In a study of over 25,000 people ages 17-93 years, researchers found that smoking just a few cigarettes a day is as harmful to the lungs as smoking a pack or more per day.
Researchers looked at the lung function in light (<5 cigarettes/day) and heavy smokers (>30 cigarettes/day). They found that FEV1 (the amount of air exhaled forcefully in one second) decline in light smokers (7.65 mL of air each year) happens at a rate comparable to heavy smokers (11.24 mL of air each year), and around five times more than former smokers (1.57 mL of air each year).
Researchers translated this finding to light smokers losing the same amount of lung function in one year as a heavy smoker may lose in nine months. Although lung function naturally declines with age, smoking can accelerate this process, putting smokers at risk for chronic respiratory conditions.
Researchers also tested the assumption that lung capacity—how much air the lungs can hold—returns to normal in a few years of quitting smoking. They found that the rate of decline does not return to normal for at least 30 years in ex- and current smokers.