Chronic obstructive pulmonary disease (COPD) is the most common lung condition and is comprised primarily of three related conditions - chronic bronchitis, chronic asthma, and emphysema. In each condition there is chronic obstruction of the flow of air through the airways and out of the lungs, and the obstruction generally is permanent and may be progressive over time. While asthma features obstruction to the flow of air out of the lungs, the obstruction is usually reversible. Between "attacks" of asthma the flow of air through the airways typically is normal. These patients do not have COPD. However, if asthma is left untreated, the chronic inflammation associated with this disease can cause the airway obstruction to become fixed. That is, between attacks, the asthmatic patient may then have abnormal air flow. This process is referred to as lung remodeling. These asthma patients with a fixed component of airway obstruction are also considered to have COPD. This category covers more than COPD, but this is the most frequent lung problems with the exception of lung cancer.