Experts from different fields of medicine have classified the severity of asthma - whether allergic or nonallergic - into four levels. These asthma severity levels provide the basis for stepwise management of the disease.

Bear in midn, however, that these levels of severity aren't permanent or static. Asthma is a condition that can change throughout your life. The primary goal of the stepwise approach is to get your asthma to the lowest classification possible. Therefore, effectively treatring your condition is crucial: Otherwise, your asthma severity may move up the classification scale to the point where you could potentially suffer from severe relentless symptoms that adversely affect your quality of life.

As described in the National Institutes of Health (NIH) Guidelines for the Diagnosis and Management of Asthma, the four levels of asthma severity are 

• Mild intermittent. Symptoms occur no more than twice a week during the day and no more than twice a month at night. Lun-function testing shows 80 percent or greater of the predicated normal value, compared to reference values based on your age, height, sex and race as established by the American Thoracic Society. In addition, your peak expiratory flow rate should't vary by more than 20 percent during episodes and from the morning to the evening. Between episodes, you may be asymptomatic (not have noticeable symptoms), and your PEFR shouls be normal. If your asthma is at this level, a worsening of symptoms is usually brief, lasting a few hours to a few days, with variations of intensity. 
• Mild persistent. Symptoms occur more than twice a week during the day, but less than once a day, and more than twice a month at night. Lung-function testing shows 80 percent or greater of the predicted normal value. Your PEFR may vary between 20 and 30 percent. If your asthma is at this level of severity, then worsening of symptoms can begin to affect your activities. 
• Moderate persistent. Symproms occur daily and more than once a week at night, requiring daily use of a short-acting bronchodilator. Lung-function testing shows a 60-80 percent range of the normal predicted value. Your PEFR can vary more than 30 percent. Symptoms can worsen at least twice a week, with episodes lasting for days and affecting your activites. 
• Severe persistent. Symptoms occur continuously during the day and frequently at night, limiting physical activity. Lung-function testing is 60 percent or less of the normal predicted value. Your PEFR may vary more than 30 percent, and frequent aggravations of your condition can develop. 

Excerpted from "Asthma for Dummies" by William E. Berger, MD