I think it's the UV protection index.
The higher the number, the longer the duration.
In the summer, you sweat a lot and the sun cream is washed away, so you have to apply it every two to three hours.
In that case, 35 or 50 is meaningless.
It lasts that long even if it's about 15.
It is more important to find something less irritating to the skin than to have a high sun protection index.
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