BBC 6 Minute English 是 BBC Learning English 出品的英语学习节目。每周一期，每期约6分钟，两位主播围绕某个话题展开对话，非常适合英音爱好者模仿学习。来源：BBC，仅用于语言学习分享
Rob Hello. This is 6 Minute English from BBC Learning English. I’m Rob.
Georgina And I’m Georgina.
Rob Brrr! It’s freezing cold outside today, Georgina! Make sure you wrap up warm.
Georgina 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.
Rob 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’.
Georgina 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?
Rob 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%?
Georgina 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%.
Rob 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.
Georgina 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.
Rob So maybe the US army’s view is out of date.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:
Tim Harford
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.
Georgina 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.
Rob 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.
Georgina 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.
Rob Here’s professor of physiology, Mike Tipton, taking up the story with BBC World Service programme, More or Less:
Professor Mike Tipton
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.
Rob Here the soldiers’ bodies were protected with insulation – thick material used to stop heat from escaping.
Georgina 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.
Rob And so the idea that half your body heat is lost through the head slowly become a popular myth.
Georgina 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.
Rob 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.
Georgina 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?
Rob OK then. Well, in my quiz question I asked how much heat the US army guide says is lost through the head.
Georgina I guessed, c) 50 to 55%, or roughly one half. Was I right?
Rob 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.
Georgina 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.
Rob 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.
Georgina 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…
Rob …and also affect your core temperature – your body’s internal temperature, including the heart and blood.
Georgina That’s all for this programme. Remember to wrap up warm for the winter…
Rob 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!
Georgina Bye!
