Most people get COPD from smoking. Cigarette smoke damages lungs. This can develop into COPD over many years.

COPD makes you work harder to breathe. Air may get trapped in the lungs. This prevents your lungs from filling completely with fresh oxygen-filled air when you breathe in (inhale). It's harder to take deep breaths, especially when you are active and start breathing faster. Over time your lungs may become enlarged, filled with air that does not transfer oxygen into the blood. These problems lead to shortness of breath (also called dyspnea). Hoarse, whistling breathing (wheezing) and a chronic cough are common. So is feeling tired and worn out (fatigue). 

The cells in the airways make more mucus than normal. The mucus builds up, narrowing the airways. This means less air travels into and out of the lungs. The lining of the airways may also become swollen (inflamed). This causes the airways to narrow even more.

The small airways are damaged and lose their stretchiness. The airways collapse when you exhale. This causes air to get trapped in the air sacs. This means that less oxygen enters the blood vessels. And less oxygen is delivered to all of the cells of your body. This makes it hard to breathe.

Cilia are small hairs that line and protect the airways. Smoking damages the cilia. Damaged cilia can’t sweep mucus and particles away. Some of the cilia are destroyed. This damage makes COPD worse.