How to stop the fog????

Bought a set of 3m Fog Free safety glasses form Home Depot....aint worth the extra $$ I was charged. Worked great up until the point that I needed to clean them....they now fog up, not as bad a s "regular" safety glasses but bad enough to where it is impossible to see. What is strange is they sometimes fog up....and sometimes they don't....Waiting for the Cat Crap to show up...