Skin has millions of years of incremental improvement at keeping stuff out. It is not unheard of for a carcinogen applied to the body to not cause skin cancer

Skin has millions of years of incremental improvement at keeping stuff out. It is not unheard of for a carcinogen applied to the body to not cause skin cancer directly but to cause cancer elsewhere because being on the body makes it likely to get in through the various holes where it then finds its way to some more susceptible tissue.