Solar ultraviolet (UV) light capable of injuring the skin is classified by wavelength into UVA I (340-400 nm), UVA II (320-340 nm) and UVB (290-320 nm). UVB is responsible for most of the erythema of sunburn. UVA has been implicated in the development of phototoxicity and photoaging. The FDA permits sunscreen manufacturers to claim broad-spectrum protection if their products block at least part of UVA II in addition to UVB.

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