Photo: Collected
A century and a half ago, the normal average human body temperature was higher than it is today. Various recent studies say this average has declined steadily over the past 150 years. But scientists are still investigating whether this is due to improvements in human health, changes in lifestyle, the ability to control the environment, or methodological errors in how temperature was measured in the past.
Extensive research on body temperature was begun by the German physician Carl Wunderlich in the 1860s. At the time, medicine faced a fundamental question about fever: was fever a separate disease, or a symptom of some other illness inside the body?
Thermometers were not yet in routine use. Against this backdrop, Wunderlich measured the body temperatures of men and women of various ages over 15 years. He collected underarm temperatures of 25,000 patients treated at a hospital in Leipzig, Germany; in all, the number of data points came to more than 1 million.
Analysing this data, Wunderlich set the average body temperature of a healthy adult at 37 degrees Celsius, or 98.6 degrees Fahrenheit. This value later became the internationally recognised standard for normal body temperature. By his reckoning, a temperature above 38 degrees Celsius, or 100.4 degrees Fahrenheit, was a sign of fever.
Other researchers before Wunderlich had reached almost the same conclusion about normal temperature. But it was the strength of his vast dataset that gave 37 degrees Celsius special weight in medicine. According to Philip Mackowiak, a medical historian and infectious disease specialist at the University of Maryland in the United States, the temperature Wunderlich set later gained special status as a universal diagnostic standard.
But recent research shows that the normal average body temperature has changed over the past 150 years. This raises the question: was Wunderlich’s original standard flawed, or has human body temperature really changed with modern lifestyles and changes in the environment?
Julie Parsonnet, an infectious disease specialist at Stanford University in the United States, said Wunderlich laid the foundation for many established ideas in medicine about normal fluctuations in body temperature. In his research he found that normal temperature varies with age, sex and time of day: women’s temperatures are higher than men’s, temperature falls with age, and it fluctuates at different times of the day.
Recent research, however, says that the average body temperature of people today is lower than that of people 150 years ago. In particular, the average temperature of American men today is about 0.59 degrees Celsius lower than that of American men in the early 19th century.
About a decade ago, Parsonnet and her research team analysed three large temperature datasets covering about 190,000 people in the United States. From this data, collected between 1862 and 2017, they found that the normal average body temperature of an adult is close to 97.5 degrees Fahrenheit, or 36.39 degrees Celsius.
According to Parsonnet, the new normal temperature for adults today is close to this level.
Several other studies carried out before and since have produced almost the same results. The downward trend in temperature has been seen not only in developed countries but also in developing countries and among indigenous populations.
Adrian Jaeggi, a biologist and anthropologist at the University of Zurich in Switzerland, said the average temperature of the Tsimane, a semi-nomadic indigenous community in the Amazon rainforest, was about 37 degrees Celsius in the early 2000s. After two decades of rapid socio-economic change, it fell to 36.5 degrees Celsius, or 97.7 degrees Fahrenheit, in 2018.
Scientists say there may be several reasons behind the fall in average temperature. Parsonnet believes the improvement in people’s overall health is one of the main ones. In Wunderlich’s time, average life expectancy was 38 years; it is now nearly double that. The prevalence of infectious diseases has fallen and living standards have risen.
Parsonnet said people in the past did not lead lives as healthy as today’s; diseases such as tuberculosis, serious dental disease, dysentery, rheumatic fever and osteomyelitis were widespread then. As she explains, the immune system burns a lot of calories fighting these diseases, and calorie burning is linked to heat production. So when the body carries a heavier burden of disease, heat production may also increase.
The effect of improved health has also been seen among the Tsimane. According to Jaeggi, their overall health has improved thanks to vaccination programmes and better sanitation. He said the use of toilets and the habit of wearing shoes have increased among them compared with 20 years ago; as a result, hookworm and other parasitic worm infections caused by walking barefoot on contaminated soil have also declined.
Thanks to modern technology, controlling indoor temperature is now easy as well. At least 88 per cent of homes in the United States have air conditioning, and in the United Kingdom a similar proportion of homes have central heating.
Parsonnet said modern people spend most of their lives in environments where the indoor temperature is controlled regardless of the weather outside. In medicine such an environment is called the ‘thermal neutral zone’, where the body does not have to spend extra energy to maintain normal temperature. In her view, without the extra work of regulating temperature, the need to produce energy falls, and so the body’s heat production may decline.
