Second-hand smoke in cars: How did the “23 times more toxic” myth turn into fact?

Methods

Our search of MEDLINE with combinations of keywords (i.e., smoking, cars, second-hand smoke, children) to identify the scientific source of the “23 times” claim yielded 19 articles. Google and Factiva searches using the MEDLINE search terms showed that the 23 times figure has been widely cited by international media, nongovernment organizations and politicians (Appendix 1, available at www.cmaj.ca/cgi/content/full/cmaj.090993/DC1).

We believe we have located all the peer-reviewed articles; however, a comprehensive search of media reports and other grey material is beyond the scope of this paper. Those examples of media reports and the inclusion of the 23 times claim in reports from nongovernment organizations illustrate the broad dissemination of the claim.

Historical timeline

In January 1998, the Rocky Mountain News, a newspaper in Denver, Colorado, reported on proposed legislation to ban smoking in cars carrying children. The bill was introduced by state Senator Dorothy Rupert, who reportedly took action quickly when “she learned that smoking was 23 times more toxic in a vehicle than in a house and 8½ times more toxic than in an aircraft because of the smaller enclosed space.” 4 The source of this figure is a November 1997 press release —by local advocates of tobacco control in support of the draft bill — that cited a 1992 study of tobacco-specific N-nitrosamines in indoor air as the reference for the 23 times figure. 28 However, that study did not make the 23 times claim as quoted in the Denver newspaper.

The 23 times estimate has evolved from its modest origins as a brief quotation in a US newspaper to its current status as evidence of the dangers of exposure to second-hand smoke in cars. The concept shifted into the academic mainstream when a 1998 Tobacco Control editorial on protecting children from secondhand smoke 6 included a passage that closely replicated the Rocky Mountain News quotation. Both the newspaper report and the Tobacco Control editorial were subsequently cited in a 2003 issue of Nicotine and Tobacco Research, 7 which further entrenched it in the peer-reviewed literature.

The real fillip for the comparison, however, was the release of the Ontario Medical Association’s 2004 position paper on children’s exposure to second-hand smoke, 5 which noted that:

[B]ased on the evidence that exposure to second hand smoke 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.

The resource cited for this information was the 1998 Rocky Mountain News report. 4

Credibility conferred by the Ontario Medical Association’s use of the statistic resulted in broad dissemination throughout Canada. It was cited in a fact sheet from the British Columbia Ministry of Health in 2005 8 and in 17 news reports, including the national newspaper The Globe & Mail. 29 The Canadian Broadcasting Corporation, in a January 2008 report, referred to the Ontario Medical Association’s reliance on “a Colorado study that suggested tobacco smoke in cars is 23 times more toxic than smoke in houses, because cars have a much smaller volume.” 30 Referral to the Ontario Medical Association’s report was not restricted to Canada; use of the figure by international media and health agencies — the US-based Action on Smoking and Health, 21 GASP (Global Advisors on Smokefree Policy) New Jersey 20 and Action on Smoking and Health Scotland 31 and in recent peer-reviewed articles on exposure to second-hand smoke 9^–11 — has further added to its credibility.

Perhaps the most explicit indication of the statistic’s broad acceptance as fact is its frequent use without reference to its derivation; for example, the claim was uncited in an Australian media report, 32 a peer-reviewed journal article 23 and a press release issued by the Australian Medical Association 24 and on the website of Action on Smoking and Health Ireland. 22 Less precise and similarly unreferenced notations that describe second-hand smoke in cars as “20 times” or “more than 20 times more toxic” than in the home are also common, particularly in Australia, where the National Heart Foundation 27 and state 25 and federal politicians 26 have made such claims to support legislation restricting smoking in cars carrying children.

The continuing appeal of the figure was underlined in early 2009, when news of “irrefutable evidence to show that a car can be 23 times more toxic than a home environment in the context of passive smoke” in a press release from Action on Smoking and Health Ireland (that cited unspecified Colorado research), 13 was subsequently repeated in the Irish Medical Times16 and the Irish Times. 17

These reports preceded the April 2009 publication of a paper in the European Respiratory Journal (which cited the 2004 report from the Ontario Medical Association 5) on possible links between breathing difficulties and exposure to second-hand smoke in cars among Irish schoolchildren. 10 On Apr. 19, the UK Sunday Times reported on the 23 times claim, 12 citing the European Respiratory Journal article, and the Times article was in turn referenced in a daily news release from Action on Smoking and Health UK 14 and on the websites of the European Lung Foundation 18 and the Oxford Health Alliance. 15

Implications

We traced the evolution of this “myth turned fact” to emphasize that only credible evidence should be presented to advance policy. Solid evidence has been the foundation of the progress made in tobacco control in recent decades. 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. 33

Despite the inaccuracy in reporting the level of magnitude of exposure to second-hand smoke in cars, policy-makers should not be deterred from enacting legislation to ban smoking in cars. Several studies on exposure to second-hand smoke have demonstrated that smoking in cars produces high and unsafe concentrations of second-hand smoke particulate 34^,35 that are comparable to or higher than the levels measured in hospitality venues that allow smoking. 36 The best available scientific evidence suggests that smoking in a car for even a short time produces levels of respirable particles that are potentially harmful to children. 34

A 2006 study on second-hand smoke in cars reported a mean concentration of respirable suspended particles measuring less than 2.5 microns in diameter at 272 μg/m³ in cars when the windows were closed and 51 μg/m³ when they were open, allowing for maximum possible ventilation. 34 Guidelines from the US Environmental Protection Agency describe concentrations of 40 μg/m³ as unhealthy for children and other sensitive groups and 250 μg/m³ as hazardous for any person. 37 These documents provide accurate measurements of the air quality in cars when someone is smoking and should replace the 23 times figure favoured by some tobacco control organizations.

A ban on smoking in cars is an extremely important public health policy that has the potential to dramatically reduce the amount of exposure to second-hand smoke experienced by children. 38 Legislation banning smoking in cars carrying children has been enacted in several states or provinces in Australia, the US and Canada.

Recommendations

We recommend that researchers and organizations stop using the 23 times more toxic factoid because there appears to be no evidence for it in the scientific literature. Instead, advocates 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. Further, we recommend citing the 2006 study by Rees and Connelly 34 as reliable evidence that the level of particulate matter found in cars where smoking is allowed exceeds that in the safety guidelines of the US Environmental Protection Agency, particularly for children.

Basic steps can be taken to avoid dissemination of inaccurate information. First, organizations publishing or communicating research findings should adopt a strict policy of only citing original sources for research findings; they should never rely on secondary citing of reports or media articles. Second, peer review processes should emphasize not only a critique of the original content of papers and reports, but also the importance of assessing accurate referencing of previously published research. Finally, the broader lesson of our study is that researchers and advocates can be highly effective partners in bringing about change in public policy, but such partnerships can be jeopardized by incomplete knowledge transfer. Researchers and advocates should not be fearful of working closely together — indeed, greater collaboration may help to ensure greater accuracy in reporting research findings. This is a shared responsibility and, as our paper demonstrates, advocates and journalists are not the only ones who can misreport research findings.