Recent investigations of temperature records from the 19th century to the present suggest that the average healthy adult now runs cooler than the classic 37 °C (98.6 °F) value that has guided clinicians for more than a century.
Historical benchmark and modern findings
In the late 1800s, German physician Carl Wunderlich collected more than one million armpit readings from roughly 25,000 patients of all ages and both sexes. From that massive dataset he calculated a mean of 37 °C for healthy adults, a figure that quickly became the universal reference for fever detection.
A decade‑old study led by infectious‑disease clinician Julie Parsonnet examined three large American datasets that together contain temperature measurements from nearly 190,000 individuals taken between 1862 and 2017. The analysis concluded that the contemporary normal for adults is closer to 36.4 °C (97.5 °F).
The downward trend is not confined to industrialised nations. Anthropologist Adrian Jaeggi reported that the Tsimané people of the Amazon rainforest, who averaged about 37 °C in the early 2000s, recorded a mean of 36.5 °C (97.7 °F) in 2018 after two decades of rapid socioeconomic change.
Potential causes of a cooler baseline
Several explanations have been offered for the observed cooling. Parsonnet points to overall improvements in public health: life expectancy has risen from roughly 38 years in Wunderlich’s era to nearly double today, and modern populations face far fewer chronic infections such as tuberculosis, dysentery and rheumatic fever. Fewer immune challenges mean less caloric expenditure, and calories are a source of body heat.
Jaeggi adds that better sanitation, widespread vaccination, the introduction of indoor toilets and the habit of wearing shoes have reduced parasitic infections like hookworm, further lowering the metabolic load.
Parsonnet also notes that most people now spend the majority of their lives in climate‑controlled environments, the so‑called thermal‑neutral zone, where the body does not need to generate extra heat to maintain core temperature.
Another line of reasoning focuses on measurement techniques. Medical historian Philip Mackowiak observed that the glass thermometers used by Wunderlich were calibrated about 2 °C higher than modern devices, which would inflate recorded values. Moreover, Wunderlich’s reliance on armpit measurements, rather than the oral readings common today, complicates direct comparisons.
Parsonnet’s team addressed the instrument‑bias concern by comparing temperature data from Union Army veterans measured between 1862 and 1930 with later datasets that used consistently calibrated thermometers. Their findings still showed higher temperatures among people born earlier, indicating that the decline is not solely an artifact of faulty equipment.
Factors that shape an individual’s baseline
Beyond long‑term trends, four physiological variables influence a person’s normal temperature. Older adults tend to run cooler because metabolic rate slows with age, according to infectious‑disease doctor Waleed Javaid. Women generally record higher readings than men, a pattern noted by Parsonnet without a clear mechanistic explanation. Body temperature follows a circadian rhythm, dipping in the early morning hours and peaking in the late afternoon, as described by rheumatologist Jonathan Hausmann. Finally, stature and body mass have opposite effects: taller individuals lose heat more readily across a larger surface area, while greater weight modestly raises temperature.
Experts caution against treating any single temperature value as a rigid diagnostic cutoff. As Jaeggi remarks, the difference between 36.99 °C and 37.01 °C is clinically insignificant, though extremes below 35 °C (96 °F) or above 40 °C (104 °F) still signal potential medical emergencies.
Steve Lopez is a Senior Editorial Columnist and Health & Public Policy reporter for News Raise. Steve focuses on healthcare advancements, medical technologies, and public health policies.