Philip Mackowiak, however, is not willing to accept improvements in health and lifestyle as the only explanation. In his view, errors in the instruments used to measure temperature in the past must also be taken into account. Mackowiak once had the chance to examine directly a glass thermometer used by Wunderlich. He found that, compared with modern thermometers, its calibration was set in such a way that it read about 2 degrees Celsius higher.
He therefore believes readings taken with that thermometer may have been 2 degrees Celsius higher than the actual temperature.
Mackowiak said measuring body temperature in those days was a complicated and time-consuming task. The mercury-filled glass thermometers were about a foot long. Before use, they had to be calibrated against fixed temperatures such as boiling water and melting ice.
Patients then had to lie still for 15 to 20 minutes, because it took quite a while for the temperature to register properly on the thermometer. According to Mackowiak, because of these limitations, Wunderlich’s average temperature may have contained errors from the outset. Moreover, Wunderlich measured underarm temperatures, whereas readings taken in the mouth are now more common. This makes it difficult to compare underarm temperatures of that era directly with today’s oral temperatures.
Research has identified the influence of age, sex, time of day, and height and weight in determining normal body temperature.
Age: Body temperature falls with age. As Waleed Javaid, an infectious disease specialist at West Virginia University, explains, the speed of metabolism slows with age, and physical activity also declines. As a result, the body uses less energy and produces less heat.
Sex: Women’s body temperatures are generally higher than men’s. Parsonnet said the menstrual cycle temporarily raises temperature; but even setting this effect aside, older women’s temperatures remain higher than men’s. Scientists are still not sure why this happens. In her view, there are differences in the natural temperature-regulating systems of women’s and men’s bodies.
Time of day: Body temperature follows the body’s daily biological rhythm, or circadian rhythm. A recent study by Jonathan Hausmann, a rheumatologist at Boston Children’s Hospital and Massachusetts General Hospital, found that temperature is lowest between 3am and 5am and peaks between 4pm and 6pm. This is thought to be due to increased physical activity during the day.
Height and weight: Height and weight also affect body temperature. As Parsonnet explains, greater height increases the body’s surface area, so more heat escapes and temperature falls. Greater weight, on the other hand, raises temperature slightly.
Parsonnet believes that although there may have been errors in the calibration of Wunderlich’s thermometer, that alone cannot explain the higher temperatures of people in the past. She and her team analysed temperature data on former soldiers of the US Union Army between 1862 and 1930. It showed that those born earlier had higher average temperatures than those born later.
Parsonnet said the researchers compared the temperatures of people born in different years measured at the same time, and also analysed temperatures of people born in the same year measured at different times. In this way, the effect of instrument calibration could be separated from the effects of age and birth year.
Their research found a link between birth year and average body temperature, meaning the difference did not arise simply because the measuring instruments changed. Parsonnet’s team later analysed two more modern datasets and got almost the same results. That data was collected between 1971 and 2017 using thermometers believed to have been calibrated to the same standard.
Researchers therefore believe real physical and environmental changes play a role in the fall in average temperature.
These recent findings have prompted calls to rethink the importance of the conventional standard for normal body temperature, 37 degrees Celsius or 98.6 degrees Fahrenheit. Two widely used and important medical references have already made changes on this point: Harrison’s Principles of Internal Medicine and ‘UpToDate’, the online medical database used by doctors.
Experts say, however, that knowing your own normal body temperature is useful, but too much weight should not be placed on a single number. In Adrian Jaeggi’s words, such temperature thresholds should not be thought of as strict dividing lines, because in practice there is no great difference between 36.99 and 37.01 degrees Celsius.
However, an abnormal fall or rise in temperature may indicate a serious physical problem. According to experts, an adult’s temperature falling below 35 degrees Celsius, or 96 degrees Fahrenheit, or rising above 40 degrees Celsius, or 104 degrees Fahrenheit, is generally of particular concern. Such temperatures may signal a serious problem inside the body.
The picture that has emerged from scientists’ research so far leaves less room to treat 37 degrees Celsius as the unchanging normal temperature of the human body. Instead, the need to assess normal temperature in light of age, sex, body build, daily biological rhythm, health status and changes in lifestyle is coming to the fore.
At a glance
According to the Parsonnet team’s original research paper published in the journal eLife on 7 January 2020, after adjusting for age, height and weight, average temperature fell at a rate of 0.03 degrees Celsius per birth decade; the study also included a dataset of 150,280 people from 2007 to 2017.
Nature reported in 2020 that another study of about 35,000 people in 2017 had found an average temperature of 36.6 degrees Celsius.
Sources
Background sources: eLife (PubMed Central), 7 January 2020; Nature, 2020
