Before the start of mass production of tobacco products
in the late 19th century, lung cancer was a rare cancer (1).
Currently, however, it is the leading cause of cancer deaths
in males and the second leading cause in females globally (2).
In 2012, lung cancer killed an estimated 1,098,700 men and
491,200 women worldwide, corresponding to 24% and 14%
of all cancer deaths in males and females, respectively (2).
The most important cause of lung cancer is tobacco
smoking, although the proportion of lung cancer deaths
attributable to smoking vary across populations, ranging
from >80% in the United States (3) and France (4) to
61% in a pooled analysis of 21 Asian cohorts (5) and 40%
in sub-Saharan Africa (6). Lung cancer is also caused by
secondhand tobacco smoke, with an estimated 21,400 lung
cancer deaths in non-smokers annually (7). Other risk
factors for lung cancer include indoor air pollution because
of unventilated combustion of coal in the household for
heating and cooking (8); outdoor air pollution (9,10);