Fears over additive-free cigarettes
Smokers who believe they are choosing the healthy option with additive-free cigarettes may find they are actually more toxic than conventional ones.
US researchers say tests showed nicotine levels were higher after smoking the additive-free cigarettes.
Researchers say teenagers in particular are increasingly choosing alternative cigarettes such as cloves, hand-rolled Indian bidis, and other additive-free cigarettes, expecting them to be better for them than the usual brands.
A survey of teenagers in the Boston area found 40% had smoked bidis at least once.
In addition to their perceived health benefits, bidis are popular with young smokers because they are cheap and easy to buy, and come in a range of flavours, such as chocolate or root beer.
Ten smokers, with an average age of 24, were asked to smoke their own brand of cigarette, a strawberry and a non-flavoured bidi and un-filtered additive-free American Spirit cigarettes.
Plasma nicotine and exhaled carbon monoxide levels were monitored.
The time taken to smoke each cigarette and the number of puffs taken were also recorded.
After smoking, participants completed questionnaires about each product.
It was found that nicotine levels were significantly higher after smoking American Spirit cigarettes or bidis, compared to when they smoked their own cigarettes.
High nicotine levels lasted longest with the American Spirit cigarette.
Exhaled carbon monoxide levels, measured 15 minutes after the cigarettes had been smoked, were less conclusive.
Levels were lower for American Spirit and the unflavoured bidi than the smoker's own brands.
But higher levels of carbon monoxide were exhaled after smoking the strawberry-flavoured bidi.
Dr Wallace Pickworth of the National Institute of Drug Abuse, who led the research, admitted numbers were small and included only occasional bidi smokers, which could have influenced the findings.
But he said: "The results indicate that, contrary to the belief of many consumers, bidi and additive-free cigarettes deliver substantial amounts of nicotine and other toxic components of tobacco smoke."
Amanda Sandford, research manager for Action on Smoking and Health: "The study explodes the myth that additive-free or 'natural' tobacco is safer than that used in conventionally produced cigarettes.
"Whilst there are concerns that some additives may increase the addictiveness of nicotine or make it easier to smoke, this study underlines the fact that 'natural' tobacco is just as hazardous as that packed with additives."