Retorts

Sunscreen Factors

In the otherwise informative article "UV -- A Burning Issue" [Feb 1993], Janine Griffin is wrong about sunscreens. The Sun Protection Factor (SPF) does not tell us how long we can stay out in the sun without burning. This is because the tests to measure SPF are carried out with UV lamps about a hundred times the intensity of sunlight, and are completed in such a short time that the all-important time factor in the protection afforded by the sunscreen is ignored.

I can illustrate this point with typical data from a recent test with which I was associated. One of the subjects in the testing panel required 10 seconds' exposure to the xenon lamp, which imitates sunlight, to receive a just detectable burn. The same subject, with the standard amount (2 mg/cm²) of sunscreen on the skin, required 240 seconds of exposure to produce the same redness. The SPF is the ratio of these two times, 240:10 = 24.

Since the exposure time is a mere four minutes, this type of test could tell us only about the protection we get beyond that time if the protective film remains fully intact throughout the longer period. In fact, it does not, and it has been known for 20 years that the protection afforded by sunscreens falls off quite rapidly with time.

The SPF therefore tells us only what we start with. A more useful application of the xenon lamp burn technique is to find the SPF at the end of a specified period rather than at the beginning. In this way, we can check on the all-important lasting qualities which vary tremendously among the different products on the market. Very few commercial sunscreens carry information on this final SPF.

The statement that sunscreens need to be applied 30 minutes before use is not true. The UV absorbance will be satisfactory right from the beginning. I suspect that the unstated logic behind such a view is that it allows a period during which the UV blockers might partly diffuse into the strateum corneum before one indulges in sweaty exercise which may wash them off the surface. In this case, it may have some value, but there is no guarantee that it will offset the inability of most commercial sunscreens to provide adequate protection for more than an hour or two.

Dr Brian Wilkins, Victoria University