Those first few puffs on a cigarette can within minutes causes genetic damage linked to cancer, says US scientists.
In fact, researchers say the "effect is so fast that it's equivalent to injecting the substance directly into the bloodstream," in findings described as a "stark warning" to those who smoke.
The study is the first on humans to track how substances in tobacco cause DNA damage, and appears in the peer-reviewed journal Chemical Research in Toxicology , issued by the American Chemical Society.
Using 12 volunteer smokers, scientists tracked pollutants called PAHs, or polycyclic aromatic hydrocarbons, which are carried in tobacco smoke and can also be found in coal-burning plants and in charred barbecue food.
They followed one particular type - phenanthrene, which is found in cigarette smoke - through the blood and saw it form a toxic substance that is known to "trash DNA, causing mutations that can cause cancer," says the study's authors.
"The smokers developed maximum levels of the substance in a time frame that surprised even the researchers: just 15 to 30 minutes after the volunteers finished smoking."
"These results are significant because PAH diol epoxides react readily with DNA, induce mutations, and are considered to be ultimate carcinogens of multiple PAH in cigarette smoke," the authors write.
Lead scientist, Professor Stephen Hecht of the University of Minnesota, says the study is unique because it examines the effects of inhaling cigarette smoke, without interference from other sources of harm such as pollution or a poor diet.
"The results reported here should serve as a stark warning to those who are considering starting to smoke cigarettes," says Hecht.
Lung cancer kills about 3000 people around the world each day, and 90 per cent of those deaths are attributable to cigarette smoking.
The research was funded by the National Cancer Institute.