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#G0621* – Second-hand smoke in cars: Is it myth or fact?

Posted by Greg Lance - Watkins (Greg_L-W) on 18/11/2011

#G0621* – Second-hand smoke in cars: Is it myth or fact?

CMAJ Analysis

CMAJ
© 2010 Canadian Medical Association or its licensors

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

Ross MacKenzie MA, Becky Freeman MSc

1

Changes to public health policy do not usually occur
simply as a result of epidemiologic research detailing
the health hazards facing a population. Policy change
requires both strategic and opportunistic advocacy to transform
research findings into health reforms.1 Successful advocacy
campaigns often require the translation of complex
research findings into short and memorable media quotes.
Managing the risks involved in either oversimplifying
research results or misreporting findings is essential to maintaining
the credibility of public health professionals. Unfortunately,
inaccurate reporting of health information is not an
uncommon phenomenon.2
While conducting research for a study on the Australian
advocacy campaign to ban smoking in cars,3 one of us (BF)
encountered many media reports that stated that second-hand
smoke was “23 times more toxic in a vehicle than in a home.”
In a subsequent exhaustive search of the relevant literature, we
failed to locate any scientific source for this comparison.
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. In this
paper, we describe how a local media report of an unsourced
statistic led to the same statistic being widely reported in the
international media and peer review literature (Figure 1).4–27
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 http://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 Nnitrosamines
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
smoke6 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.
Second-hand smoke in cars: How did the “23 times more
toxic” myth turn into fact?
Ross MacKenzie MA, Becky Freeman MSc
From the School of Public Health, University of Sydney, Sydney, Australia
CMAJ 2010. DOI:10.1503/cmaj.090993
Key points
• The suggestion that second-hand smoke is 23 times more
toxic in a vehicle than in the home is widely accepted in
the media and academic literature.
• Despite its media currency, the “23 times” claim is
unsubstantiated.
• This nonvalidated figure came to be widely reported in the
popular media and scientific publications.
• Authors and organizations publishing or otherwise
disseminating research findings should adopt a strict policy
of citing only original sources.
DOI:10.1503/cmaj.090993
Early release, published at http://www.cmaj.ca on April 12, 2010. Subject to revision.
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 20058 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 Jersey20 and Action on Smoking and Health Scotland31 and
in recent peer-reviewed articles on exposure to second-hand
smoke9–11 — 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 article23 and a
press release issued by the Australian Medical Association24
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 Foundation27
and state25 and federal politicians26 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 Association5) 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
2 CMAJ
Analysis
Action on
Smoking and
Health UK
Apr. 20, 200914
Oxford Health
Alliance
Apr. 23, 200915
European Lung
Foundation
Apr. 22, 200918
Rocky Mountain News
Jan. 10, 19984
Ontario Medical Association report
20045
British Columbia Ministry
of Health fact sheet 20058
Nicotine &
Tobacco Research
20037
Tobacco
Control 19986
Irish Medical Times
Feb 25, 200916
Tobacco
Control 200811
Cancer Causes
& Control 20099
European Respiratory Journal 200910
Sunday Times (UK)
Apr. 19, 200912
Action on Smoking and Health
Ireland
Feb. 24, 200913
Irish Times
Feb. 24, 200917
Media in Canada, Australia and Ireland19
and websites of GASP (Global Advisors of
Smokefree Policy) New Jersey,20 Action on
Smoking and Health US21 and Action on
Smoking and Health Ireland22
Figure 1: Dissemination of the claim that second-hand smoke is 23 times more toxic in cars than in homes.
Analysis
the Times article was in turn referenced in a daily news
release from Action on Smoking and Health UK14 and on the
websites of the European Lung Foundation18 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 particulate34,35
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 Connelly34
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.
This article has been peer reviewed.
Competing interests: None declared.
Contributors: Both Ross MacKenzie and Becky Freeman conceived of the
paper, conducted data collection and prepared the manuscript. Ross MacKenzie
prepared the figure, the appendix and the first draft of the manuscript.
Funding: Ross MacKenzie is funded by a 2006 research grant from the Cancer
Council New South Wales; Becky Freeman is funded by National Health
and Medical Research Council grant 396402, Future of Tobacco Control.
 
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CMAJ 3
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Correspondence to: Dr. Ross MacKenzie, School of Public Health,
Edward Ford Building (A27), University of Sydney NSW 2006,
Australia; rmackenzie@health.usyd.edu.au
4 CMAJ
Analysis

“In politics, stupidity is not a handicap.” 
Napoleon Bonaparte (1769-1821)

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