After 20 minutes:
Blood pressure and heart rate drop
After 12 hours:
The balance of carbon monoxide and oxygen in the bloodstream has returned to normal.
After two weeks to three months:
The risk of heart attack decreases, circulation improves and lungs are working better.
After one to nine months:
Respiratory problems such as coughing and shortness of breath have started to decrease. Tiny hairlike structures in the lungs, called cilia, start working again cleaning the lungs and helping prevent infection.
After one year:
Excess risk of heart disease decreases to half of that of a current smoker.
After two years:
Risk of stroke starts dropping.
After five years:
Risk of stroke can fall to about the same level as that of a nonsmoker.
After 10 years:
The risk of dying from lung cancer declines to about 50 percent of that of a current smoker. The risk of cancers of the mouth, throat, esophagus, bladder, cervix and pancreas has decreased significantly.
After 15 years:
The risk of heart disease is similar to that of a nonsmoker.
Sources: American Cancer Society; American Lung Association; National Institutes of Health