Sunscreen products contain molecules or molecular
complexes that can absorb, reflect, or scatter UV photons.
Protection against the high-energy photons of UV-B
radiation is needed to prevent erythema and sunburn.
Photoaging, on the other hand, is primarily caused by
low-energy UV-A radiation. Thus, novel sunscreens are
those that can prevent, counteract, and even repair
UV-induced skin damage. Their protective effects include
1) direct absorption of photons; 2) inhibition of chronic
inflammation; 3) modulation of immunosuppression; 4)
induction of apoptosis; 5) direct antioxidant activity (for
example neutralizing or trapping reactive oxygen species);
and 6) indirect antioxidant activity (that is, induction of
intrinsic cytoprotective reactions that lead to detoxification
of various oxidants).2
Finally, sunscreens must be safe, not
only for the humans who apply them to any exposed skin
but also for the environment, the ultimate repository of
the substance.