摘要：

One decade ago, a sun protection factor () of 15 was considered a complete blocker of ultraviolet radiation (UV). The logic behind that cutoff point was that sunscreens with this number would always prevent erythema and that preventing erythema would prevent all the ill effects of UV exposure. Today, we know that both of these assumptions were wrong and we tend to recommend higher . Consumers apply only about one-quarter to one-half thickness of the layer of sunscreen material used to measure the in the laboratory. That means that less than 50% of the number claimed on the label is spread on the consumer's skin, meaning that a sunscreen with an 30 will give the real protection of an of 15. Therefore, recommend 60 when you want a real protection of 30 ! Significant injury, DNA damage, mutations, and can and do occur also with cumulative suberythemal UV exposure. Thus, erythema induction, a criterion that defines , is not a good indicator of UV damage. We also need higher values to prevent the damage caused by suberythemal doses of UV. The value of the claimed on the label is diminished by environmental factors that are not taken into account during measurements in the laboratory, such as sweating, water immersion, rubbing off, and photodegradation. There are some misunderstandings and confusion about the mode of action of physical sunscreens. It was originally considered that, in contrast to organic sunscreens, the inorganic metal oxides (and ) acted as scatterers or reflectors of UV light, as a mirror. This is not the case with modern micronized forms of metal oxides. It has been shown that both and mobilize electrons within their atomic structure while absorbing UV radiation. Thus, although metallic oxides are not inert per se, in their coated form they are stable, non-toxic, and safe and they act as highly efficient UV attenuators. Therefore, we recommend our patients to use this type of sunscreens. We should exert all our influence upon our patients not to expose themselves to excessive sunlight, to routinely use generous layers of sunscreen agents, and to wear protective clothing. To wait for the dust to settle around the issue of the effectiveness of sunscreens in preventing , while the ideal sunscreens--topical, systemic, whatever--are at our disposal, is a luxury we cannot afford.

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