In cancers that have a high incidence among certain populations, where no germline mutation can be identified there is likely some outside factor contributing to the progression of a normal cell to a cancer cell. Lifestyle or environmental contributors to cancer have been identified that increase the risk of developing cancer. Contributors to cancer formation are called carcinogens. Carcinogens can be separated into three categories: physical and chemical carcinogen and oncogenic viruses.
We know that exposure to UV light from the sun overtime can cause skin cancer. This is a well-documented type of physical carcinogen. The UV light actually changes the structure of amino acids. This type of change can lead to cancer formation. Radiation is also considered a physical carcinogen and acts in a similar manner to UV light with respect to altering the genome. Asbestos is another very common physical carcinogen with a unique method to induce cancer. Asbestos fibers actually interfere with chromosome alignment during cellular division ultimately causing uneven numbers of chromosomes in the sister cells. Any material or particle that can induce cancer formation when implanted in a mouse model is classified as a physical carcinogen.