Behind the smokescreen

(Warning: compared to my usual posts this is a bit long and a bit grumpy)

I’m a non-smoking epidemiologist, with an academic focus on non-communicable disease. I should be in favour of reducing the public health threat of smoking, right? You bet! And I should be delighted by the BMA’s recommendations that smoking in cars should be banned? I’m not so sure…

See, it sounds like a good idea in some respects. The harms of second hand smoke are well established and they informed the legislation on banning smoking in public places. But I found myself wondering just how much harm passive smoke exposure in cars actually causes -what is the population attributable disease risk? And whether banning smoking in cars would actually do anything to reduce road accidents and lessen the burden of disease related to second hand smoke?

Media reports were lacking these details so I turned to the BMA’s 22 page publication on the matter.  And I’m still none the wiser. At best, it’s a non-systematic review in which any actual quantitative data on the effects of in-vehicle smoking on health has not been presented and/or is referenced in an oblique fashion (by referencing papers that themselves are referencing the original sources). There are two main lines of argument put forward:

(i)  Passive smoke inhalation is harmful to health, and car travel with a smoker presents a particular hazard in this regard.

(ii) The act of smoking while driving poses a risk of distraction which may cause road accidents.

Let’s unpack these arguments and examine the evidence base presented.

Starting with the section “Smoking in vehicles: health impacts”:

“…studies demonstrate that the concentration of toxins in a smoke-filled vehicle is 23 times greater than that of a smoky bar, even under realistic ventilation conditions.”

One of the sources for this figure makes the comparison between a car and a house, not a car and a bar, but is not itself the original source of that statistic. In fact the “23 times greater” stat seems to have been artfully debunked here but nonetheless persists in the literature.


 “In England an estimated 30 per cent of smokers smoke in their vehicles, and over half of all journeys made by children aged 16 and under are by private vehicle. It is likely that private vehicles are a significant source of exposure to [second hand smoke] in children.”

So, we have an estimate for prevalence of in-car smoking, and an estimate of frequency of car travel by the under 16s, but no estimate of the proportion of car journeys undertaken by children in which someone is smoking. Some big gaps here. And what does ‘significant’ mean in this context?

Later the report says:

“A study by the British Lung Foundation (BLF) showed that more than half (51 per cent) of eight to 15 year olds have been exposed to cigarette smoke when confined in a vehicle.”

The source for this is footnote in a press release. Unfortunately the methods and detailed results of the survey are only available on request so I can’t check whether those 51% of 1000 children surveyed are exposed on a daily basis or have ever been exposed to in-vehicle smoke, even if just once. So I still say the jury is out on the prevalence and magnitude of this exposure.

They make another attempt here:

“This [passive smoking exposure occurring ‘sometimes’ or ‘often’] is especially true of young people (37 per cent of 18 to 24 year olds), and of adults from lower socio-economic groups (31 per cent of those in socioeconomic groups C2DEb).”

This wording is lifted directly from this RCP report (p. 144). The numbers seem to come from a YouGov 2009 smoking survey but I can’t locate that to check how the question was asked or how many young people were surveyed. So none of the above statements are based on peer-reviewed research in the public domain.

Now for the next section: “Smoking in vehicles: road safety”

“There is some evidence that drivers who smoke are at greater risk of accidents than those who do not, with smokers about 50 per cent more likely to die in a crash.”

This references a paper which was actually an experimental study of 10 people, videoed while driving and subjected to distractions. Presumably the “50% more likely” figure comes from something referred to in the introduction, but I cannot check as the paper is in Italian. It would have helped to cite the original source of this statistic – presently it’s unclear whether the elevated mortality is due to a higher likelihood of crashing, or worse survival once involved in a crash.

“A study by Hutchens et al (2008) showed that smoking was the only unique factor among a host of associated risk factors (alcohol, marijuana, sensation seeking) with greater crash odds, independent of demographic factors and general health risk taking. The study determined that smokers were twice as likely to have had a crash as non-smokers, even after controlling for gender, race, ethnicity, geography, socio-economic status, and the length of licence held.”

This references a paper in which teenagers (not all drivers) were found to be twice as likely to have had a crash if they were smokers. This is not the same as a risk posed by smoking while driving. Indeed the authors of this study are exploring whether risky health behaviours are correlated with risky driving behaviours. As far as I can tell from a quick skim, at no point do the authors suggest that smoking while driving predisposes to accidents, rather that it is something about the risk behaviours of teenage smokers that put them at elevated risk of road accidents.

