Thursday, September 29, 2005

New Study Finds Smoking 1-4 Cigarettes Per Day is Hazardous; ACS Responds

A study published in the current issue of Tobacco Control reports that smoking 1-4 cigarettes per day is associated with increased risks of death from heart disease, lung cancer, and all causes.

The study was a longitudinal (cohort) study of more than 42,000 adults, ages 35-49, in Norway, who were followed for nearly 30 years. The adjusted relative risk among smokers of 1-4 cigarettes per day of dying from heart disease was about 2.8 and for dying from lung cancer was about 2.8 among men and 5.0 among women.

The authors conclude: "In both sexes, smoking 1-–4 cigarettes per day was significantly associated with higher risk of dying from ischaemic heart disease and from all causes, and from lung cancer in women. Accordingly, five cigarettes per day is not a threshold value for daily cigarette consumption that must be exceeded before serious health consequences occur."

In response to the study, the American Cancer Society (ACS) issued a statement, which read in part: "The finding that smoking just 1 to 4 cigarettes a day can significantly boost heart disease and cancer rates is important because many smokers, due to expanding restrictions on smoking in public places and at work, are cutting back on the number of cigarettes they smoke each day. By doing so, they often feel that they are sharply reducing or eliminating the health dangers from smoking. But this study shows that this is not the case and reiterates the important message that there is no such thing as a safe level of smoking."

The Rest of the Story

I think it's important to note, first of all, that the risk estimates reported here for smokers of 1-4 cigarettes per day are likely to be overestimates of the true risk. The reason for this is that exposure to cigarette smoking was measured at baseline but likely changed over the 30 or more years of the study follow-up. Because of the addictive nature of smoking, it is unlikely that smokers of 1-4 cigarettes per day were able to maintain that low level of smoking for 30 years. To the extent that these smokers increased their cigarette consumption, then the risks reported in the study for 1-4 cigarettes per day may really apply to a higher level of cigarette consumption.

On the other hand, there are two factors that could lead the risks to be underestimated. First, a number of nonsmokers at baseline may actually have started smoking and thus the mortality rates for nonsmokers may have been higher than they should have been. This is likely to be a small factor, because most smoking initiation begins well before age 35. In fact, the authors found that of 11,365 nonsmokers at baseline who were re-interviewed 10 years later, 10,762 were still nonsmokers.

Second (and probably more importantly), some of the 1-4 cigarette per day smokers may actually have quit smoking entirely. The authors did in fact find that of the 776 smokers of 1-4 cigarettes per day who were re-interviewed 10 years later, 237 were no longer smoking.

To determine whether the true risks of smoking 1-4 cigarettes per day are higher or lower than those reported in the study, one has to weigh the extent to which 1-4 cigarette per day smokers at baseline are likely to progress to higher levels of smoking versus the extent to which they are likely to stop smoking.

In the study, the authors made such an assessment by examining the smoking behavior of a sample of these smokers who could be re-interviewed 10 years after their initial interview. Based on interviews with about half of the 1-4 cigarette per day smokers, the authors concluded that the two effects basically counteracted each other and that there was therefore no substantial bias in either direction: "In all, we see no reason to believe that the relative risk estimates for light smokers are substantially biased."

The problem is that the sample of subjects who were successfully followed up (only 50% of the entire sample) is much more likely to have substantially lower levels of smoking than the true (entire) sample. We know from prior research of this type that smokers (and particularly heavier smokers) are less likely to be successfully re-interviewed at long-term follow-up than nonsmokers. So it is quite likely that had the full sample been available for re-interview at 10 years (and again at 20 and 30 years), there would have been a much higher extent of smoking progression among 1-4 cigarette per day smokers at baseline compared to smoking cessation among these smokers.

Nevertheless, although I think the risk estimates may be somewhat overestimated, it is unlikely that the true risks are not significantly elevated. So I think the basic study conclusions are still valid.

Now to the American Cancer Society statement. I have to say that I find that statement to be misleading, or possibly inaccurate. The statement suggests, I think, that cutting back on cigarette consumption does not "sharply reduce" the health dangers from smoking.

I don't find that statement to be accurate, and I don't think it is supported by the study findings.

For smokers of 1 pack per day who cut down to 1-4 cigarettes per day, the reduction in the relative risk for death from all causes drops from about 3.3 to about 1.5. For death from lung cancer, it drops from about 30 to about 4. And for all cancers, it drops from about 3 to 1.1 (basically, no increased risk). I do find these reductions in risk to be "sharp" reductions.

In addition, the most dramatic improvement in health to be realized from decreased cigarette consumption is probably improvement (or at least slowing the progression of deterioration) of lung function. Smokers with chronic obstructive lung disease (COPD) who are able to sharply reduce their cigarette consumption are doing themselves a great thing - they may even be saving their lives. Because for smokers with COPD, there is nothing better they can do then to quit smoking or sharply cut down on the amount they smoke.

While I obviously agree with the suggestion that quitting smoking completely is far better than continuing to smoke at lower levels, I just don't think it's accurate to imply that reducing cigarette consumption will not sharply reduce the health dangers from smoking.

The real danger here, however, is not in the possible misrepresentation of the scientific evidence. It is, rather, the possibility that in making this statement, the ACS might actually play some part in convincing smokers who would otherwise continue to cut down on the amount they smoke that it is simply not worth it because they aren't going to see any health improvement anyway. Given the addictive power of nicotine and cigarette smoking, it is far more likely that these discouraged smokers will simply continue smoking at their current amounts then that they will quit smoking entirely. And they may even increase their cigarette consumption, since it may appear from the ACS statement that the amount smoked does not relate directly to disease risk.

This is, in fact, one of the downfalls of public health messages to the public in general - the fact that in trying to emphasize the health dangers of a particular behavior, we often fail to acknowledge that there is a dose-response relationship, and that there is such a thing as risk reduction (reduction in risk associated with decreased levels of the behavior).

To deny that fact may be to deprive many individuals of a golden opportunity to change their behavior and improve their health, if not to save their lives.

1 comment:

Wonka said...

Great blog and very sensible post. However, I would have liked the authors to include some other possible confounding variables, such as, I suppose, alcohol consumption or socioeconomic status.