[ihumanable]

This is not correct, https://www.fda.gov/about-fda/center-drug-evaluation-and-res...

<quote>

There is a popular misconception that SPF relates to time of solar exposure. For example, many consumers believe that, if they normally get sunburn in one hour, then an SPF 15 sunscreen allows them to stay in the sun 15 hours (i.e., 15 times longer) without getting sunburn. This is not true because SPF is not directly related to time of solar exposure but to amount of solar exposure. Although solar energy amount is related to solar exposure time, there are other factors that impact the amount of solar energy. For example, the intensity of the solar energy impacts the amount. The following exposures may result in the same amount of solar energy:

- one hour at 9:00 a.m.

- 15 minutes at 1:00 p.m.

</quote>

[tgb]

I don't see anything in that article that actually contradicts the definition of SPF. It's basically saying:

> It's a common misconception that speed determines how fast you get to your destination, but actually distance also matters. So speed does not reflect time to destination. Instead, speed lets consumers compare the level of relative quickness and that 60mph is quicker than 6mph.

Obviously we need to hold other factors constant. It's not a useful point in when talking about speed and I don't see the point in talking about SPF. And it certainly doesn't mean that time to destination is inversely proportional to speed.

[thehappypm]

If you're standing under a UV light that's constant, I think the SPF === time works. In the real world, where the sun, ya know, rises and sets, this is actually a harmful assumption.

[tgb]

The FDA's article gives a meaninglessly useless. The multiplicative one is clear. What is the plausible case where someone gets confused by this in a way that's harmful? Would anyone ever over-estimate the time they can spend in the sun? I can only imagine under-estimating that time (which is fine) since our base reference for sunburns is always to picture the hottest day of summer at the beach at middday - the worst case scenario. The sun setting makes this better not worse!

Either way the multiplicative one is clear: 30 minutes of midday exposure with 30 SPF is approximately equal to 1 minute of midday exposure and so is minimal. That's helpful to know! And 50 is only moderately better than 30, but 30 is a lot better than 10 - the FDA page says nothing about this.

[ihumanable]

Quick someone hold the Sun's position in the sky constant for me so my incorrect understanding of SPF holds true :)

I merely want to provide a fuller explanation of what SPF is from an authoritative source, in this case the FDA.

[csours]

It's correct, but with extra steps required for comparison.