I attribute to my robust immune system to the amount of dirt I ate as a child, I was a digger in the school yard, and I liked playing in mud - while being a thumb sucker well into elementary school.
I know you're being facetious but I wanted to say, a friend of mine's kid recently got diagnosed with elevated lead levels in his blood, likely caused by eating contaminated dirt from their backyard. So... test before you try it, I guess?