How much heat do you lose from your head?

Summary

More or Less – “How much heat do you lose from your head?”

Date: March 7, 2020
Host: Tim Harford (BBC Radio 4)

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Episode Overview

This episode investigates the widely held belief that people lose most of their body heat through their heads—frequently cited as “40 to 45%”—and examines where this number comes from, whether it's accurate, and what more modern research says. Drawing on expert interviews and historical studies (including some with troubling origins), the episode uncovers both the scientific and cultural roots behind the head-heat-loss claim and offers practical guidance for cold weather.

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Key Discussion Points & Insights

The Origin of the Myth (00:16 – 01:31)

• The episode opens with reports of dangerously low temperatures in parts of the US, leading Tim Harford to recall the old advice: wear a hat because “more than 40%” of your body heat is lost from your head.

• He notes that this advice appears in sources such as the US Army Field Guide.

  Quote: “You can lose 40 to 45% of body heat from an unprotected head.” – Tim Harford quoting the US Army Field Guide (00:52)

• Recent sources dispute this figure, citing a much lower percentage.

  Quote: “The head accounts for about 7% of the body surface area and the heat loss is fairly proportional to the amount of skin that's showing.” (01:05)

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Historical Research and Its Flaws (01:12 – 02:15)

• Tim interviews Dr. Thea Pretorius, a South African academic who researched the origin of the “head heat loss” statistic.

• The earliest studies on head heat loss were conducted in the Dachau concentration camp during WWII, using unethical methods that produced unreliable data.

  Quote: “They were also interested in the, the cooling of the back of the head. The rate of core cooling was 250% quicker than what they would expect... for just the back of the head in the water.” – Dr. Thea Pretorius (01:31)

• Tim and Dr. Pretorius agree the data is scientifically worthless due to its abhorrent methods.

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The 1950s Canadian Military Study (02:15 – 03:19)

• Post-war, Canadian researchers conducted studies for the military using volunteers in Arctic gear.

• Prof. Mike Tipton (University of Portsmouth) explains that, in these studies, with the head uncovered but the body insulated, up to 50% of heat was lost from the head.

  Quote: “When you've got insulation over much of the rest of the body preventing heat loss, then obviously the percentage of heat loss from the head is going to be high … it amounted to about half of the resting heat production of the body.” – Prof. Mike Tipton (02:40)

• Tim notes this scenario popularized the "half your heat is lost through the head" myth, even in places where it doesn't apply, like South Africa.

  Quote: “I remember growing up in South Africa … That was strange to us. There is not one South African that have a desire even to cover their heads outside if they have to warm up. Especially in the 1970s.” – Dr. Thea Pretorius (03:19)

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Modern Research: Thea Pretorius's Hypothermia Studies (03:44 – 05:22)

• In the 2000s, Dr. Pretorius moved to cold Manitoba, Canada, where the US Coast Guard funded studies on whether survival suits should protect the head.

• Volunteers were submerged in cool water with and without their heads underwater.

• Findings:

  • The head is about 7% of body surface area but loses heat faster than other areas.
  • Submerging the head increased heat loss by 11%, but caused a 30% greater drop in core temperature.

  Quote: “When the head was also in the water, you were losing 11% more heat. You've only got 7% more area. So the head is disproportionate, but not dramatically disproportionate.” – Tim Harford (05:10)

• The link between head heat loss and rapid core temperature drop remains somewhat mysterious.

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The Physiology: Why the Head Loses Heat (06:02 – 07:32)

• Prof. Tipton explains: In cold environments, the body reduces blood flow to extremities (fingers, toes) to preserve core/brain heat—this process leads to frostbite risk.

• The body does not restrict blood flow to the brain/head, so it continues to lose heat at a steady rate.

  Quote: “The body sacrificing the extremities to maintain brain and deep organ function.” – Prof. Mike Tipton (06:29)

  Quote: “The body does not constrict blood flow to the brain because you need it.” – Tim Harford (06:38)

  • Hence, with hands and feet shut off, the head remains one of the primary areas of heat loss—especially when the rest of the body is insulated.

• Tipton highlights that differences in research results often stem from methodology: whether people are naked, clothed, dry, or wet.

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Practical Advice: Should You Wear a Hat? (07:32 – 08:32)

• Both experts agree: The older studies are accurate for conditions where the body is wrapped up and the head is exposed. In cold situations, wearing a hat makes a noticeable difference.

