Shedding light on sunscreen use
CMAJ 1997;157:1511
See response from: C.J. Mills
In their article "A place in the shade: reducing the risks of UV exposure" (CMAJ 1997;157[2]:175-6 [full text / texte complet]), Drs. Konia J. Trouton and Christina J. Mills provide a reasonable summary of the known interactions between ultraviolet radiation (UVR) and the skin. However, several points merit clarification.
The statement that sunscreens prevent sunburn but do not prevent other UVR damage to the skin is inaccurate. First, sunscreens do not prevent sunburn; rather, they reduce the risk of sunburn by increasing the maximum exposure time before burning occurs. Second, some sunscreens do reduce the risk of other forms of UVR-related skin damage. Solid data concerning the ability of sunscreens to reduce certain forms of UVR-related damage -- specifically carcinogenesis -- are lacking, but there is a strong theoretical basis for the use of sunscreens to reduce the combined risks of carcinogenesis and photo-aging in humans.
The advice to delay exposure to the sun until 15 to 30 minutes after application of sunscreen is long outdated. The original sun protection factor (SPF) standards were established on the basis of a 20-minute delay after application, but the investigators later stated that their theoretical reasons for the delay were invalid and that sunscreens achieve full SPF value immediately. By perpetuating the myth of delayed activation we may unnecessarily be reducing compliance with sunscreen use.
In contrast to what was stated in the article, most sunscreens now on the market do not need to be reapplied every 2 hours, keeping their potency for 4 or even 6 hours after application, unless there is heavy sweating, significant friction or exposure to water. The admonition to reapply these expensive compounds every 2 hours may push their cost and convenience beyond the reach of the average consumer.
The statement that physical barriers are preferred over chemical barriers for infants is confusing. If the authors are referring to the distinction in sunscreen ingredients between UVR-absorbing chemicals, called "chemical blockers," and UVR-reflecting chemicals, called "physical blockers," there are no compelling data to suggest that the latter are any safer than the former. If the statement is intended to suggest that barriers such as shade, clothing, parasols or hats are preferable to sunscreen, it should instead be intuitively obvious that the most effective protection is obtained from using both methods simultaneously. For example, at the beach only the combination of a hat plus sunscreen can provide adequate protection against direct UVR exposure from the sun, ultraviolet scatter from the sky and reflection from the sand.
The corrections noted here may improve the utility of this article for busy clinicians, as well as compliance among their patients.
Brian W. Gregory, BSc, MD
Director, Undergraduate Dermatology Program
University of British Columbia
Vancouver, BC
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