People living in close proximity to those who smoke tobacco may have a higher risk of cancer as a new study published in The Lancet journal has found that second-hand smoking is the tenth biggest risk factor for the disease.

Using results from the Global Burden of Diseases, Injuries, and Risk Factors (GBD) 2019 study, the researchers investigated how 34 behavioural, metabolic, environmental, and occupational risk factors contributed to deaths and ill health due to 23 cancer types in 2019.

Changes in cancer burden between 2010 and 2019 due to risk factors were also assessed. Estimates of cancer burden were based on mortality and disability-adjusted life-years (DALYs), a measure of years of life lost to death and years lived with disability.

The researchers at the University of Washington, US assumed that all persons living with a daily smoker are exposed to tobacco smoke. They used surveys to estimate the proportion of individuals exposed to secondhand smoke at work.

The study found that smoking, alcohol use, and high body mass index (BMI), were the top three risk factors for cancer. These were followed by unsafe sex, high fasting blood glucose, particle air pollution, asbestos exposure, diets low in whole grains and milk, and second-hand smoking. These factors accounted for 3.7 million deaths and 87.8 million DALYs in 2019, the researchers said.

"This study illustrates that the burden of cancer remains an important public health challenge that is growing in magnitude around the world," said Christopher Murray, Director of the Institute for Health Metrics and Evaluation (IHME) at the University of Washington. "Smoking continues to be the leading risk factor for cancer globally, with other substantial contributors to cancer burden varying," said Murray, a co-senior author of the study.

Second-hand smoke is smoke from burning tobacco products, like cigarettes, cigars, hookahs, or pipes. Secondhand smoke also is smoke that has been exhaled, or breathed out, by the person smoking. People may also be exposed to second-hand smoke in public places like bars, restaurants, and casinos, as well as in vehicles.

According to the US Centers for Disease Control and Prevention (CDC), there are more than 7,000 chemicals in tobacco smoke, including hundreds of chemicals that are toxic and about 70 that can cause cancer.

Secondhand smoke can cause health problems in children and adults, and can even be deadly. Since 1964, about 2,500,000 people who do not smoke have died from health problems caused by secondhand smoke exposure, the CDC states.