The Globe May Be Warming, but People Are Cooler
June 15, 2024
By Carol A. Kemper, MD, FIDSA
Medical Director, Infection Prevention, El Camino Hospital, Palo Alto Medical Foundation
SOURCE: Ley C, Heath F, Hastie T, et al. Defining usual oral temperature ranges in outpatients using an unsupervised learning algorithm. JAMA Intern Med 2023;183:1128-1135.
What is normal? Patients like to tell me their normal temperature is “not normal.” Our idea of a normal temperature came from the work of a German physician who screened 25,000 patients and concluded in an 1868 publication that the normal human body temperature was 98.6°F. That number has stuck with us for 150 years. Are people now cooler?
Body temperature is highly variable, depending on the time of day, menstrual cycle, body weight and height, metabolism, hydration status, etc., not to mention illness and general health. This fascinating cross-sectional study assessed oral body temperatures for adult outpatient encounters in internal medicine and family practice clinics in a single large medical system from 2008-2017.
Primary diagnosis, medications, age, sex, height, weight, time of day, and day of the month were abstracted from the electronic medical record for each encounter. A filtering algorithm was used to remove encounters with a diagnosis possibly affecting body temperature — leaving only those clinical diagnoses unrelated to temperature.
A total of 618,306 adult patient encounters were recognized, and 36% were removed by the filtering algorithm for non-included diagnosis. That left 396,195 encounters for 126,705 patients. The mean age was 52.7 years, and 57.4% were female. This data set yielded a mean oral temperature of 97.95°F (36.64°C), with the central 95% of temperatures ranging from 96.17°F to 99.19°F (35.95°C to 37.33°C). Interestingly, the mean oral temperature never reached 98.6°F (37°C) for any of the subgroups examined.
Linear regression and linear mixed-effects modeling were applied separately to women and men, and identified the hour of the day, age, height, weight, and month as independent predictors of body temperature. During the day, body temperature peaked at approximately 4 p.m. and then rapidly decreased.
Surprisingly, men more consistently experienced a drop in temperature around 45 to 55 years of age, although both men and women got cooler as they aged. There was a slight increase in temperature with increasing weight in both sexes, and a slight decrease in temperature with increased height.
The authors suggest that the current “normal” body temperature is closer to 97.9°F and our previous concept of the body being warmer at 98.6°F should be “retired.” If you believe the original 1868 findings, then body temperatures appear to have been declining, possibly from improved general health, reductions in chronic infections, and improved dental health, to name a few reasons. The idea that humans have been cooling at a rate of -.05°F every decade for the last 150 years is fascinating.
What is normal? Patients like to tell me their normal temperature is “not normal.” Our idea of a normal temperature came from the work of a German physician who screened 25,000 patients and concluded in an 1868 publication that the normal human body temperature was 98.6°F. That number has stuck with us for 150 years. Are people now cooler?
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