I was interviewed on BBC Scotland radio this morning on the subject of banning smoking in cars (listen here for 7 days, 2.17 hours in). Sleepy though I was—and coming off the back of a hilariously one-sided vox pop—I tried to make the point that the simple act of opening a window in a moving vehicle provides ample ventilation to disperse secondhand smoke.
Prof John Britton had earlier told the BBC that levels of secondhand smoke in cars are twenty times higher than in smoky bars. As a medical man, his opinion naturally trumped mine as far as the presenter was concerned. Nevertheless, it's worth finding out this '20 times higher' claim comes from.
A good place to start is a heavily referenced report from ASH (UK). It claims that:
According to a report by the Ontario Medical Association, secondhand smoke levels in cars can be 23 times greater than in a house.
ASH give a citation of this report from the Ontario Medical Association, which says:
Based on the evidence that exposure to SHS in a vehicle is 23-times more toxic than in a house due to the smaller enclosed space, the state of Colorado drafted a bill that would impose fines on adults caught smoking in cars when a child is present.
But what evidence is this? Their only reference turns out to be a news story from the Rocky Mountain News, not exactly a reliable scientific source.
ASH do, however, have another source:
A study comparing secondhand smoke particle concentrations in a vehicle with those in a bar which allowed smoking, found in-vehicle concentrations 20-times greater than inside the bar.
Again there is a citation, this time to an actual scientific journal, but the article in question does not measure secondhand smoke in cars, nor does it attempt to. It certainly doesn't give any estimate of how much more secondhand smoke is in cars than other locations, and it doesn't cite any references that might lead us to find an article that does.
And there the trail ends. Such is the game of Chinese whispers that passes for evidence-based medicine these days.
If you want to find some real science on this issue, you have to turn to an American Journal of Preventive Medicine study from 2006, which measured particulate matter (PM2.5) in vehicles.
Bearing in mind that the EPA's 'hazardous' level for 24 hour exposure is 250 ng/m3, this study found average peak concentrations of 271 ng/m3. But they did so by keeping the windows closed. When a window was opened, the level was only 51 ng/m3. This is a fraction of what would be found in a smoky bar (200-500 ng/m3) and is well within the EPA's limit (which, remember, is for 24 hour exposures, not the occasional car journey). And after smoking, levels quickly fell to the same found in a nonsmokers' car.
The study also measured carbon monoxide levels, with even less impressive results. When the window was opened, levels barely changed at all.
Another study sometimes cited carried out a similar experiment but only opened the window by 3 inches. Even with this restricted ventilation, average levels of PM2.5 were 119 ng/m3—well below the EPA's hazardous level.
All of which suggests that—if smoking in cars is a problem at all—it is one that can be simply solved by opening the window. And that, of course, is what everyone already does.
Would I agree with a law forcing people to open the window whilst smoking? I probably would, if I thought that would be the end of the matter, but we all know that it won't be because protecting people from secondhand smoke is not the purpose of the Royal College of Physicians' latest 'demands'. By calling for a total ban on smoking in cars, even when no one else is present, they have finally given the game away. This issue isn't about science and it's not about 'protecting' nonsmokers. It never has been.
UPDATE
While I was on BBC Scotland, Tony Blows was on Radio 5, making his point rather more forcefully. He called Deborah Arnott a liar which, considering she brought up the Scottish heart scam, I suppose is fair comment. F2C have the audio.
Incidentally, I was told that the producers wanted me to debate with a spokesman from an anti-smoking group but the spokesman refused! I bet Arnott now wishes she'd ducked out as well.
