Friday, July 02, 2004

Passive smoking and heart disease

'The risks of passive smoking could be twice as bad as previously feared,' according to BBC News reporting on a study by London's St George's Medical School and Royal Free Hospital. The researchers studied 4792 men, aged 40-59 years old, over a period of 20 years. They found that those non-smokers with the highest levels of cotinine (a by-product of nicotine) in their blood were 50-60 per cent more likely to suffer from coronary heart disease than those will very low levels. Professor Peter Whincup, who led the study, told BBC News, 'The effects of passive smoking are likely to be bigger and more widespread. This adds weight to the argument that we should do everything we can to minimise passive smoking exposure.'

Reports in the Guardian suggest that Labour may include a prohibition on public smoking in its next election manifesto, while doctors at the British Medical Association conference produced a giant prescription demanding a ban.

A link between passive smoking and ill-health is plausible, but has never been proven. While this study seems to use better methods than many previous ones, there are major question marks over the results.

A big problem with many previous studies has been that they simply asked a non-smoker how much their partner smoked and tried to estimate exposure on that basis. This study is better in that it actually attempts to provide some kind of objective measure of nicotine exposure. But only one sample was taken from the study subjects - between 1978 and 1980. This was then frozen and thawed for testing 20 years later.

So, at best, we have a snapshot of passive smoking exposure from 25 years ago. But these men were already well into middle age, so it doesn't give us much of a clue as to what exposure they had in the many years before the test - and the many years afterwards. Nor do we know if the reading for any particular man was a blip due to a one-off exposure close to the test. And smoking is not the only source of nicotine. Many vegetables, like potatoes, and black tea contain significant levels which could have affected the results. Surely testing would need to have been done at a number of different times in order to draw firm conclusions?

The results from this study also seem to be perverse. For example, the risks for non-smokers with 'heavy' levels of passive smoke exposure seem to be very similar to those of people who smoke between one and nine cigarettes a day - even though the cotinine levels for the smokers are, on average, 28 times higher. While the light passive smokers suffered few incidents of heart disease in the first few years of the study, in later years they suffered just as many incidents per year as the heavy passive smoking group.

The results produced in this study, described by the authors themselves as, '…modest in size with limited precision…' seem peculiar unless we are to believe that there is a relationship between smoking exposure and heart disease which increases dramatically at low levels of exposure, then plateaus at an exposure equivalent to considerably less than one cigarette per day and maintains a similar level even among active smokers. Just to add to the confusion, no significant effect was found in relation to strokes - which might have been expected if smoking exposure was causing other health problems.

The absolute risks are also worth noting: at the end of the study, 82 per cent of heavy passive smokers, now between 60 and 79 years old, had not suffered a major heart disease event. Among the light passive smokers, the figure was 88 per cent. This is by no means a tiny difference. But even if everything reported in this study were true, is it really worth adding further to the burden of legislation so that a small number of people can have an extra couple of years on the geriatric ward?

Moreover, this study is being used as an argument about what to do about passive smoking in 2004, using measurements taken over two decades ago. Since then, smoking rates have declined, smoking in many workplaces and on public transport has been banned already, and ventilation is often much better in those places that are still smoky. For the vast majority of people, exposure will have fallen dramatically already. Yet, that hasn't stopped this study being used to increase calls for more prohibitive legislation.

Passive smoking and risk of coronary heart disease and stroke: prospective study with cotinine measurement, British Medical Journal, 30 June 2004

BBC NEWS | Health | Passive smoke risk 'even greater'

Monday, June 28, 2004

Nice quote from John Mortimer

"I refuse to spend my life worrying about what I eat. There is no pleasure worth forgoing just for an extra three years in the geriatric ward."

An epidemic of epidemiology

I've used the 50th anniversary of Hill and Doll's first report on their classic smoking study to restate some of the ideas from John Brignell's new book, The Epidemiologists: Have They Got Scares For You. Like his previous book, Sorry, Wrong Number, it's an excellent review of what is wrong with the science behind much of the health panics we read about today. It's only a shame that it's not with a bigger publisher that could give the book a wider audience.

spiked-science | Article | An epidemic of epidemiology