You could be stepping on mold.
Mold grows where there is moisture present. Moisture in the air can be felt, for example humidity. But there is also a small amount of moisture in things you wouldn't think had it, such as wallpaper, back splash in the kitchen, and floors. All hardwood floors have a small amount of moisture to keep them they way they are. That was the case at this home. We found high level of moisture under their hardwood floors and we had to remove them. We can do it all! Call us when you need us, SERVPRO of Park Cities is here to help!