You can't compare the straight percentage, a 98% filter lets through twice as much as a 99% filter.

In both cases though the level of UV will be easily tolerated, which is the entire point. UV index is a linear scale, so more SPF has rapidly diminishing returns even in places with a UV index of 15+.

That the duration of protection is independent of SPF makes this particularly true. There are only a handful of places in the world where atmospheric conditions might give a very high SPF marginal benefits.

True, so the important factor is - how does this map to your chances of getting skin-cancer?

The most reasonable answer is to look at the transmission percentage, not the blocking percentage.

No it isnt. Two photons is twice as much transmission than one photon, but both cases are totally insignificant.

How? Sorry I'm confused by that statement.

If you have a "100 unit" light bulb, and a material that blocks 98% of the light it emits, 2 units of light are getting through. If you have a material that blocks 99% of the light, only 1 unit - half as much - is getting through.

(This is why the SPF scale is inverted/measures transmittance. SPF 50 sunscreen theoretically allows through 1/50th of the UVB radiation (or whatever wavelengths are specified by your local regulator).)

98% = 2 units of UV reaching the skin

99% = 1 unit of UV reaching the skin

Thus 98% filtering lets in 2x as much as 99% filtering

But what about the sunscreen with ingredients that are carcinogenic before you even need to consider UV protection?

Yes that would be serious so I suppose in an actual specific case regarding some specific real ingredients in products, we could discuss that.

There have been cases of benzene being detected in sunscreen. It's not an intentional ingredient, just one that is common in industrial manufacturing. I don't think that's what the parent was worried about though.

https://www.ewg.org/sunscreen/beware-of-benzene-shining-a-li...

No, benzene was specifically what I was thinking of to the point that I assumed it was so well known that it wasn't question as being a thing any more. Just like asbestos in baby powder

That’s thankfully no longer really a thing - the world has realised that there is no such thing as asbestos-free talc, so baby powder is now mostly corn-starch AFAICT.

You mean in the US, right? Because J&J were known to continue to sell their talc based products internationally.

It’s certainly stopped here in Aus too, and there was a UK documentary about it a while ago implying it’s known there as well.

Beyond that, no clue.

If you drive gas car there’s far more benzene around you than in sunscreen.

Okay, and? If you can’t see a difference in benzene as a byproduct vs an ingredient people lather in their skin and rub in sold as a way to protect against skin cancer while giving you chances of a different cancer as something totally different, then your being deliberately obtuse and not contributing to this conversation in any meaningful way.

My point is benzene is all around you. You willingly breathe it vs accidental trace amounts in a vital product.

IIRC Robocop predicted this.

I mean, that’s a whole separate question really. Alongside which constituents may be long-lasting and harmful to (for example) marine life.

> The actual difference between (say) SPF 30 and 50 is not a lot, 96.7% UV filtering vs 98% but I’m not 100% sure how that translates to actual rates of cancer.

Counterintuitively, higher SPF matters a _lot_. The difference is in the _durataion_ of the protection and in the amount of sloppiness you can afford while applying the cream.

Suppose that for you the half-life for the sunscreen is 1 hour. SPF 30 cream would thus decay to SPF 7 in 2 hours, providing little protection. But an SPF 90 cream would still offer quite reasonable SPF 25 protection.

The same applies to sloppiness. SPFs are measured in perfect conditions, with a prescribed amount of the cream spread evenly. So the higher the SPF, the more mistakes you can make while applying it.

Would different SPF sunscreens have same half-life? I have not dig into it, but I would think there is a few mechanism or chemicals and those would have different halflifes.

Certainly, but the same principle still applies. Higher SPF will provide more headroom for a given chemistry.