We purchased potting soil from this Home Depot last year that was one of the worst purchases we ever made. The soil had Fungus Gnats! It was terrible. As soon as we used the soil to pot our indoors plants the bugs came flying. They invaded our home. It was a constant bombardment of little black gnat flying at your face.
Since we try to be environmentally friendly and would not like to breathe in bug spray ourselves, we bought sand to cover the soil in the hopes of killing the larvae. Not sure if that worked or not because the bugs continued to fly around the house. We finally realized; They were coming right out of the bag! We opened the cabinet door where the bag was being stored. They were in the cabinet!
The gnats killed our poor plants we had for years. It was a terrible mess. Never again.