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The SPF number shows the level of protection from UVB rays.
An SPF of 30 blocks 96,7 % of UV rays so only 3,3% will get through.
However, only recently was found UVA should be the focus of sun protection as is the UVA that is responsible for 95% of photoaging and skin cancer.
The most important thing to look for on a sunscreen label is “broad spectrum” which means that you will be protected against UVA and UVB.