As the Climate Warms, Homo sapiens Cools: How’s That for Irony?
Updated: Feb 14
“Humanity has but three great enemies: fever, famine, and war; of these by far the greatest, by far the most terrible, is fever.”
Sir William Osler
“Fever itself is Nature's instrument.”
As you might imagine, new findings on what constitutes a normal body temperature would catch the eye of an infectious diseases physician, as evaluating patients with elevated temperatures, that is, fever, is a constant part of our practice. Thus when a publication appeared in the online journal eLife on January 7, 2020 entitled: Decreasing human body temperature in the United States since the Industrial Revolution, I was intrigued.
97.5F (36.6C) is the new normal. The study, led by Julie Parsonnet, an infectious diseases specialist at Stanford University, involved an analysis of body temperatures in three data sets. The first data set was from 23,710 Union Army veterans of the American Civil War, whose temperatures were measured between 1860 and 1940. The other data sets spanned 1971 to 1975 and 2007 to 2017. In total, her research team analyzed 677,423 temperature measurements. They concluded that on average, American body temperature has declined by 0.03C per decade over the past 150 years. And they propose that the mean body temperature should now be considered 97.5F (36.6C), not the commonly believed 98.6F (37C).
All kinds of questions were raised in critiques of their work about the accuracy and consistency of the temperature measurements during these three time periods, the methods used (oral, rectal, or axillary temperatures), time of day (temperature is lower in the morning), and representative age groups (in particular, the very young—who normally have somewhat higher temperatures, and the very old—temperatures usually run lower in the elderly).
It all started with Wunderlich. Despite the criticisms of the Parsonnet study, their conclusions were based on data from the largest number of people since the landmark study of the German physician Carl Wunderlich in 1851. His data, drawn from millions of axillary temperatures of 25,000 patients in Leipzig, established that 98.6F (37C) was the normal human body temperature.
Based on the results of a more contemporary study of body temperatures, I already considered Wunderlich’s conclusions outdated. That study, carried out by Philip Mackowiak, a leader in fever research, and his colleagues at the University of Maryland, involved 148 healthy men and women aged 18 through 40 years. The results of their study were published in 1992 in the Journal of the American Medical Association, in an article entitled “A Critical Appraisal of 98.6F, the upper limit of the normal body temperature, and other legacies of Carl Reinhold August Wunderlich.” They demonstrated that 96.8F (36.8C) was the average temperature of their subjects and that 99.9F (37.7C) was the upper limit of the normal temperature range.
So what? Why are the results of the Parsonnet study worthy of our attention? First and foremost, I believe, is because of their biological implications, that is, that the body temperature of our species has been steadily declining for more than a century. The mechanism underlying this phenomenon was unexplained by the Stanford investigators. They speculate that perhaps in the mid-19th century, people had higher temperatures because they were harboring low-grade infections—a common driver of elevated body temperature. Or they suggest the decrease in our normal body temperature is a reflection of changes over the past century and a half in our environment or of the foods we eat.
Clinical significance. Another reason the Parsonnet study is important is because of its clinical implications. As everyone would appreciate, an elevated body temperature—by Mackowiak’s definition, temperatures over 99.9 (37.7C)—deserves attention because it may signify a fever. But before such temperatures can technically be called a fever, the elevation must be caused by a resetting of the thermoregulatory center in the brain, usually by cytokines (proteins) released from activated cells of the immune system. (Elevated body temperatures that are called hyperthermia are seen in conditions such as heat stroke and hyperthyroidism in which cytokine levels are normal.)
Fever is a cardinal sign of many infectious diseases—a sign that all parents worry about when detected in their children, or for that matter, in themselves. For example, along with other vital signs (blood pressure, pulse, and respiratory rate), fever may be a tip-off of the diagnosis of sepsis, a medical emergency that kills nearly 270,000 thousand Americans per year (see the November 3 germgems post—Sepsis: What Everyone Needs to Know).
But it is also important for everyone to know that “fever is your friend.” (Some microbes can’t stand the heat, and some aspects of immunity work better at temperatures in the fever range). It’s not the fever, but rather it is the underlying cause—usually an infection—that Sir William Osler alluded to as one of our “three great enemies.” Amazingly, the 17th century English physician Thomas Sydenham understood that fever is “Nature’s instrument.” More contemporary studies have demonstrated that fever is a host defense mechanism.
Where’s the irony? While we don’t know why our body temperatures are cooling, we do know that our species plays a major role in warming the Earth by generating greenhouse gases resulting in climate change. The facts that the body temperature of Homo sapiens is getting cooler at the same time that our planet is warming is a supreme irony.