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The reactions
of human skin to UVA radiation (330-400 nm)
Sakamaki, Takeshi1,
Tian, Wei Dong1, Moyal, Dominique2,
Chardon, Alain2 and Kollias, Nikiforos1
Wellman Laboratories of Photomedicine, Massachusetts General Hospital,
Harvard Medical School, Boston MA1
L'Oreal Research, Clichy, France2
Abstract-
The pigment responses of human skin to broad band UVA radiation include
immediate pigment darkening (IPD) and at doses exceeding tenfold the
IPD threshold dose an immediate pigment forms that persists for days
to weeks termed persistent pigment darkening (PPD). We studied the immediate
pigment reaction and the persistent pigment reaction (PPD) at various
times after exposure and at threshold and superthreshold levels. Studies
were conducted (a) to determine the threshold UVA fluences to produce
IPD and PPD, (b) the dependence of the threshold fluence and supethreshold
pigment on fluence rate for both IPD and PPD, (c) the dependence of
IPD and PPD on dose fractionation and (d) the dependence of the PPD
response on wavelength. Twelve subjects were recruited for each study
and gave written informed consent. The light source was a filtered Xenon
arc with emission 330-400 nm. The threshold fluence for IPD was 0.7plusmn:]0.3
J/cm2 and for PPD was 10.9plusmn:]3.4 J/cm2 .
Superthreshold reactions ( >30 J/cm2 ) included long lasting
erythema for 80% of the volunteers tested. The threshold fluence for
IPD induction increased with fluence rate while the threshold fluence
for PPD induction was constant with fluence rate. The intensity of persistent
pigmentation produced at superthreshold fluences was greatest for the
lowest fluences and decreased in intensity with increasing fluence rate.
Dose fractionation at superthreshold fluences produced lower intensity
of pigmentation than the intensity of pigment produced with a single
uninterrupted dose. The results on IPD threshold fluence agree with
earlier studies. The threshold fluence to produce pigmentation that
persists for more than 24 hours was found to be stable for times greater
than 1.5 hours after exposure. For a great number of volunteers, exposure
to UVA at superthreshold doses for PPD, resulted in a mild erythema
response often confused with pigment that persists for weeks. The PPD
threshold fluence was found to increase slowly with wavelength in the
UVA. It is curious that while UVA effects are believed to be oxygen
dependent the threshold PPD fluence is not dependent on fluence rate
i.e. reciprocity holds.
Keywords: UVA,
pigmentation, skin responses, IPD, PPD
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