12 hours after quitting
The carbon monoxide level in your blood drops to normal.
(US Surgeon General’s Report, 1988, p. 202)
2 weeks to 3 months after quitting
Your circulation improves and your lung function increases.
(US Surgeon General’s Report, 1990, pp.193, 194,196, 285, 323)
1 to 9 months after quitting
Coughing and shortness of breath decrease; cilia (tiny hair-like structures that move mucus out of the lungs) start to regain normal function in the lungs, increasing the ability to handle mucus, clean the lungs, and reduce the risk of infection.
(US Surgeon General’s Report, 1990, pp. 285-287, 304)