> That rage grew when she learned the sunscreen she had been using for years was unreliable and, according to some tests, offered next to no sun protection at all.

If it wasn't working at all, wouldn't you notice getting sun burned?

Initially i thought it was going to be something advertised as spf 30 but actually 15. However spf 4 or less seems so low it should be noticable i would assume.

▲

jandrewrogers 4 days ago | parent [-]

SPF doesn’t mean what people think it does. The level of protection is something like (1 - (1 / SPF)), such that the difference in marginal protection between SPF 15 and SPF 30 is literally only a few percent. While SPF 4 sounds “low”, it is already providing you 75% of the maximum possible protection.

The returns on protection are very much diminishing by SPF 30.

▲

gruez 4 days ago | parent | next [-]

As others have mentioned, the difference between 75% (SPF 4) and 96% (SPF 30) might seem small, but the latter implies you can stay in the sun 7.5 times as long before getting sunburnt. That's significant. Moreover sunscreen rapidly loses effectiveness, so having "extra" protection might be worth it, especially if you don't reapply every 2 hours or after sweating/swimming, which what most sunscreens recommend.

▲

jandrewrogers 4 days ago | parent | next [-]

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.

▲

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.

▲

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.

▲

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.

▲

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.

▲

jiggawatts 4 days ago | parent | prev [-]

It's like an error rate. If you write code where 99 of the lines of code are correct out of a 100, your code is twice as robust as a programmer writing 98 correct lines.

▲

wahnfrieden 4 days ago | parent | prev | next [-]

However across millions of people over lifetimes may offer substantial increase in incident reduction no?

	▲	willsmith72 4 days ago \| parent [-]
		Sure but this is talking about n of 1

▲

bawolff 4 days ago | parent | prev | next [-]

I mean, that is what i thought it meant.

So SPF 4 you are letting 25% of the sun through. I would assume that would be enough to still be sun burnt on a high uv index day if you spend most of it on the beach.

▲

93po 4 days ago | parent | prev | next [-]

what a tremendous failure on a regulation level

▲

DoctorOetker 4 days ago | parent | prev [-]

What is of interest is not the blocking percentage, but the transmission percentage.

According to WikiPedia:

"For example, "SPF 15" means that 1⁄15 of the burning radiation will reach the skin, assuming sunscreen is applied evenly at a thick dosage of 2 milligrams per square centimeter[67] (mg/cm2)."

so assuming a linear dose response relationship (obviously oversimplified) when not using the sunscreen 15 times more instantaneous random damage is incurred compared to when using the sunscreen.

This does not translate directly into the rate of cancers though: just like the final damage of a meteorite storm isn't proportional, even though the instantaneous damage is.

Suppose a meteorite strikes a hospital, lots of damage. Then years later a meteorite strikes a school, lots of damage. Obviously if both happen in quick succession more damage will occur.

But if the whole human population takes up sunscreen use, selective pressure on cellular coping mechanisms will be relaxed, and eventually future generations won't be as resilient against sunburn. So just live your life, and don't allow scaremongers to separate you from your money, or thus indirectly scare you into doing your job for them.