Since the beginning we have been breathing in dust and smoke. Our lungs remove it using mucus to trap and the little hairs (name escapes me) in the bronchial passages to "pump" the solids and mucus out and down the gullet. As long a this system isn't disabled or overwealmed all is fine.
This is what causes chain smokers (I smoke ) problems as nicotine puts the hairs to sleep and it takes about ten minutes for them to start working again.