“Elliot et al (2006) found cigarette use was positively and significantly associated with traffic incidences – including crash incidence – among young drivers.”

Again, this is a study of correlations between risky health behaviours and risky driving behaviours.

“A 1996 study by Lang et al had shown cigarette use to be a key predictor of single vehicle crashes for females during the first two years of driving.”

This is another study of risk behaviours in adolescents.The main message I’m getting so far is that we shouldn’t allow teenage smokers to drive.

“It is important to note that the Highway Code does not make it a specific offence to smoke while driving, any more than it is currently an offence to use audio or navigation systems, or eat while driving. …Alongside the evidence base highlighting that smoking and [second hand smoke] in a vehicle is harmful to health, the BMA policy to ban smoking in vehicles would help to ensure that drivers are not distracted by the act of smoking while driving and this may reduce trauma, health service usage and deaths due to road traffic accidents, although further research is required.”

In all there are a number of gaps between the presented evidence and the postulated effects on health. The authors do concede that “further research is required” with respect to the road safety issue but do not acknowledge the limitation that this places on the strength of their recommendations. There is a lot of supposition about the magnitude of exposure to second hand smoke in vehicles and the likely consequences, without any proposal to quantify this in cohort studies or other research. The evidence that is presented is misleading in places.

The idea that smoking in confined spaces like cars poses a respiratory risk for non-smoking passengers, and may cause an additional distraction hazard is definitely plausible. But in the absence of quantitative estimates of the harms of in-vehicle smoking and the possible benefits of removing this hazard, I think the BMA have overstepped the mark by calling for a change to legislation. A greater benefit to child health could probably be had by legislating against transporting children over 10 by car for distances under one mile: think of the cardiovascular benefit, the reduction in vehicle emissions and reduced number of road vehicle crashes (although probably tempered by the unfortunate hazards of being a pedestrian/cyclist). But as I don’t have the numbers to back that up I’m not going to make a formal recommendation and send out a press release.

Let me be clear – I have no pro-smoking agenda. The only agenda I want to push is one of evidence-based medicine and policy. That politicians misrepresent data to support their arguments is no surprise, but I feel deeply disappointed that an organisation like the BMA would do something as drastic as call for changes in the law when the evidence does not yet support the claimed extent of the hazard nor the likely benefits of such an intervention. Failure to base such recommendations on firm evidence undermines the credibility of public health advice and serves to give more credence to the ‘nanny state’ accusations that are levelled at far more robust recommendations.

I’d love to see smoking rates reduced to nothing and an eradication of all smoking-related illnesses, but I also want to be able to trust that the finite resources and influence that medical professionals have at their disposal are being used in the most effective way. I want to know that lobbying is based upon a solid evidence base supporting a causal relationship, not just ecological studies, surveys and postulation. I want proposals to be honest about what they are, and not pretend to be evidence-based when the evidence is thin and maybe not even the right evidence to be reporting. Is that too much to ask?


25th Nov: Update on this post here.



  1. Very nice commentary. As to smoking being a distraction, from personal experience it’s not the smoking, but ferreting about for the fag packet and the lighter, adjusting the windows to get the flame to stay alight etc. etc. which is distracting!
    But seriously – the suggestion that smokers are more prone to accidents probably can be demonstrated at a psychological level. To continue to smoke (as I do) in the face of all the evidence suggests that one has an attitude to danger (or to gambling) which might also translate into being less than ‘mumsy’ at the wheel.

  2. Excellent overview Ben and thanks for raising this – I did Tweet the URL but forgot to name check you 😦

  3. I should just add that the above post doesn’t mean that I’m in favour of smoking in cars. If the BMA had said something like “It’s well established that smoking and second hand smoke have associated health risks. We would like to reduce the UK smoking rates by making smoking more inconvenient and normalising the smoke-free environment, and would thus urge for a review on issue of smoking in private vehicles” (or something along those lines) then I’d have been 100% ok with that. But instead there’s a pseudo-scientific review of selected evidence and a call for laws to be changed.

    It seems weird to make such a proposal when we don’t even know the extent of the problem – 79% of people in the UK don’t smoke, 70% of English smokers don’t smoke in their cars and of the 30% who do, we have no idea how many do it when they have passengers, and how often. It may be that an intervention targeted at those who persist may have more of an effect than a blanket ban which is practically unenforceable. Meanwhile, the time and resources put in to lobbying on this (with no guarantee of success) might be better deployed tackling public health issues with interventions we *do* know work.

