Are all smokers the same? Clearly not. Some smoke 60 a day, some 20 a day and some, one or two, and not every day.
So when we talk about the risks of smoking, what do we mean? They cannot be the same for all smokers. Sir Richard Peto, Britain's foremost expert on smoking, a decade ago suggested all smokers eventually end up as 20-a-day smokers. But this is not the case.
Occasional or social smokers exist - but they are rare. They are defined in two ways: either as not smoking every day or as smoking an average of less than one cigarette a day.
Surveys suggest that between 10 and 18 per cent of smokers smoke five or fewer cigarettes a day. The surveys are now 20 years old but there has been little recent research.
Professor Robert West, an expert on smoking at University College London, is updating a study of light smokers he made with a colleague, Peter Hajek, in 1995. They found occasional smokers were predominantly middle class, better educated and viewed smoking as a social activity. However, they were still addicted. About 80 per cent of occasional smokers find they cannot stop when they try. But this challenges our understanding of addiction. How can you be "occasionally" addicted?
Professor West says: "Nicotine addiction is not just about keeping the level of the drug topped up to avoid withdrawal symptoms. One way addiction works is by forming an association between situations where a person would typically smoke, which then creates the impulse to smoke when they find themselves in that situation again. A lot of daily smokers report very strong situational cravings."
Not all occasional smokers are the same, however. Some used to be 20-a-day smokers who are trying to give up while others will be 20-a-day smokers in a few months' time. Relatively few are stable occasional smokers.
Switching from daily smoking to occasional smoking is possible - but it's hard. Most smokers can't do it for the same reason that most alcoholics can't return to controlled drinking. It is all or nothing.
Yet some can remain occasional smokers - neither increasing their consumption nor reducing it.
Figures suggest there are high numbers in North America, though this is unexplained. What protects them from escalation into dependence on nicotine and prevents the development of tolerance and withdrawal discomfort? There may be genetic factors, family influences or what experts call "dimensions of social adjustment that may accompany higher levels of educational attainment".
Occasional smokers pose another question. What are the health risks? Clearly they must be smaller than those faced by daily smokers - but they are not as low as you might think.
Smoking causes a wide range of diseases, but the key ones are cancer and heart disease. With cancer, the risks are proportionate to the amount smoked.
"The excess risk of cancer for a daily smoker is 15-fold higher than for a non-smoker," Professor West says.
Heart disease is different - the risk is not proportional to the amount smoked. It is much higher, proportionally, for the first one or two cigarettes - hence the risk from passive smoking.
There is some fascinating evidence that demonstrates this. Helena, a small city in Montana, banned smoking in public in June 2002, only to have the ban rescinded six months later.
During the ban, heart attack admissions to the local hospital fell by 40 per cent. After the ban was lifted, the heart attack rate increased to its former level.
Following publication of this research, the Centres for Disease Control and Prevention in Atlanta warned that even 30 minutes of exposure to other people's smoke might be enough to trigger a fatal heart attack in people at risk of heart disease.
Scientists have been puzzled by this disproportionate risk. A non-smoker who lives with a person who smokes 20 a day has one-third of the risk of their partner, even though they are actually exposed to only 1 per cent of the smoke.
Laboratory evidence suggests this is because toxins in tobacco smoke peak at low levels of exposure, increasing the stickiness of the blood (the tendency of the platelets to aggregate) and inflaming the arteries, increasing the risk of thrombosis - a blood clot that can trigger a heart attack. What applies to passive smokers, of course, also applies to occasional smokers.
Professor West says the mechanism is similar to that which puts people with heart disease at risk on days of high pollution. "Inhaling particulates has an immediate, same-day effect on people whose vasculature [the state of their arteries] is compromised."
Figures published last month show smoking continues to decline among young people. Nevertheless, 27 per cent of school pupils had tried smoking and 5 per cent smoked at least one cigarette a week, girls more than boys. Most of these will become 20-a-day smokers.
Pupils who were regular smokers were likely to show signs of dependence on the habit. About two-thirds reported that they would find it difficult not to smoke for a week, while almost three-quarters said they would find it difficult to give up smoking altogether. Almost two-thirds of regular smokers had tried to give up.
The proportion of pupils who thought it was OK for someone of their age to try smoking to see what it is like has steadily decreased from more than half in 1999 to around a third in 2010.
Of course, it is not a good idea to smoke. Apart from anything else it makes your legs fall off (I find this morsel of information works especially well with children). More people undergo amputations because the circulation in their limbs has been destroyed by smoking than for any other reason.
But if you have to smoke, it is better to smoke occasionally rather than regularly and it is better to smoke a little than a lot.