Lung cancer has become an epidemic since the 1930s. Identified risk factors include:
Host factors
Cigarette smoking
The incidence of lung cancer in the 20th century has largely mirrored the rise and fall in cigarette smoking.
The risk of lung cancer in smokers is ten times that of non-smokers. However, the incidence is expected to plateau in the next few decades because of a fall in the number of smokers.
Even so, lung cancer remains on the list as one of the major killers because of the sheer number of people who smoke, whether cigarettes, cigars, pipes or beedis (cigarettes made of unprocessed tobacco wrapped in leaves.)
With the advent of low-tar filter cigarettes, the type of lung cancer has shifted from mostly squamous cell to adenocarcinomas, which occur more peripherally. This is probably because of deeper and more frequent inhalations of smoke.
The lung cancer risk is determined by the number of cigarettes smoked, the duration of smoking and the age at which one starts smoking.
Family history
The risk of lung cancer rises 1.7-fold with a positive family history and the risk becomes 3.6-fold if two or more relatives had the disease.
Genetic factors
Familial clustering of lung cancer is associated with certain gene markers. For instance, the presence of TP53 variants in smokers increases the risk three-fold in comparison to non-smokers.
Another marker on chromosome 15 comprises three genes coding for nicotinic acetylcholine receptor subunits, which might increase the carcinogenic effects of nicotine or render the person more liable to nicotine addiction.
People with a single copy of the marker have a 30 percent higher risk, whereas two copies carry a 70-80 percent increased risk.
Menthol cigarettes
Menthol cigarettes date back to the 1920s, and are used most extensively by African-Americans.
The use of menthol cigarettes may lead to cigarette smoking, greater tendency to smoking addiction, and may also intensify exposure to toxins in cigarette smoke.
Estrogen-progestin formulations
Women who smoke in addition to taking estrogen plus progestin formulations, as opposed to estrogen alone, have a higher risk of lung cancer.
Processed meat
Cured meat, deep-fried meat and meat chili all increase the risk of lung cancer.
Alcohol intake
More than 30 g of alcohol a day is linked to a higher incidence of lung cancer.
Radiation
Many people who have had radiotherapy to the chest or breast, or have undergone CT scans, may have a higher risk of lung cancer.
Age
Lung cancer incidence increases with age.
Supplements
Supplementation with high-dose β-carotene increases the risk of lung cancer in smokers.
BMI
Lean people tend to have a higher risk of lung cancer. However, this is expected to be a confounding factor and low body weight is not thought to cause lung cancer. Instead, these populations tend to smoke and drink more, which may affect the risk.
Sex
Lung cancer is currently the leading cause of female deaths from cancer in the USA. More men than women still die from it, however, because of the higher incidence of smoking among males. This gap is rapidly narrowing at present.
Race
African-American and white women have a similar lung cancer incidence. Among males, there is a 47 percent hike in both incidence and mortality among African-American men.
However, young African-Americans have begun to quit smoking in large numbers, which means this gap is likely to narrow over time.
Why more African-Americans with lung cancer die may reflect many factors, such as a later stage at diagnosis, the lack of high-quality specific treatment of lung cancer and possibly a greater susceptibility to lung cancer in response to smoking.
People of Native American, Asian and Hispanic origin have lower incidence and mortality rates in comparison to the communities mentioned above.
In Asian populations, the tumor characteristics, such as certain epidermal cell receptors, are beneficial in terms of survival and treatment response.
Socioeconomic Status
Poverty, low-income occupations, and lower education levels are associated with increasing lung cancer incidence rates all over the world, irrespective of the socioeconomic status of the country as a whole.
In China, for instance, there was an amazing six-fold variance between the lowest and highest income groups. Low socioeconomic class is an indicator for later diagnosis and a worse prognosis.
Confounding factors include increased smoking, impoverished or unhealthy diets, coupled with occupational or general exposure to other inhaled carcinogenic agents because of the poor socioeconomic profile.
Environmental factors
Passive smoking
Also called secondhand smoking, this term refers to the inhalation of smoke from another person’s cigarette, and is responsible for 1.6 percent of lung cancers.
