Normal human body temperature, also known as normothermia or euthermia, is a concept that depends upon the place in the body at which the measurement is made, and the time of day and level of activity of the person. There is no single number that represents a normal or healthy temperature for all people under all circumstances using any place of measurement.
Different parts of the body have different temperatures. Rectal and vaginal measurements, or measurements taken directly inside the body cavity, are typically slightly higher than oral measurements, and oral measurements are somewhat higher than skin temperature. The commonly accepted average core body temperature (taken internally) is 37.0 °C (98.6 °F). The typical oral (under the tongue) measurement is 36.8±0.7 °C, or 98.2±1.3 °F. In Russia and former Soviet countries, the commonly quoted value is 36.6 °C (97.9 °F), based on an armpit (axillary) reading. Although some people think of these numbers as representing the normal temperature, a wide range of normal temperatures has been found. In adult men and women the normal range for oral temperature is 33.2–38.2 °C (92–101 °F), for rectal it is 34.4–37.8 °C (94–100 °F), for the Tympanic cavity it is 35.4–37.8 °C (96–100 °F) and for axillary it is 35.5–37.0 °C (96–99 °F).
The time of day and other circumstances also affects the body's temperature. The core body temperature of an individual tends to have the lowest value in the second half of the sleep cycle; the lowest point, called the nadir, is one of the primary markers for circadian rhythms. The body temperature also changes when a person is hungry, sleepy, or cold.
Gabriel Fahrenheit originally used human body temperature as a reference point for his temperature scale, defining it to be 96°. Later redefinition of his scale to use the boiling point of water as a reference point caused the numerical value for normal body temperature to drift.
In 1861, Carl Reinhold August Wunderlich released data on the armpit, or axillary, temperatures of twenty five thousand people, and reported the mean to be 37.0 °C (98.6 °F), with a range of 36.2 °C (97.2 °F) to 37.5 °C (99.5 °F).
Temperature control (thermoregulation) is part of a homeostatic mechanism that keeps the organism at optimum operating temperature, as it affects the rate of chemical reactions. In humans the average oral temperature is 36.8 °C (98.2 °F), though it varies among individuals, as well as cycling regularly through the day, as controlled by one's circadian rhythms with the lowest temperature occurring about two hours before one normally wakes up.
Body temperature normally fluctuates over the day, with the lowest levels around 4 a.m. and the highest in the late afternoon, between 4:00 and 6:00 p.m. (assuming the person sleeps at night and stays awake during the day). Therefore, an oral temperature of 37.2 °C (99.0 °F) would, strictly speaking, be normal in the afternoon but not in the morning. An individual's body temperature typically changes by about 0.5 °C (0.9 °F) between its highest and lowest points each day.
Temperature is increased after eating, and psychological factors also influence body temperature.
Many outside factors affect the measured temperature as well. "Normal" values are generally given for an otherwise healthy, non-fasting adult, dressed comfortably, indoors, in a room that is kept at a normal room temperature (22.7 to 24.4 °C or 73 to 76 °F), during the morning, but not shortly after arising from sleep. Furthermore, for oral temperatures, the subject must not have eaten, drunk, or smoked anything in at least the previous fifteen to twenty minutes, as the temperature of the food, drink, or smoke can dramatically affect the reading.
Children develop higher temperatures with activities like playing, but this is not fever because their set-point is normal. Elderly patients may have a decreased ability to generate body heat during a fever, so even a low-grade fever can have serious underlying causes in geriatrics.
Normal body temperature may differ as much as 1.0 °F between individuals or from day to day.
A temperature setpoint is the level at which the body attempts to maintain its temperature. When the setpoint is raised, the result is a fever. Most fevers are caused by infectious disease and can be lowered, if desired, with antipyretic medications.
An organism at optimum temperature is considered afebrile or apyrexic, meaning "without fever". If temperature is raised, but the setpoint is not raised, then the result is hyperthermia.
Hyperthermia is an acute condition which occurs when the body produces or absorbs more heat than it can dissipate. It is usually caused by prolonged exposure to high temperatures. The heat-regulating mechanisms of the body eventually become overwhelmed and unable to deal effectively with the heat, causing the body temperature to climb uncontrollably. Hyperthermia at or above about 40 °C (104 °F) is a life-threatening medical emergency and requires immediate treatment. Common symptoms include headache, confusion, and fatigue. If sweating has resulted in dehydration, then the affected person may have dry, red skin.
