A BMJ report on the review — led by Allan Hackshaw, a professor at University College London in the United Kingdom — reveals that even if you smoke around one cigarette per day, your risk for stroke and coronary heart disease is "much greater than expected."
He and his colleagues calculated that the risk from smoking about one cigarette per day is around "half that for people who smoke 20 per day."
The findings challenge a widely held view that smoking just a few cigarettes per day is "relatively safe."
Instead, the report concludes that: "No safe level of smoking exists for cardiovascular disease."
'Most preventable cause of premature death'
"Tobacco is the only legal drug that kills many of its users when used exactly as intended by manufacturers," declare the World Health Organization (WHO).
The latest WHO estimates reveal that more than 7 million people worldwide die every year from using tobacco. This figure includes around 890,000 deaths from exposure to second-hand smoke.
In the United States, smoking is the "most preventable cause of premature death," according to the American Heart Association, who state that smoking reduces "tolerance for physical activity and increases the tendency for blood to clot."
Smoking raises the risk of developing many persistent health problems, including atherosclerosis, a condition in which fatty deposits build up inside arteries, thereby raising the risk of stroke and coronary heart disease. Consequently, the risk of heart disease in smokers is double that of non-smokers.
Risk higher for men, even higher for women
Prof. Hackshaw and colleagues were prompted to carry out their review by occasional single studies and a review of five studies reported 20 years ago that found that the risk of coronary heart disease from smoking fewer than five cigarettes per day was higher than expected.
So, for their systematic review and meta-analysis, they used data from 141 published studies to calculate the risk of coronary heart disease and stroke for those who smoke one, five, and 20 cigarettes per day compared with never-smokers.
Using data from all the studies, they calculated that smoking about one cigarette per day is linked to a 48 percent raised risk of coronary heart disease and a 25 percent raised risk of stroke in men.
However, when they only used data from studies that had adjusted their results to take into account the effect of other factors that can increase these risks, they found that they went up to 74 percent and 30 percent, respectively.
The analysis for women revealed even higher estimates of risk associated with smoking about one cigarette per day: 57 percent for coronary heart disease and 31 percent for stroke.
Again, when analyzing only studies that took into account confounding factors, these figures went up to 119 percent and 46 percent, respectively.
Smoking one or two daily carries large risk
The researchers also drew a comparison between smoking a few and smoking 20 cigarettes per day.
They found that compared with never smoking, smoking about one cigarette per day carries 40–50 percent of the risk for coronary heart disease and stroke that is associated with smoking 20 per day.
"We have shown," they argue, "that a large proportion of the risk of coronary heart disease and stroke comes from smoking only a couple of cigarettes each day."
They suggest that many people may be surprised by these findings. "But there are also biological mechanisms that help explain the unexpectedly high risk associated with a low level of smoking," they add.
'Only complete cessation is protective'
"Any assumption that smoking less protects against heart disease or stroke has been dispelled," notes Kenneth Johnson, adjunct professor at the University of Ottawa in Canada.
In an editorial linked to the review, he discusses its significance to public health and concludes, "only complete cessation is protective and should be emphasized by all prevention measures and policies."
He urges regulators who are dealing with new "reduced risk" tobacco products to take note because "any suggestion of seriously reduced coronary heart disease and stroke from using these products is premature."
"The take-home message for smokers is that any exposure to cigarette smoke is too much."
Prof. Kenneth C. Johnson