Properly applied SPF 50 sunscreen blocks 98 percent of UVB rays; SPF 100 blocks 99 percent. When used correctly, sunscreen with SPF values between 30 and 50 offers adequate sunburn protection, even for people most sensitive to sunburn.
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Higher SPF sunscreens contains higher concentrations of chemical sunscreen ingredients. It is scientifically shown that chemical (organic) sunscreen ingredients can be absorbed into the skin, and when struck by UV radiation, these can become quite aggressive free radicals, increasing the risk of damage.
But suffice it to say that if you mix sunscreen, the maximum SPF you'll get is the higher of the two you started with, she said. And you'll likely end up diluting it instead. In fact, if the two sunscreens use different active ingredients to protect you from the sun, they could end up compromising one another.