While the appearance of occasional dark gray spots in your sputum or phlegm may be related to air pollution, it is also well known that smoking makes the natural cleaning mechanisms of airways inefficient, by removing the fine hair like structures (cilia) which act as brooms.
Dust and pollutants may therefore accumulate in the airways and be coughed out on and off. There is no need to take a medicine for this problem but regular steam inhibition will help clear the airways of phlegm.