For much of this century dermatologists have advocated the use of sunlight in the treatment of acne although it is not known which is  the  most beneficial wavelength range. The spectrum  of artificial  sunlight is narrower than that of natural radiation, which may explain the comparatively poor response to  artificial ultraviolet radiation (UVR). The possible reasons for the positive effect of sunlight are varied. There is no doubt that the camouflaging effect produced initially by  the erythema and  subsequently by  the  pigmentation  is  of marked help, particularly in psychological  benefit to the patient. In vitro, UVR is known to have several effects on bacteria  and  could  therefore influence  the  surface bacteria, while it can penetrate the lower epidermis and upper dermis and may therefore have an effect  on the bacteria  located  deeper  in  the  sebaceous glands. Another effect of sunlight on the  skin is  scaling.  This may enhance the removal of follicular corneocytes but it could be argued that excess scaling may even potentiate ductal obstruction. These  suggestions are speculative.

  Although many patients comment that their acne is better in  the  summer, this  is  not so  in  all  cases. According to one report, although some 60 per cent of patients noticed  an improvement, 20 per cent did not notice any difference,  and  in the other 20 per cent  their acne  was aggravated during the  summer months.2 Closer questioning of these patients revealed that the clinical deterioration was often associated with hot and humid environments. There  may, however, be other explanations. In  the rabbit ear model for comedogenicity,32 UVB radiation enhances the 'comedogenic' potential of squalene. UVB converts squalene to its oxide, which is much  more comedogenic than  squalene it.