The duration of protection is independent of SPF. There is no implication that you can stay in the sun longer with a higher SPF (FWIW, the packaging more or less makes this clear). The only thing SPF represents is a marginal reduction in total UV flux during the protected period.

Anything over SPF 30 buys you approximately no additional protection.

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DoctorOetker 4 days ago | parent [-]

The instantaneous damage is directly inversely proportional to SPF.

Using no sunscreen is SPF 1 (at 2 milligrams per square cm). Sunscreen SPF 2 would correspond to halving the rate of instantaneous damage.

SPF 30 compared to SPF 4 would indeed give (30/4)=7.5 times lower rate of instantaneous damage.

The SPF scale is more sensible than your blocking percentage scale.

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jandrewrogers 4 days ago | parent [-]

The dose response is not linear, there is no “instantaneous damage” below some threshold. Your argument assumes something that isn’t true.

As with most things, the dose makes the poison.

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gruez 3 days ago | parent [-]

>The dose response is not linear, there is no “instantaneous damage” below some threshold. Your argument assumes something that isn’t true.

Source? All things being equal, I'd expect half the UV damage by going from 98% UV protection to 99% UV protection. That's significant even though the protection only went up by only 1%. Moreover as I mentioned in my previous comment, even if you assume that 2% UV exposure (from 98% protection) basically never results in skin cancer, that figure is only achieved if you use sunscreen perfectly, which no one really does.

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aydyn 2 days ago | parent [-]

Melanin production is stimulated by UV so over time the response is absolutely non linear.

	▲	DoctorOetker 2 days ago \| parent [-]
		Yes part of the linear instantaneous damage will start being absorbed by the melanin.