Objective:
The rapid increase in lung cancer in nonsmoking women puzzled the public. The 3.4 folds increase for age 65-69 rate in 35 years was translated into a 13-fold increase in total lung cancer cases in women in Taiwan. More than 90% came from nonsmokers. Without smoking as a cause, how they came about was a mystery, and many people blamed it on air pollution.

Methods:
Smoking rates and age-specific lung cancer incidence in Taiwan for the last 35 years were compiled from governmental statistics, and compared between male and female.

Results:
Male smoking rate was 65% in 1975 in Taiwan. Growing up as a girl, she was surrounded by men with 2/3 smoking, such as her father, her brothers, her husband, and her co-workers. With crowded indoor spaces, not to be exposed to second-hand smoking would be exceptional for any woman. The continued increase of age-specific lung cancer rates from 1981-2016 in women patterned after the increase in men. Rates for women were half of men, incompatible with air pollution theory. Starting 2009, male rates declined in the old age, with similar decline in women.

Conclusions:
Given the similarity in stepwise pattern of age-specific rates increase in lung cancer in the last 35 years between women and men, the high second-hand smoking exposure, resulting from 2/3 of men surrounding women smoking, and the recent decline occurred in sync between men and women, main cause for lung cancer in nonsmoking women in Taiwan should be second-hand smoking, and not air pollution.