It was a beautiful day – blue sky, sun shining – but I wasn’t fooled. It was Melbourne in the Spring, and I knew there would be an icy breeze. I put on a jumper, coat and scarf, and headed out to meet a friend. When I reached the cafe, I saw her basking in the sunlight, wearing only a T-shirt. Huh? “Aren’t you freezing?” “No,” she replied smugly, eyeing my woollen layers. “I’m black, I absorb more heat than you.” Huh? That can’t be right.
Do black people really absorb more heat than whites?
Pop this question into Google, and you’ll be insulted. But, aside from responders wincing at the derogatory descriptions of “black and white” skin tones, it’s a reasonable question. After all, when you wear a black coat or sit in a black car you get hotter than doing the same activity in white. So why would skin be any different? Well, unsurprisingly, it’s complicated.
Oft sited – amongst grandfathers and milk-bar ladies – as evidence that darker skin absorbs more light and therefore heat, than lighter skin is this anecdote: In places with hotter climates, such as Nigeria, you see darker skin compared to people from colder climates, such as the pale Britons. But, if darker skin did absorb more heat, wouldn’t it be better to have dark skin in Britain, so your skin could grasp what tiny light is left? Hm.
What makes skin darker?
Human skin colour is largely determined by a pigment called melanin, which is made by tiny melanin factories called melanosomes, that sit inside skin cells – called melanocytes. Melansomes make more melanin when they are larger and not clumped together. And overall, the more melanin they produce, the darker the skin.
But other pigments also affect skin colour – such as carotene, which makes our skin orange coloured. And hemoglobin, which transports oxygen throughout the body, can turn our skin reddish or blueish depending on whether there is oxygen in our blood.
How does skin absorb light?
Light moves in waves, which can be short and powerful, like X-rays, or long waves, such as microwaves. Today, we aren’t too concerned with just any light waves – we want the ones that make us feel hot, and these are infrared lightwaves. According to NASA, they are about the size of a pin head. And when they reach our skin, or the pavement or just about anything, they get absorbed – and the energy from the light is transformed into heat.
In general, darker objects absorb more infrared light than lighter objects, and become hotter because they are transforming more light into heat.
Does darker pigmented skin absorb more light than whiter skin?
According to anthropologist Nina Jablonski, “there is essentially no difference in absorption of infrared light between dark and light skin” (Annu. Rev. Anthropol. 2004: 33:585-623). Funny thing is – I’m not sure I believe her. The two papers she cited didn’t directly measure the amount of infrared light being absorbed on the skin (which these days can be measured using an instrument called a reflectance spetrophotometry). And from first principles I can’t see why darker pigments would be any different to darker coats or cars – in terms of the light wave absorption.
Perhaps, what Jablonski meant to say, is that if there is a difference in the light being absorbed in the skin, it’s not directly translating into extra body heat. Humans of all shades and sizes have the same core body temperature – 37 degrees Celsius. Through exercise and seasonal changes, this body temperature doesn’t change because humans can release heat by increasing blood flow and sweating. Studies have shown that the heart rate and sweat rate of African Americans and Europeans, matched for height and weight, is the same when they exercise. So if darker skin is absorbing more heat where is it going?
Possibly into the melanocytes. There is anecdotal evidence, along with medical records from the Korean War that darker individuals, with heavily pigmented skin, are more susceptible to frostbite than lighter individuals. Animal studies show that melanocytes are killed by freezing than other skin cells. So, it’s possible that while more light is being absorbed, the larger melanocytes are somehow using the heat, so it’s not getting transferred to the rest of the body – but this is a lot of speculation on my behalf.
So, darker skin might absorb more light waves than light skin– we don’t really know, but going from first principles, they probably do. However, this doesn’t mean these lucky individuals are hotter in the sun because the energy from absorbed the light waves isn’t converting into body heat. Where it is going – I’m not sure. But it does seem that this is no black and white issue.
If it’s not about heat, why is some skin darker than others?
There is a strong link between pigmentation and locations that receive high doses of UV radiation. The more UV radiation hitting an area, the darker the skin.
UV radiation – which is another form of light wave, that is shorter and more powerful than infrared – can penetrate the skin, causing sunburn, DNA damage, and skin cancer. Melanin acts as a natural barrier to UV radiation by absorbing the UV radiation, but then very effectively scatters them – so they don’t harm the DNA of skin cells below. Some academics believe that darker skin naturally confers a sun protection factor (SPF) of 10 – 15.
And why bother with whiter skin then?
It’s still up for debate. The most popular reason for why lighter skin is that it allows Vitamin D production. Vitamin D is needed to absorb calcium absorption and a deficiency can lead to weak bones and impaired locomotion. The production of this handy vitamin is triggered by UVB radiation. So, when people live in areas that have low doses of UV radiation, the last thing they need is melanin scattering it.