None. Having any mold in your crawl space becomes a major health hazard to anyone living in your home. Even the smallest amount of mold releases spores into the air, which are eventually carried into your living space through a process known as the Stack Effect. Once these spores are released, mold can begin to grow exponentially if no steps are taken to protect your crawl space. Long story short, any amount of mold in your crawl space is a bad thing.