• Dr. Pretorius shares a personal anecdote about wearing a "toque" (Canadian for winter hat) even in a hot tub during -15°C weather.

  Quote: “Don't underestimate the heat that you lose through the head, because it makes a difference.” – Dr. Thea Pretorius (08:02)

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Notable Quotes & Memorable Moments

• “You can lose 40 to 45% of body heat from an unprotected head.” – Quoting US Army Field Guide (00:52)
• “The head accounts for about 7% of the body surface area and the heat loss is fairly proportional to the amount of skin that's showing.” (01:05)
• “But it doesn't do that to the head.” – Tim Harford, discussing vasoconstriction (06:48)
• “Don't underestimate the heat that you lose through the head, because it makes a difference.” – Dr. Thea Pretorius (08:02)

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Segment Timestamps

• 00:16 – 01:31: Introduction of the "40% of heat lost from head" claim; origins in military advice
• 01:31 – 02:15: Dark origins: WWII Dachau experiments
• 02:15 – 03:19: 1950s Canadian military studies; myth spreads globally
• 03:44 – 05:22: Modern research by Dr. Pretorius; findings on heat loss and hypothermia
• 06:02 – 07:32: Prof. Tipton explains physiology and methodology influence on research
• 07:32 – 08:32: Practical takeaway—wear a hat in the cold

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Conclusion

Despite the persistent myth of “losing half your heat through your head,” the truth varies by situation. You lose a proportion of heat through your head relative to its surface area unless the rest of your body is well-insulated—in which case, the head may account for a greater share of heat loss. In practical terms: in cold climates or situations where your body is protected but your head is exposed, wearing a hat really does help keep you warm.

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“Don't underestimate the heat that you lose through the head, because it makes a difference.”
– Dr. Thea Pretorius (08:02)

Transcript

[A]

Hello and welcome to More or Less on the BBC World Service. We are your friendly guide to the numbers that explain our lives. And I'm Tim Harford. It's been chilly this week, hasn't it?

[B]

Dangerously cold temperatures are expected to impact millions of people this weekend across the northern United States.

[A]

It's going to be bitterly cold in the Northeast, so minus 22 in Caribou. That is rivaling pretty much anything that I've seen this month. And that raises an age old question. Should I wear a hat? How much heat do you lose from your head anyway? When I was growing up, I was told it was more than 40%. And the Cold Weather Survival Chapter of the US Army Field Guide.

[B]

Always keep your head covered. You can lose 40 to 45% of body heat from an unprotected head.

[A]

But is that view out of date? Because these days a spot of light googling produces a very different result.

[C]

The head accounts for about 7% of the body surface area and the heat loss is fairly proportional to the amount of skin that's showing.

[A]

That's a big difference. Which view's right? I spoke to a South African academic, Dr. Thea Pretorius, who's studied the history of the science of heat loss. She told me that the modern hunt for answers has started in a very grim place.

[B]

So it did start in the dark times of the Second World War and it was done in the concentration camp in Germany, Dachau. They would literally leave them in brutally cold water. I think the temperature was like 2 or 3 degrees C that they left them in to see how long it takes before they die. They were also interested in the, the cooling of the back of the head. The rate of core cooling was 250% quicker than what they would expect it for. Just the back of the head in the water.

[A]

You could lower people's core body temperature. If you cooled the back of their head, that was the case, you could

[B]

lower it much faster.

[A]

The methods were abhorrent and the data worthless. But questions about heat loss from the head persisted. Professor Mike Tipton of the University of Portsmouth says researchers in Canada tried to answer them in the 1950s.

[C]

Those experiments were done for the military and the people were wearing sort of Arctic warfare clothing.

[A]

And that sounds really warm. Arctic warfare sounds proper warm.

[C]

Yeah. I mean, it's going to be the kind of stuff that you'd go out on a really cold day because the question was how important is the head now? To also provide some equipment for us, a hat or something, some form of insulation and in that scenario, of course, when you've got insulation over much of the rest of the body preventing heat loss, then obviously the percentage of heat loss from the head is going to be high. And at minus 4 degrees Celsius, it amounted to about half of the resting heat production of the body.

[A]

And so the idea that half your heat is lost through your head slowly became popular wisdom, even. Even in a hot country where it made little sense.

