during the inhalation of air to the lungs. it goes through your nose, which has hairs that trap the dust and other particles. from there if the irritants are still in your body system they will be stuck to the mucus in your pharynx and trachea.

the body releases these irritants by a sneeze, or a cough.

but if the irritants can get past the first two defenses, the white blood cells in your lungs can attack those irritants.

No, Nicotine is a carcinogen.

Substances in the urine that irritate the bladder may cause the bladder muscle to malfunction. The presence of a urinary tract infection and the ingestion of excess caffeine can act as irritants.

They help to clear such irritants from the respiratory tract

of course. If onychorrhexis is caused by exposure to detergents and chemicals or other irritants that can cause drying of the nail, stopping the exposure to these irritants will eventually let the nail grow out to be normal again.