Lung cancer - risk factors

A study published in December 2011 estimated that over 80% of lung cancers in the UK in 2010 were caused by smoking.1 This included around 950 cases of lung cancer in non-smokers that were attributed to environmental tobacco smoke (ETS) exposure. The total population attributable fraction for smoking is higher for men (87%) than women (84%), which reflects the higher smoking prevalence among men in the past.1 (see smoking statistics for details of smoking prevalence in the UK and abroad).

Current smokers are 15 times more likely to die from lung cancer than life-long non-smokers. 2 Risk of developing lung cancer is affected by level of consumption and duration of smoking. 2

Compared with non-smokers, those who smoke between 1-14 cigarettes a day have eight times the risk of dying from lung cancer and those who smoke 25 or more cigarettes a day have 25 times the risk. However, risk is more dependent on duration of smoking than consumption: smoking one pack of cigarettes a day for 40 years is more hazardous than smoking two packs a day for 20 years. 3,4.

There is also evidence that starting to smoke at a young age carries additional risks of lung damage. 5

Smoking cessation has very significant health benefits even for people who have been smoking for many years.

A lifelong male smoker has a cumulative risk of 15.9% for dying from lung cancer by age 75. For men who cease smoking at ages 60, 50, 40 and 30 years, their cumulative risk of dying from lung cancer falls to 9.9%, 6.0%, 3.0% and 1.7% respectively ( Figure 4.1). 6

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Smoking cessation has the same health benefits for women with the cumulative risk of dying from lung cancer up to age 75 being 9.5% for life-long smokers, and falling to 5.3% and 2.2% for women who stopped smoking around age 60 and 50 respectively.

Data from other European countries support these results leading to the conclusion that giving up smoking in middle age avoids most of the subsequent risk of lung cancer. 7

Meta-analyses have shown that exposure to ETS at home or at work among non-smokers increases risk by about a quarter. 8, 9 Heavy exposure doubles risk. 9  The European Prospective Investigation into Cancer and Nutrition (EPIC) -- a large, Europe-wide study -- has shown that heavy exposure to ETS (daily for many hours) in childhood increases risk of lung cancer by almost four times in people who have never smoked. 68 Case-control studies have confirmed the link with exposure to ETS in childhood, showing risk increases of 47-125%.69,70

See the smoking page for more detailed smoking statistics.

Radon is considered to be the second most important cause of lung cancer after tobacco. A naturally occurring radioactive gas, radon is a known human carcinogen and increased risks of lung cancer were first observed in uranium miners with high radon exposure levels. 11, 12

Radon can also accumulate in homes and other buildings at much lower levels. 13 Some of the highest natural levels in the UK have been found in the southwest, but levels well above average have been found in other parts of the UK.

A study of indoor radon exposure estimated that it may be responsible for 9% of lung cancer deaths in European countries. 14 A study published in December 2011 estimated that around 5% of lung cancer cases in the UK in 2010 were linked to radiation exposure, almost three-quarters of these cases (around 1,375 cases) were due to radon in the home.75 Of the total radon-associated lung cancers, around 13% occured in people who had never smoked, with the remainder being caused by the combination of radon exposure and smoking.75

Exposure to radon multiplies the risk of lung cancer for both smokers and non-smokers by the same amount but its effect is much greater for smokers as their risk of lung cancer is already much higher. For example, at an indoor concentration of 800 Becquerels (Bq)/m ³, the absolute risk of lung cancer by age 75 among non-smokers was an estimated 0.93%, while for smokers it was 21.6%. 14The average indoor radon concentration in Europe is estimated as 59 Bq/m ³.

Other sources of radiation exposure contributing to the burden of lung cancer include radiotherapy (which was estimated to cause around 320 cases of lung cancer in the UK in 2010), diagnostic radiation (around 125 cases) and natural (background) radiation (around 85 cases).75

Several industrial carcinogens, for example, arsenic and polycyclic hydrocarbons as well as some occupations including non-ferrous metal production and painting, have been linked to lung cancer. 15-18 There is evidence that heavy occupational exposure to diesel exhaust increases lung cancer risk 19, 20 and the large American prospective Agricultural Health Study Exposure suggests that exposure to herbicides and insecticides increases lung cancer risk. 21, 22

