The Evolving Relationship of Social Class to Tobacco Smoking

The Evolving Relationship of Social Class to Tobacco Smoking
and Lung Cancer
Michael J. Thun
The article by Menvielle et al. (1) in this issue of the Journal fits into
the epidemiological tradition of testing how well one scourge
explains another. Its goal is to quantify the extent to which educational and socioeconomic differences in smoking in Western
Europe account for social class differences in lung cancer occurrence. Complicating this effort, however, is that the relationships
between social class, smoking, and lung cancer have not been static
but have evolved over the last 50 years, following historical trends
in the dissemination of manufactured cigarettes (2). In Europe,
dependence on manufactured cigarettes spread from men to
women, from North to South, and from higher to lower social
classes (2–4).
Vestiges of this demographic progression can be seen in
studies conducted early in the 20th century that showed no
association between low socioeconomic status and higher risk of
lung cancer (5). No disparity in lung cancer was seen among
men in England and Wales in 1910–1912 and 1930–1932,
jnci.oxfordjournals.org
among women in Sweden in the 1960s, or in older birth cohorts
in Finland from 1971 to 1985. Not until the 1970s were lung
cancer incidence and death rates consistently higher among
men with lower social status (2,5), especially in Northern
Europe (2,5).
Very different socioeconomic patterns in cigarette smoking
and lung cancer are still seen in Southern Europe, where the
social class gradient is reversed among women and much
weaker among men than in the North. Menvielle et al. observe
this among participants in the European Prospective
Affiliation of author: American Cancer Society, Atlanta, GA.
Correspondence to: Michael J. Thun, MD, MS, American Cancer Society,
Atlanta, GA 30303-1002 (e-mail: [email protected]).
DOI: 10.1093/jnci/djp005
© The Author 2009. Published by Oxford University Press. All rights reserved.
For Permissions, please e-mail: [email protected].
JNCI
|
Editorials 285
Investigation into Cancer and Nutrition. Men and women from
the five countries representing Northern Europe (United
Kingdom, Norway, Sweden, Denmark, and the Netherlands)
were more likely to smoke if they had completed college than if
their education had stopped at primary school. In contrast,
women from Southern Europe (Spain, Italy, and Greece) were
more likely to smoke if they had progressed further in their
education. For men, the educational gradient was in the same
direction, but weaker in Southern Europe than in Northern
Europe.
In this context, it is extremely difficult for Menvielle et al. to
disentangle the historical and birth cohort effects of lifetime smoking on lung cancer risk from any other factors that may have contributed to risk. The authors’ conclusion that “self-reported
differences in smoking consistently explain approximately 50%” of
the educational inequalities in lung cancer is contingent on the
quality of the individual measurement of lifetime smoking and the
ability of the study to distinguish fixed relationships from those
that are dynamic and evolving.
The design of the study is straightforward. The analyses are
based on approximately 391 000 men and women recruited in nine
countries (excluding France) in Western Europe between 1992
and 1994. At enrollment, respondents 40–65 years of age were
asked to report their highest level of education, smoking status
(current, former, never), number of cigarettes currently smoked
per day, age at starting, years of smoking, and various dietary factors including fruit and vegetable consumption. A total of 1631
incident cases of lung cancer were identified in follow-up through
2004 or 2006.
Although the study design is simple, its results are difficult to
compare with those of cohort studies in North America because
educational levels are defined differently in Europe and because
the educational gradient in lung cancer risk is expressed in
terms of relative index of inequality (RII). In Europe, a primary
school education corresponds to the first 5–9 years of education. This varies slightly among countries, ranging from grades
1–5 in Germany and Italy to grades 1–9 in the United Kingdom.
The RII approach is useful in comparing these countries
because it allows for the different distributions of educational
attainment that exist among these countries. However, the
results cannot be compared with related findings in the United
States because the RII compares risk at the very bottom of the
educational hierarchy (zero percentile) with that for the very
top (100th percentile). These values are approximately 30%
larger than the educational gradients typically reported in the
United States, which compare the average risk in the lowest
educational category to the average risk in the highest educational category (6).
