I've been doing hardwood floors for over 16 years... We've caught our garbage dumpster on fire a few times over the years due to stain rags and polyurethane dust. The first time they gave us a pass, subsequent fires cost us $250 by the trash company and who knows how much if the fire department shows up. It's a real problem that some don't believe in. All it takes is a small rag soaked in oil and about 80-90F air temp and it can start to smolder.