The normal human body temperature is generally considered to be 37°C, and any reading above this level is commonly regarded as a sign of fever.
The normal human body temperature is generally considered to be 37°C, and any reading above this level is commonly regarded as a sign of fever. However, new scientific research suggests that the average human body temperature has gradually declined over the past 150 years. According to the findings, the average temperature of a healthy adult today may be around 36.4°C, lower than the traditionally accepted 37°C. Scientists are now trying to understand why this change has occurred. Is it the result of improved health, changing lifestyles and modern conveniences, or were there shortcomings in the methods used to measure body temperature in the past?
In the 19th century, German physician Carl Wunderlich was investigating an important medical question: Was fever a disease in itself or a symptom of another illness? At the time, thermometers were not routinely used to measure body temperature. To find an answer, Wunderlich conducted research over approximately 15 years, measuring the body temperatures of 25,000 patients. The study included men and women of different ages, and more than one million temperature readings were recorded.
Based on this extensive research, he established 37°C (98.6°F) as the average body temperature of healthy adults. He suggested that a temperature above 38°C (100.4°F) could be considered a fever. His 37°C benchmark subsequently became widely accepted around the world as the standard for identifying normal body temperature and diagnosing fever. According to Philip Mikoyuk, a medical historian and infectious disease specialist affiliated with the University of Maryland, some researchers had reached similar conclusions before Wunderlich. However, the scale of the data he collected was so extensive that his benchmark became a global medical standard.
According to Julie Parsonnet, an infectious disease specialist at Stanford University, Wunderlich made several important discoveries about natural variations in human body temperature. His research found that women's body temperatures were slightly higher than men's, that temperature tended to decrease with age, and that it varied at different times of the day. However, subsequent studies have found that people today have lower average body temperatures than those who lived 150 years ago. For example, the average body temperature of men in the United States has fallen by approximately 0.59°C compared with the early 19th century. About a decade ago, Parsonnet and her team analysed three major datasets containing body temperature records for approximately 190,000 people in the United States between 1862 and 2017.
The study found that the average body temperature of adults today is approximately 97.5°F, or 36.39°C. Several subsequent studies have also supported this trend. Interestingly, the decline is not limited to developed countries. According to Adrian Jaeggi, a biological anthropologist at the University of Zurich in Switzerland, the average body temperature of members of the semi-nomadic Indigenous Tsimané population living in the Amazon rainforest was around 37°C in the early 2000s. By 2018, it had fallen to approximately 36.5°C. Researchers say the population experienced rapid changes in its lifestyle and socioeconomic conditions during this period.
Scientists have proposed several possible explanations for the decline in average human body temperature. Parsonnet believes that one of the main reasons is the overall improvement in human health. According to her, average life expectancy in Wunderlich's time was around 38 years, whereas today it has nearly doubled. In the past, people frequently suffered from diseases such as tuberculosis, serious dental infections, dysentery, rheumatic fever and bone infections. The immune system uses more energy to fight such illnesses, and this energy consumption also generates heat. When the body is constantly fighting infections and diseases, its energy requirements increase, potentially affecting body temperature.
Similarly, improvements in vaccination, sanitation and healthcare have positively affected the health of the Tsimané population in the Amazon rainforest. Researcher Adrian Jaeggi says the use of toilets and wearing shoes have also become more common in the area. These changes have reduced infections caused by parasites such as hookworms, which can enter the body when people walk barefoot on contaminated soil.
Experts also point to climate-control systems in modern homes as another important factor. At least 88 per cent of homes in the United States now have air conditioning, while a similar proportion of homes in the United Kingdom have central heating. According to Parsonnet, much of our lives are now spent in environments where the body does not need to expend as much energy to keep itself warm or cool. In scientific terms, this is known as the thermal neutral zone—an environment in which the body does not need to use additional energy to maintain its temperature.
Not all scientists agree that improved health and changing lifestyles are the only reasons for the decline in body temperature. Medical historian Philip Mikoyuk believes that inaccuracies in the thermometers used in the past may also account for some of the difference. He recalls examining a glass thermometer from Wunderlich's era and noticing that its measurement scale was approximately 2°C higher than that of modern thermometers. This could mean that readings obtained with the older instrument were higher than the actual temperature. According to Mikoyuk, measuring body temperature in those days was not as straightforward as it is today. Mercury-in-glass thermometers were approximately one foot long and had to be calibrated using fixed temperature points, such as boiling water and melting ice.
Patients also had to lie still for 15 to 20 minutes because the thermometers took considerable time to register a temperature. Another difference was the method of measurement. Wunderlich measured temperature by placing the thermometer in the armpit, whereas oral temperature measurement is also commonly used today. These differences make direct comparisons between measurements from the two periods difficult.
Experts say that body temperature is not constant and can be influenced by several factors, including age, sex, time of day and physical build. Age: Body temperature generally decreases as people grow older. According to Waleed Javaid, an infectious disease specialist affiliated with West Virginia University, metabolism slows with age and physical activity tends to decline. This affects energy consumption and heat production.
Sex: Women's body temperatures are generally slightly higher than men's. Parsonnet says this difference has also been observed among older women, although scientists have yet to fully understand the reason.
Time of day: Body temperature naturally fluctuates throughout the day. Research by Jonathan Hausmann, a rheumatologist affiliated with Boston Children's Hospital and Massachusetts General Hospital, found that body temperature is generally at its lowest between 3 a.m. and 5 a.m. and reaches its highest level between 4 p.m. and 6 p.m. One possible explanation is the increase in physical activity during the day.
Height and weight: Physical build also affects body temperature. According to Parsonnet, taller people have a relatively larger body surface area, which can increase heat loss. By contrast, higher body weight may cause a slight increase in body temperature.
Despite possible inaccuracies in older thermometers, Parsonnet and her team have defended their findings. They analysed historical body temperature records of soldiers who served in the American Civil War between 1862 and 1930. The analysis also found that people born earlier had higher average body temperatures than those born in later years. According to the researchers, because people born in different periods had their temperatures measured during the same era, it was possible to distinguish the effects of thermometer differences from those associated with age or year of birth.
The researchers also examined two additional datasets collected between 1971 and 2017, in which relatively modern and consistent thermometers were used. These results also showed a downward trend in body temperature. Following these findings, some authoritative medical reference sources have reconsidered the idea that 37°C should be regarded as the universally normal body temperature for every person. These include the renowned medical textbook Harrison's Principles of Internal Medicine and the online medical reference resource UpToDate.
However, experts say that no single number should be treated as a definitive measure of human health. A slight variation in body temperature does not necessarily indicate illness. According to researcher Adrian Jaeggi, there is no meaningful difference between 36.99°C and 37.01°C that would justify reaching entirely different conclusions about a person's health. However, a body temperature below 35°C or above 40°C in adults may indicate a serious medical problem and requires medical attention. The central message of the research is that normal body temperature is not the same for everyone and can change over time. Therefore, when assessing a person's health, doctors should consider other symptoms and the individual's overall medical condition rather than relying solely on a single thermometer reading.
