The 45% of heat lost through the head came out of an arctic suit

The US Army survival manual, Field Manual 21-76, has been repeating since 1970 that you should cover your head because 40 to 45% of the body's heat escapes through it. The number came out of arctic trials the army itself ran, in the nineteen-fifties, and there the set-up matters more than the result: the volunteers were put into a survival suit of the kind that closes at the wrists and the neck, and taken out into the cold with one single part of the body bare, the head. Then it was measured where their heat went. It went out through the head, of course. That was the only exit anyone had left open.

From then on the figure travelled by itself. It jumped from the manual to the winter features, from the features to household wisdom and from there to the counter of the sports shops, and along the way nobody went back to ask about the suit.

Take the suit off and the number collapses. With those same volunteers in swimming trunks, the head would have accounted for around 10% of the loss, which is roughly what its size entitles it to: an adult's head is about that fraction of the skin's surface. That 45% was the measure of the gap the suit left open.

Rachel Vreeman and Aaron Carroll, two US physician-researchers, put this in writing in 2008 in the BMJ, a British medical journal, in a review of winter myths titled Festive medical myths. Their correction gave the sentence back its proper size: any uncovered area takes most of the loss when everything else is well wrapped. With the body covered and the head bare, a very high fraction can indeed go out that way. That describes your clothes, not your head.

Meanwhile the 45% goes on doing its work, and it is not harmless. Whoever believes it buys an expensive hat and goes out with an open jacket, convinced they have covered half the problem; or wraps up a child from head to toe except the hands and considers the head sorted. The myth puts the hat in the wrong place in the reasoning.

That leaves the part the myth gets right by accident, and it is worth understanding because it is the only part that is any use. The head is usually exactly what people leave uncovered when they go out. The face, moreover, is full of cold receptors, so the sensation arrives sooner and harder than on an equally exposed thigh. And when you are cold the body shuts down the blood supply to hands and feet long before it does so to the scalp, so the head keeps arriving warm at the surface and keeps handing that heat to the air. The hat works. It works because of the skin it covers.

Out of all this comes one instruction, and it fits into four steps you can do standing in the hallway, before opening the door.

First, count the skin you are actually leaving bare. Count the head if you have no hat, the neck if the coat does not close at the top, the wrists showing between sleeve and glove, the ankles between trousers and shoe, and the face, which is going to be uncovered anyway. What you are wearing open does not count as covered.

Second, start with the biggest patch on that list. If you are in a closed coat, thick trousers and boots, that patch will be the head and the neck, and the hat will do a great deal, exactly as in the trial. If your jacket is open, the biggest patch is your chest and no hat will fix it: zip up before you look for anything else.

Third, correct for wind. Moving air strips off the warm layer you have next to the skin and passes through porous fabrics, so on a windy morning an outer layer that blocks the wind is worth more than another jumper underneath. And check the openings — cuffs, collar, waist — because that is where the warm air you had already made escapes.

Fourth, correct for water. A garment wet with rain or sweat conducts heat far better than the same garment dry, however thick it is; if you are going to walk fast, leave with one layer fewer than your body asks for in the doorway and close it again when you stop.

The next time somebody throws the 45% at you, ask for the missing fact and ask how those volunteers were dressed.