Below is an instruction from Human. Write a response.
    ### Instruction:
    What causes COPD ?
    ### Response:
    Smoking Most cases of COPD develop over time, from breathing in fumes and other things that irritate the lungs. Some of the things that put you at risk for COPD include smoking, environmental exposure, and genetic factors. Cigarette smoking is the most common cause of COPD in the United States (either current or former smokers).
    