What external factor is known to cause cancer?

Chemical exposure is known to be a significant external factor that can lead to cancer development. Many chemicals have been identified as carcinogens, meaning they have the potential to cause cancer in living tissues. Examples include substances such as benzene, formaldehyde, and certain pesticides. These chemicals can damage the DNA in cells, leading to mutations that may promote the growth of cancerous tumors.

In addition to direct chemical exposure, the process by which these substances initiate cancer often involves complex interactions with cellular mechanisms, including inflammation and oxidative stress. The long-term exposure to chemical agents, especially in occupational settings or through environmental pollutants, significantly increases the risk of developing various forms of cancer, such as lung, bladder, and skin cancers.

While genetics and immunologic conditions are also important contributors to cancer risk, they are typically classified as intrinsic factors rather than external. Similarly, infections can cause cancer but are a different category of risk factor since they often involve a biological agent rather than a chemical element. Therefore, the link between chemical exposure and cancer has been extensively studied and is well-documented within oncological research.