For more than a century, 37 degrees Celsius has been regarded as the average body temperature of a healthy person. However, data from more recent studies suggest that people today may be slightly cooler than their predecessors.
The foundation for the classic figure was laid in the 1860s by the German physician Carl Wunderlich. Over 15 years, he collected the temperatures of 25,000 patients and accumulated more than one million measurements. Based on them, he identified 37 degrees as the average temperature for healthy adults, while accepting a temperature above 38 degrees as a sign of fever.
Today’s Average Is Lower
Julie Parsonnet’s team at Stanford University analyzed three large datasets containing repeated measurements from nearly 190,000 people in the United States. The data cover the period from 1862 to 2017. The results show that American men today have an average body temperature about 0.59 degrees Celsius lower than men in the early 19th century.
A similar change has been observed outside developed countries as well. The average temperature of the Tsimane—a semi-nomadic Indigenous population in the Amazon rainforest—was around 37 degrees in the early 2000s. By 2018, it was already around 36.5 degrees.
Better Health May Be Part of the Explanation
Parsonnet links the lower readings to improved health. During Wunderlich’s time, average life expectancy was 38 years, whereas today it is almost twice as high. Infections such as tuberculosis, dysentery, rheumatic fever and osteomyelitis were more widespread.
When the immune system fights infections, it uses more energy, which also increases heat production. Among the Tsimane, vaccines, improved hygiene, the use of toilets and wearing shoes have limited some infections, including those associated with walking barefoot on contaminated soil.
Modern lifestyles may also play a role. Air conditioning is available in at least 88 percent of homes in the United States, while a similar share of homes in the United Kingdom have central heating. According to Parsonnet, people spend much of their lives in a temperature zone in which the body does not have to actively produce heat to maintain a constant temperature.
How Reliable Is the Comparison with the 19th Century?
Medical historian and infectious disease specialist Philip Mackowiak points to another possible explanation—old thermometers. When examining a glass thermometer used during Wunderlich’s time, he found that the device had been calibrated approximately 2 degrees higher than modern thermometers.
Measurements were more difficult at the time: mercury thermometers were about 30 centimeters long, and patients had to lie still for between 15 and 20 minutes. In addition, Wunderlich measured temperature in the armpit, whereas oral measurements are more commonly used today. This makes a direct comparison difficult.
Parsonnet’s team, however, also found a decline in more recent data collected between 1971 and 2017 using thermometers expected to have similar calibration. According to scientists, body temperature is also influenced by age, sex, time of day, height and weight.
This does not mean that 37 degrees is now an incorrect value for every person. Adrian Jaegi warns that temperature ranges are not sharply separated. Readings below 35 degrees or above 40 degrees in adults, however, may be a sign of a more serious problem.
Sources: bbc.com