Why Is 98.6ºF Our Normal Body Temperature?

The term normal body temperature refers to the core body temperature of an adult, healthy human being which is empirically found to be 37ºC or 98.6ºF. Though the normal core body temperature varies slightly from person to person and oral temperatures within 98.2ºF (+/- 0.8ºF) are considered normal. Moreover, it has been observed that the body temperature of every individual varies slightly during different times of the day. Assuming that an adult person sleeps during the night and works through the day; the body temperature is slightly lower in the morning and reaches its peak during afternoon and evening and then gradually goes down in the night. The variation is usually in the range of +/- 0.8ºF. The normal temperature of the same individual also shows the slight variation on two consecutive days.

The credit for first systematic record of normal human body temperature goes to German Physicist Carl Wunderlich in 1861. He has taken the temperature of a large number of people (some literature claim it to be 1 million) and found that 37ºC was the average oral temperature. In the Fahrenheit scale, this converts to 98.6ºF and makes the figure look more precise than intended. The widely accepted range of normal body temperature by medical science is between 97 ºF (36.1ºC) to 99ºF (37.2ºC) when measured orally. The temperatures below or above them are conditions of hypothermia and hyperthermia respectively. The health condition was arising out of wide variations viz. Below 96ºF and above 101ºF need clinical intervention and may be fatal at times if left untreated.

The temperature of the human body also varies from place to place in the body. The oral temperature commonly referred in every day parlance is taken by placing the thermometer under the tongue. This is accepted and most convenient way to take body temperature. The internal temperature of the body, which is slightly higher at 99.6ºF (37.6 ºC) is a more accurate representation since it is taken from internal cavities like rectum ( or even vaginal temperature in females) but is obviously not the convenient method for taking temperatures of adults. Skin temperature also referred as axillary temperature, taken by placing the thermometer in the armpits, shows a slightly lower readings of 97.7ºF (36.5ºC). Tympanic temperature, taken from eardrums, is also one of the less used methods of recording body temperature and shows higher readings slightly. There is an established general correlation between these temperatures taken from different body sites. The tympanic and rectal temperatures are slightly higher than oral temperatures while skin temperatures are shade lower.

The normal body temperatures show a slight variance with race, sex and age. Higher body metabolism results in higher temperatures. This accounts for higher temperature recorded in the late afternoon which is attributed to higher metabolic as well as the muscular activity. Similarly, as the age advances, the elderly people show lower body temperatures due to reduced metabolic activity.