ARTICLE
There is increasing awareness that exposure of skin to UVA radiation
cannot be considered to be without risk.
The amount of UVA reaching the surface of the earth is approximately
20 times greater than that of UVB. Moreover, unlike UVB rays, UVA rays
are not attenuated by the ozone layer, they pass through clouds and glass
and are emitted at a constant rate throughout the day from the sunrise
to the sunset. Finally while 90% of UVB radiation is blocked by the stratum
corneum, over 50% of UVA radiation is capable of penetrating deep into
the cutaneous structures as far as the papillary and reticular dermis.
It has been shown that UVA induces damage in cellular DNA, which is
clearly different from that induced by UVB. Thus, although the UVB radiations
are directly absorbed by DNA, the UVA require photodynamic reactions in
which endogenous chromophores contribute to the formation of reactive
oxygen species liable to induce DNA alterations. In addition to these
effects there is more and more evidence that chronic exposure to UVA can
induce immunosuppression at the skin site.
Besides possible tumor induction, photoaging may be involved with UVA
exposure. UVA transmission to the dermal tissue is much greater than that
of UVB. Thus the connective tissue and cells that synthesize dermal matrix
proteins are targets for UVA radiation. The UVA generated reactive oxygen
species can cause cross links of proteins, inactivation of enzymes, cell
membrane alterations and release of proteases, collagenase and elastase
which clinically results in wrinkling, furrowing and sagging of the skin.
Moreover, as Pr. Kligman mentioned in one of his numerous papers on the
mechanisms involved in photoaging, "Predicting chronic effects of UV based
only on acute exposures is completely misleading. Even small amounts of
UVA received chronically is damaging".
As you will see during this symposium, recent studies have clearly demonstrated
that even small suberythemogenic amounts of UVA radiation can cause significant
chronic photodamage to human skin both at the epidermal and dermal level.
Whether the use of low concentration sunscreen in daily care products
can prevent all or part of these deleterious effects is a question we
will try to answer during this symposium.
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