Tobacco's role in increasing the chance of lung cancer is one of the most widely known of tobacco's harmful effects on human health.
What many people, smokers and nonsmokers alike, may not know is that tobacco use increases risks of cancer at many sites in the body in addition to the lungs. These include the head and neck, (covering cancers of the oesophagus, larynx, tongue, salivary glands, lip, mouth, and pharynx), urinary bladder and kidneys, uterine cervix, breast, pancreas, and colon. Each of these is described below, together with related links.
Lung cancer - the big one
As noted at the outset, the paper describing the association between tobacco use and lung cancer stands as a classic in public health. On average, smokers increase their risk of lung cancer between 5 and 10-fold and in developed countries, smoking is responsible for upwards of 80% of all lung cancers. Using American data, 24% of men who smoke can expect to developing cancer during their expected life time.
Recently, the spread of tobacco use to developing countries has led to papers describing similar patterns there. Thus, in a report from India, roughly two-thirds of all patients with lung cancer were smokers, using either cigarettes and/or bidis, hand-rolled tobacco. Among 54 female patients, only 5% were smokers, reflecting both the low prevalence of tobacco use among women and the cancer-causing effects of environmental tobacco smoke.
In a study of 1 000 000 deaths in China, lung cancer risk was two to four times higher among men who smoked compared to men who did not smoke and this association was generally consistent over both rural and urban areas.
Lung cancer remains a disease with a dismal prognosis. Although one-year all-stage survival is reported to have increased from 32% in 1973 to 41% in 1994, five-year survival has remained unchanged at 14%. Early detection has been promoted as a potentially valuable intervention but its cost-effectiveness puts it beyond the reach of all but the most wealthy health care systems, and even then, pales in comparison to the cost-effectiveness of comprehensive programs and policies to reduce tobacco consumption.
Head and neck
Tobacco use has long been linked to head and neck cancers, particularly in tissues through which inhaled tobacco smoke must pass. For oral cancers, men who smoke have a 27-times higher rate of oral cancer than men who do not smoke. For laryngeal cancer, rates are 12 times higher among smokers.
Part of the explanation for tobacco's effects may come from mutations in a gene called p53 - mutations which are far more common among smokers with squamous cell head and neck cancers than among nonsmokers. In addition, the pattern of mutations in nonsmokers reflected endogenous mutations, likely to arise spontaneously, while those in the genes of smokers were changes more likely to arise from an external mutagenic agent. p53 is thought to be a tumour suppressor gene, meaning that mutations in this gene leave the body less well-equipped to prevent the growth of tumours.
Urinary bladder & kidneys
In the Western world, tobacco use is the single most important cause of bladder cancer, accounting for an estimated 40-70% of all cases. Smokers' risks of bladder cancer are 2-3 times higher compared to nonsmokers. Despite the fact that the bladder is not exposed directly to tobacco smoke, polyaromatic hydrocarbons, known to be carcinogenic, may well be absorbed into the blood and transported to the bladder where the bladder cells are then unable to withstand that carcinogenic effects of these compounds.
Tobacco's effect on cervical cancer was only recognized recently, in part because women who smoke may have other risk factors for cervical cancer, particularly exposure to human papilloma virus which increases risks of cervical cancer. Nevertheless, there is now general consensus that cigarette smoking increases risks of cervical cancer, particularly among women smoking as many as 40 cigarettes daily, and is responsible for approximately 30% of cervical cancer deaths in the USA.
For women in developed countries, rates of breast cancer have been rising over the last few decades, eerily tracking with the rapid increase in female smoking that occurred in the middle of the 20th century. Data on links between breast cancer and smoking have, however, been mixed, and this has led to conflicting health messages. Among Danish women interviewed at the time of mammography, smoking for more than 30 years was associated with a 60% higher risk of breast cancer and onset at an average of eight years earlier, when compared with nonsmokers.
Given that breast cancer incidence is soon to be eclipsed by lung cancer incidence among women, further data clarifying the role of smoking in causing breast cancer should be of value in strengthening efforts to inform and assist women to quit smoking.
Smoking is estimated to be responsible for 30% of pancreatic cancer. Similar to bladder cancer, carcinogens inhaled by the smoker are thought to enter the blood stream and reach the pancreas via the blood and also bile, secreted by the liver to aid digestion. Pancreatic cancer prognosis remains very poor with 5-year survival less than 5% in most reports.
Again, like bladder and pancreatic cancer, colon cancer risk is increased among smokers, presumably due to the transport of carcinogens to the colon from inhaled or swallowed tobacco smoke. Data supporting this association come from several longitudinal studies in which groups of people are followed over many years to record the occurrence of various illnesses.
Based on data from both male and female health professionals in the USA, smoking appears to double the risk of colon cancers. Most colon cancers begin as polyps, precursor growths for cancer. Risk of cancer increases with polyp size and there is a dose-response relationship with increasing years of tobacco use associated with larger polyps and, after 35 years of smoking, colon cancer.