Sunscreens have improved immensely in the past 40 years thanks to advances in formulation and a deeper understanding of sunlight and exposure. As a natural extension of progress, however, new pain points have emerged; these are discussed in brief in this commentary from world-renowned sunscreen expert Brian Diffey, Ph.D., of Newcastle University. Continue reading on Page DM1 of the April digital magazine.

Exposure Myths

Misunder­standings about environmental sun exposure are common in the dermatol­ogical and cosmetic literature. It is a fact¹ that both UVA and erythemal UV (largely UVB) vary over the course of a clear day in an approximate bell-shaped fashion beginning at sunrise, reaching a peak around noon and ending at sunset. Yet it is not uncommon to read in the literature phrases such as: UVB intensity declines from noontime apex, but UVA intensity remains relatively constant throughout the day.²

Likewise, measurements show unequivocally that UVA and UVB both show a daily variation throughout the year that peaks in the summer months and reaches a minimum in mid-winter, with a summer to winter variation that becomes more marked as we move further from the equator. For example, in New York (latitude 40.8°N), the mid-winter and mid-summer UV indices on a clear day are 2 and 10, respectively, while the corresponding UVA ambient levels over a winter and summer day are about 40 J/cm2 and 170 J/cm2, respectively. Nevertheless, we find statements such as: It is useful to remember that the level of UVA reaching the earth’s surface is very similar in both summer and winter.³

Both of these examples from the dermatol­ogical literature are clearly wrong, and misinfor­mation such as this may have contributed to the promotion of the need for year-round sun protection even at northerly latitudes.

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Published by Cosmetics & Toiletries, April 3, 2020