Body Temperature

What is body temperature?

Body temperature is a measure of the body's ability to generate and get rid of heat. The body is very good at keeping its temperature within a narrow, safe range in spite of large variations in temperatures outside the body.

When you are too hot, the blood vessels in your skin expand (dilate) to carry the excess heat to your skin's surface. You may begin to sweat, and as the sweat evaporates, it helps cool your body. When you are too cold, your blood vessels narrow (contract) so that blood flow to your skin is reduced to conserve body heat. You may start shivering, which is an involuntary, rapid contraction of the muscles. This extra muscle activity helps generate more heat. Under normal conditions, this keeps your body temperature within a narrow, safe range.

Where is body temperature measured?

Your body temperature can be measured in many locations on your body. The mouth, ear, armpit, and rectum are the most commonly used places. Temperature can also be measured on your forehead.

What are Fahrenheit and Celsius?

Thermometers are calibrated in either degrees Fahrenheit (°F) or degrees Celsius (°C), depending on the custom of the region. Temperatures in the United States are often measured in degrees Fahrenheit, but the standard in most other countries is degrees Celsius.

What is normal body temperature?

Most people think of a "normal" body temperature as an oral temperature of 98.6°F (37°C). This is an average of normal body temperatures. Your temperature may actually be 1°F (0.6°C) or more above or below 98.6°F (37°C). Also, your normal body temperature changes by as much as 1°F (0.6°C) throughout the day, depending on how active you are and the time of day. Body temperature is very sensitive to hormone levels and may be higher or lower when a woman is ovulating or having her menstrual period.

A rectal or ear (tympanic membrane) temperature reading is 0.5 to 1°F (0.3 to 0.6°C) higher than an oral temperature reading. A temperature taken in the armpit is 0.5 to 1°F (0.3 to 0.6°C) lower than an oral temperature reading.

What is a fever?

In most adults, an oral temperature above 100°F (37.8°C) or a rectal or ear temperature above 101°F (38.3°C) is considered a fever. A child has a fever when his or her rectal temperature is 100.4°F (38°C) or higher.

What can cause a fever?

A fever may occur as a reaction to:

• Infection. This is the most common cause of a fever. Infections may affect the whole body or a specific body part (localized infection).
• Medicines, such as antibiotics, narcotics, barbiturates, antihistamines, and many others. These are called drug fevers. Some medicines, such as antibiotics, raise the body temperature directly; others interfere with the body's ability to readjust its temperature when other factors cause the temperature to rise.
• Severe trauma or injury, such as a heart attack, stroke, heat exhaustion or heatstroke, or burns.
• Other medical conditions, such as arthritis, hyperthyroidism, and even some cancers, such as leukemia, Hodgkin's lymphoma, and liver and lung cancer.

Can a low body temperature be dangerous?

An abnormally low body temperature (hypothermia) can be serious, even life-threatening. Low body temperature may occur from cold exposure, shock, alcohol or drug use, or certain metabolic disorders, such as diabetes or hypothyroidism. A low body temperature may also be present with an infection, particularly in newborns, older adults, or people who are frail. An overwhelming infection, such as sepsis, may also cause an abnormally low body temperature.

Can a high body temperature be dangerous?

Heatstroke occurs when the body fails to regulate its own temperature, and body temperature continues to rise. Symptoms of heatstroke include mental changes (such as confusion, delirium, or unconsciousness) and skin that is red, hot, and dry, even under the armpits.

Classic heatstroke can develop without exertion when a person is exposed to a hot environment and the body is unable to cool itself effectively. In this type of heatstroke, the body's ability to sweat and transfer the heat to the environment is reduced. A person with heatstroke may stop sweating. Classic heatstroke may develop over several days. Babies, older adults, and people with chronic health problems have the greatest risk of this type of heatstroke.

Exertional heatstroke may develop when a person is working or exercising in a hot environment. A person with heatstroke from exertion may sweat profusely, but the body still produces more heat than it can lose. This causes the body's temperature to rise to high levels.

Both types of heatstroke cause severe dehydration and can cause body organs to stop functioning. Heatstroke is a life-threatening medical emergency, requiring emergency medical treatment.

Body temperature is checked to:

• Detect fever.
• Detect abnormally low body temperature (hypothermia) in people who have been exposed to cold.
• Detect abnormally high body temperature (hyperthermia) in people who have been exposed to heat.
• Help monitor the effectiveness of a fever-reducing medicine.
• Help plan for pregnancy by determining if a woman is ovulating.

Take your temperature several times when you are feeling well to find out what is normal for you. Check your temperature in both the morning and evening, since body temperature can vary by as much as 1°F (0.6°C) throughout the day.

Wait at least 20 to 30 minutes after smoking, eating, or drinking a hot or cold liquid before taking your temperature. Also wait at least an hour after vigorous exercise or a hot bath.

