Lung cancer is cancer of the trachea (windpipe), bronchus (airway) or lung air sacs (alveoli). It is now the leading cause of cancer deaths worldwide in both men and women. Survival rates vary depending on the cell type of the cancer and whether the disease was diagnosed at an early or late stage. Research has shown that, on average, only 12.6% of people with lung cancer are alive 5 years after diagnosis. Lung cancer was a rare disease at the start of the 20th century, but increases in exposure to tobacco smoke and other triggers of the disease (e.g. air pollution) have contributed to a pandemic in the 20th and 21st centuries.
Evidence that active smoking increases risk of lung cancer
- People who smoke tobacco are 11 times more likely to develop lung cancer than those who do not smoke
- The risk of developing lung cancer in smokers compared to never smokers is similar in both men (9 times more likely) and women (12 times more likely)
- The risk of developing lung cancer from smoking is higher when looking at studies conducted in Europe (15 times more likely) compared to studies conducted in the rest of the world (9 times more likely)
- Lung cancer risk increases when people smoke more cigarettes per day and for a longer period of time
What evidence was used?
The present study, designed to update previous systematic reviews of evidence relating to the risk of developing lung cancer from active smoking, commenced in January 2013. Initially, the two most recent systematic reviews were identified that included literature published over the period prior to 2008 (Gandini 2008, Lee 2012). A further search was then performed looking for studies published in 2008 and after.
A total of 34 studies were found, which looked at the relationship between active smoking versus never smoking and the risk of developing lung cancer.
The studies found out whether the people they were investigating were active smokers either by asking them (in 32 studies) or by also measuring levels of smoke in their body (in 2 studies).
Thirteen out of the 34 studies were conducted in Europe.
The quality of the 34 studies included (as judged by the Newcastle-Ottawa scalescore) gave an overall median score of 7 (range 3-8). Before the quality was evaluated, scores of <7 had been selected as indicative of lower quality and scores of ≥7 acceptable as of high quality. Therefore, 19 of the 34 studies were judged to be of high quality.
Detailed findings and data
People who smoked were 11 times more likely to develop lung cancer compared to those who had never smoked (pooled relative risk ratio 10.92, 95% confidence interval 8.28 to 14.40. Click here to see a forest plot of the findings - Figure A
Men were found to have similar risks of developing lung cancer from smoking (9 fold increase) as compared to women (12 fold increase). Click here to see a forest plot of the findings - Figure B
To explore the impact of geographical location, the analysis was grouped based on where the studies were conducted. Studies conducted in European countries had evidence of larger risks of developing lung cancer from smoking (15 fold increase) compared to studies conducted elsewhere (9 fold increase). Click here to see the analysis. Click here to see a forest plot of the findings - Figure C
To explore the impact of the methodological quality of the 34 studies included in the review, the analysis was grouped into higher (≥7) versus lower (<7) quality studies according to the Newcastle Ottawa scale score. Similar increases in the risk of developing lung cancer in smokers were seen in the higher quality studies (10.5 fold increase) and the lower quality studies (11 fold increase). Click here to see a forest plot of the findings - Figure D
Numbers of cigarettes
The majority of the included studies assessed whether there was an association between the incidence of lung cancer and cigarette consumption, using either the number of pack years of smoking (defined as the number of packs smoked per day multiplied by the number of years smoked - thus one pack year is equivalent to 365 packs smoked each year) (13 studies), duration of smoking (5 studies) and the number of cigarettes smoked per day (17 studies). Individual studies also looked at the age since starting smoking (2 studies), the total number of cigarettes smoked in a person’s lifetime (1 study) and levels of nicotine in toenails (1 study).
All of the studies reported the incidence of lung cancer was consistently greater in those with higher cigarette consumption. A pooled analysis of the 13 studies reporting pack years found consuming less than 20 pack years of cigarettes resulted in a significant 3 fold increase in developing lung cancer; consuming 20-40 pack years of cigarettes resulted in a 7 fold increase; consuming 40-60 pack years resulted in an 11 fold increase; and consuming more than 60 pack years resulted in a 12 fold in increase of lung cancer. Relative Risk Ratio of lung cancer by pack years of smoking
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Evidence that passive smoking increases risk of lung cancer
- People who are exposed to passive smoke are 1.41 times more likely to develop lung cancer
- Most of the evidence on passive smoking and lung cancer risk is based on data from women non-smokers who are exposed to passive smoke from their smoking husbands
- The risk of developing lung cancer from passive smoking is similar in studies conducted in Europe and elsewhere
What evidence was used?