[B]

I remember growing up in South Africa and my initial degree was in kinesiology. And we would read in our Northern American textbooks that we should cover our heads when we warm up. That was strange to us. There is not one South African that have a desire even to cover their heads outside if they have to warm up. Especially in the 1970s.

[A]

In the early 2000s, Thea Pretorius moved from sunny South Africa to the city of Winnipeg, Manitoba, in the middle of the Canadian prairie. What was it like, as someone from a warm country going and investigating chill in this incredibly cold place? Did you find it a bit bracing, the Canadian winter?

[B]

Yes, because in Manitoba, it's like minus 54 for some periods of the winter. It's extremely cold.

[A]

It was cold at work, too. The US Coast Guard had commissioned research on how much heat is lost from the head. They wanted to know whether it was worth designing life jackets and survival suits which kept people's heads out of the water. So Thea Pretorius and her colleagues began submerging volunteers. They'd do an hour in a cool tub wearing speedos with their heads out, then with their heads submerged, breathing through a tube. They repeated the experiments with the volunteers wrapped up in dry suits. The key finding, if the back of

[B]

the head is in the water, you cool faster and you become hypothermic faster. You've got less time to rescue.

[A]

The surface area of the head is just 7%, but she found it loses heat at a higher rate.

[B]

And the added heat loss was 11%, but the core temperature drop was about 30%.

[A]

So when the head was also in the water, you were losing 11% more heat. You've only got 7% more area. So the head is disproportionate, but not dramatically disproportionate.

[B]

But what was it doesn't explain the 39% that you lose in core temperature.

[A]

The connection between heat loss from the head and core temperature remains a mystery, but that part of Dr. Pretorius study is often ignored. Instead, her study has been cited to support the idea that we only lose around 10% of heat from our heads. But for Professor Mike Tipton, if you want to know whether to wear a hat when you're going outside on a cold day, that older 1950s study still stands. That's the one involving soldiers on dry land in Arctic warfare suits losing 40 to 45% of their heat through their heads.

[C]

They looked at the change in tissue insulation in the finger compared to the head. And as the. The temperature cooled, the tissue insulation of the finger increased by a factor of six because blood flow to the extremities was shut down. And that's the body's reaction to protect the deep body tissues by essentially sacrificing the extremities, vasoconstricting, decreasing blood flow to the extremities.

[A]

Is that why you get frostbite?

[C]

Yes, exactly. The body sacrificing the extremities to maintain brain and deep organ function.

[A]

But nobody gets frostbite of the brain.

[C]

No.

[A]

Because the body does not constrict blood flow to the brain because you need it.

[C]

Exactly. And when they measured the vasoconstriction or the heat loss from the head, it remained constant throughout.

[A]

So when it's cold, your body will cut off blood flow to the extremities to protect you from heat loss. But it doesn't do that to the head.

[C]

Yes. Which means, of course, that even in the scenario where you're naked in a cool environment, you're still going to be losing relatively more heat through your head than you would be through, for example, the same surface area of your hand because you've not vasoconstricted the head. I mean, often when you get scientists disagree or come up with different results, they'll use the sort of get out of jail free card of. Oh, well, it's differences in the methodology. Actually, in this case it is. It's a methodological difference that has produced the difference in the findings rather than a specifically physiological difference.

[A]

Yes.

[C]

I mean, the paper has subsequently been criticized for all kinds of different reasons, but actually, if you accept what they were doing, look, in a sort of common exposure scenario of people working, wearing quite a lot of clothing, but with the head exposed, it's perfectly reasonable piece of research and a perfectly reasonable finding.

[A]

Dr. Thea Pretorius has no argument with that. She remembers life in Canada and learning the word locals use for hats, toques.

[B]

We had a hot tub outside and we would still go into the hot tub when it was minus 15 degrees C. And I remember that we would still be so cold that we would wear toques while sitting in the 40 degrees sea water, the heads feel cold. So what I want to say is don't underestimate the heat that you lose through the head, because it makes a difference.

[C]

I thought Canada

[A]

was cold. Dr. Thea Pretorius speaking to me from her home back in South Africa. And thanks also to Professor Mike Tipton. And that is all we have time for on More or LESS this week. And if you have any questions about nonsensical numbers or suspect statistics, contact us at more or lessbc.co.uk. we will be back next week and until then, goodbye. More or Less was presented by Tim Harford and produced by Leonie Robertson and Lizzie McNeil.