Smoking just one to four cigarettes a day almost triples a smoker's risk of heart disease and lung cancer, reveals a large study in Tobacco Control.
The impact is stronger for women, the study shows, and quashes the cherished notion that "light" smokers escape the serious health problems faced by heavier smokers.
The researchers tracked the health and death rates of almost 43,000 men and women from the mid 1970s up to 2002. All the participants were aged between 35 and 49 at the start of the study, when they were screened for cardiovascular disease and diabetes.
Although a significant proportion of the light smokers increased their daily consumption, this had not exceeded 9 cigarettes a day. And almost as many had given up as had increased their consumption.
Taking account of risk factors likely to influence the findings, the data nevertheless showed that light smoking endangered health. The steepest risk occurred between nought and four cigarettes a day.
Compared with those who had never smoked, those who smoked between 1 and 5 cigarettes a day were almost three times as likely to die of coronary artery disease.
While there was little difference in the risk of dying from any type of cancer, this was not the case for lung cancer.
Men who were light smokers were almost three times as likely to be killed by lung cancer. And women were almost five times as likely to die of the disease as their non-smoking peers.
Light smokers also had significantly higher death rates from all causes - 1.5 times - than those who had never smoked, with the death rates corresponding to the number of cigarettes smoked every day.
As the light smokers had smoked for fewer years than the heavy smokers, the researchers analysed the projected impact of smoking at this level for five years.
This indicated that the risk of death from coronary artery disease would have been 7% higher, and the risk of lung cancer would have been 47% higher in women.