Identify a common risk factor for lung cancer.

The presence of smoking or exposure to secondhand smoke is widely recognized as a significant risk factor for lung cancer. Tobacco smoke contains over 7,000 chemicals, many of which are carcinogenic. When inhaled, these substances can cause mutations in the DNA of lung cells, leading to abnormal cell growth and the development of cancer. Moreover, secondhand smoke, which affects non-smokers who are around smokers, can also increase their risk of developing lung cancer, underscoring the dangers of tobacco use beyond the individual smoker.

In contrast, regular exercise and a high-fiber diet have been associated with various health benefits, including reduced cancer risks for some types of cancer, but they are not considered direct risk factors for lung cancer. Similarly, being under the age of 30 typically does not increase the risk of lung cancer; in fact, lung cancer is most commonly diagnosed in older adults, particularly those aged 65 and above. Thus, smoking and secondhand smoke stand out as a well-established risk factor for lung cancer.