from BBC Learning English. I'm Rob.

And I'm Georgina.

Brrr! It's freezing cold outside today,

Georgina! Make sure you

wrap up warm.

I'll put my woolly hat on. When I was

growing up I was told that you lose

half your body heat from your head.

Oh, don't believe that, Georgina!

It's just a popular myth - you know,

something people think is true

which actually isn't, 'like bulls get

angry when they see the colour red',

or 'goldfish only have a three-second memory'.

Oh... I thought red really did

make bulls angry! But you're right,

there is some disagreement over

the age-old question: should I wear

a hat when it's cold outside?

In this programme, we'll be asking

how much body heat we lose from

our head and discovering that

a simple answer isn't so easy to

find. But first, it's time for my quiz

question. And let's start by asking

someone who knows all about

surviving in the cold - the US army.

According to the 'Cold Weather

Survival' chapter of the US army

field guide, how much heat is lost in

the cold through an uncovered head?

Is it: a) 30 to 35%?, b) 40 to 45%?

or c) 50 to 55%?

You might say it's just a popular myth,

Rob, but I still think half your body heat

is lost from the head, so

I'll say c) 50 to 55%.

OK, Georgina - we'll come back

to that later. Anyway, whichever answer

is correct, the US army obviously

thinks a large percentage of body heat

escapes through the head.

But that may not be the whole picture.

Over the years, experiments to

measure body temperature in

the snowy wastelands of Canada

and Alaska have given wildly

different results - mostly because

of variations in the methods used,

for example, whether the

volunteer's head was covered or

not, and whether they were dry

or submerged in water.

So maybe the US army's view is

out of date. And here's some surprising

information that Tim Harford, presenter

of BBC World Service programme,

More or Less, found after a quick

search on Google:

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 human body's surface area

means the total area of skin on

its outer surfaces - that's the head,

chest - or torso, plus the arms and legs.

According to this view, heat

loss - meaning the total amount of

heat transferred away from something

through its surface, is proportional

to body surface area. In that case,

a 50% heat loss from the head - which

only makes up 7% of the body's

surface area - seems like an overestimation.

In the 1950s, other military experiments

were carried out in Canada on soldiers

wearing artic warfare clothing - the kind

of super-warm thermal clothes you

might wear in sub-zero temperatures -

but with nothing to cover their heads.

Here's professor of physiology,

Mike Tipton, taking up the story

with BBC World Service

programme, More or Less:

The question was: how important

is the head to also provide some

equipment, a hat or 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.

Here the soldiers bodies were

protected with insulation - thick

material used to stop heat from escaping.

Since their heads were exposed

to the cold, around half of their body

heat escaped that way when resting -

not moving or doing anything active.

And so the idea that half your body

heat is lost through the head slowly

become a popular myth.

But before you throw your woolly

hats in the bin, there's another

consideration to bear in mind;

one that concerns your core

temperature - that's the internal

temperature inside your body,

including the blood, heart,

and other vital organs.

When the head is allowed to

get cold and the body is well insulated,

the body's core temperature drops

rapidly due to the circulation of blood.

Wow! This question really is

blowing hot and cold - now I have no

idea how much heat is actually lost

from the head! Why don't you just

tell me the answer, Rob?

OK then. Well, in my quiz question

I asked how much heat the US army

guide says is lost through the head.

I guessed c) 50 to 55%, or

roughly one half. Was I right?

Well, you were warm, Georgina -

but not right. In fact the army field

guide says, b) 40 to 45% - but

as we've seen in this case,

cold facts are hard to come by.

Let's recap our vocabulary then,

because we're still not sure if

wearing a hat to keep warm is

just a popular myth - something

people believe to be true but

which actually is not.

It seems that heat loss - the

total heat transferred away from

something, is linked to the surface

area or total area of the body's

outer surfaces when exposed

to the cold.

But wearing insulation - material

used to stop heat from escaping,

may change the body's resting

temperature - its temperature

when not moving and at rest...

... and also affect your core

temperature - your body's internal

temperature, including the heart and blood.

That's all for this programme.

Remember to wrap up warm for the winter...

And maybe pop a woolly hat in

your pocket to wear, just in case!

See you again soon for more

trending topics and vocabulary

here at 6 Minute English.

Bye for now!

Bye!