Cause

COPD is most often caused by smoking. Most people with COPD are long-term smokers, and research shows that smoking cigarettes increases the risk of getting COPD:²

• Some studies show that up to half of long-term smokers older than age 60 get COPD.³
• Smoking both tobacco and marijuana increases the risk of COPD more than smoking either one.⁴

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 hard to breathe through the narrow airways, so you feel short of breath.

Long-term (chronic) mucus production and inflammation over many years may lead to permanent lung damage and may make it more likely that 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 trouble entering the air sacs. And carbon dioxide has a harder time getting out of the lungs.

Other causes

Other possible causes of COPD include:

• Long-term exposure to lung irritants such as industrial dust and chemical fumes.
• Preterm birth that leads to lung damage (neonatal chronic lung disease).
• Inherited factors (genes), including alpha-1 antitrypsin deficiency, a rare condition in which your body may not be able to make enough of a protein (alpha-1 antitrypsin) that helps protect the lungs from damage. People who have this disorder and who smoke generally start to have symptoms of emphysema in their 30s or 40s. Those who have this disorder but do not smoke generally start to have symptoms in their 80s.