The present review started in January 2013, and was designed to identify and update previous systematic reviews of studies that assessed the effect of passive smoking on lung cancer.
Three systematic reviews were identified that included literature published over the period up to 2007. An updated search for additional original studies was then performed using 2007 as the starting date up to January 2013. From these searches, eight studies assessing the effect of passive smoking on the incidence of lung cancer in non-smokers were identified and included. A further seven studies assessing the effect of passive smoking on the risk of death from lung cancer were also identified and included. Thus a total of fifteen studies have been included in the present updated review.
Two of the included studies used a nested case control design; the remaining thirteen studies used a cohort design. Two cohorts were used in more than one study; Shanghai Women’s Health Study cohort (Fowke 2011; Wen 2006) and the European Prospective Investigation into Cancer and Nutrition (EPIC) cohort (Vineis 2007; Chuang 2011). To prevent double counting of data, the results from the Wen 2006 and Vineis 2007 papers were used in the main analyses; therefore the main results are based on data from 13 studies.
Four studies were conducted in Europe, with the remaining studies being conducted in either the USA (3 studies), New Zealand (1 study), China (3 studies), Korea (1 study) or Japan (3 studies).
Eleven of the studies assessed the effect of passive smoking in women only, whilst four studies assessed the effect in men and women. The majority of studies recruited participants over the age of 40 years.
Studies measured participants’ passive smoking by asking them in fourteen studies and through testing urine in one study.
The methodological quality of the fifteen studies, as judged by the Newcastle-Ottawa scale score, gave an overall median score of 6 (range 4─8). Before the quality was evaluated, scores of <7 were designated indicative of low quality and scores of ≥7 were designated acceptable as high quality.; Six (40%) of the 15 included studies were judged to be high quality.
Detailed findings and data
People who were exposed to passive smoke during adulthood were 1.41 times more likely to develop lung cancer compared to never smokers unexposed to passive smoke (relative risk ratio 1.41, 95% confidence interval 1.21 to 1.65). Click here to see a forest plot of the findings - Figure A
Two studies using the same cohort (European Prospective Investigation into Cancer and Nutrition (EPIC)) found that daily exposure to passive smoke during childhood was associated with between a 1.30 and 1.00 (doubling) increase in the risk of lung cancer (Chuang 2011; Vineis 2007). However, this finding was non-significant.
To explore the impact of geographical location, the analysis was grouped based on where the studies were conducted. Studies conducted in European countries showed similar risks of developing lung cancer from passive smoke exposure as compared to studies conducted elsewhere. Click here to see a forest plot of the findings - Figure B
To explore the impact of the methodological quality of the 13 studies included in the review, the analysis was grouped into higher versus lower quality studies. Similar increases in the risk of developing lung cancer in participants exposed to smoke were seen in the higher quality studies and the lower quality studies. Click here to see a forest plot of the findings - Figure C
The results from the studies were grouped by the sources of the exposure to passive smoke (any source, household, husband and work). Exposure to passive smoke from husbands or work were significantly associated with increased risks of lung cancer in women never smokers 1.36 times and 1.57 times increase in risk, respectively). Click here to see a forets plot of the findings - Figure D
Numbers of cigarettes
Six of the included studies looked at the effect of intensity of exposure to passive smoke (De Waard 1995; Kurahashi 2008; Jee 1999; Cardenas 1997; Garfinkel 1981; Hirayama 1984). A meta-analysis of four studies from Korea, Japan and the USA found a 1.46 times increased risk of lung cancer in women whose husbands smoked >20 cigarettes per day (relative risk ratio 1.46, 95% confidence interval 1.10 to 1.44) (Kurahashi 2008; Jee 1999; Garfinkel 1981; Hirayama 1984).
In another study, a significant dose─response relationship was seen in the number of cigarettes smoked per day by the husband and the risk of death from lung cancer (p=0.03), where a significantly increased risk of death from lung cancer was seen in women exposed to 40+ cigarettes per day (relative risk ratio 1.9, 95% confidence interval 1.0 to 3.6) (Cardenas 1997).
Similar increased risks were also seen in women whose husbands smoked for more than 30 pack-years (Kurahashi 2008).
However, in one study, a three-fold increase in the risk of lung cancer was seen in women whose husbands smoked for more than 30 years (relative risk ratio 3.1, 95% confidence interval 1.4 to 6.6) (Jee 1999).
One study found similar increased risks of lung cancer in women who were exposed to lower (9.2-23.4 ng/mg) and higher levels (23.4-100 ng/ml) of passive smoke from any source compared to those unexposed (<9.2 ng/mg) (measured using urinary cotinine levels) (De Waard 1995).
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