Passive smoking lowers blood pressure in girls, study reveals

Before retiring to bed last night, I cast a weary, cynical eye on a brief story from The Independent which reported:

Boys who inhale second-hand tobacco smoke at home may experience significant levels of raised blood pressure, a study has found.

But in girls, passive smoking appeared to be associated with a lowering of blood pressure.

These results, if true, suggest that there are dramatic differences between the male and female bodies that make them respond very differently to secondhand smoke. They also imply that there is a new and unlikely form of treatment for girls with high blood pressure: blow smoke at them.

I'm joking, of course, although it is as valid to draw that policy conclusion from the data as it is for the study's author to claim that her work provides "further incentive for governments to support smoking bans". Actually, neither policy would have any effect because the changes in blood pressure were negligible in both cases—a 1% increase/decrease either way, which even the author admits is meaningless for individual health.

An alternative interpretation is that the study found no effect of passive smoking on blood pressure but that by stratifying the results, the researcher was able to find one by chance (in this case, by splitting the group into genders). Or it may be something else; the study only compared cotinine readings with blood pressure readings after all.

The study's author naturally favours the first interpretation:

"These findings support studies suggesting that something about female gender may provide protection from harmful vascular change," said Dr Jill Baumgartner.

Whether there are any biological reasons to explain these contradictory findings, I don't know (... paging #carlvphillips ...). I couldn't possibly comment because—once again—this study has not been published  and it has only been presented at a conference before being press released around the world. It's a PhD dissertation by a student of Population Health and Environment & Resources.

There is an abstract available, however, which shows the increase in boys is not significant while the decrease in girls is significant. Take both genders together and the overall effect of passive smoking on blood pressure is to lower it, although it is unclear whether that association is significant. The abstract also shows a complete lack of a dose-response relationship which is a bit surprising if tobacco smoke was the true cause of the correlation.

What I do know is that it is misleading to report this study under the headline used by both the Daily Mail and The Guardian:

Passive smoking raises blood pressure in boys, study reveals

In fairness to the Mail, they mention the conflicting finding for girls towards the top of the article. The Guardian, to its shame, mentions it towards the bottom. Ben Goldacre recently mentioned an interesting lay experiment which showed how few people bother reading the whole article before making their minds up. Not very many, apparently, which gives The Guardian the horrible distinction of being even worse at reporting science news than the Daily Mail in this instance. (Neither were helped by the study's press release, which only briefly mentions the findings for females.)

We know that the great majority of newspaper readers only ever read the headline, so let's hope the same is not true of blog readers, otherwise the title of this post—which is as technically correct as the Mail and Guardian headlines*—will also give people a mischievously one-sided view of the story.


* Actually, rather more correct since I have at least focused on the statistically significant finding.

(Michael Siegel and Taking Liberties have more on this story.) 




UPDATE:


Carl V. Phillips has a definitive critique on this study over at Ep-ology. Well worth reading.

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