Despite media reports to the contrary, there is no evidence to support the assertion that smoking in cars is 23 times more toxic than in other indoor environments, say researchers writing in the Canadian Medical Association Journal (CMAJ).

The authors say citing the inaccurate statistic has the potential to turn the public against efforts to ban smoking in automobiles. And If you're going to dictate to people how they should behave, not making things up is an important precursor.  

The CMAJ article describes how a local media report of an unsourced statistic — that "second-hand smoke was "23 times more toxic in a vehicle than in a home" — led to widespread reporting of the figure in international media and peer-reviewed literature.

However, there appears to be no scientific evidence to support this claim. "In a subsequent exhaustive search of the relevant literature, we failed to locate any scientific source for this comparison," write the authors. The 23 times estimate has evolved from being a brief quotation in a US newspaper to entering the academic mainstream in 1998 when a Tobacco Control editorial closely copied the previous quote. Both the newspaper report and the editorial were then cited in an issue of Nicotine and Tobacco Research.

Researchers and organizations should stop using the 23 times more toxic "fact" because of the lack of evidence in scientific literature, the authors suggest. Instead, advocates of the ban should just make sweeping generalizations about the risks of secondhand smoke exposure and mention children if possible. Something like, "exposure to second-hand smoke in cars poses a significant health risk and...vulnerable children who cannot remove themselves from this smoky environment must be protected," would be a good substitute for the bogus statistic.

"The biggest danger of inaccurately interpreting research on smoking in cars for the sake of a snappy media sound bite is to lose favor with an overwhelmingly supportive public and to provide ammunition for opponents of tobacco control," the authors conclude.

Citation: Ross MacKenzie, Becky Freeman, 'Second-hand smoke in cars: How did the "23 times more toxic" myth turn into fact?', Can. Med. Assoc. J., Apr 2010; doi:10.1503/cmaj.090993