Letters

Emerging tobacco hazards in China

Is assumption of no association between smoking and other causes of death valid?

Double standards apply with importation of tobacco into developing countries

UK authors' reply

Is assumption of no association between smoking and other causes of death valid?

EDITORLiu et al used the term "proportional mortality study" to describe their method of comparing the smoking habits of 0.7 million adults who died of neoplastic, respiratory, or vascular causes with those of a reference group of 0.2 million who died of other causes in China.¹ The term can be confusing as it is used only for proportional mortality ratio analysis in standard epidemiology textbooks.² We suggest that the study can be more easily understood if it is described as a case-control mortality study.

An important assumption in such analyses is that the other causes of death should be unrelated to the exposure "not only in the sense of causation but also in terms of `self-selection' for the exposure and the diagnosis and certification of the underlying cause of death."³ Liu et al validated this assumption by showing that the smoking rates of the male and female reference groups were only slightly higher than those of the surviving spouses of the people who had died. However, they did not elaborate whether this similarity was true for each city or rural area in China, and, if it was not, why.

Could this similarity be a feature of populations in which the tobacco epidemic is at an early stage? The authors' assumption may not be valid in other studies (such as our Hong Kong study⁴) or future studies that use a similar design. One potential confounding factor is social class, which is often associated with both smoking and mortality, and it may lead to an association between smoking and other causes of death. Studies elsewhere have observed some association between smoking and other causes of death (for example, in the American Cancer Society's cohort the mean annual mortality from other medical causes was 39/100 000 men in never smokers and 81/100 000 in current smokers)⁵; choosing such other causes as referents would underestimate the risks from smoking.

It is fairly easy to define a priori which are the other causes of death for smokers as relations between smoking and many diseases are known, but it is difficult to define them when other risk factors (such as alcohol consumption) are studied in relation to mortality. Information on smoking (and confounders and other risk factors) in another control group randomly selected from the surviving population should be collected for validation; if the results do not support the assumption, classical case-control analysis comparing the dead and the living is necessary.

T H Lam, Professor. 
S Y Ho, Researcher. 
Department of Community Medicine, University of Hong Kong, Hong Kong, China commed{at}hkucc.hku.hk

^a Competing interests: None declared.

Double standards apply with importation of tobacco into developing countries

EDITORSmoking is a scourge that, although universal in distribution, ravages the economies of developing countries both directly and indirectly. As a non-medical person, I acknowledge with admiration the moral and economic purpose behind studies such as those by Liu et al and Niu et al. ^{1 2} I wonder, however, about the lack of speculation in the papers, let alone recommendations, on the possible measures that governments, health bodies, and non-governmental associations should undertake to combat what is obviously a healthcare disaster. Given the press coverage that high impact papers such as these attract and that the BMJ's readership extends to the non-medical world, Lopez in particular missed the opportunity to put this right in his editorial.³

In the electronic responses to these studies Pletten attempts to rectify this.⁴ But his triumphalismthat China should learn from the United States' experience of dealing with tobaccodisplays the ignorance that individuals with his views have of the enormous contribution made by the United States to the importation of tobacco into developing countries. More sensitive people in the Western world would find disturbing the fact that cigarette packets intended for sale in the West bear health warnings such as "cigarette smoking kills" and "cigarette smoking causes cancer" whereas those intended for sale in the developing world bear warnings diluted of impact, such as "cigarette smoking may be injurious to health" and "cigarette smoking may damage your health"both in English and in the language used locally. The ethics, or lack thereof, of the parties concerned is obvious.

Medical researchers are often in a powerful position when it comes to influencing healthcare decisions and should use this for the public good. Now that these papers have proved the obvious, perhaps we should do something about it.

Yonden Lhatoo, Editor. 
Asia Television, Hong Kong, China ycharlie{at}hk.super.net

^b Competing interests: None declared.

UK authors' reply

EDITORThree different types of study have led to virtually identical conclusions about smoking and death in China ^{1 2} :

(1) A case-control study in which the smoking habits of one million people who had died were compared with those of 300 000 who had not¹

(2) A prospective study of 250 000 adults, 10 000 of whom had died²;

(3) What we chose to call a proportional mortality study, in which the smoking habits of 700 000 adults who had died of neoplastic, respiratory, or vascular disease were compared with those of a reference group of 200 000 adults who had died of other causes.¹

To avoid confusion between the second and third of these, we are reluctant to adopt Lam's suggestion of calling the third a case-control mortality study, but the choice of terminology is not very important. What chiefly matters is the results: already there are almost a million deaths a year from smoking in China, and eventually there will be two or three million a year. These facts were not appropriately widely accepted until these studies were done, and their wide acceptance may well be achieved more rapidly if (despite Lhatoo's concerns) the findings are presented without any strong recommendations other than that they should be widely known. Both papers are available in translation in the February 1999 Chinese language edition of the BMJ.

Richard Peto, Professor. 
Zheng-Ming Chen, Reader. 
Jillian Boreham, Senior research officer. 
Clinical Trial Service Unit, Nuffield Department of Clinical Medicine, Radcliffe Infirmary, Oxford OX2 6HE gale.mead{at}ctsu.ox.ac.uk

^c Competing interests: None declared.