Flea collars do contain chemicals that are toxic to humans if absorbed through the skin or blood and if eaten.
So you want to make sure the flea and tick collars are not chewed on by a child, toddler etc if they are near the dog or cat wearing the flea collar.
My son when he was 2 years old would wrestle with our cat that had a flea and tick collar on and I would catch him trying to bite the flea and tick collar.
I finally had to remove the flea and tick collar from the cat when my 2 year old son was near the cat because it would've likely made my 2 year old son sick if he had continued biting it.
But as long as the flea and tick collars are not worn by humans or eaten by humans the flea and tick collar is generally safe.
If you do wear a flea and tick collar which I did and still sometimes do on my wrists when out in the woods just be aware that you could be exposing yourself to toxic chemicals.
However if you do wear a flea and tick collar don't wear it all day and wash your skin off afterwards.
And if you handle a flea and tick collar with your bare hands be sure to wash your hands off afterwards.
Humans can use flea collars to keep fleas and ticks away from them.
However if a human uses a flea collar they do so at their own risk as the chemicals that are used in flea collars can be toxic to humans.
The chemicals in flea collars that can be toxic to humans can be absorbed into the blood through the humans skin so it's not recommended to wear a flea collar as a human.
However when I was out in the woods I put a flea collar on both my wrists before to prevent ticks from getting onto my wrists.
I would reach down for something or go through some brush and then ticks would get onto my wrists or arms.
But when wore a flea collar on those arms I no longer had any problems with ticks getting on there.
I haven't gotten sick from it but I didn't wear the flea collars on my wrists all the time but only when out in the woods.
And I made sure to wash my arms and wrists off really good after removing the flea and tick collar.