Mold is the worst indoor air pollutant today! The source mold needs to grow abundtantly is moisture. So, if your crawlspace is moist, it will have mold. In modern homes with moisture issues, mold becomes an epidemic.Your wooden floor joists, sub-floor, and ducting could be a breeding ground for millions of mold spores. You protect every other part of your home from moisture, while leaving the crawlspace completely unchecked.