98.6°F No Longer the Average Human Body Temperature, New Study Claims

Researchers at Stanford University have found that average human body temperature has steadily decreased over the past two centuries, challenging a long-held standard established in the 19th century.

The study, led by Dr. Julie Parsonnet, analyzed nearly 200 years of data and found that the widely accepted average of 98.6°F (37°C) is no longer accurate.

Extensive data analysis

The team reviewed 618,306 oral temperature readings from adult patients collected between 2008 and 2017. Advanced computer algorithms were used to remove data influenced by illnesses, ensuring the accuracy of the analysis.

The findings reveal that normal body temperature ranges between 97.3°F to 98.2°F (36.2°C and 36.8°C), with an overall average of 97.9°F (36.6°C).

“Most people, including many doctors, still think everyone’s normal temperature is 98.6°F (37°C),” Parsonnet said. “In fact what’s normal depends on the person and the situation, and it’s rarely as high as 98.6°F (37°C).”

Temperatures decline over time.

The study also found that the average body temperature in the U.S. has dropped by at least 0.09°F (0.05°C) every decade since the 19th century.

Researchers attributed this decline to improved living conditions, better public health, and advancements in healthcare, which have reduced chronic infections and long-term health issues. “Physiologically, we are simply different from what we were in the past,” Parsonnet explained.

Rethinking diagnostic standards

Lead author Catherine Ley noted that previous studies relied on a fixed average of 98.6°F (37°C) as a cutoff point. “Instead of thinking about a temperature distribution, which is what the initial study showed, we’ve taken a mean of 98.6°F (37°C) and used it as a cutoff value,” Ley said.

The Stanford team argues that understanding individual temperature ranges could improve diagnostic accuracy.

The decline in body temperature has implications for diagnosing conditions like fever. Experts suggest that identifying deviations from a person’s normal temperature could help detect illness earlier. Slight changes may indicate the onset of the disease before other symptoms appear.

Factors influencing body temperature

Temperature naturally fluctuates throughout the day, influenced by factors such as age, sex, weight, height, and the time of day. Older adults often have slightly lower temperatures due to reduced muscle mass and slower metabolism.

In contrast, women typically have slightly higher temperatures because of hormonal changes, such as those during the menstrual cycle.

Persistent deviations outside the healthy range of body temperature, experts caution, may signal underlying health conditions, such as infections or metabolic disorders.

The findings challenge conventional medical practices that rely on the 98.6°F (37°C) average human body temperature as a diagnostic tool. Researchers say individualized temperature measurement could become a critical part of modern healthcare.