Exposure to sun poses risk of skin cancer even in the dark, study finds

Damage to skin cells continues for hours
after spending time in the sun, according
to research that uncovers a new link
between sun exposure and cancer.
The discovery that some of the most
serious damage to skin cells may be
occurring in the dark raises the prospect of
new “evening after” lotions that would
help limit the effect and reduce the risk of
skin cancer.
Vitamin E was identified by scientists as a
promising candidate for helping to
“quench” the after-effects of a hot day at
the beach.
Unexpectedly, the source of the “dark
damage” was found to be melanin, the
pigment in skin cells that normally acts as
a shield against ultra-violet (UV) radiation.
Prof Douglas Brash, who led the research
at Yale University, said: “If you look inside
adult skin, melanin does act as a shield.
But it is doing both good and bad things.”
The latest work reveals that UV light
produces a cascade of chemical reactions,
including the production of a “super
bleach”, which reacts with melanin
causing one of its electrons to be “excited”.
The melanin then deposits its extra energy
in the surrounding tissue. If a strand of
DNA happens to be nearby, it can absorb
the energy causing the double helix strand
to bend, preventing the genetic code from
being read correctly.
“Melanin participates in the DNA-
damaging part of this reaction only for a
few minutes, maybe only a few
microseconds,” said Brash.
Exactly the same type of damage – called a
cyclobutane dimer (CPD) – is already
known to occur directly during sun
exposure as the UV rays hit DNA strands
and scramble the letters of the genetic
code into mutations. The more mutations
skin cells accumulate over time, the higher
the likelihood that one of them will turn
out to be cancerous.
However, until now, the damage was
thought to stop as soon as a person took
shelter in the shade.
The latest study, published in the journal
Science, monitored skin cells to reveal that
mutations carried on appearing for about
four hours after a session of sun exposure
equivalent to a “just perceptible” sunburn.
When the scientists investigated further
they discovered melanin’s crucial role in
this process.
In the study, the scientists showed that
potassium sorbate, a widely used food
preservative, was effective at blocking the
ongoing damage, although this isn’t “what
you’d want to put on your skin”, according
to Brash. Vitamin E, which was also
effective, would be a better candidate and
might explain why it is so effective in
suncreams and moisturisers. It was
previously thought that Vitamin E simply
worked by blocking UVB rays.
Áine McCarthy, of Cancer Research UK
said, “The discovery that UV radiation can
continue to harm our DNA hours after
exposure raises the possibility of
developing future products that might
reduce this ‘dark damage’. For now, the
best way to cut your risk of skin cancer is to
enjoy the sun safely and avoid sunbeds.”
About 100,000 people are diagnosed with
non-melanoma skin cancer each year and
about 13,500 with melanoma, which is
rarer but causes far more deaths.
The findings could help explain the
different risk factors for the two different
types of skin cancer – melanoma is
associated with brief intense sun
exposures, the kind that cause blistering
sunburn, whereas other skin cancers are
normally linked to long-term moderate
exposure. It is possible that more “dark
damage” may occur depending on whether
sunlight is absorbed over a longer time
period.