Asthma affects people in different ways and to various degrees. There are four levels of severity used to classify different types of asthma:

• Mild intermittent. Symptoms appear up to two days a week and two nights a month.
  
  
• Mild persistent. Symptoms appear more than two nights a month and more than twice a week, but not more than once in a single day.
  
  
• Moderate persistent. Symptoms occur every day and more than one night a week.
  
  
• Severe persistent. Symptoms occur continuously throughout the day and frequently at night.

Asthma is most commonly triggered by allergens, such as dander, pollens, dust and molds. Known as allergic asthma, this form of asthma is found in approximately half of all asthma patients. But other stimuli also can trigger asthma symptoms or a full-blown asthma attack. Forms of asthma triggered by factors other than allergens include:

• Exercise-induced asthma (EIA). Asthma that occurs during vigorous physical activity. The condition usually manifests itself a few minutes after a person begins exercise. EIA afflicts a large percentage of asthmatics, and also can occur in individuals who usually have no other asthma symptoms.
  
  
• NSAID-sensitive asthma. Severe asthma attacks that occur as a result of taking nonsteroidal anti-inflammatory drugs (NSAIDs), including aspirin and ibuprofen. People with this condition should consult a physician, who may recommend an alternative pain reliever, such as one that contains acetaminophen.
  
  
• Nocturnal asthma. Asthma symptoms that manifest themselves primarily at night. It tends to occur between midnight and 8 a.m. and seems to be especially prevalent around 4 a.m.
  
  
• Adult–onset asthma. Asthma that makes its first appearance in an adult after age 20. It is more common in women than in men.
  
  
• Occupational asthma. Asthma that is triggered by conditions in the workplace. The main triggers for occupational asthma are airborne contaminants or allergens and extremes in temperature or humidity levels. It is most likely to occur among those who work with animals or animal–based products and in industries such as food service (especially bakery), chemicals, electronics, gardening, metalworking, painting, printing, plastics, rubber and textiles.