The overblown risks of second-hand smoke

Throughout Europe bans on smoking in all public places have either been implemented or are under consideration. It is widely assumed that there is a strong scientific basis for claims for the health risks of passive smoking. But is there?

Shortly before his death in 2005, UK scientist Sir Richard Doll published his 50-year follow-up of the key original study showing the link between tobacco and lung cancer: its headline conclusion was that cigarettes cut smokers' life expectancy by ten years.

But in claiming a similar level of scientific authority for the dangers of passive smoking, epidemiology may have surrendered to the demands of public health propaganda.

The Swedish toxicologist Robert Nilsson, while accepting that numerous studies appear to show that passive smoking causes a statistically significant increase in risk, has questioned its epidemiological significance.

He offers estimates of the annual incidence of cancer in a population of 100,000 resulting from various environmental factors: unknown (177), diet (135), smoking (68), other lifestyle factors (45), sunshine (23), environmental tobacco smoke (2).

By contrast, in a population that consumes Japanese seafood (which contains arsenic) this will cause 12 cases of cancer, where there are traces of natural arsenic in drinking water (five cases); eating mushrooms (three cases).

In other words, Nilson found that the risk of passive smoking is comparable with that of environmental agents that are generally regarded as an insignificant threat to health.

But perhaps the most fundamental defect of the presentation of the risk of passive smoking is the failure to distinguish between relative and absolute risk.

In a critical commentary, Raymond Johnstone, the Australian medical research scientist, notes that the annual death rate from lung cancer among the non-smoking wives of non-smoking men is around six per 100,000, whereas among the non-smoking wives of smoking men the corresponding figure is eight per 100,000. This may be reported as an increased (relative) risk of 33%.

Yet in absolute terms it amounts to an absolute (or exposure) risk of one in 50,000, which is, for practical purposes, negligible.

His conclusion is: "The most that one can say about the alleged link between passive smoking and lung cancer is that if there is one, then it is so small that it is difficult to measure it accurately and the risk, if any, is well below the level of those to which we normally pay attention."

The alarming estimates of deaths attributable to passive smoking result from multiplying miniscule risks of dubious validity by vast population numbers - an effective propaganda device but statistical sharp practice.

The intense moral fervour and political commitment now driving the campaign against passive smoking has created a climate inimical to serious scientific inquiry.

In 2003 the British Medical Journal published a study of 120,000 adults in California over a 40-year period, which concluded that "the results do not support a causal association between environmental tobacco smoke and tobacco-related mortality, though they do not rule out a small effect".

The authors were subjected to a barrage of personal attacks and insinuations of dishonesty. In response, they pointed out the selective reporting of the anti-smoking campaigners and their attempts to suppress divergent data. They noted that "what is most dangerous is the willingness to distort the truth to defend one's position, claiming all along that science and righteousness are on one's side".

The drive to impose restrictions on smoking has not been in the least inhibited by expert doubts about the validity of the evidence on which it is based. Indeed, as medical historian Virginia Berridge has observed, "the coalition advocating those restrictions pre-dated the evidence".

Yet, as she acknowledges, "by the mid-1990s, there was widespread agreement that the epidemiological evidence on passive smoking was at least debatable".

When Doll was asked about passive smoking in 1998, he indicated that though he thought it was harmful, he found that "the quantitative relationship" was "very weak", suggesting that this view was more grounded in loyalty to the anti-smoking cause than his confidence in the figures.

And when he was interviewed on BBC Radio in February 2001, Doll said that "the effects of other people smoking in my presence are so small that it doesn't worry me".

• Dr Michael Fitzpatrick is a general practitioner in North-East London and the author of several books, including MMR and Autism (Routledge, 2004). He is a regular columnist for the British Medical Journal.

Commentary feature in which the author takes a look at research which claims that passive smoking is an insignificant risk. Article is part of a European Voice Special Report 'Healthcare'.