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The subject of SPF (Sun Protection Factor) testing evokes varied responses from the scientific and regulatory communities. Related discussions are provocative, often complex and sometimes exasperating. However, testing cannot be addressed without everyone weighing in, and this includes the testing lab voice and the scientific and regulatory communities.
“[SPF] is obtained by dividing the energy required to produce erythema [sunburn] on product protected fair [Skin types I, II, and III] human skin, by that required to produce erythema on the human subject’s unprotected skin,” explained Dennis Lott, VP technical affairs, Tanning Research Laboratories, manufacturer of Hawaiian Tropic. “Protected skin is that which has received an application of 2 mg/cm2 of SPF product.”
Lott said worldwide test methods expose small areas of the protected and unprotected skin to five to seven increasing doses of ultraviolet (UV) energy produced by an artificial light source called a solar simulator. The first small area on the protected and unprotected skin to exhibit erythema 24 hours after exposure is called the “minimal erythema dose (MEDp)” and MEDu respectively. “An MED is typically defined as an unambiguous redness reaching the borders of the exposure subsite,” said Lott. “The SPF for the individual taking the test is then determined by dividing the amount of energy required to produce erythema on the protected site by that of the unprotected site.” Simply put: SPF = MEDp/MEDu.
The final SPF value is determined by averaging the values obtained from either a 10- or 20-subject panel. “In the United States, a 20-subject panel is used, and the result is statistically massaged in an attempt to determine the point at which 95% of the population will obtain the SPF result,” Lott added.
“Two of the most glaring problems with the SPF test are the fact that the endpoint is not protection, but sunburn, and the fact that only sunburn is measured,” Lott said. “In many individuals, the first reaction to the UV energy may not be erythema but melanogenesis (tanning). For example, subjects frequently show reactions to the UV energy at a lower energy dose than that judged to be erythema.”