Just about everything and anything. I would love to list ingredients that I could guarantee won’t cause your skin to have an allergic reaction, but there is no single ingredient or combination of ingredients that can live up to that sweeping claim. Why not? Because everyone is biochemically different, each of us has a unique chemical makeup, and the endless paradoxical differences in the way our bodies perform is why we can react so differently when exposed to the same thing.
Because of the almost limitless combination of ingredients in all sorts of cosmetic formulations, it is truly impossible to know if, when, or how anyone’s skin will react to any cosmetic. Your only recourse, and this is not the best news, is to keep experimenting until you find what works for you.
If you do get a reaction, stop using the product immediately. The next step would be to try an over-the-counter cortisone cream, which can offer almost immediate relief. If the reaction persists, you would need to consult your physician.
Finally, be patient. If you do have an allergic reaction, wait until it subsides before you venture out to try something new or different. Pare down to the absolute basics, usually just cleanser and a touch of moisturizer over very dry areas, try a bit of over-the-counter cortisone cream to reduce irritation, and stay out of the sun.