TOBACCO AND CANCER
These days, at least in America and Europe, we are all aware that to keep cancer at bay avoiding tobacco smoke is as important as sensible eating. The estimate for the USA is that 30% of cancers are caused by smoking, with poor diet accounting for 25%. Ninety per cent of lung cancers are attributable to smoking and the World Health Organization has estimated that in the twentieth-century tobacco-associated diseases killed 100 million people (Fig. 1).
1. The effect of smoking on the lungs
The seminal UK study that conclusively linked smoking to lung cancer was published in 1950 by Richard Doll and Austin Bradford Hill. That and subsequent reports certainly influenced attitudes, leading in the ensuing 50 years to the progressive prohibition of smoking in public places and a gradual decline by about half in the number of UK smokers. In the USA, things are a bit more idiosyncratic with some states that have banned smoking in all enclosed public places but others having no state-wide prohibition.
Perhaps the most depressing aspect of the smoking saga is that the first statistical evidence linking lung cancer and cigarette smoking was published not by Doll and Hill in 1950 but 90 years ago in the 1920s. By 1935 the German physician Fritz Lickint felt able to write there was ‘no longer any doubt that tobacco played a significant role in the rise in bronchial cancer’ and to coin the term ‘passive smoking’. Because those pre-war studies were carried out in Germany and published in German they have tended to be ignored.
The gloomy reading provided by the WHO figures for twentieth-century deaths due to smoking is as nothing compared to their predictions. The figure of 5.4 million a year that tobacco use kills now (that’s one every six seconds) will rise to over 8 million (a year) by 2030. Of the more than one billion smokers, over 80% live in low- and middle-income countries. Currently, there are 200,000 tobacco-related deaths in Africa and it is there and in other under-developed regions that cigarette smoking is being heavily promoted by the manufacturers. It is the people of these regions who will, if this trend continues, contribute 80% of the 8 million dead.
The effects of smoking are not confined to the lung: almost all the major cancer types are between two and six times more likely to develop in smokers than in non-smokers (i.e. mouth, pharynx, larynx, bladder, oesophagus, pancreas, stomach, liver, cervix, kidney and myeloid leukaemia). The carcinogenic effects of tobacco arise because specific chemicals in nicotine can cause mutations that disable the function of critical genes. As well as initiating tumours, tobacco smoke can also act as a tumour promoter by causing chronic inflammation.
The fact that tobacco smoke is carcinogenic and the problem of involuntary (passive) inhalation, recognised so many years ago by Licking, has been confirmed by numerous studies and has now prompted legislation in many countries banning smoking in public places. The most convincing evidence on passive smoking comes from long-term studies of with smokers, for which pleasure their lung cancer risk goes up by 20 to 30%. Although it is clearly not possible to quantify how many carcinogens involuntary smokers inhale, the evidence is that exposure to cigarette smoke at work can increase the risk by up to 20%.
It is perhaps surprising that most studies have found no link between smoking and breast cancer, or indeed prostate or endometrial cancer of the uterus. Surprising because chemicals in tobacco smoke have been shown to cause breast cancer in rodents and these compounds have also been detected in both breast tissue and in breast milk. The most persuasive of such studies compared 58,515 women with the disease with 95,067 who were disease free, a data set large enough to separate smoking from those of alcohol. Almost inevitably, there are other surveys that suggest there may be a link. The California Environmental Protection Agency (2005) concluded that, in women who were mainly pre-menopausal, passive smoking could be associated with breast cancer and the US Surgeon General’s (2006) report described the evidence as ‘suggestive but not sufficient’. The age at which women start smoking does appear to be very significant in that smoking within five years of the first menstrual cycle almost doubles the risk before menopause. This may be because teenage breast tissue that is still developing is more sensitive to smoke carcinogens.
Obviously, it is best never to start smoking but if you have succumbed all is not lost. Yet another major survey conducted by Richard Doll and his colleagues has shown that giving up smoking will improve your chances of avoiding lung cancer. The earlier the better, of course, but even those who cease after the age of 50 reduce their risk by over 60%.