Secondhand smoke induces the cancerous mutation of genes that are different from those affected among active smokers, increasing the risk of lung cancer among nonsmokers, according to a recent study by the National Cancer Center Japan.

Researchers say the study has shown how passive smoking causes genetic mutations, providing further evidence of the harmful effects of secondhand smoke. They also call for stricter domestic laws to ban smoking in all indoor spaces in line with international standards.

Currently, restaurants and bars in Japan are allowed to have smoking areas under certain conditions, which could cause patrons to inhale tobacco smoke involuntarily.

In the , published in the Journal of Thoracic Oncology in February, researchers analyzed the DNA sequences of 291 female nonsmokers, of whom 213 had been exposed to secondhand smoke, and 122 female smokers. They all underwent surgery for lung adenocarcinoma at the National Cancer Center Hospital in Tokyo between 2011 and 2017.

Lung adenocarcinoma is the most common type of nonsmall cell lung cancer that affects cells secreting substances such as mucus.

The team found that gene mutations associated with lung cancer were different among smokers and nonsmokers participating in the study.

Nonsmokers who had been exposed to tobacco smoke saw an increase in the expression of the APOBEC3B gene, caused by APOBEC proteins. Such an increase was not seen among smokers.

Specifically, in the genes that had mutated in nonsmokers who had been exposed to secondhand smoke, 15.6% of the mutations involved the accumulation of APOBEC proteins in the tumor cells, as opposed to 7.32% in those who had not been exposed.

The mutations of this type were unevenly distributed in the tumor cells, the researchers added, noting that this may create ways for cancer cells to survive in the body under different conditions and boost their survivability by making existing drugs less effective.

Lung cancer causes about 76,000 deaths in Japan each year, and about 1.8 million deaths worldwide.

The International Agency for Research on Cancer, an intergovernmental agency that forms part of the World Health Organization, classifies involuntary or passive smoking as Group 1, the highest risk level, meaning that a causal relationship between exposure to passive smoke and cancer has been established.

An earlier study by the National Cancer Center showed that the risk of people exposed to passive smoke developing lung cancer was 1.3 times higher than those who weren＊t exposed. For lung adenocarcinoma, the risk for people with a history of passive smoking is twice the level seen for those without, the researchers said.