YSM Issue 93.2

THE NEW 98.6 DEGREES
How and why human body temperature has lowered
COUNTERCOUNTER
BY
KELLY
FARLEY
KELLY
POINT
IMAGE COURTESY OF ROY PERRY
When you go to the doctor, the first measurement
taken—whether it’s an annual check-up or a sick
call—is your body temperature. Over the years, the
method has varied, from mercury thermometers under the
armpit to infrared thermometers scanned over the forehead. But
the standard indicator of health has remained the same since
1851, when it was first reported by the German physician Carl
Reinhold August Wunderlich. As you’ve probably heard before,
the normal human body temperature is 98.6 degrees Fahrenheit.
Julie Parsonnet and her colleagues at Stanford, however, have found
evidence that disrupts this paradigm: they recently reported that that
human body temperature has steadily decreased over the past two
centuries to a current average of 97.9 degrees Fahrenheit. In the most
comprehensive analysis to date, their study examined hundreds of
thousands of temperature measurements from three databases ranging
from the end of the Industrial Revolution to present day. In the end,
they found a constant decrease in temperature from decade to decade.
Parsonnet’s study is important not because it shows that human
body temperature is lower but because it shows that it has dropped
since the nineteenth century. Medical professionals have known for
the past few decades that healthy human body temperature is lower
than the 98.6 degrees Fahrenheit standard. A Russian pharmacy
chain founded in 1991 is even named 36.6—the Celsius equivalent
to 97.9 degrees Fahrenheit—in honor of the more accurate lower
temperature. Scientists previously assumed that 98.6 degrees
Fahrenheit must have been wrong at the time of measurement
as well, blaming differences in historical measurement methods
and instrument calibration. According to Parsonnet, the original
number may not have been wrong at all. It is just no longer accurate
for modern humans. The question is: why?
“Temperature is a marker of metabolism,” Parsonnet said. With
lower body temperatures, our metabolism must be slower. Perhaps
this change is caused by our more temperate environments.
With heating and air conditioning, modern Americans live
in the “thermoneutral zone” of sixty-four to seventy-two
degrees Fahrenheit in which our bodies do not have to increase
metabolism to keep warm. With inactive modes of transport and
sedentary desk jobs, we also move less, further suppressing our
metabolic rate and possibly explaining the rise in obesity.
Parsonnet prefers an alternate explanation: our cleaner
environments. Thanks to sewage systems, hand sanitizer, antibiotics,
and modern infrastructure, we have decreased the rates of formerly
widespread infections, such as syphilis, rheumatic heart disease, and
tuberculosis. Thanks to vaccination, we have minimized infectious
diseases of the past and have hopes to apply the same to infectious
diseases of the present. All of this leads to decreased inflammation,
which in turn leads to lower metabolism and thus a lower body
temperature. It is uncertain whether this decreased inflammation is
true everywhere; the modern temperatures in the study were collected
in the United States. Moving forward, Parsonnet is interested in
examining temperatures in developing countries as well.
If the “normal” human body temperature is lower, does this
challenge the way we approach fever and medical diagnosis?
Though taking your temperature may be the first thing your doctor
does, it is never the last. Think about the family history, blood
pressure readings, throat swabs, blood draws, and everything else
that goes into a sick call. We are drawn towards binaries: sick versus
healthy, feverish versus normal. But temperature varies from
person to person and even within a person over the course of a day.
The real question is not why our 98.6 degrees Fahrenheit
standard was wrong but rather why we rely on it so heavily
when every person is an individual. There is no overall “normal”
human standard. Instead, “There is a ‘normal’ for each person
that depends on their age, sex, weight, height, and the time
of day their temperature was measured,” Parsonnet said. Her
team is already working on an algorithm that determines what
is abnormal for an individual patient at any particular time. In
the age of big data and personalized medicine, we are beginning
to see patients as individuals instead of averages. Today, we are
surprised that the 98.6 degrees Fahrenheit standard has been
replaced by 97.9 degrees Fahrenheit. In the future, we may be
surprised that we relied on an average at all. ■
Barondess, J. (2014). Scanning the Chronic Disease Terrain: Prospects
and Opportunity. Transactions of the American Clinical and Climatology
Association, 125(2014), 45-56.
Fischer, K. (2020, January 20). Forget 98.6 degrees Fahrenheit. Humans Are
Cooling Off — Here’s Why. Healthline.
Protsiv, M., Ley, C., Lankester, J., Hastie, T., & Parsonnet, J. (2020). Decreasing
Human Body Temperature in the United States Since the Industrial
Revolution. eLife 9e49555.
28 Yale Scientific Magazine September 2020 www.yalescientific.org