Photostability

A sunscreen that is photostable has a constant SPF during UV exposure, while a sunscreen that is not photostable (photolabile) has an initial SPF much higher than the labeled SPF, and its SPF diminishes during UV exposure. The SPF measured in human subjects by means of a solar simulator represents the cumulative effective UV dose applied at the time when the cumulative effective UV dose transmitted to the skin reaches one MED. Although a sunscreen with any SPF value can be photostable or photolabile, photostability is desirable because a photostable sunscreen is more likely than a photolabile sunscreen to maintain its labeled SPF value in outdoor sunlight, which generally has a different spectrum from that of a solar simulator. The solar spectrum tends to degrade photolabile sunscreens more effectively than the spectrum of the typical solar simulator. A photostable sunscreen product also requires a lower percentage of active ingredients and thus pro- ^ vides more efficient protection. Perhaps a greater concern is that during the a process of photodegradation, photolabile sunscreens may produce free radicals on J

t the skin. The example that follows illustrates in vitro photostability determinations for a hypothetical SPF 30 sunscreen product.

We can model the in vivo behavior of sunscreens on the skin by using an in vitro system, in which the surface of a sunscreen film on a collagen substrate is irradiated and the transmitted UV dose is monitored over time. When the transmitted UV dose in MEDs reaches 1 (1 MED = 20 effective mJ/cm2), the applied dose is the product SPF. This is analogous to the UV dose at which the MED occurs on human skin. An "ideal" sunscreen has a constant SPF and transmits a constant fraction of the applied dose, while a sunscreen that is photolabile has a diminishing SPF and transmits an increasing fraction of the UV dose. Representative graphs of the applied effective UV dose vs the transmitted effective UV dose for ideal and photolabile ("unstable") sunscreens are shown in Figure 4.

10 20 30

10 20 30

0 10 2D 30

Figure 4 Transmitted and applied UV doses for (A) "ideal" and (B) "unstable" (photolabile) sunscreen products rated SPF 30.

0 10 2D 30

Figure 4 Transmitted and applied UV doses for (A) "ideal" and (B) "unstable" (photolabile) sunscreen products rated SPF 30.

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