Mold. It's monsoon season here, on a good day I can feel the rain coming (due to spores lifted up and transported by wind before the rain arrives here). In that case I directly pop an antihistamine. On bad days I'm too late with that, which results in itchy eyes (mostly), sneezing, snotting, etc. Monsoon season is bad, rest of the year I have little issues unless I walk into a mold infested place.
I'm still doing that at old age, as part of my daily routine.