A note about sunscreen layer thickness and its application to our skin

It wasn’t until I began testing sunscreen did I realize how the application of sunscreen really affects its performance. If you’re like me, you probably don’t want to leave a white layer of sunscreen on your skin, and instead just use as little sunscreen as possible and rub it in until its invisible on our skin (face especially). This has big implications for sunscreen blocking performance however.

After testing a few sunscreen formulations, I found more viscous and heavier sunscreen tends to settle in a thicker layer compared to “watery” sunscreen. The thicker layer typically results in more UV blocking ability and a thinner layer results in less UV blocking ability. When testing, the thicker sunscreen layer tends to produce better UV blocking performance results.

Its important to apply a thicker layer of sunscreen when applying it to your body.

It turns out the thinner you apply the sunscreen the less effective it really is. When I test sunscreen and the layer is generally what we would use on our own skin (when its almost transparent or barely visible), the sunscreen protection is greatly reduced.

But it’s worse for physical based sunscreen (like zinc oxide or titanium dioxide)…

This reduction in effectiveness isn’t as pronounced with chemical (organic) sunscreen, but with physical (inorganic) sunscreen using physical blockers such as zinc oxide or titanium dioxide, the blocking protection is GREATLY reduced. This actually makes a lot of sense when one thinks about it.

Since chemical sunscreen is distributed evenly throughout the sunscreen substrate, chemical or organic sunscreens will apply much more evenly and effectively. On the other hand, sunscreen relying on physical particles tend to clump, are spread thin, and tend to be dispersed unevenly the thinner the layer of sunscreen applied.

If you are concerned about a white cast on your skin…

I recommend using a chemical based sunscreen if aesthetics are a concern. Physical based sunscreen formulations using zinc oxide or titanium dioxide if used in (higher percentages) typically leave a white cast on a persons skin (especially seen with flash photography). As such, a person tends to apply smaller amounts of sunscreen to the face and other visible parts of the body. Not only is a smaller quantity of sunscreen applied, but it is also rubbed in, or distributed in a manner which leaves a very thin layer of sunscreen on the skin so as not to be visible. Again, this thinner layer will provide much less protection from ultraviolet irradiance.