UVA Protection: An
Update
Joe Stanfield
Suncare Research
Laboratories, LLC
December, 2007
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The longer
ultraviolet wavelengths in sunlight are known as UVA radiation (320-400 nm).
Until a few years ago, the shorter wavelengths, known as UVB (290-320 nm),
were considered the most important contributor to skin damage resulting from
excessive sun exposure. New studies have provided evidence of greater UVA
involvement in skin tumor development, suppression of immune function and
premature aging, than previously realized. [1] UVA
comprises 90-95% of terrestrial radiation, and penetrates deeply into the
dermis, whereas the shorter UVB wavelengths, are up to a thousand times more
effective in producing sunburn, but penetrate the more superficial epidermis
only. UVB is associated primarily with direct effects on cellular DNA, while
UVA is associated with indirect damage to the cells through production of
free radicals. [2] Sunlight is always a mixture of UVA
and UVB, and laboratory tests that separate the effects of UVA from those of
UVB can distort the relative importance of UVA protection. Perhaps the most
important reason for "balanced" UVA and UVB protection is to assure
that sunscreen products protect over the broad range of solar spectra that
occur with varying sun angles due to latitude, season and time of day. Marketed
sunscreen products offer vastly improved protection against solar UVA, yet
the FDA has not accepted a method for quantifying UVA protection. Proposed
methods include measurement of sunscreen critical wavelength, the persistent
pigment darkening test, the Boots Star Rating and the UVA Index, which was
recently developed by European investigators. Each has its own advantages and
disadvantages. The FDA was expected to select a method and open a public
comment period, late in 2006. The selected method will probably be based on
one or more of those listed above. The
Critical Wavelength method proposed by Diffey [3] requires mathematical integration of the in vitro
product absorbance spectrum from 290 to 400 nm to determine the wavelength
below which 90 percent of the cumulative area of the absorbance curve
resides. If that wavelength is 370 nm or greater, the product is considered
"broad spectrum," which denotes balanced protection throughout the
UVB and UVA ranges. The PPD
protection factor test [4] is a laboratory evaluation on
human subjects that yields the ratio of the UVA dose required to produce a
very light skin darkening response, with and without a sunscreen product on
the skin. The response is evaluated 2 to 4 hours after administration of UV
doses. The PPD
test produces rapid results with moderately low doses of UVA. Product UVA
protection may be categorized based on the UVAPF. The PPD response is stable
and reproducible; however its clinical significance is questionable, because
the action spectrum for PPD is not defined for wavelengths shorter than 320
nm, and the response is masked during outdoor sun exposure by other skin
responses to UV. Thus it is impossible to relate the PPD protection factor
directly to the degree of protection is sunlight. |
The Boots
Star Rating [5] involves in vitro measurement of the
product absorbance spectrum from 290 to 400 nm and calculation of a ratio of
average UV absorbance to average UVB absorbance. The ratio is used to assign
products among 5 categories of "broad spectrum," protection,
similar to the Diffey Critical Wavelength method. The UVA
Index is the ratio of the in vitro persistent pigment darkening factor
(UVAPF) to the labeled in vivo SPF.[6] To determine the
UVA index the sunscreen is applied to a substrate, and its spectral
transmission is measured for wavelengths from 290 to 400 nm. The SPF is
calculated and the transmission spectrum is corrected to match the in vivo
SPF. The sample is then irradiated with a nominal UV dose to compensate for
any lack of photostability. Finally, the UVAPF is
calculated and used to calculate the UVA Index Although
the UVA Index is cumbersome to determine, and the methodology is not optimum,
it has been endorsed by European regulatory authorities, and has become the
predominant method for demonstrating adequate UVA protection of sunscreen
products. A UVAPF/SPF ratio of 0.33, along with a critical wavelength
of 370 nm is considered acceptable. The FDA
has published a proposed Sunscreen Monograph Addendum (See Link) that
outlines a UVA rating system based on "stars" similar to the Boots
Star Ratings (See above). To label a product with one or more stars, the
sunscreen must be tested for protection against PPD (See above). For one star
(Low) the protection factor must be 2 to under 4; for 2 stars (Medium)the protection factor must be 4 to under 8; for 3 stars
(High) the protection factor must be 8 to under 12 and for 4
stars(Highest)the protection factor must be 12 or more. In addition to the in
vivo test, the FDA proposal also requires an in vitro test on roughened
quartz plates that measures the ratio of the area under the absorbance curve
from 340 to 400 nm (UVAI) to that under the full UV absorbance curve (290-400
nm). The measurement is made after irradiation with 2/3 the labeled SPF in MEDs (1 MED = 20 effective mJ/cm2). For 1 star
(Low) the ratio must be 0.20 to 0.39: for 2 stars (Medium) the ratio is 0.40
to 0.69; for 3 stars (High) the ratio is 0.70 to 0.95 and for 4 stars
(Highest) the ratio must be greater than 0.95. Both the in vivo and the in
vitro tests must be performed and the lower rating of the two methods is
used. We now
have considerable experience in determining the UVA index and the FDA rating,
as well as critical wavelength and Boots Star Rating of our clients'
formulas. Lack of photostability is a major problem for sunscreens
containing Avobenzone, particularly in combination
with Octinoxate. There are successful strategies
for stabilizing Avobenzone, including the use of
stabilizers such as diethylhexyl 2,6-naphthalate and
avoiding Octinoxate [7], but
achieving photostable, broad spectrum sun
protection is a significant challenge in the U.S. at present. We also
have considerable experience in assessing the photostability
of our clients' new and existing formulas, and have developed a simple test
of sunscreen photostability [8]. |
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References 1. Fourtanier A, Bernerd F, Bouillon C, Marrot
L, Moyal D, Seite´S,
Protection of skin biological targets by different types of sunscreens. Photodermatol Photoimmunol Photomed 2006; 22: 22-32 2. White Paper: Urgent
need for UVA sunscreen active ingredients to protect 3. BL Diffey. A method for broad spectrum classification of
sunscreens. Int J Cosmet Sci 16:47-52, 1994. 4. Chardon A, Moyal D, Hourseau C. Persistent
pigment darkening as a method for the UVA protection assessment of
sunscreens. In: Protection of the Skin Against Ultraviolet Radiations, Rougier A, Schaefer H, eds. John Libbey
Eurotext, Paris 1998, pp. 131-136. 5. Brown MW. Of
Cassiopeia and five stars. 2010-A Sun Odyssey, 6. Wendel V, Klette E, Gers-Berlag H. A new
in vitro test method to assess the UVA protection performance of sun care
products. SÖFW 127:12-30, 2001. 7. Cole C, Chu M, 8. Stanfield JW.
Optimizing in vitro Measurement of Sunscreen Protection. SÖFW Journal July
2006, 132:19-22 |
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