Based on this logic, someone using SPF 15 who would ordinarily burn after 15 minutes of exposure would instead take 3 hours and 45 minutes to be affected the same extent. And an SPF of 100 implies that that same person would be protected for more than 24 hours. Here's the formula:
SPF x the time it
takes to burn = time needed to receive the same dose of UV that you'd have gotten if you hadn't used sunscreen.
But as scientists are beginning to point out, SPF is a rather
incomplete and somewhat misleading index. The key factor you want to pay attention to isn't really time -- it's the completeness of your protection.
One example of how this perspective on sunscreen tampers with SPF is that as the numbers increase, you're not really buying substantially better
protection. As our own Derek Thompson explained previously:
Each minute wearing SPF 30 sunscreen lotion, you get a 1/30th, or 3.33%, of UV exposure that you would get without the lotion. Subtracting that 3.33
percentage points from 100, you're protected from 97% of UVB rays with SPF 30 sunscreen.
Buying SPF 80 improves on SPF 30, but not by much. The stronger product blocks another 1.75 percentage points of UVB radiation. Upgrading again to SPF 100
blocks 99 percent of UVB rays, but compared to SPF 80, it's really only a quarter of a percentage point better.
When all these products are functionally the same, it becomes absurd to say that one will wear out within hours while the other gives users license to
spend the whole day outdoors on a single application. Yet that's precisely what manufacturers imply, with routine claims that their products are waterproof
or that they offer "all-day protection."
And it's not just how much of the sun's ultraviolet radiation gets blocked, either -- it also matters what kind. We know that UV comes in three different flavors so far: there's UVC, which is filtered out naturally by the earth's atmosphere; there's
UVB, the kind that causes sunburns, melanoma, and basal cell skin cancers; and then there's UVA, which ages your skin and causes most cases of squamous cell carcinoma (the second most common form
of skin cancer).
In the United States, commercial sunscreens have generally done a great job at blocking UVB rays. But they've been less effective at stopping UVA. The FDA has approved only one major ingredient, avobenzone, that's been shown to combat UVA radiation. There are others, such as a
compound called Tinosorb, but it isn't available here.
These limitations haven't stopped sunscreen manufacturers from liberally pasting "broad spectrum protection" on many of their product labels, which tends
to mislead consumers into thinking that they're safer than they really are. It creates a kind of moral hazard where people who don't understand or who
failed to follow the instructions potentially wind up taking even greater risks with their bodies than they would if the labels were more measured.