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Why do I have to reapply sunscreen?

Cubelixa’s question…I’ve been wondering about this for quite a while now. What happens to my sunscreen if I apply it to my face in the morning and then go to work (I’m an office girl)? If I don’t do sport and sweat a lot, take a bath or rub my face with my hands, do I still have to re-apply it several times throughout the day or will it protect me just fine the entire day? What happens to it?

The Right Brain’s response:

It’s true that sunscreen needs to be replenished if it’s removed by washing or sweating or rubbing. But that’s not the only reason it needs to be reapplied. Chemical sunscreens actually “wear out” over time.

Sun damaged sunscreens

UV absorbers work by trapping high energy photons in their structure. This process excites the absorbing molecule and causes it to re-emit the energy from the photon in a harmless form. But the sunscreen molecule can only go through so many “absorb and re-emit” cycles before it begins to deteriorate. This is more common in strong sunlight and it’s why you have to reapply more lotion to maintain your Sun Protection Factor. If you’re in the office as you describe, it’s less likely to be a problem for you. U

This WebMD article provides some general details on sunscreen application that may be helpful. And here’s another source that explains that although this photodegradation of of UV absorbers is not fully understood that it can be reduced by adding stabilizing components. For instance, there is some evidence that the addition Mexoryl SX can reduce the photodegradation of Parsol 1789. Sunscreen breakdown can also be slowed down with the addition of other ingredients such as iron chelators or vitamins C or E.

This also helps to explain why you shouldn’t mix sunscreens with other lotions, you may be inadvertently messing up a formula that has been balanced to protect the photo-stability of the active ingredient.

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