It's always a combination of genetic predisposition and external effects working together (except, of course, in very rare cases of lethal-dose exposures to carcinogens). Usually external effects just increase the chances of mutations, and then fate and your genetics role the dice.

Not to forget Chemicals like PVCs plasticizers, solvents like benzene in paints, bis-phenol A in plastic bottles, flame retardants in our home that we breathe whenever we sit down in something soft, insecticides(air in cities is super polluted of them) and diesel particles.

Those things are extremely dangerous, but we have not natural sensors to detect and avoid exposure to them.

I bought a geiger detector and it surprised me to find places with thousands of times more radiation than others.

In Dawkins' "The Ancestor's Tale" he mentions that rodents have poor DNA repair relative to other mammals.

Is it a joke? They've a trunk.