  4. Interesting…. the link I provided to the report has now broken. Foolishly I hadn’t saved it but it’s still available via Google cache here:

    Digging around I have managed to find another link here:
    But now the report has been edited to say “Further studies demonstrate that the concentration of toxins in a smoke-filled vehicle could be up to 11 times greater than that of a smoky bar.” So the 23 times error has gone.

  5. Thank you for that.

    I was half expecting you to say that one of the research papers cited by the BMA was on display in the bottom of a locked filing cabinet stuck in a disused lavatory with a sign on the door saying ‘Beware of the Leopard’.


  6. Steve Powell · · Reply

    Thanks for this, you raise some good points – still, the BMA does cite four actual peer-reviewed studies on smoking in cars, and as these are indeed overall quite worrying I thought I would put the links here to save others time in googling. is an experimental study from New Zealand which suggests that smoking in cars with a window open produced levels of microparticles comparable to those in bars in which smoking is allowed, and smoking in a car with the windows closed produced levels around ten times as high; in each case the levels dropped back to near zero after a few minutes.–Car_Smoke.pdf is a rather more thorough article which reports levels of microparticles after one cigarette in a car comparable to those in bars (actually, this is the report on which the article is based, as the article itself is subscription-only, yuk). There is a third similar article which produced dangerous levels closer to those in the NZ study. The fourth relevant study is on airflow and is most worrying because it goes into the medium-term effects; how long do the negative effects linger:

  7. Yet the BMAs Head of Science and Ethics in response to direct questions claimed that the 23 times number was based on peer reviewed evidence and scientific consensus. She did so very publicly and very assuredly on Radio 4s Today programme.

    I think that says everything anyone needs to know about the ethics of the BMA.

    If we did introduce legislation to ban smoking in cars with children present which is what the BMA are really trying to achieve then it is extremely unlikely that we would ever be able to design a study that could measure the benefits. How could we? We have no idea of the harm if any that is occurring. So why make a scientific case in the first place?

    I don’t smoke and have no vested commercial interests in this argument. I am just tired of the abuse of both my intelligence and science by health campaigners. I have no problem with people campaigning on moral grounds, I would just prefer them not to claim scientific credibility and above all else, to please stop lying.

  8. Anthony Williams · · Reply

    On a lighter note, if 51% of the Children are exposed to SHS by only 30% of smokers, then the loss of fertility due to smoking has been shown by the figures to be a MYTH, just as we know the rest of it is.

  9. When you put it like that, the numbers *are* a bit of a head-scratcher! 🙂

    Thanks for sharing those links, Steve. It’s a shame that they stopped short of trying to do some modelling of the extent of the problem in the UK (incorporating actual estimates of numbers of journeys in which passengers are exposed to smoke) instead of the slightly woolly statements with big gaps in between.

    The more I think about it, the less I’m inclined to think that the BMA reports’ questionable presentation of the data is deliberate obfuscation – the way it reads suggests to me inexperience on the part of whoever was charged with reviewing the literature and writing the report. But that should have been picked up by internal fact checking/editing and it certainly shouldn’t have been released in this form.

    1. The history of public health campaigning suggests that you are erring on the side of kindness by portraying the BMA as incompetent rather than disingenuous but it should be remembered that the BMA is a Trade Union and not an expert scientific body so perhaps there is some merit in that argument. Apologies if the use of the L word was a bit heavy for the blog, it’s just that I haven’t really got to grips with all the fancy ways of describing people who are not telling the truth.

  10. Daniel I. Shostak · · Reply

    Extremely interesting and challenging conversation.

    Currently, in public health, medical care, and public policy disciplines there is a growing and accelerating movement towards ‘evidence based’ decision making. While I believe that the concept of EBDM is conceptually ‘reasonable,’ its current state is not.

    The BMJ material is disappointing in its ‘cherry picking’ of limited evidence. In this particular instance, BMJ is not providing ‘evidence,’ but using its political capital to legitimate and further the issue. I believe this to be reasonable as I think it is in the best interests of the public to reduce secondary exposure to smoke. Nor is it reasonable to believe that the policy does any direct harm. This is not EBM/EBDM, but the more traditional, interest based, and still successful route to public health policy making.

    The original post and the commentary points to several hurdles to EBM/EBDM:
    -Many policy analysts/politicians/lobbyists have little knowledge or regard for EBDM; let alone ability to apply or perform it.
    -Those who want EBDM have yet to convince possible users that it is preferable/better than existing policy processes.
    -Often, there is scant actual evidence upon which to deploy EBDM in public health policy.
    -Given the above and other considerations, there remains a lack of consensus as to what information constitutes evidence.

