Despite the widely-held belief that the normal body temperature is 98.6 ᵒF (37.0 ᵒC), it is becoming increasingly clear that the average body temperature among adults (at least in the U.S.) is actually lower than 98.6 ᵒF (37 ᵒC).

The concept of a single normal body temperature dates way back to the 1800’s, based on measuring axillary temperatures by mercury thermometers. ¹ However, a 2001 systematic literature review of 20 studies (1935-1998) of normal body temperature measured in adults found the following mean temperatures: oral 97.5 ᵒF (36.4 ᵒC), rectal 98.4 ᵒF (36.9 ᵒC), tympanic 97.7 ᵒF (36.5 ᵒC), and axillary 97.3 ᵒF (36.3 ᵒC ).  A British study involving >35,000 patients also found a lower mean oral temperature of 97.9 ᵒF (36.6 ᵒC). ² A 2020 US study of a cohort of >150,000 adults (2007-20017) found a mean oral temperature of 98.1 ᵒF (36.7 ᵒC) in men and 98.2 ᵒF (36.8 ᵒC) in women; these values were lower than that of an earlier cohort (1971-1975). ³

So is the discrepancy between the body temperature in 1800’s and the more recent era due to the differences in measurement techniques or the population? In other words, are we cooling off?

The weight of the evidence suggests that our bodies are cooling!³  The study of an 1860-1940 cohort—presumably using similar thermometer techniques —found a gradual drop in the mean temperature during that period alone. Since axillary temperature (accounting for some of the values in the earlier cohort) is about 1 ᵒC lower than that of oral temperature, the magnitude of the drop in mean temperatures over the past 150 years is likely higher that those reported. ³

Potential explanations for our cooling bodies over the past 2 centuries include reduction in the population level inflammation due to improved standard of living, sanitation, lower incidence of chronic infections. improved dental hygiene, and cooler ambient temperatures. ³

 

Fun Fact: Did you know that in 1851 Carl Wunderlich, a German physician, obtained millions of axillary temperatures from 25,000 patients in Leipzig and thereby established the standard body temperature of 98.6 ᵒF (37 ᵒC)? ᵒ

 

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References

1. Sund-Levander M, Forsberg C, Wahren LK. Normal oral, rectal, tympanic and axillary body temperature in adult men and women: a systematic literature review. Scan J Caring Sci 2002;16:122-128. https://www.ncbi.nlm.nih.gov/pubmed/12000664
2. Obermeyer Z, Samra JK, Mullainathan S. Individual differences in normal body temperature: longitudinal big data analysis of patient records. BMJ 2017;359:j5468. https://www.ncbi.nlm.nih.gov/pubmed/29237616
3. Protsiv M, Ley C, Lankester J, et al. Decreasing human body temperature in the United States since the industrial revolution. Human Biology and Medicine, Jan 7, 2020. DOI: 10.7554/eLife.49555. https://www.researchgate.net/publication/338433061_Decreasing_human_body_temperature_in_the_United_States_since_the_industrial_revolution