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What is the importance of the percentage of active ingredients in sunscreen?

Asked by Annick 3 Rep.

I am religious about wearing sunscreen, a minimum of 30SPF but often 50SPF.  However, I do not understand how sunscreens can have the same SPF, but different percentages of the same active ingredients.  For example, one sunscreen I love – because of its wonderful texture and smooth application –  has 4% zinc oxide and 4% titanium oxide.  Other mineral sunscreens I use have higher percentages of zinc oxide (between 10-21%), which I understand from reading is the more effective of the two oxides.  Yet the SPF on them will be the same (30%) as the 4% zinc, 4% titanium one.  Since I never sit in the sun and find myself, for better or worse, indoors most of the day, every day, I feel comfortable wearing the lower percentage sunscreen.  However, for those who are outside more, and on those few days when I am as well, isn’t it more prudent to wear the sunscreen with the higher percentage of active ingredients?  Will it protect more?  It would just seem logical that as long as an SPF is 30 or above, the more important measure is the percentage of active ingredients, so it is more important to read the label on the back of the bottle, than the front of the bottle.  Please correct me if I’m wrong; this subject continues to be confusing.  But having already had two very early stage melanomas removed, I really want to get it right.  Thank you.

Annick answered
    Dr. Schultz 1.26K Rep.

    Thanks for your question. I’m happy to hear that you wear sunscreen every day! The SPF of a sunscreen is determined by active user testing, therefore, no matter what the percentages of ingredients, it’s been demonstrated to provide the amount of protection , i.e. the spf listed on the bottle. If the one with a higher active ingredient provided  better protection, it would have a higher SPF. The FDA has this one figured out so you don’t have to try to calculate the ingredients… your protection depends on the rated SPF, applying enough and reapplying immediately after swimming or sweating and every 2 to 3 hours otherwise.

    Dr. Schultz edited answer

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