Do chemical sunscreens become irritants with UV exposure only?

[Guest guest]

I have been testing a liquid foundation that has a chemical sunscreen

on a small area of my face for about 3 weeks now - and have not had

a reaction to it - but I also have not had any sun exposure. I was

encouraged that I had found a makeup with sunscreen that wasn't

irritating for me. (I hadn't been able to find a makeup with only

physical sunscreens that works for me either.)

Then this weekend, I was in the sun for about 30 minutes, and the

following day the test area had some irritation and red spots.

Could my reaction be from the sunscreen being " activated " and broken

down into different chemicals by exposure to UV light? (Or do

chemical sunscreen pretty much " activate " upon application - with or

without UV exposure.) Thanks!