Overexposure to the sun has many adverse effects, most obviously visible acute injury (sunburn), but immunosuppression can also occur. Chronic overexposure may cause wrinkling, pigment changes, and damage to DNA. The latter is a more serious consequence in that it can precipitate premalignant actinic keratoses, basal and squamous cell carcinoma, and malignant melanoma. The ultraviolet (UV) light that injures the skin is classified into three wavelengths: UVA I, UVA II and UVB. Of these, UVB and UVA II are the major causes of sunburn. UVB causes more damage, but both types can cause photoaging and have been linked to cancer in animals. Most topical sunscreens contain combinations of organic chemicals that absorb various wavelengths of UV light. The U.S. Food and Drug Administration (FDA) permits manufacturers to claim UVA protection if their products block at least part of UVA II. Medical Letter consultants reviewed the effectiveness of commercially available sunscreens.

All commercially available sunscreens include agents that absorb a specific type of UV light or some combination of wavelengths. The abbreviation SPF, or sun protection factor, is commonly advertised on sunscreen products. SPF is the ratio of the time required to produce minimal erythema on skin protected by a sunscreen to the time required to produce the same erythema without the sunscreen. Many factors decrease the effectiveness of the sunscreen, including wind, heat, humidity and altitude. The SPF does not provide any information on protection against UVA radiation, and long-term UVA damage can occur with repeated sunburns. Sunscreen is not typically applied consistently to the skin, and therefore does not often achieve the claimed SPF. Because of this inconsistency, multiple applications may be necessary to maintain protection during prolonged exposure.

The systemic and cutaneous effects of sun-screen have been widely studied. Sunscreen can potentially decrease cutaneous synthesis of vitamin D, especially in the elderly. However, studies of patients with xeroderma pigmentosa did not confirm this finding. Regular use of sunscreen for six months prevented development and increased remission of actinic keratoses. Regular use of sunscreen has also been shown to delay development of new actinic keratoses. Retrospective studies have suggested that there is an increased risk of melanoma in adults and an increased development of nevi in children who use sun-screen. However, these studies are limited by lack of control of factors such as fair skin or use of less protective clothing; therefore, no definitive conclusions can be drawn.

Medical Letter consultants recommend regular use of a sunscreen with a high SPF that includes protection against UVA radiation. Sunscreens currently on the market are unlikely to increase the risk of any cutaneous malignancy, but some persons may still overestimate the effectiveness of sunscreens and overexpose their skin to the sun.