Try opening another side a bit...Well right now the sides are covered in thick plastic to keep out the wind, snow and rain as much as possible. But, they've been covered for over a month now and I've just started noticing a smell. The top of the run, about 3 inches, is open all the way around (no plastic) to let fresh air in. I have been using pine shaving...and it was fine. But just recently it's become very smelly.