forehead thermometers, also called temporal artery thermometers, use an infrared scanner to measure the temperature of the temporal artery in the forehead.

Pros of forehead thermometers:

• Forehead thermometers record temperature quickly without needing to touch the body.
• They do not cause physical discomfort.
• They can be used for children and adults.
• Germs and viruses can't be passed on since they do not touch the body.

Cons of forehead thermometers:

• May cost more than other thermometers.
• May be less accurate than other types of thermometers, especially when used in direct sunlight or cold temperatures. They also can be inaccurate if the forehead is sweaty or if the thermometer is held too far away from the forehead.

Thermometers by age

Healthcare professionals may recommend certain types of thermometers based on a person’s age.

• Under 3 months. Use a rectal contact thermometer or a contactless forehead thermometer.
• Ages 3 to 6 months. Use rectal and armpit contact thermometers. Or use a contactless forehead thermometer.
• Ages 7 months to 3 years. The rectal, ear and forehead contact thermometers work well for this age group. Or use a contactless forehead thermometer.
• Age 4 years and older. Contact thermometers for the mouth or ear work well. Or use a contactless forehead thermometer.
• Age 65 years and older. Use a mouth or ear thermometer. Do not use any type of forehead thermometer.

Talk with your healthcare professional about what may be right for you or your child.