About 40 per cent did not know of the existence of small holes in the filters of low-tar cigarettes, designed to let in air and thereby lessen tar and nicotine intake; while two thirds had no idea that blocking the vents with lips or fingers would almost double the yields.
In theory, low-tar cigarettes (defined as less than 10mg) should be better for you: the fewer toxic chemicals inhaled, the fewer tar deposits form in the lungs. Which is why, thanks to a series of voluntary agreements between government and the tobacco industry, tar yields generally have been falling over the past 30-odd years - from about 35mg per cigarette in the Fifties to the maximum limit, introduced this year by EU directive, of 12mg. Nicotine yields have declined too: today's typical low-tar brand might have a yield of about 6mg of tar and 0.5mg nicotine. A good proportion of the tobacco smoked is highly processed, and thought to be less biologically active than untreated tobacco.
There is little doubt that the drop in tar levels (as well as a fall in tobacco sales) has been a crucial factor in the decline in lung cancer rates in Britain. Over the last 30 years, for example, rates of the disease among young men have fallen by about two thirds. Research has also found that smokers of low-tar cigarettes run a slightly lower risk of heart disease. So is switching to low tar a healthy option, as 30 per cent of the smokers appear to believe? Far from it.
For one thing, the "smoking" machines used by the industry to measure tar and nicotine yields bear little resemblance to a real live smoker. As the US survey suggests, many smokers may unwittingly block the air vents on the filter simple because they do not know of their existence. More crucially, research shows that those who switch to low-tar brands compensate by "oversmoking" - inhaling more deeply, puffing more frequently or smoking more cigarettes - in order to get the nicotine fix they are used to. Behavioural scientists believe that because of this compensating behaviour, nicotine intakes may be similar to those of 30 years ago, when people were smoking far stronger brands.
In addition (and as the tobacco giant the Liggett Group admitted last week, to the consternation of the rest of the industry) smoking cigarettes is a potentially lethal activity, whatever the tar yield. Smokers of low-tar brands may be at relatively lower risk of lung cancer - but tobacco remains the largest cause of all cancer deaths in the UK. Low-tar brands may also mean a slightly lower risk of heart disease - but smokers in their thirties and forties still have five times as many heart attacks as non-smokers, with the difference between low- and medium-tar cigarettes very small indeed: for heart attacks below the age of 60, about 75 per cent in medium-tar smokers and 70 per cent in low-tar smokers were caused by tobacco. Tobacco-related deaths may be down by a fifth in the last decade, but one in four smokers will still die in middle age, and about half of all cigarette smokers will eventually be killed by their habit, either from lung cancer, heart disease, lung disorders such as bronchitis and emphysema, or some two dozen other tobacco-related diseases.
Researchers are at pains to stress there is no such thing as a safe cigarette and the best thing any smoker can do is give up the weed completely. As one eminent researcher has put it - switching to a low-tar brand is like jumping from the 25th floor of a tower block, rather than the 30th.Reuse content