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SPF Test Debate Sizzles

By: Nancy Jeffries
Posted: October 10, 2008, from the February 2006 issue of GCI Magazine.

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Lott said even trained technicians have trouble agreeing on the first unambiguous MED. “It has been reported that technicians’ MED choices vary by as much as 25%, making the resultant SPF vary from lab to lab or from the same lab if judged by different technicians,” said Lott. “Applying a completely uniform film of product over the protected skin site is impossible, and also can cause significant variation in the onset of the first MED. Differences in the solar simulators can produce UV spectra that vary significantly from lab to lab.” He said UV spectra richer in UVA energy compared to UVB energy may tend to make the response from a Type II or III skin type melanogenesis rather than erythema. All these factors cause difficulty in grading subjects’ responses and can produce differences in the final SPF obtained.


Lott said an adaptation can be made that will solve, or at least improve, problems—the pass/fail method. “The basis for the method is simply that no subject response to the UV exposure on the product-protected site is allowed for the sub site to be graded as passing. Thus, the product must truly protect against any UV-induced reaction to pass at a labeled SPF,” said Lott.

The method uses the same parameters as the present SPF method, as well as the same solar simulator, product dosage and number of exposures given to the unprotected site to obtain the MEDu. “The primary difference lies in the UV exposures given to the protected site. The protected site is given four equal doses of UV energy each corresponding to the expected product SPF. Twenty-four hours after exposure, the protected site is examined and each sub site is judged to ‘pass’ if there is no discernible reaction to the UV energy or ‘fail’ if there is any visible reaction, be it melanogenesis or erythema,” said Lott. The test is given to a minimum of five subjects with the goal of obtaining 20 pass or fail values. Based on the values obtained, the product can only pass if, based on binomial statistic tables, the product has at least a 95% chance of being equal to or greater than the tested value.

“The present SPF test is only a measure of erythemal energy that primarily comes from UVB, and the pass/fail test does not differentiate between a response caused by UVB or UVA,” said Lott. “For this reason, it is truly a measure of broad spectrum [UVB and UVA] product protection.” A product that does not have a high amount of UVA protection will not yield as high values with the pass/fail as with the existing SPF methods. “This is important because there is a great deal of worldwide debate today on what UVA test method is to be used, yet almost everyone acknowledges that product users recognize SPF,” continued Lott. “This leads to confusion as to how UVA protection should be labeled to convey the degree of UVA protection yet not detract from the SPF message.”

Lott said advantages of pass/fail include use of fewer human subjects, less potentially harmful UV exposure, endpoint protection, recognition of uneven product application, measurement of UVA and UVB protection, and simple grading.

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