Sorry not to give you a one time reason. They can come out with a thought a smell. In a room. I have been very lucky I have not had a bad attack as I feel tthem coming. when i start to feel a slight bit below the heart more of the diaframe I start my breathing and they go away for the most part. When I have one I cannot breath thru I take a half tab then I take the rest of the pill. I do know that it is the brain resepters that are not balanecd can cause it