Light smokers damage their lungs just as much as addicts who smoke two packets of cigarettes daily, doctors have claimed.
The researchers at Columbia University warned that lighting up a few of times each day is much riskier than a lot of people think.
They tested the lung function of 25,000 volunteers, a mixture of smokers and ex-smokers, as well as adults who had never smoked.
Results showed the lung function of heavy smokers – defined as smoking 30 a day – declined at a rate of 11.24ml per year more than non-smokers.
In comparison, the rate of decline was around 7.65ml per year among the lighter smokers, who had fewer than five cigarettes a day.
The researchers calculated that light smokers lose the same lung capacity over one year that their heavy smoking counterparts do in nine months, the Daily Mail reports.
Lead author Dr Elizabeth Oelsner said everyone should be strongly encouraged to quit smoking, no matter how many cigarettes per day they are using.
The results also showed the decline in lung function caused by smoking does not return to normal within a few years of giving up the habit.
Ex-smokers see a decline in function of 1.57ml per year and this does not return to zero for at least 30 years, the researchers claimed.
The results of the study were published in The Lancet Respiratory Medicine.