Coughing is a natural way for the airways to clear irritants. Coughing can be caused by: 1. Irritants (like dust, pollution, or chemicals in your workplace or home) 2. Smoking 3. A cold, flu, or allergies  4. Lung infections 5. Heartburn 6. Some medicines  Coughing may also be a sign of lung disease (like asthma or chronic obstructive pulmonary disease-COPD) and lung cancer. Talk to your doctor if you:  have had a cough for more than 3 weeks (persistent cough) have a new cough that concerns you or  notice changes to your existing cough.