Hi, its simple to understand that how asthmatic patients can t or becomes difficult to breath. Our tracheobronchial tree consists of smooth muscle fibers, and any muscle get contracts or spasm with effect on cold and just opposite to warm or hot, it relaxes Now, if any one who is already have their tracheobronchial muscles allergic to spasm( like in asthma)and in addition when they get cold air,this smooth muscles begin to contract leading to constriction of tracheobronchial tree turned into unable to get air exchange and difficulty in breathing. Inhalers makes this muscles relaxed or dilatation in medical terms within seconds. That s the beauty of allopathic drugs. Hope u understand. Good bye