A sunscreen's labeled sun protection factor may overestimate protection at temperate latitudes: a human in vivo study

We lack comparative data on sunscreens with comparable sun protection factors (SPFs), but with different levels of UVA protection, especially against cumulative erythema from repeated suberythemal exposure. Thus, we compared the protection from cumulative sunburn with two sunscreens labeled SPF 6, but with different UVR-absorbing properties, one that absorbs the UVB waveband and the other that absorbs UVB+UVA wavebands. We simulated sunlight typical of temperate latitudes to expose skin daily to suberythemal doses for 13 consecutive days. The study population consisted of eight fair-skinned sun-sensitive healthy young adults. Erythema was assessed by eye and objectively, and the SPF of each sunscreen was modeled with changes in solar UVR with time of day and latitude. The broad-spectrum sunscreen gave much better protection against cumulative erythema than the UVB sunscreen. The explanation for this is that UVA makes a greater contribution toward sunburn at temperate latitudes than under the laboratory conditions in which SPF is tested and assigned. The data support the current trend toward broad-spectrum sunscreen protection. They also show that labeled SPF is much more reliable with broad-spectrum sunscreens because SPF with primarily UVB sunscreens is dependent on time of day and latitude.