Sunscreens

Abstract

Most of the damaging effects of sunlight result from cutaneous ultraviolet radiation (UVR) exposure. UVR (100–400 nm) comprises only 5–10% of terrestrial sunlight, of which UVB (280–315) forms about 5% and UVA (315–400) the remaining. Both UVB and UVA irradiation can have similar profound deleterious effects on human skin, but though the relative amount of UVB reaching the earth is small it is responsible for most of the harmful effects of sunlight such as sunburn, skin cancers and photoageing. People with fairer skins are more prone to develop these effects. To protect against them, however systemic, orally ingested sunscreen agents have been sought but no satisfactory ones so far found, although α-carotene has been used with limited results in porphyrias, hydroxychloroquine in certain other photodermatose and antioxidants in general, all without marked success. Thus the most efficient products so far have proved to be a variety of topically applied creams and lotions containing organic or inorganic chemical agents that absorb or reflect UVR. They may be classified as:

• chemical sunscreens (organic chemical absorbers)

• physical sunscreens (inorganic chemical absorbers)

• combination sunscreens.

Chemical sunscreen agents act as UVRabsorbing molecules or chromophores, taking up UVR and converting it to harmless low-energy longer wavelengths, mostly in the infrared region. Physical sunscreens in addition scatter and reflect light. There are however very few chemicals that can absorb both UVB and UVA efficiently at concentrations suitable and permissible in commercial preparations and hence sunscreens often comprise combinations of multiple agents so as to provide a broad spectrum of protection. Organic chemical absorbers are generally compounds with an aromatic ring structure conjugated with a carbonyl group, with an electron-releasing amine or methoxyl group substituted in the ortho or para position of the ring. Derivatives of paraaminobenzoic acid (PABA), cinnamates, salicylates, camphor, anthranilates, benzophenones and dibenzoylmethanes are the seven major groups of these agents currently used in sunscreen formulations. Such agents with a high optical density or molar extinction coefficient at the wavelength of maximum UVR absorption, the so-called absorption maximum _(Xmax) are likely to give maximum protection at the lowest concentrations. The sun-protection factor (SPF) is the most widely accepted way of measuring this protection, and is the ratio calculated by dividing the UVR dose required to evoke minimal erythema on sunscreen-protected skin by that required on unprotected skin. Substantivity (water resistance) on the other hand is the ability of a sunscreen to remain on the skin when subjected to environmental conditions likely to remove it, such as swimming or sweating; this is measured by determination of the SPF after often two 20-min immersions of the sunscreen-applied test site in moving water.