Asthma is caused by an oversensitivity of the lungs and airways to certain environmental and emotional "triggers". In an asthma attack, muscles spasm in the bronchi of the lungs. This causes these small airways to narrow and constrict, making it difficult to exhale. Because the passages are narrowed and air flow reduced, mucus also builds up in the lungs and this makes it even more difficult to breathe. Asthma symptoms vary greatly in frequency and degree. Some asthmatics have an occasional episode that is mild and brief; otherwise they are symptom free.