Cigarette-smoking, as everyone now recognizes, is injurious to health. Only a few beyond the fringe would argue against the proposition that smoking is linked to lung cancer. And, in the wake of the Framingham study, it is clear that cigarettes make for an unhealthy heart as well. According to the British Heart Foundation, if the entire population of the UK were to kick the smoking habit 10000 fewer men and women of working age would die from heart attacks every year - and that figure is, by the way, higher than the number of smoking-related deaths from any other disease, including lung cancer and chronic bronchitis. Put simply, people who smoke 20 cigarettes a day are twice as likely to have a heart attack than people who smoke none. If you are over 50 the risk is a staggering 10 times greater.

So far, however, we have been talking about smoking in 'general' terms only: that such and such a cigarette consumption leads to a certain level of heart disease. What a number of researchers have been trying to find out is precisely how cigarette smoke causes heart problems. Probably the most important constituents in smoke in this respect are carbon monoxide (a toxic gas emitted from, among other sources, automobile exhausts) and nicotine - the active stimulant and the ingredient that gives smokers their pleasure.

Nicotine, being a stimulant, increases the production of the hormone adrenalin, which makes the heart beat faster and raises blood-pressure. Carbon monoxide, on the other hand, has a depressive effect on the vitality of the blood, because it combines with the pigment - haemoglobin - in the red corpuscles and impairs their capacity to transport oxygen.

The combined effects seem to be that the blood is likely to clot too freely, forming thromboses. This problem arises because smoke tends to interfere with the formation of blood platelets. A study carried out on the blood of 73 patients with coronary heart disease, as compared with a control group of 55 normal individuals, showed that the CHD group mostly had larger and denser platelets. The implication, which I find perfectly reasonable, is that the bigger the platelets the greater the likelihood of abnormal degrees of clotting, so that blockages in the coronary arteries are created.

Platelets are probably formed by the break-up of larger cells in the various blood vessels branching into the lungs after blood has been pumped there from the right ventricle for oxygenation. Smoke in the lungs impairs this fragmentation so that, instead of normal-size platelets being produced, large, sticky aggregates are generated. At the same time the nicotine is quickening the pace of the blood-flow and the carbon monoxide is lowering the oxygen content of the blood. Cumulatively, these conditions are ideal for the creation of blockages.