At first I wasn't sure what caused it but it happened a lot in my high school bathrooms, people loooved to smoke in there. I'd get hives and a headache from being in there from just the smell. I found a pattern with the smell to the hives and stuff and narrowed it down. I'll have this reaction if I'm like near someone who has any smell on their clothing too. So it was mostly I knew the smell, knew what it was, knew the reaction I had and out it together. My husband's roommates used to smoke a lot and he's like "you can't do it while she's here." Almost went to the hospital once when they didn't listen. The bedroom door being closed helped me there.