Recent studies indicate that we've been cooling off since the 19th century when 98.6 degrees Fahrenheit was established as the average body temperature of humans.
For centuries we've believed that the normal body temperature of a human being is 98.6°F. This standard was set about 150 years ago by a German physician after analyzing a million temperatures from 25,000 patients, helping generations of parents determine the severity of their child's illness. However, recent research indicates that 98.6°F is no longer the average body temperature of Americans today. Researchers have determined that Americans' average, normal body temperature has dropped about 0.05 degrees Fahrenheit per decade, based on their birth year.
A research article titled Decreasing human body temperature in the United States since the Industrial Revolution, published in eLife last month, shows we've been cooling off since the 19th century when 98.6°F was established as the norm. In fact, not only has the average body temperature of Americans dropped since German physician Carl Reinhold August Wunderlich's study in 1851 established this number as the standard, but it has also dropped since the 1970s. Speaking to Chicago Tribune, Dr. Julie Parsonnet, one of the authors of the study and professor of medicine at Stanford University, said, "People are stuck on the 98.6 number, but that number has always been wrong. There’s never been a real number because people vary."
However, despite recent studies, there are still some unknowns when it comes to the continued decrease in body temperature. For their study, Dr. Parsonnet and her fellow researchers Myroslava Protsiv, Catherine Ley, Joanna Lankester, and Trevor Hastie the compared body temperature records from three time periods. They examined data from a study that recorded the body temperatures of Civil War veterans from the mid-1800s through 1930, the Centers for Disease Control's records from the 1970s, and the temperatures of patients who visited Stanford health clinics between 2007 and 2017.
A comparison of all this data indicated that the body temperature of men born in the 2000s is about 1.06°F lower than that of men born in the early 1800s. Meanwhile, the body temperature of women born in the 2000s is on average 0.58°F lower than that of women born in the 1890s. This shows that "it's not just an ancient change," said Parsonnet. These findings also disprove previous theories that advancements in thermometers or means of calculating research data explain the change in average body temperature over the years.
However, we are yet to determine what exactly is causing the continued decline or what this drop could mean moving forward, said Parsonnet. A plausible explanation at this point could be that the metabolic rates of Americans have slowed with people growing taller and heavier over the past decades. Another factor, according to Parsonnet, could be modern medicine's elimination of certain diseases, like syphilis, tuberculosis, and periodontal diseases. At the time the original 98.6-degree normal was established, a majority of the population would've been fighting these diseases and therefore experiencing inflammation and higher temperatures, she added.
Parsonnet emphasized the need to determine the exact cause of the continued decline in body temperature as it could have implications on lifespan. "We are having human cooling, and we don’t know what that means, but it’s good to know that it’s happening," she said. Meanwhile, doctors aren't too concerned with evidence of declining temperature. Dr. Edward Ward, an emergency medicine physician at Rush University Medical Center, stated that he doesn't give much weight to any established body temperature standard. Instead, he chooses to focus on the extremes, taking a temperature above 100°F as an indicator of fever and that below 94 degrees as a sign of hypothermia.
"It’s not surprising that there will be changes (in normal body temperature) since the Industrial Revolution. As an ER doctor, I’m looking for abnormalities," said Ward. He also pointed out that there's "a difference between having what is medically considered a fever and feeling feverish. If someone is normally 96 and then suddenly they're 99, they probably feel uncomfortable." Parsonnet stated that this approach further emphasizes that people should pay heed to how they feel and not simply go by the number on the thermometer.