I was told to use a cut up flea collar in the vacuum bag to kill any fleas, not attached to pets. Fleas breed in cycles, so it is possible to kill adult fleas, just to hatch another bunch later, possibly in your vacuum bag. I thought the flea collar thing was just an extra precaution for fleas that may find their way into your home. The pets are obviously protected by something like Frontline. I never though that a single cut up collar would be toxic. If it is so toxic, why is it okay to put on an animal?