Non-smokers who have a smoker at home have a 20 to 30 percent increased risk of lung cancer, compared to those without exposure to home smoking.
Children who lived with passive smoke had a 3.6 times higher incidence of lung cancer as adults.
Pollution
Automobile emissions contain polycyclic aromatic hydrocarbons and metals such as arsenic, nickel, and chromium.
These metals are also present in many factories, and arsenic may be ingested in drinking water. They induce oxidation stress and inflammation, which leads to a hypercoagulable state as well as autonomic dysfunction, together contributing to up to 11 percent of lung cancers. Diesel exhaust also contains carcinogens.
In developed countries, indoor pollution by radon in the form of soil-derived gas increases the risk, especially with active or passive smoking. This is much higher in occupations such as uranium mining.
In poorer nations, indoor air is polluted by combustion products from unprocessed solid fuels, notably soft coal (a fossil fuel) and biomass fuels such as wood, used for cooking and space heating.
In China alone, 600 000 deaths from lung cancer over 30 years could be avoided by cutting solid fuel use in half.
Occupational risk factors
Exposure to carcinogens at the workplace, such as chrysotile asbestos and crystalline silica, as well as inhaled radioactive particles in uranium miners or nuclear plant workers, increases the incidence of lung cancers.
Coke oven workers inhale benzopyrene in tar and soot. All of the former work synergistically with tobacco smoke.
Geographic patterns
Within the USA, Kentucky has three times as many lung cancer patients for any given age group as Utah.
Lung cancer rates also shift across national boundaries, in both men and women. This is linked to smoking patterns, but with a 20-year lag period.
The incidence of smoking in both men and women is highest in Europe and North America but lowest in Africa.
Lung cancer not related to smoking is a unique feature of women’s health in China. Chinese women have high rates of lung cancers related to inhalation of fumes other than tobacco smoke, such as cooking gas or wood smoke.
In Chinese men, however, the rate of smoking has shot up ten-fold over a mere 40 years, with a corresponding and continuing increase in lung cancer incidence.
Thus a global view shows that tobacco smoking, and consequently, lung cancer, has shifted its focus from Western countries to the so-called Third World nations, and particularly to Asia.
Today, more lung cancers are diagnosed in these countries than in developed nations.
Protective factors
Food supplements
Fruits and vegetables contain antioxidants which protect against the development of cancers.
The most important among these are carotenoids, such as lutein, zeaxanthin, lycopene and β-cryptoxanthin, as well as vitamin C and E.
Total carotenoid levels in diet and serum show a 20% to 30% lower risk from the highest to the lowest exposure groupings.
Isothiocyanates are thought to underlie the protective effect of cruciferous vegetable intake, such as cabbage, broccoli and Brussels sprouts. They may induce glutathione S-transferase which exerts anti-cancer effects.
However, cigarette smoke lowers the serum levels of most antioxidants.
Physical activity
People who participate in high to moderate exercisers have a 13-30% lower risk, even with heavy smoking.
Quitting smoking
Abstinence at any age lowers the risk and reduces the number of precancerous lesions. The effect increases with the duration of abstinence. The risk remains elevated above baseline for years, however.
Lung cancer in people who have never smoked
The incidence of lung cancer in never-smokers ranges from about 5 to 21 per 100 000 in the age group between 40 and 80 years.
In other words, about 20 percent of lung cancer patients, accounting for approximately 300 000 deaths, have never smoked.
The main risk factors include passive smoking, exposure to occupational or domestic carcinogens (including nickel, wood or soft coal smoke, chromates, arsenic and automobile exhaust), and radon exposure. However, these are absent in a significant proportion of cases.
More men than women, and more African-Americans, are affected, as are Asians living in Asia, in comparison to individuals of Caucasian origin.
Survival with lung cancer
The 5-year survival rate depends mostly on the stage at which the cancer is diagnosed, ranging from 52 percent to 4 percent in stage 1 and stage 4 respectively.
References
Further Reading
40% of US cancer cases linked to lifestyle choices