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Science Exposed—What Should the Minimum Recommended SPF Be to Avoid Sunburn?
By: Brian Diffey, PhD DSc, Newcastle University, UK
Posted: January 26, 2012, from the February 2012 issue of Cosmetics & Toiletries.
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To say that SPF 15 is sufficient “if applied adequately” is impractical because it generally will not happen. Anyone who has ever actually tried applying sunscreen at 2 mg/cm2 over a large part of the body finds it virtually impossible as the sunscreen runs off of the skin, making it difficult to achieve a cosmetically pleasing application. This does not even factor in the cost of applying sunscreen adequately, since whole-body coverage for a single application at 2 mg/cm2 in an adult requires around 30 mL—meaning consumers will use a standard 200 mL bottle every two to three days if reapplied at least every 2 hr, as both the FDA and NICE recommend.
So in this author’s view, the advice that sunscreen should possess a labeled SPF of 15 or higher is not in the interest of public health, especially since it is often overlooked that the labeled SPF means roughly 50% of subjects in the laboratory assay achieved this level of protection or higher but equally, about 50% achieved lower levels of protection—and this is when the product is applied at 2 mg/cm2.
The minimum labeled sunscreen SPF recommended to avoid sunburn should instead be 30, since these products will deliver a protection factor of around 10 to most people who use the product. This should be sufficient to prevent sunburn under almost all circumstances. The same cannot be said if the recommendation is SPF 15, where generally only around fivefold protection will result, and where in instances of prolonged sun exposure, sunburn will and does result—the so-called sunscreen-sunburn paradox.5–9
Finally, remember that sunscreens are commonly used during vacations, where sun exposure occurs daily for as many as a two weeks. The time course of erythema is such that a sub-erythemal response could be the result at the end of a single day’s sun exposure on sunscreen-protected skin, but a definite erythema might be seen a few days into the vacation if the same daily sub-erythemal exposure is delivered over several days.10
Consequently, it would be wise to err on the high side of SPFs if sunburn is to be avoided in almost every eventuality, so that public health agencies are not faced with disgruntled consumers who followed their advice but still ended up with sunburned skin.