# Why the Average Human Body Temperature Is Gradually Declining Scientific evidence suggests human physiology is cooling over the last 150 years. - **Source:** [Folha de S.Paulo](https://www.folha.uol.com.br) - **Published:** 2026-11-09 - **Topic:** [science](https://brifing.si/c/science) - **Original article:** - **On Brifing:** https://brifing.si/s/b696ebf9-eea4-4eb2-83b2-a05bce3a5ce3 ## The 37°C Standard In the 1860s, German physician Carl Wunderlich established 37°C as the standard human body temperature. He derived this figure from over one million axillary measurements taken from 25,000 patients. This benchmark became a universal diagnostic tool, defining fever as any reading exceeding 38°C for over a century of medical practice. ## Evidence of Cooling Recent research indicates that the average human body temperature has steadily declined since the 19th century. Studies analyzing data from 1862 to 2017 show that modern adults typically register closer to 36.4°C. This shift is observed across diverse populations, including indigenous groups experiencing rapid socioeconomic and environmental changes. ## The Health Hypothesis Experts suggest that improved public health is a primary driver of this cooling trend. Historically, chronic conditions like tuberculosis and osteomyelitis caused persistent inflammation, which consumes calories and generates heat. As modern medicine and sanitation reduced these systemic burdens, the body's baseline metabolic heat production has likely decreased significantly. ## Environmental Thermal Neutrality Modern living environments play a crucial role in regulating our internal temperature. With widespread access to air conditioning and central heating, humans spend more time in a thermal neutral zone. Consequently, the body exerts less energy to maintain homeostasis, resulting in lower overall heat production compared to previous generations. ## The Calibration Debate Some historians argue that the original 37°C figure was flawed due to instrument calibration errors. Philip Mackowiak noted that 19th-century mercury thermometers were often calibrated two degrees higher than modern devices. However, researchers like Julie Parsonnet maintain that the cooling trend persists even when controlling for these specific measurement discrepancies. ## Tsimané Population Study The Tsimané, a semi-nomadic indigenous community in the Amazon, provide a unique case study. Their average temperature dropped from 37°C to 36.5°C between 2000 and 2018. This rapid change correlates with increased access to modern healthcare, footwear, and sanitation, which significantly reduced the prevalence of parasitic infections and chronic inflammation. ## Methodological Challenges Comparing historical data with contemporary readings is complex due to varying measurement techniques. Wunderlich measured temperatures in the axilla, whereas modern clinical practice favors oral readings. Despite these differences, longitudinal studies confirm a downward trend by comparing individuals born in different eras while using consistent, modern calibration standards for recent data. ## Redefining Clinical Norms Major medical references, including 'Harrison's Principles of Internal Medicine', have begun to adjust their clinical guidance regarding normal temperature ranges. Experts emphasize that 37°C should not be viewed as a rigid threshold. Small variations are normal, and clinical focus should remain on significant deviations rather than minor fluctuations around the historical average. ## Clinical Significance While the average has shifted, extreme temperatures remain critical indicators of underlying health issues. Readings below 35°C or above 40°C in adults typically signal serious physiological distress. Understanding this long-term cooling trend helps clinicians better interpret patient data, moving away from a static, century-old reference point toward a more nuanced biological understanding. ## Sources covering this story - https://redir.folha.com.br/redir/online/emcimadahora/rss091/*https://www1.folha.uol.com.br/ciencia/2026/11/por-que-o-corpo-humano-pode-estar-esfriando.shtml — --- _Summary written by Brifing from the original reporting. Copyright in the original article belongs to its publisher; read the full piece at the link above._