We rely on sunscreens to protect against erythema induced by sunlight. A sunscreen's sun protection factor (SPF) is measured in a highly controlled situation based on FDA requirements, which include applying the sunscreen in a layer with a thickness of 2 mg/cm2. These investigators performed a variety of studies to determine whether sunscreens are used as recommended and if the intended effect is achieved.
Studies of sunbathers at beaches in Denmark found that 65% used at least one sunscreen. Users applied the sunscreen in a layer 0.5 mg/cm2 thick. The median SPF used was 5 to 6 and was independent of skin type. Forty-three percent of users applied sunscreen only after arriving at the beach, and less than half reapplied it after swimming.
Comment: These and other previously performed studies make clear that sunscreens are not used as intended by regulatory agencies or the industry. Sunscreens are applied in too thin a layer, immediately before sun exposure (as opposed to one or two hours before, which is important for effectiveness, especially with PABA-containing sunscreens), and are rarely reapplied after swimming. The lesser protection provided by a thin layer of sunscreen cannot be compensated for by using a higher SPF: A sunscreen with an SPF of 50 applied at 0.5 mg/cm2 yields an SPF of only 2.7. The authors suggest that, rather than trying to change people's habits, changing the SPF test to use a sunscreen thickness of 1 or 0.5 mg/cm2 may be preferable.
Published in Journal Watch Dermatology March 1, 1998
Wulf H-C et al. Sunscreens used at the beach do not protect against erythema: A new definition of SPF is proposed. Photodermatol Photoimmunol Photomed 1997 13 129-132.
- Medline abstract (Free)
To ensure that your Reader Remark is not formatted as one long paragraph, precede new paragraphs with either a blank line or an indentation.