Fever part 1: What is it?

The normal human body temperature is thought to average 98.6 F. We all remember this from elementary school, and I’ll bet many of you remember the old glass thermometers, the one your mom used with the little red line right at 98.6. If you could top that, you got to stay home! But, like so many things we were told in elementary school, the truth is more complex and murky.

The classic 98.6 is based on a book by Carl Reinhold (sometimes his name is given as “Carl Reinhold August Wunderlich,” a wonderful name that I hope comes up in casual conversation this week). In 1868 he published The Course of Temperature in Diseases, in which he hand-calculated the averages of about a million measurements in 25,000 patients, coming up with 37 C (=96.8 F). He also declared that based on his observations, 38 C (100.4 F) was the upper limit of the normal temperature, essentially defining “fever” for the first time. It turns out that his thermometers weren’t calibrated very well, and were probably off by at least 1 or 2 degrees, but he gave it a good try—especially considering that his thermometer was a foot long, and took twenty minutes to register a stable measurement. (I don’t know where he put that in his patients, and I’m not sure I want to know.) More recent research pegs the average temperature at 98.2 F, but even this varies at least one degree between individuals. One’s own temperature can also vary at least one degree based on the time of day (normal temperatures are lowest first thing in the morning, unless you’re ovulating.) There is also evidence that carefully measured temperature averages vary between human races and genders (women tend to run hotter than men—no surprise there.) Thought it’s not technically correct, 98.6 F (37 C) still remains widely accepted as the “normal” human body temperature for everyone at any time.

If 98.6 F is the traditional (though inexact) definition of “normal”, then what’s a fever? There isn’t a universal definition. Most pediatricians consider a rectal temperature above 100.4 to be a fever; in adults, the number 100 is more often used, usually referring to an oral temperature (though in the elderly, normal “resting” temperatures may considerably lower than 98.2 F). Measuring rectal temperatures becomes more difficult past a few months of life, so often an armpit, oral, or forehead temperature is measured. To be clear in communicating with your pediatrician, say the number that the device recorded, followed by the method you took it: “Junior was 100.8 degrees measured orally.” Don’t add or subtract degrees to “correct” the temperature, just tell us what the number is. In most cases outside of the newborn period, the exact number is not actually very important, but we do like to have a general idea of how high the fever was.

Fever occurs in children most commonly from infections, but can be a result of many other rarer problems (such as adverse reactions to medicines, inflammatory arthritis, cancer, and thyroid disease). Fever can also occur as part of “heat stroke,” when dehydration combined with exposure to heat overwhelms the body’s capacity to control its temperature. Victims of heat stroke feel warm and dry—not sweaty—and are often delirious or sleepy.  This is a true medical emergency that can lead to kidney failure, brain damage, and death. It’s the only health condition where fever itself contributes to harm.

This is the first post in a little series I’m writing on fevers. In future posts, we’ll explore what fevers are for, why parents don’t need to fear fevers, and a super-simple “pediatric insider” action plan for parents to follow when their child runs a fever. Stick around!

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Tags: child, fever, temperature, thermometer

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