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Two-photon fluorescence intensity images of cells
deep in the epidermis showing reactive oxygen species activity
following sunscreen application to the skin surface. Reactive
oxygen species can react with cellular components, leading to skin
damage and increasing the visible signs of aging.
UV filters (octylmethoxycinnamate, benzophenone-3
and octocrylene) widely used in sunscreens generate reactive
oxygen species (ROS) in skin when exposed to ultraviolet radiation,
augmenting the ROS that is naturally produced.
(Images by K. Hanson, UC Riverside)
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When sunscreen is applied on the skin, however,
special molecules – called UV filters – contained in the sunscreen,
cut down the amount of UV radiation that can penetrate the skin. Over
time, though, these filters penetrate into the skin below the surface
of the epidermis, the outermost layer of skin, leaving the body
vulnerable to UV radiation.
Led by Kerry M. Hanson, a senior research scientist
in the Department of Chemistry at UCR, the researchers report that
three UV filters (octylmethoxycinnamate, benzophenone-3 and
octocrylene), which are approved by the Food and Drug Administration
and widely used in sunscreens, generate ROS in skin themselves when
exposed to ultraviolet radiation, thus augmenting the ROS that is
naturally produced. The researchers note that the additional ROS are
generated only when the UV filters have penetrated into the skin and,
at the same time, sunscreen has not been reapplied to prevent
ultraviolet radiation from reaching these filters.
Study results will appear in an upcoming issue of
Free Radical Biology & Medicine. An advance copy of the paper
is available online on the journal's Website.
"Sunscreens do an excellent job protecting against
sunburn when used correctly," said Hanson, who works in the laboratory
of Christopher Bardeen, an assistant professor of chemistry at UCR. "This
means using a sunscreen with a high sun protection factor and applying
it uniformly on the skin. Our data show, however, that if coverage at
the skin surface is low, the UV filters in sunscreens that have
penetrated into the epidermis can potentially do more harm than good.
More advanced sunscreens that ensure that the UV-filters stay on the
skin surface are needed; such filters would reduce the level of
UV-induced ROS. Another solution may be to mix the UV-filters with
antioxidants since antioxidants have been shown to reduce UV-induced
ROS levels in the skin."
In their research, Hanson and colleagues used
epidermal model tissue and applied sunscreen to the surface to test
the effect of sunscreen penetration on ROS levels in the deep
epidermis. A two-photon fluorescence microscope allowed them to
visualize ROS generation occurring below the skin surface. The ROS
activity was detected using a probe molecule whose fluorescent
properties change upon exposure to ROS. On comparing images taken
before and after the skin was exposed to UV radiation, they found that
ROS generation in the skin increased after sunscreen penetration.
About 95 percent of the visible signs of aging are
associated with UV exposure. About 90 percent of a person's total
life-time UV exposure is obtained before the person is 18 years of
age. Only a few UV-filters are available that block "UV-A," the
wavelengths that penetrate more deeply into the skin, all the way into
the dermis where collagen exists.
"For now, the best advice is to use sunscreens and
re-apply them often – the Skin Cancer Foundation recommends every two
hours, and especially after sweating or swimming, which can wash away
sunscreen – to reduce the amount of UV radiation from getting through
to filters that have penetrated the skin," Bardeen said. "This, in
turn, would reduce ROS generation."
Next, the researchers plan to investigate the
effect of smog on ROS generation in the skin. |
