I wonder how much cancer is caused by all the crap that is in our food and drink? We really are supposed to eat a caveman style diet but who does? God knows I don’t.
One risk factor is bad genes that get to express themselves because conditions that would have killed a person before have now become treatable, chronic conditions.
Another risk factor is excessive radiation exposure (sun light, for example) that damages dna in cells in the right way that it doesn't kill them, but does damage replication inhibitors.
Physical trauma is another risk factor, as in the case of asbestos. Asbestos fibers are both fine enough and strong enough to penetrate cells down to the nucleus and can damage dna.
Oxidative stress is yet another risk factor, which can be caused by environmental pollutants, including junk in our food, or cigarette smoke, but it is actually a normal part of metabolism. The cells actually oxidize (burn) glucose in a controlled manner, and your cells are equipped to handle the process, but the more they replicate, and the more they oxidize, the more worn out the telomeres that keep dna from unraveling can become. In other words, some cancer is simply related to aging.
But you can minimize oxidative stress by reducing the amount of metabolization that occurs in your cells, which is one reason why I am a proponent of periodic fasting.