Chronic obstructive pulmonary disease (COPD) is most often caused by smoking. Nearly everyone with COPD (80% to 90%) has been a long-term smoker, and research supports the fact that smoking cigarettes increases the risk of developing COPD.¹ At least 10% to 15% of long-term smokers develop COPD with symptoms; some studies indicate up to 50% of long-term smokers older than age 45 develop COPD.² This may be in part due to inherited factors (genes) and exposure factors that can affect your risk of developing COPD.¹

See a graph on how smoking affects the ability to breathe.

COPD is often a mix of two diseases, chronic bronchitis and emphysema. Both of these diseases are caused by smoking. Although you can have either chronic bronchitis or emphysema, people more often have a mixture of both diseases.

Chronic bronchitis

Almost all people with chronic bronchitis are, or have been, tobacco smokers. Over time, tobacco smoke and other lung irritants can lead to inflammation in the airways of the lungs (bronchial tubes). As a result, the airways produce more mucus than they normally would. Inflammation and excess mucus cause coughing and narrow the airways. It is difficult to breathe through the narrow airways, making you feel short of breath.

Long-term (chronic) mucus production and inflammation over many years may lead to worsening and permanent lung damage and may make it more likely you will get lung infections.

Emphysema

In emphysema, tobacco smoke and other irritants can damage the elastic fibers in the lungs. These stretchy strands of tissue are needed for normal lung function. They allow the lung tissue to stretch when you breathe in and help pull the lungs back to their normal size and shape as you breathe out. When the elastic fibers are damaged:

• The tiny air sacs (alveoli) at the end of the bronchial tubes are damaged. These air sacs are where the blood exchanges carbon dioxide (a by-product of metabolism) for oxygen. When air sacs are damaged or destroyed, their walls break down and the sacs become larger. These large air sacs move less oxygen into the blood. After air sacs are destroyed, they cannot be replaced.
• The smaller airways in the lungs (bronchioles) tend to collapse when you breathe out, trapping air in the alveoli. As a result, oxygen-rich air has difficulty entering the air sacs and the bloodstream.

See an illustration of bronchitis and emphysema.

Other causes

Other possible causes of COPD include:

• Long-term exposure to lung irritants such as industrial dust and chemical fumes.
• Low birth weight and having repeated lung infections.
• Inherited factors (genes), including alpha1-antitrypsin deficiency, a rare condition in which your body may not be able to make a protein (alpha1-antitrypsin) that helps protect the lungs from damage. People with this disorder who smoke generally develop the symptoms of emphysema in their 30s or 40s. Those who have this disorder but do not smoke generally develop symptoms in their 80s.

COPD exacerbations

A COPD exacerbation is a rapid, sometimes sudden, and prolonged worsening of symptoms (cough, amount of mucus, and/or shortness of breath). A COPD exacerbation can be mild to life-threatening, and you may have to go to the hospital. They are most commonly caused by infections and air pollution. Infections may be caused by viruses or bacteria. Infections may affect the airways in the lungs (bronchitis) or the lungs themselves (pneumonia).