I've been using the bags for almost two months. 5 bags in an 8x8x6 room with 8 plants. First grow and wanted a sealed environment since outside air has been record high temps all summer. Two bags developed a mold on the inside, one became completely covered in it. Also noticed black mold like bloches on some dying leaves. Hasnt been a very successful first grow so i dont completely blame it on the bags. Humidity in the room was very high until I finally got a good used dehumidifier. The company has replaced the bags through the shop I bought them from (25$ each) with very little hassle. They said there is no way anything but co2 and o2 passed through that patch, but I find that hard to believe. So keep an eye out for any mold and get them outta there if you do. I break up the block once it has become covered in white stuff and the bags are almost always fully inflated. After seeing Spencer's readings I wonder if I might be overdoing it.