Sun protection factor
A so-called sun protection factor (SPF or SPF) is written on every sun cream. But what does it mean?
The sun protection factor indicates how much longer you are theoretically can stay in the sun before getting burned. In other words: what fraction of the UV radiation still reaches the skin. For example, if you know that you could be in the sun for 10 minutes without sunscreen before you burn yourself and you put on a sunscreen with SPF 30, you could theoretically stay in the sun for 300 minutes before you get sunburned. So only 1/30 of the UV radiation would get through. The above 10 minutes are used as a Self-protection time of the skin designated. In practice, this is very difficult to determine because it depends on the strength of the UV radiation and this changes over the entire day. You can find more about this in the corresponding article. It is important that you know that a sunscreen with protection factor 50 protects you twice as well as one with SPF 25, because only half as much UV radiation penetrates your skin.
Attention: The sun protection factor stated on the sun cream is determined in the laboratory with a comparatively large amount of sun cream. Studies have shown that most people use significantly less sunscreen per area and thus only achieve about half the sun protection factor.
Since sun-a-wear measures the strength of UV radiation, the app can calculate the UV dose and can also include applied sunscreen. This allows it to specify the remaining protection time of the skin more precisely. To be on the safe side, the app only uses half the protection factor of the sun cream, which in our opinion is more realistic than taking the full SPF.
Applying sunscreen again
Do health authorities recommend putting on sunscreen again after a period in the sun? But why is that and should I do the same with a cream that is advertised with all-day protection?
In principle, reapplication is recommended if the protective effect of the sun cream is impaired and the advertised sun protection factor is no longer given. There are mainly 3 reasons for this:
- Sweating: When we sweat, the sweat removes part of the sun cream and it can no longer protect these areas.
- Friction: The friction of sand, terry towels or clothes on the skin can also remove sunscreen.
- Water: When swimming, the water can remove sunscreen from the skin. If a sunscreen is labeled as waterproof, it should still have the advertised protection even after 30 minutes in the water.
So you see: In most situations in life it makes sense to repeat the creaming, because at least friction and sweat in the sun can hardly be avoided.
The sun-a-wear app can remind you to reapply sunscreen if it determines that the protection may have worn off and you are still in the sun. For example, the sensor detects when it was in the water and thus allows the app to decide whether a notification would be useful.
By the way: What actually happens if you put on sunscreen a second time? Does the protection factor double? No! A renewed application of sunscreen only renews the protection. That means it doesn't matter whether you put on 1 or 10 sunscreen, the sun protection factor is a maximum of 30.
A common misconception about the protection factor
And finally, a short correction to a widespread misconception. Statements like: “50s sunscreen is pure money-making. 30s sunscreen absorbs 96% of UV radiation and 50 absorbs 98%. Should we pay so much more for this 2% more protection? ” Let's look at where this statement comes from and why it doesn't make sense. The table below shows a calculation example on a day on which we are exposed to a UV dose of 1000 UVIm.
| Unprotected skin | Sun protection factor 30 | Sun protection factor 50 | |
|---|---|---|---|
| Sun protection factor | 1 | 30 | 50 |
| Proportion of UV radiation that is blocked | 0% | 96.3% (29/30 are blocked) | 98% (49/50 are blocked) |
| Proportion of UV radiation that comes through | 100% | 3.3% (1/30 comes through) | 2% (1/50 comes through) |
| UV dose that comes through | 1000 UVIm | 33 UVIm | 20 UVIm |
So you can see that the first part of the statement is correct, but the conclusion is wrong. If you look at the UV dose that penetrates your skin you can see that you get more than one and a half times the dose with the 30 sunscreen than with the 50.
Further reading:
Sources:
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