The point is not that you are eating dirt. The point is that you are constantly surrounded by small amounts of things that are not good for you. Take a plane? Eat a banana? You are exposed to radiation. Drink water in certain parts of the country? You are exposed to small amounts of uranium. Is any of that good? No, but it's unavoidable to be exposed to small amounts of naturally occuring toxins and it's not worth worrying about.