    I believe that EBDM for public health/population health/prevention policy suffers a more difficult burden than medical care EBM. There are incentives for the health industries and governments to develop medical ‘evidence.’ However, there are few incentives to do the necessary research for public health. It is even possible, that there are interests that actively oppose public health evidence building.

    Many public health initiatives of the past have been reactive to ‘visible’ manifested health emergencies. I think that most of us in public health policy wish to expand policy beyond these emergencies in order to be more pro-active.

    At this time, public health policy makers are torn between the customary, political approach of policymaking, proposing ideas that appear to be common sense and of relatively low cost. Alternatively, we can switch to EBM/EBDM in the face of policy makers without sufficient understanding of it and insufficient evidence.

    In today’s difficult economic times where research funding is increasingly difficult to obtain, I do not believe that second hand smoke exposure in autos would top the list of priority public health issues for research to build evidence. This issue, along with many others, does not merit the effort to do proper evidence development. The political, partially informed process remains best route to expanding public health/population health/prevention policy.

    Those of us who encourage EBM/EBDM need to develop campaigns that demonstrate EBM/EBDM as superior to existing policy-making processes and to educate policy makers. In the meantime, we need to continue to support ‘common sense’ policy initiatives that are likely to improve population health, despite the poor evidence base.

    Daniel I Shostak, MPH MPP
    Strategic Affairs Forecasting LLC

  11. Jonathan Bagley · · Reply

    There have been studies based on data from accidents among the general population The one most widely quoted was funded by the AAA assoc. for traffic safety in 2001. Smoking was found to be negligible: see below

    Specific distraction % of drivers
    Outside person, object, or event 29.4%
    Adjusting radio/cassette/CD 11.4%
    Other occupant 10.9%
    Unknown distraction 8.6%
    Moving object in vehicle 4.3%
    Other device/object 2.9%
    Adjusting vehicle/climate controls 2.8%
    Eating and/or drinking 1.7%
    Using/dialing cell phone 1.5%
    Smoking related 0.9%
    Other distractions 25.6%
    The study can be found here

    I fear that, just as with passive smoking, this will be swamped by junk studies produced by anti tobacco campaigners. This seems to be the only avenue they have have in attempting to ban smoking in cars with no passengers. I find these conclusions consistent with my own anecdotal experience. I can’t remember ever coming close to an accident through smoking, but on several occasions after fiddling with the radio, I’ve thought, “There but for the grace of God”.

  12. ‘The harms of second hand smoke are well established and they informed the legislation on banning smoking in public places.’

    The risks are minimal (at best), the information presented to the public, health workers and parliament was cherry picked and spun in a manner that appeared to wildly exaggerate the risk of potential harm to non smokers. Why don’t they simply stick with the same old tired propaganda already cited? A car is, to all intents and purposes, an enclosed space. And, as already decreed, there is NO safe level of exposure to ETS (by definition presumably outdoors as well). Oh yea, major sticking point – a private car is an entirely private space. As are houses, which are next on the list…..

  13. harleyrider1978 · · Reply

    Michael Mcfadden said it best on the second hand smoke nonsence:

    the “threshold” theory and the “no threshold” theory.

    The threshold theory argues that below a certain level any exposures to carcinogens literally “disappear” because they get swallowed up in all the random chaos of how the body works at a microscopic level. The “no threshold” theory says that even the smallest possible event, such as a one second exposure to a small beam of sunlight, could be the “trigger” that later produces a deadly cancer. The “no threshold” theory applied in meteorology would say that yes, a single flap of a single butterfly’s wings in the backwoods of Australia could have “caused” Hurricane Katrina. The “threshold” scientists say that such an idea is nonsense. The Antismokers’ “no safe level” argument is a “no-threshold” argument: they’re saying even very small exposures to ETS could have as big an effect on lung cancer as a butterfly’s wings in Australia could have on US hurricanes.

    Until Fanatacism is removed from public health, nothing they say or claim should be regarded as truthful!

    Junk Science on second hand smoke has destroyed public healths name and credibility. A new disrespect for the laws passed by the government that erode civil liberties cannot be repeaied until government gives up nanny state politics and goes back to a foundation of freedom and liberty!

    1. harleyrider1978 · · Reply

      I stated this part:

      Until Fanatacism is removed from public health, nothing they say or claim should be regarded as truthful!

      Junk Science on second hand smoke has destroyed public healths name and credibility. A new disrespect for the laws passed by the government that erode civil liberties cannot be repeaied until government gives up nanny state politics and goes back to a foundation of freedom and liberty!

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