How long does SPF 50 last, and when to reapply?
A sunscreen with SPF 50 lasts around 2 hours. It should be reapplied at least every two hours or slightly over 2 hours.
But doesn’t SPF 50 last a lot longer than two hours?
The 50 in SPF 50 tells us how much protection the sunscreen offers—how much UVB radiation it blocks.
The 50 in SPF 50 does not tell us how often to reapply the sunscreen.
A common misconception about SPF 50 is that since it offers more protection than SPF 30 and SPF 15, many incorrectly assume its protection lasts longer. Every sunscreen should be reapplied every two hours, irrespective of its SPF number.
The SPF number tells us the intensity with which the sunscreen protects our skin.
SPF 50 protects our skin from 98% of the UVB rays that hit our skin.
SPF 20 protects our skin from 97% of the UVB rays that hit our skin.
SPF 15 protects our skin from 93% of the UVB rays that hit our skin.
However, every sunscreen, regardless of its SPF number, must be reapplied every two hours. How fast the sunscreen’s effect wears off depends on our skin’s resistance to the sun’s radiation, geographic location, and the time of day.
If you live in an intense sun-radiation area (hot!), it is safer to reapply every two hours, especially at peak times. For other areas, you can reapply every three hours. But if you would like to be safe, stick to every two hours.
A quick note: SPF stands for sun protection factor.
So, what does the 50 in SPF 50 measure?
The 50 measures how much UVB radiation the sunscreen blocks. UVB radiation is the sun’s radiation that can cause skin cancer and age skin.
SPF 50 blocks 98% of the sun’s UVB radiation from affecting the skin. Another way of saying this is that SPF 50 allows 2% of the sun’s UVB radiation to enter and affect our skin – the rest it blocks.
It’s important to note that there’s another type of sun radiation, UVA, which also poses a threat, causing skin cancer and aging skin. While SPF measures the protection against UVB, it’s not enough. Look for the letter ‘pa’, usually mentioned next to the SPF number. The plus signs following ‘pa’ indicate the protection against UVA radiation, completing the full picture of sun protection.
Is SPF 50 better than 30?
Yes, SPF 50 is better than SPF 30.
SPF 30 allows 3% of the sun’s UVB radiation to enter and affect our skin.
SPF 50, however, allows 2% of the sun’s UVB radiation to affect our skin.
By examining these numbers, it becomes clear that SPF 30 provides 50% less protection than SPF 50. We should all be aware of this contrast in effectiveness when it comes to our sun protection choices—having said that, if you don’t have access to SPF 50, SPF 30 is good enough. Just don’t go any lower than 30.
What about SPF 15 versus SPF 50?
Yes, SPF 50 is way better than SPF 15.
SPF 15 allows 7% of the sun’s UVB radiation to enter and affect our skin.
SPF 50, however, allows 2% of the sun’s UVB radiation to affect our skin.
So, based on these figures, we can say that SPF 50 offers close to 200% more protection than SPF 15.
How do sunscreen manufacturers arrive at the 50 number in SPF 50?
Once they formulate a sunscreen, manufacturers must test it to ensure it has the correct SPF number.
To do this test, human volunteers expose some of their bare skin to a UV lamp in a lab. The time it takes to burn is noted.
The SPF number is calculated by comparing the time it takes for the bare skin to burn with the time it takes for the skin covered with the tested sunscreen to burn. This ratio determines the SPF number.
A few common questions…
Does SPF 50 mean it lasts 50 minutes?
No. Regardless of its SPF number, every sunscreen must be reapplied every two hours. Its effect lasts two hours.
The 50 in SPF 50 indicates how intensely sunscreen protects the skin. The 50 in SPF 50 indicates that it protects our skin from 98% of the sun’s UVB radiation that hits our skin.
If you want to find out how we arrived at 98% for SPF 50, check out this post about SPF calculation.
Does SPF 50 last all day?
No. Again, as mentioned above, sunscreen should be reapplied every two hours, regardless of its SPF number.
Is SPF 50 overkill?
SPF 50 is not an overkill. Anything above SPF 50 could be an overkill. I will write about why SPF 50 is not overkill in this section. The following section will explain why a higher than 50 SPF is pointless.
SPF 50 protects our skin from 98% of UVB radiation that hits our skin. This means it allows 2% of UVB radiation to affect our skin.
SPF 30 protects our skin from 97% UVB radiation while allowing 3% UVB radiation to affect our skin.
Based on this, SPF 50 offers 50% more protection than SPF 30.
I will take that extra 50% protection :). I would buy SPF 50 over SPF 30.
Now, let me compare SPF 50 with SPF 15:
SPF 50 protects our skin from 98% of UVB radiation that hits our skin. This means it allows 2% of UVB radiation to affect our skin.
SPF 15 protects our skin from 93% UVB radiation while allowing 7% UVB radiation to affect our skin.
Based on this, SPF 50 offers 250% more protection than SPF 15.
That’s a huge difference in protection.
Is high SPF pointless?
There are two reasons why sunscreens above 50 are not preferred. In some countries, the regulatory bodies actively discourage sunscreens above 50 or 60.
Here are the 2 reasons behind this: