The air around us is full of harmful substances. This is why breathing through your nose is healthier than breathing through your mouth. The hairs and mucuous membranes of your nose gather all the harmful substances before they can reach the lungs. 

Even the air that you breathe out during exhalation is not always clean. When sneezing or coughing, pathogenic viruses can spread into the environment. 

Smoking is harmful to the respiratory system. Tobacco smoke contains many kinds of toxic substances that can damage your lungs.