1. Home>
2. Specialties>
3. Dermatology>
4. Summary and Comment

Do SPF-30 Sunscreens Really Have SPFs of 30?

Sunscreen products are routinely tested indoors against a solar simulator which has an emission spectrum similar to that of noontime summer sunlight. When outdoor tests have been conducted, sunscreen efficacy has invariably been found to be lower. Environmental and methodologic factors play some role in this difference, but the greatest variable is the light source itself. The spectral output of sunlight varies with the position of the sun above the horizon; when the sun is low in the sky, the ratio of UVA to UVB increases significantly. It is im-possible to test high-SPF sunscreens (SPF 25 to 50) in natural sunlight because there is insufficient sunlight in a single day.

In this study, the predicted efficacy of several sunscreens was compared with the efficacy as measured in natural sunlight at different solar angles. In all cases, the theoretical estimate of effectiveness was lower in sunlight than in an indoor solar simulator. The difference was greatest for high-SPF products. For example, the SPF value of a sunscreen labeled SPF-45 was 45 when tested with a solar simulator, 30 in natural sunlight, and 11 when the sun was at a relatively low angle. An SPF-15 sunscreen was proportionately more effective, with a maximum noonday sunlight SPF of 13 and a minimum of 7.

Comment: This study confirms the common belief that sunscreens do not protect as well outdoors as they do in laboratory experiments. All SPF values obtained with the spectra of natural sunlight were lower than those obtained with a solar simulator; the higher the product's SPF, the greater the potential variance for natural sunlight protection. The reason for the difference between labeled and measured efficacy is the reduced intensity of UVA in the solar simulators currently used. Solar spectra have more than twice as much UVA as solar simulators. Testing sunscreens using solar simulators that mimic midday summer sun gives a fairly reliable measure of the SPF when these products are actually used in midday summer sun. However, the labeled SPF may pose a threat for outdoor activities in the winter because SPFs are highly unpredictable when the sun is lower in the sky but suntanning and burning are likely (as in high-altitude skiing).

— JS Dover

Published in Journal Watch Dermatology January 1, 1995

Citation(s):

Sayre RM et al. Changing the risk spectrum of injury and the performance of sunscreen products throughout the day. Photodermatol Photoimmunol Photomed 1994 10 148 -153.

• Medline abstract (Free)