In a medical setting, mild hyperthermia is commonly referred to as heat exhaustion or heat prostration; severe hyperthermia is called heat stroke. Heat stroke may come on suddenly, but it usually follows the untreated milder stages. Treatment involves cooling and rehydrating the body. This may be done through moving out of direct sunlight to a cooler and shaded environment, drinking water, removing clothing that might keep heat close to the body, or sitting in front of a fan. Bathing in tepid or cool water, or even just washing the face and other exposed areas of the skin, can be helpful.
With fever, the body's core temperature rises to a higher temperature through the action of the part of the brain that controls the body temperature; with hyperthermia, the body temperature is raised without the consent of the heat control centers.
In hypothermia, the body temperature drops below that required for normal metabolism and bodily functions. In humans, this is usually due to excessive exposure to cold air or water, but it can be deliberately induced as a medical treatment. Symptoms usually appear when the body's core temperature drops by 1-2 °C (1.8-3.6 °F) below normal temperature.
Basal body temperature is the lowest temperature attained by the body during rest (usually during sleep). It is generally measured immediately after awakening and before any physical activity has been undertaken, although the temperature measured at that time is somewhat higher than the true basal body temperature. In women, temperature differs at various points in the menstrual cycle, and this can be used for family planning.
Core temperature, also called core body temperature, is the operating temperature of an organism, specifically in deep structures of the body such as the liver, in comparison to temperatures of peripheral tissues. Core temperature is normally maintained within a narrow range so that essential enzymatic reactions can occur. Significant core temperature elevation (hyperthermia) or depression (hypothermia) that is prolonged for more than a brief period of time is incompatible with human life.
Temperature examination in the rectum is the traditional gold standard measurement used to estimate core temperature (oral temperature is affected by hot or cold drinks and mouth-breathing). Rectal temperature is expected to be approximately one Fahrenheit degree higher than an oral temperature taken on the same person at the same time. Ear thermometers measure eardrum temperature using infrared sensors. The blood supply to the tympanic membrane is shared with the brain. However, this method of measuring body temperature is not as accurate as rectal measurement and has a low sensitivity for fevers, missing three or four out of every ten fevers in children. Ear temperature measurement may be acceptable for observing trends in body temperature but is less useful in consistently identifying fevers.
Until recently, direct measurement of core body temperature required invasive insertion of a probe -- a procedure that is not clinically possible -- so a variety of indirect methods have commonly been used. While the rectal or vaginal temperature is generally considered to give the most accurate assessment of core body temperature, particularly in hypothermia, its recording is disliked by patients and medical staff alike. In the early 2000s, ingestible thermistors in capsule form were produced, allowing the temperature inside the digestive tract to be transmitted to an external receiver; one study found that these were comparable in accuracy to rectal temperature measurement.
The temperature reading depends on which part of the body is being measured. Measurements are commonly taken in the mouth, the ear, the anus, or the armpit. In females, the vagina can also be used. The median daytime temperature among healthy adults are as follows:
Normal human body temperature varies slightly from person to person and by the time of day. Consequently, each type of measurement has a range of normal temperatures. The range for normal human body temperatures, taken orally, is 36.8±0.7 °C (98.2±1.3 °F). This means that any oral temperature between 36.1 and 37.5 °C (96.9 and 99.5 °F) is likely to be normal.
Problems of various measurements
It is claimed that changes in core body temperature are reflected sooner and more accurately in the ear than at other sites.
Influencing factors on other areas where temperatures are taken:
There is a risk of injury from cracking the original glass thermometers if too much force is applied by the teeth to hold them in place and the alcohol or mercury contents are poisonous. This is avoided by the use of electronic thermometers which are made from solid plastic and use a metal (thermocouple) sensor.
A plastic thermometer strip placed on the forehead gives an approximate local reading, which depends to a great extent on ambient air temperature and local circulation effects. Using a thermometer to record the temperature under the armpit is less affected by surrounding air temperature, but is still prone to diverge from true core temperature if there are alterations in blood circulation.
Since the year 2000, small ear thermometers have become available and it is thought that the eardrum closely mirrors core temperature values. These work by detecting the infrared heat emission from the tympanic membrane and a measurement is quickly taken within one second making them popular for use with children. While the electronic display of the temperature value is easier to read than interpreting the graduation marks on a thermometer, there are some concerns for the accuracy of ear thermometers in home use.