YOU COME inside from a day at the beach, step into the shower, then towel
yourself dry and smooth on after-sun lotion. If, like many a holiday-maker,
you鈥檝e spent just a bit too long in the sun, it鈥檚 not a comfortable experience.
The water feels scalding hot, the towel seems to be made of wire wool, and
applying lotion is like rubbing salt into an open wound. And if the evening gets
cool, don鈥檛 even think about slipping on a woollen cardigan.
According to Clifford Woolf, a neurobiologist at Harvard Medical School in
Boston, these unpleasant symptoms of sunburn are there for your own good. When
common sense fails to keep us out of the sun, the body resorts to pain to
prevent us from damaging ourselves further. But how exactly does this pain come
about after you鈥檝e finished roasting on the beach? What puts the burn into
sunburn?
鈥淪unburn isn鈥檛 a burn in the sense of heating skin to a noxious level,鈥� says
Woolf. At the heart of the matter lies the skin鈥檚 response to ultraviolet (UV)
light. As you slow-cook in the sun, your skin is zapped not only by visible
light and heat, but also by two types of UV radiation. UV-A, with wavelengths
just shorter than visible violet light, has been linked to skin cancer. But the
shorter-wavelength UV-B is most dangerous and causes sunburn鈥攐r UV burn as
it鈥檚 known in the trade. UV radiation triggers the production of highly reactive
oxygen free radicals in the skin, and these have to be mopped up before they
wreak too much havoc.
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Lobster look
At the first signs of damage the immune system leaps into action, releasing a
cocktail of chemicals to start the cleanup process. This inflammatory mixture,
which includes serotonin, prostaglandin-E2 and hydrogen ions, increases
blood flow to the areas under attack, delivering immune cells and chemicals to
where they are needed. This makes the skin look red, a bit like when you blush,
but only the areas that have been exposed. Unlike with cuts and sprains, where
the inflammatory response immobilises the injured area with widespread swelling
and stiffness until it has healed, the more localised response means you end up
with a distinct line where your swimsuit ends.
So it鈥檚 the body trying to sort out the damage already done that explains the
lobster look. But what about the pain that goes with it? This is all down to the
skin鈥檚 sensory nerve fibres, which come in two main flavours. A-fibres, which
respond to sensations such as touch, vibration, pressure, temperature or pain,
mostly have a very low threshold for activation. They fire off a signal at the
slightest provocation. C-fibres, on the other hand, have a much higher
activation threshold, and their response generally registers as pain.
Our reaction to something noxious鈥攁 hot pan, say鈥攃an be divided
into two phases, explains Woolf. A rapid, instantaneous pain, mediated by the
A-fibres, is followed by a slower but longer-lasting C-fibre pain. Each fibre
stretches from the spinal cord to the skin, where it branches to form a number
of sensory endings. Whether or not a pain signal is transmitted depends on
whether tiny channels in their membranes are open or closed, in most cases to
the influx of sodium ions. This is dictated, in turn, by the presence or absence
of potentially harmful stimuli.
Woolf鈥檚 group, in collaboration with Simon Tate at Glaxo Wellcome in
Stevenage near London, has studied one of these pain-specific channels, called
the tetrodotoxin-resistant (TTX-R) sodium channel, found mainly in the C-fibres.
Woolf鈥檚 team, and others, has discovered that the TTX-R channel is opened by,
among other things, hydrogen ions and prostaglandin-E2. This allows
sodium ions to flood into the C-fibres, kicking off a pain signal. The fibres
also release neuropeptides, which set off a cascade of reactions to draw more
blood and water into the affected areas鈥攚hich is why the frazzled skin is
not only red, but a tiny bit swollen too.
But the TTX-R channel is only half of the story. Two years ago, David Julius
and colleagues at the University of California in San Francisco identified a
channel in the C-fibres called the capsaicin receptor. Capsaicin is the stuff
that puts the 鈥渉ot鈥� in red hot chilli peppers, and, as the receptor鈥檚 name
implies, it responds to this spice. But the receptor鈥檚 main job, Julius has
shown, is to detect painful levels of heat. Whereas some channels open at low
temperatures, the capsaicin receptor snaps into action only at the temperatures
we register as painful, about 43 掳C to 45 掳C, says Julius.
Or at least that鈥檚 what normally happens. Like the TTX-R channel, the
capsaicin receptor can also be opened by the inflammatory cocktail. In sunburnt
skin the inflammatory signals cause the receptor to become sensitised, perhaps
by a slight tweaking of its shape or by a change in how it passes on its
signals. 鈥淚ts activation threshold could then be lowered to about 35 掳C鈥�,
says Julius, 鈥渟o a much cooler stimulus, such as a tepid shower, would be enough
to trigger a pain signal鈥�. He speculates that touch receptors might also become
sensitised, explaining the tenderness of sunburnt skin. The capsaicin receptor鈥檚
high activation threshold might also explain why you don鈥檛 feel any pain while
you鈥檙e actually in the sun: 鈥淚t is very unlikely the skin would get hot enough
to activate the capsaicin receptors directly,鈥� says Julius.
But while the hapless holiday-makers are nursing their sore skin, the natives
are turning golden brown. Bronzed bodies don鈥檛 usually burn because melanin, the
pigment in tanned and dark skin, prevents UV from penetrating further than the
top layer of skin cells. However, tanned skin may heat up more, because it
reflects less visible light. 鈥淭his reinforces the notion that the burn in
sunburn is not a heat burn, but an inflammatory burn,鈥� says Woolf.
But if you don鈥檛 fancy spending hours on a sunbed before hitting the beach,
help is at hand. According to Bernhard Przybilla and colleagues at the Ludwig
Maximilian University in Munich, vitamins C and E also reduce the sunburn
reaction. These vitamins are antioxidants, and Przybilla鈥檚 group believes they
soak up oxygen free radicals before they do too much harm鈥� another good
reason to eat plenty of salad in the summer. But if greens aren鈥檛 your thing,
Julius says, 鈥渁spirin can be quite effective.鈥�
Perhaps the best way to avoid any pain is to remember the 鈥渟hadow rule鈥�,
suggested by Thomas Downham from the Henry Ford Medical Center in Taylor,
Michigan. When your shadow is shorter than your height, he says, the intensity
of UV radiation from the sun is more likely to cause sunburn. If you plan to
enjoy the sun this summer, hiding in the shade when the shadows are short could
just stop you from feeling the burn.