SMOKERS are further risking their health by inhaling their own second-hand smoke, a study has shown.
Passive smoking has already been proven to be a hazard for people living or working alongside smokers.
But its dangers to smokers were thought to be negligible compared with those inhaling tobacco smoke directly.
Scientists in Italy conducted a study on newspaper vendors, who tend to work in isolation.
"Newsagents were chosen because they work alone in small news stands, meaning that any tobacco smoke in the air they breathe is strictly correlated to the number of cigarettes smoked by that newsagent," said researcher Maria Teresa Piccardo from the National Cancer Research Institute in Genoa.
"We studied the contribution of environmental tobacco smoke made to carcinogen exposure in 15 active smokers," said Ms Piccardo.
For someone smoking 14 cigarettes a day, re-inhaling exhaled second-hand smoke resulted in an exposure to cancer-triggers equivalent to smoking an extra 2.6 cigarettes, the study found.
"Both active and passive smoking contributions should always be considered in studies about the health of active smokers," Ms Piccardo added.
The research is published online today in the journal Environmental Health.