Several different types of thermometers are available:

• Electronic thermometers are plastic and shaped like a pencil, with a display window at one end and the temperature probe at the other end. They work by measuring how well electricity travels through a wire. Electronic thermometers are used in the mouth, rectum, or armpit. They are easy to use, easy to read, and are accurate. If you buy an electronic thermometer, check the package for information about its accuracy. See a picture of an electronic thermometer.
• Ear thermometers are plastic and come in different shapes. They use infrared energy to measure body temperature. The small cone-shaped end of the thermometer is placed in the ear, and body temperature shown on a digital display. The results appear within seconds. Some models also show the corresponding oral and rectal readings. See a picture of an ear thermometer.
• Forehead thermometers use skin temperature to determine body temperature. Some have a soft disc that are pressed against the forehead and show the temperature on a digital display. Other types are thin pieces of plastic with numbers on them. You press the strip against a person’s forehead, and the temperature makes some numbers change colors or light up. These thermometers are not as accurate as electronic and ear thermometers.
• Disposable thermometers are thin flat pieces of plastic with colored dots and temperature markings on one end. The color of the dots shows the temperature. Disposable thermometers can be used in the mouth or rectum. A patch form can be used on a baby's skin to measure temperature continuously for 48 hours. These thermometers are safe and accurate within 0.2°F (0.1°C). They do not contain glass, latex, or mercury. You can reuse the thermometer during an illness and then throw it away.
• Pacifier thermometers are shaped like a baby’s pacifier but have a display that shows the temperature. You place the pacifier in your child’s mouth to measure temperature. These thermometers may take longer to get a reading and are not as accurate as other types.

Glass thermometers containing mercury are no longer recommended. If you have a glass thermometer, contact your local health department for instructions on how to dispose of it safely. If you break a glass thermometer, call your local poison control center immediately.

Before taking a body temperature, review the instructions for how to use your thermometer. Methods of taking a temperature are described below.

How to take an oral temperature

Oral is the most common method of taking a temperature. To get an accurate temperature, the person must be able to breathe through the nose. If this is impossible because of a stuffy nose or lack of cooperation, use the rectum or armpit.

1. Place the digital or disposable thermometer under the tongue, just to one side of the center, and close the lips tightly around it.
2. Leave the thermometer in place for the required amount of time. Time yourself with a clock or watch. Some digital thermometers give a series of short beeps when the reading is done.
3. Remove the thermometer and read it.
4. Clean a digital thermometer with cool, soapy water and rinse it off before putting it away.

How to take a rectal temperature

This is the location to measure body temperature most accurately. It is recommended for babies, small children, and people who cannot hold a thermometer safely in their mouths. It is also used when getting the most accurate measurement is essential.

1. Apply a lubricant jelly or petroleum jelly, such as Vaseline, on the bulb of the thermometer so that you can insert it easily.
2. When measuring the temperature of babies or small children, turn the child face down on your lap or on a flat covered or padded surface, such as a bed. Choose a quiet place so that the child won't be distracted or move around too much.
3. Spread the child's buttocks with one hand and gently insert the bulb end of the rectal thermometer about 0.5 in. (1.25 cm) to 1 in. (2.5 cm) into the anal canal with your other hand. Don't force it into the rectum. Hold the thermometer in place with two fingers close to the anal opening (not near the end of the thermometer). Pressing the child's buttocks together will help keep the thermometer in place.
4. Leave the thermometer in place for the required amount of time. Some digital thermometers give a series of short beeps when the reading is done. Time yourself with a watch or clock.
5. Remove the thermometer and read it. A rectal temperature reading may be as much as 1°F (0.6°C) higher than an oral temperature reading.
6. Clean a digital thermometer with cool, soapy water and rinse it off before putting it away.
7. Do not use a thermometer to take an oral temperature after it has been used to take a rectal temperature.

See a picture of how to take the rectal temperature of a baby.

How to take an armpit (axillary) temperature

Taking a temperature in the armpit may not be as accurate as taking an oral or rectal temperature.

1. Place the thermometer under the arm with the bulb in the center of the armpit.
2. Press the arm against the body and leave the thermometer in place for the required amount of time. Time yourself with a watch or clock.
3. Remove the thermometer and read it. An armpit temperature reading may be as much as 1°F (0.6°C) lower than an oral temperature reading.
4. Clean a digital thermometer with cool, soapy water and rinse it off before putting it away.

How to take an ear temperature

Ear thermometers may need to be cleaned and prepared for use. These steps can be followed when using an ear thermometer; however, follow the instructions for your specific model.

1. Check that the probe is clean and free of debris. If dirty, wipe it gently with a clean cloth. Do not immerse the thermometer in water.
2. To keep the probe clean, a disposable probe cover should be used. Use a new probe cover each time you take an ear temperature. Attach the disposable cover to the probe.
3. Turn the thermometer on.
4. For babies younger than 12 months, pull the earlobe down and back. This will help place the probe in the ear canal. Center the probe tip in the ear and push gently inward toward the eardrum.
5. For children older than 12 months and for adults, pull the earlobe up and back. Center the probe tip in the ear and push gently inward toward the eardrum.
6. Press the "on" button to display the temperature reading.
7. Remove the thermometer and throw away the used probe cover.