The strengths of the study are its size, the availability of prospectively collected information on smoking and diet for each
participant, and the quality of the follow-up information, which
includes data on histological subtypes of lung cancer. The analyses
provide strong evidence that variations in the consumption of
fruits and vegetables, at least as measured by food frequency questionnaires, account for almost none of the educational gradient in
lung cancer risk.
286 Editorials
|
JNCI
The main limitations of the study are two: 1) the information
on smoking behavior is based on a single, relatively short questionnaire that covers only some of the parameters of lifetime
smoking, and 2) the study population is drawn from countries
that represent different phases of the changing relationship
between cigarette smoking and social class. Together, these
limitations leave considerable room for misclassification of lifetime smoking behavior. For example, the analyses cannot consider the number of cigarettes smoked per day at various points
in the past, the use of pipes or cigars instead of cigarettes, or
other product variations such as filtered vs nonfiltered. The
intensity with which each cigarette was smoked cannot be measured adequately by questionnaires. One could hypothesize that
less affluent smokers may smoke each cigarette more intensively
than those with more economic resources. Misclassification of
smoking behavior may also be somewhat greater when questionnaires are administered to less educated subjects vs more educated subjects (7).
In some analyses, the authors combine data in a manner that
reduces the contrast in risk between never- and ever-smokers or
that aggregates geographic areas with very different relationships
between smoking and social class. For example, figure 1 combines
never-smokers with occasional smokers, light smokers, and “exsmokers who stopped a long time ago” to improve the precision of
the rate estimates among those considered to have minimal smoking. This does add precision, but it inflates the apparent risk among
never-smokers. Furthermore, Tables 3, 5, and 6 and Figure 1 combine the data from Northern and Southern Europe. Although these
analyses control for the center at which participants were enrolled,
the consequence of combining regions with very different socioeconomic relationships is to underestimate the contribution of
smoking to the educational gradient in lung cancer.
In conclusion, it is extraordinarily difficult to quantify the
degree to which social class differences in tobacco smoking
account for the socioeconomic differences in lung cancer in a
setting in which all of these factors have changed dramatically
over time. Menvielle et al. are almost certainly correct that some
fraction of the socioeconomic differences in lung cancer are due
to factors other than smoking. Their efforts to quantify this
fraction are laudable, even if the collective impact of factors
other than smoking ultimately proves to increase lung cancer
risk by a lower factor (perhaps 1.4) rather than by 2.0 or more.
They are also correct that in the near term and for the foreseeable future, the most effective approach to reducing both the
socioeconomic disparities and the overall burden of lung cancer
is to implement measures that we already know are effective in
reducing tobacco use.
References
1. Menvielle G, Boshuizen H, Kunst AE, et al. The role of smoking and diet in
explaining educational inequalities in lung cancer incidence. J Natl Cancer
Inst. 2009;101(5):321–330.
2. Mackenbach JP, Huisman M, Andersen O, et al. Inequalities in lung cancer mortality by the educational level in 10 European populations. Eur J
Cancer. 2004;40(1):126–135.
3. Huisman M, Kunst AE, Mackenbach JP. Educational inequalities in
smoking among men and women aged 16 years and older in 11 European
countries. Tob Control. 2005;14(2):106–113.
Vol. 101, Issue 5
|
March 4, 2009
4. Graham H. Smoking prevalence among women in the European community 1950–1990. Soc Sci Med. 1996;43(2):243–254.
5. Faggiano F, Partenan T, Kogevinas M, Boffetta P. Socioeconomic differences in cancer incidence and mortality. In: Kogevinas NPM, Susser M,
Boffetta P, eds. Social Inequalities and Cancer. Lyon: IARC Scientific
Publications; 1997:65–176.
jnci.oxfordjournals.org
6. Kinsey T. Jemal A. Liff J. Ward E. Thun M. Secular trends in mortality
from common cancers in the United States by educational attainment,
1993–2001. J Natl Cancer Inst. 2008;100(14):1003–1012.
7. Wagenknecht LE, Burke GL, Perkins LL, Haley NJ, Friedman GD.
Misclassification of smoking status in the CARDIA study: a comparison of
self-report with serum cotinine levels. Am J Public Health. 1992;82(1):33–36.
JNCI
|
Editorials 287