Myth: We lose most heat from our heads

This week Kat Arney's been getting hot over, rather than under, the collar...

Kat -   If you're listening to the Naked Scientists here in the UK, you've probably noticed that it's getting chillier. The nights are drawing in, the heating is going on, and you're busy raiding the wardrobe for gloves, scarves and  - of course - a nice warm hat. That's because we've all been told that we lose anywhere between half and 90 per cent of our body heat through our heads - depending who you listen to. Well, it turns out you shouldn't listen to anyone telling you that, because it simply isn't true.

It's hard to pin down exactly where this myth arose, but apparently the US Army Field Manual claims that 40 to 45 per cent of body heat is lost through the head. This is probably the result of early military experiments exposing people to very cold temperatures, while wearing various warm and cosy garments. As you might expect if you're all bundled up with your head exposed, you're going to lose most heat up top. But it does make sense. You'll only be warmer with a hat on, if the rest of you is also wrapped up. If you're wandering about in the buff with nothing but a bobble hat on, you can expect some funny looks - and also that the hat won't keep you warm.

So just how much heat is actually lost through the head, compared to the rest of the body? Well, for a start we need to think about the shape - and more specifically, surface area to volume ratio. We know from physics that smaller and thinner things with a high surface area to volume ratio will lose heat more quickly than bigger rounder things. The head is more or less spherical, which is a kind of shape that retains heat well (unlike thinner, flappier bits like hands and feet), and it only makes up around 7 per cent of the surface area of the body. And just to complicate the issue, people have different amounts of hair on their heads, which also acts as an insulator. To try and figure out how this all fits together, a Canadian researcher called Thea Pretorius started dunking unlucky volunteers in chilly pools, with or without their heads sticking out into warmer air. She discovered that dunking the head in the water only increased heat loss by about 10 per cent, suggesting that heat loss from the head is roughly proportional to the rate at which it's lost from other parts of the body.

But there's a twist. She's also found that the rate at which the core temperature cools down is similar if the whole body (minus the head) is cooled down, or if only the head is cooled. Intriguingly, cooling the body triggers a shivering reaction, which helps to warm you up, while cooling just the head doesn't. So, while it's not true that we lose more heat through our heads, having a cold head does cool down the rest of your body - in fact, a cold head increases core cooling by nearly 40 per cent. So, it's not so much keeping the heat in by wearing a hat on your head that keeps you toasty, as stopping the cold air getting to your head that makes a difference. What's more, making sure you keep your feet and hands warm with socks and gloves will definitely make a difference, as these are thinner body parts that will lose heat more rapidly than something solid and spherical like the head. So, as the temperature drops this winter, use science - rather than hearsay - to keep you warm and co