How to take a forehead temperature

1. Press the soft cup of the thermometer or the entire plastic strip firmly against a dry forehead.
2. Hold the thermometer in place for the required amount of time. Time yourself with a watch or clock.
3. Read the temperature before removing the thermometer.
4. Clean the thermometer with cool soapy water and rinse it off before putting it away.
5. Forehead thermometers are not as accurate as electronic and ear thermometers. If your baby is younger than 3 months or your child's fever rises higher than 102°F (39°C), recheck the temperature using a better method.

How to use a pacifier thermometer

1. Put together all of the pieces of the pacifier if you need to. Some pacifier thermometers can be used as regular pacifiers and need to have the temperature part attached.
2. Let your child suck on the nipple for the required amount of time. Time yourself with a watch or clock.
3. Remove the pacifier and read the temperature.
4. Clean the pacifier with cool, soapy water and rinse it off before putting it away.
5. Pacifier thermometers are not as accurate as electronic and ear thermometers. If your baby is younger than 3 months or your child's fever rises higher than 102°F (39°C), recheck the temperature using a better method.

Taking your temperature by mouth is only mildly uncomfortable, since you must keep your mouth closed and breathe through your nose while the thermometer is in place.

Taking a rectal temperature can be slightly uncomfortable but should not be painful.

Taking your temperature with an ear thermometer causes little or no discomfort. It is not inserted very far into the ear, and it provides a reading in only a few seconds. For this reason, the ear thermometer is widely used in health professional's offices and hospitals. However, it may be less accurate than rectal thermometers.

Taking your temperature with a plastic strip thermometer feels like having an adhesive bandage on your forehead. Although it causes very little discomfort, it is not as reliable as other methods, so another kind of thermometer should be used if the plastic strip shows an abnormal temperature.

There is very little risk of complications from taking a temperature.

When taking a rectal temperature, do not insert the thermometer into the rectum more than 0.5 in. (1.25 cm) to 1 in. (2.5 cm). Further insertion can be painful and may damage rectal tissues.

Body temperature is a measure of the body's ability to generate and get rid of heat.

Rectal and ear (tympanic membrane) temperatures are normally as much as 1°F (0.6°C) higher than oral temperatures; armpit temperatures, however, may be as much as 1°F (0.6°C) lower than oral temperatures. Forehead temperatures also are usually lower than oral or rectal temperatures. If your oral temperature is 99°F (37.2°C), your rectal or ear temperature may be about 100°F (37.8°C) and your armpit temperature about 98°F (36.7°C). When you tell your health professional about your temperature measurement, be sure to mention whether it was taken on the forehead or in the mouth, rectum, armpit, or ear.

Inaccurate temperature readings can be caused by:

• Not keeping your mouth closed around the thermometer when taking an oral temperature.
• Not leaving a thermometer in place long enough before reading it.
• Not putting the proper thermometer in the right place.
• Not following the instructions for proper use that come with the thermometer.
• A weak or dead thermometer battery.
• Taking an oral temperature within 20 minutes after smoking or drinking a hot or cold liquid.
• Taking a temperature by any method within an hour of exercising vigorously or taking a hot bath.

• Thermometers with a digital display usually need a battery. If your thermometer uses a battery, make sure it is working before taking a temperature.
• Body temperature is only one way of monitoring your health. Besides temperature, other basic measurements to monitor your health include your pulse, breathing rate (respiration), and blood pressure. These basic measurements are called your vital signs.
• A fever can make you feel uncomfortable. To treat the discomfort of a fever, wear light clothing and use light blankets or other bedding. Drink cool liquids. A bath or shower with lukewarm (not cool) water can lower body temperature. Cool or cold water can cause shivering and can cause the blood vessels near the skin to contract, which will raise the body temperature further.
• Fever-reducing medicines can lower body temperature. Unless a fever is high enough to call a health professional, fever-reducing medicine is not necessary but may help you feel more comfortable. When a fever causes discomfort, use acetaminophen (such as Tylenol) or ibuprofen (such as Advil or Motrin). Aspirin also reduces fever but should not be given to anyone younger than age 20 because of the risk of Reye's syndrome.
• When reading medical information that mentions body temperatures, note whether the temperature is listed as an oral or rectal temperature. Many books and other information about children's health list all body temperatures as rectal temperatures, because this method is preferred for measuring body temperature in a young child. If a body temperature is listed but neither oral nor rectal is specified, you may assume it is an oral temperature.
• Glass thermometers containing mercury are no longer recommended. If you have a glass thermometer, contact your local health department for instructions on how to dispose of it safely. If you break a glass thermometer, call your local poison control center immediately.

Other Works Consulted

• Auwaerter PG (2007). Approach to the patient with fever. In LR Barker et al., eds., Principles of Ambulatory Medicine, 7th ed., pp. 457–465. Philadelphia: Lippincott Williams and Wilkins.

• El-Radhi AS, Barry W (2006). Thermometry in paediatric practice. Archives of Disease in Childhood, 91(4): 351–356.