Good question! I make my own compost too. Dr. Joe Falkinham says commercial potting soil is rich in peat and contains high numbers of NTM-1 million per gram. He suggests that we use an N95 mask to avoid dust inhalation. Of course, he made that recommendation prior to COVID 19 and he doesn't address compost. I've tried wearing an N95 mask while weeding and it was tough. I do wear a surgical mask now and try to limit my gardening to early morning when it's not so hot. I guess not matter what, we need to be cautious.