UK and US researchers warn not to mix sunscreens. They tested five non mineral based sunscreens and found that when applied over zinc oxide based ones the UVA protection factor was quickly reduced. The team suggested that people should be careful to avoid mixing Zinc oxide with other sun screens
This could happen when after swimming or being in the sun for a longish period a new layer from somebody else’s sunscreen was applied. Or when applying sunscreen over a makeup containing an SPF .
