Doctors may have to rethink what the normal human body temperature is. It may not be 98.6 F or 37°C anymore, according to a recent study that suggests that body temperatures among Americans have dropped since the 1800s.

The body temperatures in both men and women have constantly decreased by 0.03°C per decade of birth. The substantive and continuing shift in body temperature provides a framework for understanding changes in human health and longevity over 157 years, says the study.

“Humans are changing physiologically over time: we are taller, fatter and cooler. We don’t know what this means. We do know that small changes in temperature –global climate change as an example — can have profound implications. Whether this is also true for humans is unknown,” Julie Parsonnet from Stanford University and the corresponding author of the study tells MEA WorldWide (MEAWW).

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In 1851, a German physician named Carl Reinhold August Wunderlich established the standard for normal human body temperature — 37°C or 98.6 °F — after measuring the body temperatures from 25,000 patients in Leipzig.

Over the years, however, studies have recorded a steady change in body temperature. For instance, a 2002 study gathered evidence from 27 other studies, showing a drop in the standard body temperatures. Further, a British study conducted on more than 35,000 patients found that the average oral temperature was 36.6°C — and not 37°C.

But scientists, including Dr. Parsonnet, were not sure what triggered this change. Before pinning down the reason, Dr Parsonnet and her team wanted to rule whether differences in body temperatures can stem from the method of obtaining temperature — armpits or oral — or the quality of thermometers were skewing the results.

Additionally, Dr. Parsonnet, who works on infectious diseases, had another reason to conduct this study. “I know from my research that some infectious diseases are disappearing from the population at a pretty dramatic rate. Infections that were normal in people in the 19th century are now largely gone. I wanted to figure out whether this had an impact on who we are physiologically,” she says.

To document the changing body temperatures over the years, the research team collected data from 1862 to 2017 — from temperatures of American civil war veterans in the 1800s to a more recent record. They saw that the temperatures have been steadily falling, based on the decade of birth, since the mid-19th century. They also saw that young people and women showed higher body temperatures.

The results suggest that a decline is body temperatures may be driven by longer lifespans. Back then, in the 19th century, the average life expectancy was 38. But with the advent of modern medicine, humans are a lot healthier, living up to an average of 79 years. A decrease in infections means that our bodies are less inflamed. This loss of inflammation is bringing our body temperatures down.

Dr. Parsonnet acknowledges that her study has shortcomings. The team could not check whether the temperatures of Civil War veterans — who fought from 1861 to 1865 — were accurate. “The thermometers were different and the doctors taking the temperatures may not have done an accurate job,” she says.

“But the temperatures of civil war veterans were similar to Dr. Wunderlich measurements. Also, the most recent temperatures were just done in California whereas the others were more national samples. There may be geographic differences that we can’t add to the equation,” she adds. “But our findings in California do match recent national surveys in Europe.”