PARADIS: This again is Alexa Friedman from EWG, a consumer watchdog.
FRIEDMAN: We know from, you know, various different studies and reports that there's, like, this issue of inconsistent testing - even the same formulation. So you can measure an SPF 50 at one lab and an SPF 30 at another.
RASCOE: That's really surprising. You'd think it would be a consistent measurement.
PARADIS: I know. Yeah. The discrepancies are notable. Anna Wang, the soft-matter squishy scientist, said there is an alternative SPF international standards test which uses plastic instead of skin to evaluate sunscreen. But she says this isn't necessarily a more accurate method because...
WANG: Skin is actually a really tricky material to try and emulate. It's, like, see-through to some wavelengths and not to other wavelengths. It's a mixture of, like, proteins and lipids. But there's also water inside, and it's also rough, and it has hairs. And the fact that it absorbs some of the ingredients - that is not going to be emulated by pieces of plastic.
PARADIS: Even though the discrepancies in SPF may seem dramatic, the fact is that the difference between an SPF 30, for instance, and an SPF 50 really isn't that much. Like, without getting too much into the math, it's the difference between blocking 97% of the sun's UVB rays versus 98%. And the reality is, sunscreen is really just a thin film on your skin, and it gets rubbed off. So whether it's an SPF 50 or an SPF 30, either way, it's typically not going to last more than a couple hours.
WANG: I'm also going to compromise the sunscreen in that time by sweating, by moving around, by being in the wind, by being on sand. It's much better just to reapply.
PARADIS: And one thing that almost any skin care pro will tell you is that no one actually ever uses enough sunscreen, and you should reapply every couple hours anyway.
RASCOE: That definitely sounds like a PSA to me. That's information I can use. Because growing up, you know, and even now, I mean, usually we put on that sunscreen once at the beach. And we don't put it on no more, you know (laughter)? Like...
PARADIS: (Laughter) I relate to that, for sure.
RASCOE: You know, you just spend a whole day out there. But it makes sense. And it is important because it's protecting us from skin cancer.
PARADIS: Yeah. It does. But the way we know that is also an interesting story. It turns out the strongest evidence comes basically from a longitudinal study, one done over a number of years.
DAVID WHITEMAN: And really, there's only been one fully powered randomized trial looking at sunscreen as an exposure and skin cancer as an outcome.
PARADIS: This is David Whiteman, a medical epidemiologist at QIMR Berghofer Medical Research Institute in Brisbane, Australia.
WHITEMAN: That was done in a little town north of Brisbane here, a little sugarcane town called Nambour.
PARADIS: It's called the Nambour Trial - the kind of research that takes a huge amount of resources and people and money and time. It started in the 1990s. And the investigator there, Adele Green, recruited more than 1,600 people in the town to participate.
WHITEMAN: And then they were randomly assigned to - one half were given free sunscreen and very strict instructions how to use it, and constant badgering to get them to use it. The other half of the town were not given sunscreen. They were told they can use sunscreen if they wished to. Just do what you normally do.
PARADIS: Both groups got free skin checks for up to a decade - checking out every little spot, looking for three types of cancers.
