Monday 18 July 2011

Second hand smoke and ADHD

Of all the modern day vices that society possesses like alcohol and drugs of abuse, tobacco smoking enjoys one the oddest relationships. Odd because despite a considerable body of evidence linking smoking to various health problems including those related to early mortality, combined with smoking being linked to a considerable health economics burden, people are still free to buy tobacco and smoke as much and as often as they choose to.

Yes, you might say that Governments have restricted various things in relation to smoking such as their taxed price, their availability to minors (supposedly), where a person can and cannot smoke, and even highlighting what smoking does to a person. The choice to smoke or not however still remains.

In recent times the health message on smoking has seemed to shift from emphasising the risk to the person smoking to the potential risks to those around the smoker with the rise of second-hand smoke. Here in the UK we have the recent 7-steps campaign, encouraging smoking parents for example to protect their kids by taking 7 steps outside from the backdoor when lighting up. I'm not sure how successful this initiative might be with the famous English weather whose default = rain.

I say all this because an article has appeared in the journal Pediatrics suggesting that second-hand smoke exposure might up the risk for various behavioural conditions, particularly attention-deficit hyperactivity disorder (ADHD). The full-text paper by Kabir and colleagues is available here. I don't want to go through all the papers methods and findings but the summary is here:

  • From a total sample size of approximately 90,000 children aged from birth to 17 years old, data for 55,358 children (aged 0-11 years) were analysed based on their exposure to second-hand smoke and their receipt (or not) of various neurobehavioural diagnoses such as learning disability, ADHD and conduct disorders. 
  • Based on this 2007 data, approximately 6% of children were deemed to be exposed to second-hand tobacco smoke. 
  • Learning disability, as reported by parents, was present in 8.2% of participants, 5.9% had ADHD and 3.6% had a behavioural or conduct disorder.
  • Adjusted prevalence figures showed that children exposed to tobacco smoke were quite a bit higher for all neurobehavioural conditions examined when compared with non-exposed children. The disparity in adjusted prevalence terms (exposed vs. not exposed) were: conduct disorder (8.7 vs. 2.8), ADHD/ADD (13.0 vs. 5.5), learning disability (15.1 vs. 7.2).
  • Outside of second-hand smoke exposure, various other factors were 'predictive' of an increased prevalence of the neurobehavioural conditions included. Poverty seemed to be the largest influence, following on from the various research linking poverty to smoking.

There are a few other details which have been picked up in the various media interest about this paper, but I have to say I am not altogether convinced that some of the reported points are the most important findings from this data. I would also add that as per the authors discussions, the data do not definitively show that smoke is the one and only risk factor; indeed adding a sentence which reads: "Other unmeasured potential confounders, such as lead exposure, cannot be ruled out". Mmm, takes me back to the US crime figures post.

I do think we have to be very careful when drawing firm conclusions from such fishing papers, particularly where 'risk' is used. Does this data alone make a case for banning smoking in homes where young children grow up? I'm afraid that I can't answer that question, although this is not the first study to suggest such an association between parental smoking and risk of ADHD. I would perhaps also add that such early exposure to tobacco smoke might also be one (of many) explanations why people with ADHD/ADD might be more prone to smoking themselves.

I would however think that data such as this on the link between second-hand smoke and potentially life-threatening conditions like asthma, perhaps makes for a more compelling public health message for Governments to work on about the dangers of second-hand smoke for children.


  1. Does correlation imply causation?

    It just occured to me...the seriousness of smoking and it's relation to early death...perhaps it's teratogenic in the womb. (Teratogenic is my favorite word since the genetic twin study implied that environment was partially responsible for autism.)

  2. Thanks for the comment. Correlation certainly does not imply causation, although I like to think that in some cases, many individually derived studies showing the same correlation might be a good roadmap towards a relationship between two (or more) factors.
    Ah yes, teratology I remember the term well. An extensive paper summarising some of the teratogenic factors 'associated' with developmental conditions like autism is this one by Taylor and Warner Rogers (full-text) :

  3. Second hand smoke can be very dangerous specially for kids, you should try not to smoke in your home and vehicles, try using electronic cigarettes, they are smokeless and also safer than the traditional ones.

  4. I have seen many people smoking in their cars when kids are present, in my opinion they are not worthy to have children when they can't take good care of them.


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