Smoking damages the body in minutes rather than years, according to research in the US.
The report, published in Chemical Research in Toxicology, shows that chemicals which cause cancer form rapidly after smoking.
Scientists involved in the small-scale study described the results as a stark warning to people considering smoking.
Anti-smoking charity Ash described the research as "chilling" and as a warning that it is never too early to quit.
The long term impact of smoking, from heart disease to a range of cancers, are well known. This study suggests the damage begins just moments after the first cigarette is smoked.
Faster than you might think
The researchers looked at the level of chemicals linked with cancer, polycyclic aromatic hydrocarbons (PAH), in 12 patients after smoking.
A PAH was added to the subject's cigarettes, which was then modified by the body and turned into another chemical which damages DNA and has been linked with cancer.
The research shows this process only took between 15 and 30 minutes to take place.
Professor Stephen Hecht, from the University of Minnesota, said: "This study is unique, it is the first to investigate human metabolism of a PAH specifically delivered by inhalation in cigarette smoke, without interference by other sources of exposure such as air pollution or the diet.
The results reported here should serve as a stark warning to those who are considering starting to smoke cigarettes."
Martin Dockrell, director of policy and research at Ash (Action on Smoking and Health), said: "Almost everybody knows that smoking can cause lung cancer.
"The chilling thing about this research is that it shows just how early the very first stages of that process begin - not in 30 years but within 30 minutes of a single cigarette for every subject in the study.
"The process starts early but it is never too late to quit and the sooner you quit the sooner you start to reduce the harm."
The research was funded by the US National Cancer Institute.
http://www.bbc.co.uk/news/health-12193602
