Hey, 98.6, It’d Be Good to Have You Back Again!

Everyone knows that the normal human body temperature is 98.6 degrees Fahrenheit (37 degrees Celsius), but it’s really more complicated than that single number. This temperature number was originally established in the mid-1800s based on millions of temperature measurements on thousands of people. The canonical 98.6 number is actually an average value with the normal range really being 97.2-99.5 degrees Fahrenheit, a fact that most people don’t seem to be aware of. Furthermore, at the individual level our normal body temperature is not a static number. Our bodily temperature increases throughout the day, rises with increasing body mass index (BMI), can vary with ambient temperature, and decreases as you age. Still, the 98.6 number has served us well as a general and consistent average for over a hundred years across the wide spectrum of humans of all races and ages.

Interestingly, 21^st century studies indicate that the 98.6 number is no longer accurate. As far back as 2002, studies on contemporary human populations found the average human body temperature to be lower than 98.6 degrees. A recent large British study of over 35,000 individuals found the average temperature in that cohort to be only 97.9 degrees Fahrenheit (36.6 Celsius). These current deviations from the 98.6 number were mostly attributed to better and more precise modern measurements compared to what was available in the 1800s. However, a new study in the journal eLIFE challenges this assertion and makes a strong case that the average human body temperature has actually decreased over the last 150 years. This comprehensive study looked at recorded temperatures in three large cohorts from the years 1860-2017. The combined cohorts contained roughly 190,000 individuals and nearly 700,000 body temperature measurements. They found a consistent, slow decrease in human temperature over that period that was not attributable to any increase in measurement accuracy.  Based on these data, the average temperature for men is now 97.5 degrees and for women it is 98.0 degrees. Although the explanation for this decrease is unknown, the authors speculate that it may reflect decreased levels of inflammation in modern humans. Our ancestors of even 100 years ago likely had much higher rates of chronic infections in the general population from parasites, tuberculosis, malaria, gingivitis, and other viral and bacterial infections. Even low level infections raise inflammation which subsequently raises body temperature, with the combined effect possibly large enough to raise the population’s average temperature slightly. Alternatively, it may be as simple as the fact that most of us live and work in conditioned spaces with heating and cooling. Since we are mostly inside and not at the mercy of outside ambient temperatures all day, our bodies may be less influenced by our environment than they were before universal heating and cooling.

While the above decrease is small, it can have some important ramifications. Everyone should be aware of their individual normal temperature and the normal range of personal variation, as deviations from this pattern can reflect subtle disease issues. Also, as more and more of us record our daily biometric information through phones, Fitbits, smartwatches, and other devices, there may be the opportunity to collect, aggregate, and analyze human temperature data in conjunction with other health information. Such a large data pool could be mined to detect unknown correlations and to look for predictive effects on health, disease, and longevity. So although it would be nice to have the simple 98.6 number back again, maybe our new appreciation of the complexity of human temperature will actually be a medical boon in the future.