For how we live.
A high number does buy measurably more protection — in one real-use trial far fewer people burned on the SPF 100 side. But how much you apply, and whether you reapply, moves the needle more than the label.
In the one randomised trial to test it, adults who used sunscreen daily showed no detectable increase in skin ageing over 4.5 years — 24% less than those who used it as they pleased.
No cream, pill or gel reverses a sunburn or shortens it. Cool compresses, moisturizer and painkillers ease the days it lasts; the burn itself is UV damage that heals on its own.
Most skin cancers rarely kill; melanoma, about 1 percent of them, causes most of the deaths. There is no proven benefit from routine whole-body screening — which puts the focus on changing moles.
Two ingredients carry most of the real evidence. The rest of the multi-step routine adds cost and, when actives are layered too hard, a damaged skin barrier.
The FDA's own trials found every chemical filter tested crossing a regulatory threshold after a single application. That threshold triggers safety studies; it is not a finding of toxicity.
SPF measures how much ultraviolet exposure it takes to burn, not how long you can stay outside. The randomised cancer-prevention evidence rests largely on 1,621 adults followed for more than a decade.
Bemotrizinol, a UV filter available in Europe and elsewhere since the early 2000s, cleared US regulatory review this month — more than a decade after a manufacturer first flagged it to the FDA.