Introduction

The risks of cigarette smoking have been investigated fairly thoroughly, but most studies have used a cut-off point of 10, 15, or 20 cigarettes per day. Smokers who have difficulty in quitting often say, "I only smoke 2-3 a day" or, "I don't inhale", and reason that they are not at increased risk.

A 20-year study of men and women living in Denmark has just been reported, which analyzes so-called 'light smoking' and cardiac events. Although the inhabitants and lifestyle of Copenhagen differ in several respects from those in other countries, the study findings are of interest to us all. Here's a summary.

Method

Between 1976 and 1978 the Copenhagen City Heart Study enrolled 14,223 people out of a population of about 90,000 living in a defined area in Copenhagen. They were followed up until December 1997, with fatal and non-fatal first myocardial infarctions (MIs) recorded, as well as the cause of all deaths. Only subjects over 30 were included, to ensure that smoking habits were well established at baseline. People with ischemic heart disease or insufficient data on cardiovascular risk factors were excluded.

Questionnaires and lab tests were done at baseline to determine existing cardiovascular risk factors. These included blood pressure, blood lipids, body mass index (BMI), educational level, household income, alcohol intake, physical activity, and family history of diabetes and MI.

Smoking status was recorded as never, former, or current smokers. Current smokers reported the amount and type of tobacco smoked, and whether smoke was inhaled or not. A cigarette was equated to 1 gram, a cheroot to 3 grams, and a cigar to 5 grams of tobacco.

Categories were created for the cardiovascular risk factors. In assessing the independent contribution of smoking to MI and all-cause mortality, the Cox proportional hazards model was used, with analyses separately by gender.

Results

A total of 2,074 subjects were excluded because of being under 30, having ischemic heart disease, or missing data on risk factors; this left 6,505 women and 5,644 men for analyses. During the 20-year follow-up period, 476 women (7.3%) and 872 men (15.5%) had an MI, with a 40% mortality rate. All-cause mortality deaths were 2,305 women (35%) and 2,883 (51%) men.

More men smoked than women (70% vs. 58%), they smoked more (measuring grams/day), and a greater proportion inhaled (77% in men vs. 70% in women). Women smoked cigarettes more than men (81% vs. 57%), although cheroots were favored by both women and men (17% and 14%, respectively).

After adjusting for the risk factors listed, there were dose-response relationships between smoking, both with and without inhaling, and both MI and all-cause mortality in both sexes. These relationships were all statistically significant, except for MIs in non-inhaling men.

The relative risks (RRs) for MI and all-cause mortality in women who inhaled were 1.65 and 1.70, respectively. For men who inhaled, these RRs were 1.39 and 1.60, respectively.

In women, 6 or more cigarettes¹ a day for non-inhalers and 3 or more cigarettes a day for inhalers increased the risks of MI or all-cause mortality significantly. In men who inhaled, 6 or more cigarettes a day increased both these risks significantly.

The RRs for these levels of smoking are set out in the table:

	Women	Women	Men	Men
	Inhalers	Non-inhalers	Inhalers	Non-inhalers
Myocard. Inf.
3-5 cigarettes1	2.14*	1.07 (ns)	1.03 (ns)	1.28 (ns)
6-9 cigarettes	2.44*	1.58*	2.10*	0.87 (ns)
Mortality
3-5 cigarettes	1.86*	1.17 (ns)	1.24 (ns)	0.99 (ns)
6-9 cigarettes	1.86*	1.24*	1.76*	0.86 (ns)

* = significantly increased risk     (ns) = not significant

Comment

This study shows that smoking down to as low as 3 cigarettes (grams of tobacco) a day for women and 6 cigarettes a day for men are associated with a significantly increased risk of both MI and all-cause mortality. The relative risks were approximately 50% higher in female smokers, compared with males. This finding, together with similar results from other studies² , reinforces the fallacy of "cutting down" as an escape from smoking hazards.

Inhalation -- or non-inhalation - is not commonly included as a factor in examining the risks of smoking. In this study, non-inhaling women who smoked more than 10 cigarettes a day had twice the risk of an MI and a 60% greater risk of death than never-smokers.

Why should women be at a greater disadvantage than men when it comes to smoking? One explanation may be that because of the known anti-estrogenic effect of smoking, women may be 'twice-hit', as the authors of the study put it; one hit comes from the increased risk due to atherosclerosis and hemostasis caused by smoking itself, while the anti-estrogen action of smoking removes the supposed natural 'protective' effect of estrogens.

It must be remembered that this study was done in Demark, where lifestyles and smoking habits are not necessarily the same as in other countries. For instance, the cardiovascular risk factors showed that men had a better risk profile with regard to total cholesterol, alcohol consumption, physical activity, income and education, while women had better triglyceride, blood pressure, BMI and diabetes risk factors. This picture would probably not be found in many countries. And cheroots are not widely smoked in the USA, for instance. Nevertheless, the findings emphasize that no female smoker is safe -- not even those who only smoke 3 or 4 cigarettes a day, or don't inhale!