Yes flea collars can be harmful to humans especially when you get the pesticide residue on your hands and skin because the pesticide on the flea collars that get on your skin can be absorbed into your skin.
The pesticides can then leach into your blood and make you really sick or even possibly cause death.
When you put a flea collar on a dog or cat make sure to wash your hands afterwards before touching food, touching your mouth, touching your eyes etc.
The flea collar is safe for dogs and cats as long as you use the correct type of flea collar on the animal.
Just don't try putting a flea collar on yourself because you could become sick or even die as a result.