Asthma is classified as intermittent (mild) and persistent (mild, moderate or severe). Patients presenting one or less weekly episodes and near normal lung function have mild intermittent asthma; those presenting over two weekly episodes or signs of airflow obstruction between symptom episodes have persistent asthma. In mild persistent asthma there is more than one weekly episode but not on a daily basis, with near normal lung function. In moderate persistent asthma symptoms occur daily with mild to moderate airflow obstruction; in severe persistent asthma patients present with frequent nighttime symptoms and moderate to severe airflow obstruction. Severity of the disorder may change over time and any patient may suffer a severe exacerbation