Thursday, 6 March 2014

Smoking bans in the home: the next logical step

Never ones to let the grass grow under their feet, the tobacco prohibitionists have predictably got their eyes on banning smoking in the home as the 'next logical step' after banning it in cars. Hence this reporting from the BBC's number one 'public health' collaborator Michelle Roberts of this rather feeble study...

Passive smoking causes lasting damage to children's arteries, prematurely ageing their blood vessels by more than three years, say researchers.

I was going to delve into the study itself, but Lee Jones has written a superlative post about it already which I warmly recommend.

What does the study actually show? The researchers looked at two groups of Finnish and Australian people, asked whether their parents were smoking in 1980 and 1983, and measured their carotid arteries during 2004-2006. If the parents didn’t smoke, the average catorid IMT ranged between 0.611mm and 0.653mm, while with both parents smoking it ranged from 0.625 to 0.669. Note, to begin with, it takes both parents smoking to see any results at all. If only one smoked, there is no difference at all between the results. Even with both smoking, the differences in IMT are obviously extremely slight, with a considerable overlap in the range of sizes recorded. Moreover, when a small group of the Finnish participants were measured three months apart, to check the validity of the IMT measurements, they found that the measurements varied by between 0.01 and 0.09mm. In some cases, then, the sheer inaccuracy of the measuring instruments would suffice to erase any difference between the smoker/ non-smoker groups. Bearing in mind that only 6% of the sample of Finns had two parents who smoked, this is a serious problem. The authors concede on p.7: “it is possible that the level of IMT and the difference in IMT between exposure groups differs to that shown here. As such, the results should be interpreted with caution”. This despite the shrill demand for “all measures” to be deployed just one page earlier, and equally strident remarks in the media.

Moreover, the article does not even try to quantify, in absolute or relative terms, the enhanced risk of heart disease implied by their findings. Note that their findings only relate to the thickness of arteries; they don’t say what the health effects are. They merely use the scary “aged by 3.3 years” line. Why might this be? Perhaps because although there is growing evidence of an association between the thickness of the carotid artery and heart disease, the link does not appear a very strong one. The most widely cited study I could find on this showed that an IMT of less than 0.5mm was associated with a rate of cardiovascular disease of about 0.1 per 100 person-years. The rate increased to about 0.2 per 100 person-years between 0.5mm and 0.75mm, and about 0.8 over 0.75mm. So, there is an association, but it’s not a very strong one, and in the range the passive smoking study is interested in, the difference appears pretty miniscule; it’s where the artery hardens to over 0.75mm that we see big changes. The other paper shows that build up of calcium – “plaque” – in the arteries is actually a much better predictor of cardiovascular disease.

So, in other words: the passive smoking study shows minor differences in the thickness of arteries that may not even exist, and these minor differences are not particularly strongly associated with a higher rate of heart disease. And yet, on this basis, the lead author insists that anyone even imagining that they might have children must stop smoking, her research team supports “all measures” to prevent passive smoking as a “priority”, and public health alarmists garner more ammunition for their campaign.

Do read the whole piece because he also makes some wider points about the notion that policy should follow 'The Science' which echo much of what Jamie Whyte says in his excellent IEA book Quack Policy.



3 comments:

Jonathan Bagley said...

If it takes both parents smoking to detect a difference, rather than total smoking exceeding some level, the difference in the samples is likely due to random variation or random measurement inaccuracy - not any effect.

Michael J. McFadden said...

Worth noting: The IMT change measured here was .015mm. According to the data at http://tinyurl.com/HomeSmoking, a change 7x as great, .1 mm, correlates with a 10% increased risk in adults.


So, IF these figures are accurate, and *IF* the “adjustment models” used to arrive at them are also accurate, and IF those increases persist through the post-40/50 heart attack years, and IF the figures are actually a result of causality rather than simple correlation with a third variable…
**IF** all the above assumptions are true THEN the most damning statement that can be made is that kids exposed to 15 to 20 years of daily smoke will have slightly over a 1% greater chance of having an adult heart attack at some point sooner than otherwise.


That’s probably about the same risk as one extra fast-food burger a month, but it’s headlined as “irreversible damage” that will kill your children. Scaremonging a bit?


- MJM

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