Temperature is a measure of an organism’s ability to generate and get rid of heat. The human body has mechanisms to maintain its internal temperature within a relatively narrow, safe range despite relatively large variations in temperatures in which the body exists.
The purpose of maintaining body temperature within a relatively narrow range is to promote and sustain life.
Thermometers are used to measure body temperature. They are calibrated in either degrees Fahrenheit (°F) or degrees Celsius (°C). Temperatures in the United States are typically measured in degrees Fahrenheit. The standard in most countries of the world is degrees Celsius.
When humans become too warm, blood vessels in the skin increase in diameter (dilate). The purpose is to carry the excess heat to the surface of the skin. In turn, this causes the body to begin to perspire. As the perspiration evaporates, it helps to cool the body. When the body becomes too cold, the blood vessels decrease in diameter (contract) so that blood flow to the skin is reduced in an attempt to conserve body heat. This often causes people to start shivering. This involves rapid, involuntary contractions of muscles. Shivering helps to generate additional heat through muscle activity. Under normal conditions, these activities maintain human body temperature within a narrow range that is healthy for the organism.
Body temperature can be measured in many locations. The mouth, ear, armpit, and rectum are the most commonly used places. Temperature can also be measured on the forehead.
Body temperature is checked for several reasons.
- To detect fever.
- To document an abnormally low body temperature(hypothermia) in people who have been exposed tocold.
- To document an abnormally high body temperature(hyperthermia) in people who have been exposed toheat.
- To monitor the effectiveness of a fever-reducing medicine (antipyretic).
- To determine when a female is ovulating, thereby increasing the probability of becoming pregnant.
Preparation for taking a body temperature consists of ensuring that the thermometer is clean and disinfected.
Aftercare consists of ensuring that a thermometer is clean and disinfected. Electronic thermometers must be turned off to conserve their batteries.
Taking a body temperature involves little risk. Inserting a thermometer into the rectum can occasionally be painful. Breaking a thermometer that contains mercury causes exposure to a toxic substance (mercury).
Most people consider a normal body temperature to be an oral temperature of 98.6 degrees Fahrenheit. This is more correctly an average of body temperatures. A person’s body temperature varies during each 24 hour period. A normal range encompasses temperatures that are 1°F (0.6°C) above or below 98.6 degrees F. Some variation is due to fluctuations in physiology nd cellular metabolism. Bodily activities (or lack) can temporarily increase (or decrease) body temperature. Body temperature is very sensitive to hormone levels and may be higher or lower when a female is ovulating during her menstrual cycle.
A rectal or ear (tympanic membrane) temperature reading is 0.5 to 1 degree F (0.3 to 0.6 degrees C) higher than an oral temperature reading. A temperature taken in the armpit is 0.5 to 1 degree F (0.3 to 0.6 degrees C) lower than an oral temperature reading.
In adults, an oral temperature above 100 degrees F or a rectal or ear temperature above 101 degrees F is considered to be a fever. Children are considered to have a fever when their rectal temperature is 100.4 degrees F or higher.
Abnormally low body temperature is called hypothermia. It is always serious and can be life-threatening. Hypothermia can occur after exposure to cold, when a person is in shock, or after alcohol or drug usage. Metabolic disorders, such as hypothyroidism or diabetes can trigger hypothermia. An infection involving the entire body (sepsis) can cause hypothermia. Infections in older adults, newborn infants or other frail persons may be accompanied by hypothermia.
Perforations of the colon due to inserting a rectal thermometer too far have been reported. These are uncommon. The number of deaths associated with taking a temperature is essentially zero.
There are no alternatives to obtaining a body temperature.
Fever— An abnormally elevated body temperature, usually defined as being 101 degrees Fahrenheit or more
Hypothermia— An abnormally low body temperature, usually defined as being 90 degrees Fahrenheit or less
Sepsis— An infection involving the entire body
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L. Fleming Fallon, Jr, MD, DrPH