Occupational exposure to silica can result in silicosis with an increased risk for lung cancer, but without silicosis there is no increased risk. 23 The body of evidence supports an increased risk of lung cancer with exposure to asbestos in non-smokers, and that risks are especially high in those who smoke, who also have past exposure to asbestos. 24

It has been estimated that around 21% of lung cancers in men in the UK and around 4% in women are linked to occupational exposures.76,77 Around 40% of the total occupation-attributable cases were caused by asbestos, around 17% by silica and around 13% by diesel engine exhaust. 77

Outdoor air pollution is also thought to make a small contribution to the lung cancer burden. 25 Evidence is strongest for an increased risk of lung cancer caused by exposure to nitrogen oxides, particularly exposure to traffic fumes. 26-28 Nitrogen oxides are produced as a by-product of combustion. In urban areas, traffic fumes are the main source of outdoor exposure. Use of solid fuel in the home for cooking or heating has been associated with an increased risk. 29, 30

A family history of lung cancer in a first-degree relative is associated with a two-fold increased risk, independent of smoking. 31 If both cancers are diagnosed before the age of 60, the risk ratio is almost five-fold. 32

The association between family history and risk may be stronger in black individuals than white. 33

A meta-analysis found that people who participated in higher levels of recreational physical activity have a lower risk of lung cancer. 34 Since that analysis, results from the EPIC study have shown a lower risk with sports in men and cycling in women, 35  and a reduction in risk with overall recreational physical activity in non-smokers.71 

In two other studies, women who were current or ex-smokers and participated in high levels of physical activity had a reduced risk of lung cancer. 37, 38 However, another subsequent study did not show a link between physical activity and lung cancer risk.36  Further research is needed to clarify the relationship of physical activity with lung cancer risk.

Treatment for Hodgkin’s lymphoma increases lung cancer risk by 2.6-7-fold. Risk ratios are higher in smokers than non-smokers and with radiotherapy compared to chemotherapy. 39 Increased risk of lung cancer has also been shown after treatment for non-Hodgkin’s lymphoma, 40 and up to 30 years after diagnosis with testicular cancer, which is linked to radiation to the chest as part of the treatment. 41

A diet rich in vegetables has been associated with a reduced risk of lung cancer, 42-44 but the protective effect may be limited to smokers: 45-48 however, evidence remains mixed. 49

Higher fruit consumption has also been associated with a small reduction in risk. 48 The role of dietary vitamin C is as yet unclear and there is a suggestion that it may be a marker for other protective dietary factors. 50, 51 A study published in December 2011 estimated that, in 2010, around 9% of lung cancers were linked to people eating less than approximately two to three portions a day (one portion = 80g) of fruit.78 However, that the link between diet and lung cancer is less certain than that of, for example, smoking.74,78

A higher intake of beta-cryptoxanthin may be associated with a reduced risk. 50 Both vitamin C and beta-cryptoxanthin are found in brightly coloured fruit. Conversely, two randomised controlled trials showed an increased risk of lung cancer with beta-carotene supplementation in smokers or those with exposure to asbestos. 52, 53 Higher intakes of selenium, copper and zinc have all been associated with a reduced risk. 54, 55    According to the EPIC study, higher circulating levels of vitamin B6 and methionine are associated with a reduction in risk of lung cancer (approximately a 50% reduction in risk for highest versus lowest levels of each).72    A combined analysis of two Norwegian double-blind, placebo controlled trials showed a 21% increase in cancer incidence in individuals receiving folic acid plus vitamin B12, mainly driven by an increase in lung cancer incidence in the supplemented group. 67

The close association between smoking and alcohol intake is a possible confounding factor in studies of lung cancer risk and alcohol and no strong link has been established. 45,56-60

Significant increases in risk of lung cancer have been reported in people with HIV and AIDS even after accounting for smoking, 61, 62 although one study showed an association in men only. 63

People with antibodies to Chlamydia pneumoniae have an increase in risk. 64 Chlamydia pneumoniae is an infectious bacteria associated with a number of diseases including pneumonia. An increased risk of lung cancer has been shown in people with systemic lupus erythematosus, 65 and Klinefelter syndrome 66, in both cases less than two-fold.

A systematic review of published studies showed that risk of lung cancer is almost doubled for people with a previous diagnosis of tuberculosis (TB), after taking into account smoking history. The risk increase persisted for more than 20 years after TB diagnosis.73