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Science and Math

What is “Normal” Body Temperature?

“Never express yourself more clearly than you can think.” — Niels Bohr

I’ve been reading Ben Orlin’s new book, Math for English Majors. Ben always has a brilliantly philosophical way of looking at numbers and math, and it got me thinking about how we often misunderstand precision.

I’ve always been fascinated by the way we handle precision—how we assume that if we know one number exactly and another is only “about right,” the overall result is still only “about right.” It’s a reminder that even when we want precision, the world often delivers something a little fuzzier.

In his book Proofiness, Charles Seife tells a great joke about a guide at the natural history museum. The guide informs visitors that the T. rex skeleton is 65,000,038 years old. Naturally, someone asks, “Wow, how were they able to date it so precisely?” The guide replies, “Well, it was 65 million years old when I started working here, and that was 38 years ago.”

Like Ben, and most people in the U.S., I learned that normal body temperature is 98.6°F and a fever starts at 100.4°F. These numbers sound incredibly precise, right? But let’s think about where they come from.

Normal body temperature traces back to the work of the German physician Carl Reinhold August Wunderlich in the 19th century. He conducted an extensive study of body temperatures and set the baseline for what was considered normal at the time: 37°C.

But normal body temperature actually varies more than originally thought. The value 37°C can range slightly (typically between 36.1°C and 37.5°C) with a proper fever starting at 38°C. These numbers feel approximate, like “About 37°C.” But when they’re converted into Fahrenheit we get much more exact sounding 98.6°F (37°C) for normal body temperature and 100.4°F (38°C) for a fever.

As it turns out, the original Fahrenheit scale was not very precise. When Daniel Fahrenheit created the Fahrenheit scale in the early 1700s, 0°F was defined by using a mixture of ice, water, and ammonium chloride (a type of salt), water was 32°F, human body temperature was 96°F, and the boiling point of water was 212°F. At the time, thermometers were just being developed, and precise measurements were hard to come by. Without widespread access to reliable instruments, Fahrenheit needed a point that was easy for anyone to reproduce. Human body temperature, which remains fairly constant under normal circumstances, was a useful choice.

Math for English Majors P. 34.

It’s ironic that in our attempt to make things more precise, we’ve ended up with numbers that seem more exact than they are. What started as a rough “about 37°C” was converted into a more specific 98.6°F. And the irony doesn’t stop there—Fahrenheit’s original scale wasn’t all that precise. He was actually using human body temperature as a fixed point on his scale. So the Celsius measurement of “about 37°C” is actually more correct than the fake precise 98.6°F.