An analysis article in the CMAJ (Canadian Medical Association Journal) concludes there is no evidence to support the fact that smoking in cars is 23 times more toxic than in other indoor environments.

The 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 Mr. Ross MacKenzie, School of Public Health, University of Sydney, Australia and coauthor. "Given that the issue of banning smoking in cars is gaining traction internationally, use of this media-friendly tobacco control “fact” presents potential problems of credibility."
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. This type of “scientific misuse” has unfortunately become an increasingly popular trait among tobacco control organisations.
"The biggest danger of inaccurately interpreting research on smoking in cars for the sake of a snappy media sound bite is to lose favour with an overwhelmingly supportive public and to provide ammunition for opponents of tobacco control," write the authors.
The authors conclude that researchers and organisations should stop using the 23 times more toxic "fact" because of the lack of evidence in scientific literature. "Instead, advocating of smoking bans in cars should simply state that exposure to second-hand smoke in cars poses a significant health risk and that vulnerable children who cannot remove themselves from this smoky environment must be protected." (pi)