I recently went on vacation with a new sunscreen (Equate Sport Sunscreen with spf50). I have never been allergy to sunscreen or so I thought. I came back from the vacation with red itchy rash. My daughters used the same sunscreen and nothing happened to them.
Rash only appeared in the areas where the sunscreen was applied. I still have it as I write this. I am taking Claritan. The spots seem to have shrunk a little.
I have also been investigating the ingredients a little more carefully. Previously I have used a NO-AD brand with spf50 and Ocean Potion anti aging sunblock spf 30 and none of those brands gave me a reaction such as the one I have right now.
The ingredient that might have done it would have to be Octocrylene. I believe that the sunscreen applied in the shade would not have given a rash but that the sun reacting with the sunscreen and that particular active ingredient did it. Sort of like a photodermititis.
For some of the people on this forum who have mentioned water might have been the catalyst to the rash, the sun might have added as well to the contribution.
For now, I am itchy and waiting for the rash to clear up before I try out my theory.