It's one of the first studies of its kind. Researchers compared the levels of tobacco specific nitrosamines in cigarette butts and in smokers from several countries. They found not all cigarettes deliver the same amount of carcinogens to people.
Nitrosamines are the main carcinogenic component of tobacco, and this study found that US brands contain about triple that of brands from Australia, Canada or the UK.
The report is published in Cancer Epidemiology Biomarkers and Prevention.