Flea collars are often purchased for pets at the first sign of the first flea, or even as a prophylactic. The problem is they expose both people and pets to toxic vapors 24 hours a day. The substances the collars are imbued with may be absorbed directly into the bloodstream. Your pets may develop dermatitis on their necks from the collars. Cats are especially susceptible. Humans can also get rashes.
In general, flea collars are not particularly helpful. They may kill a flea here and there in your pet’s fur, but they will not touch the masses of fleas hiding in your carpet or yard. Also, flea collars are largely ineffective on large dogs since the vapors tend not to reach the tail, where fleas gather.
Some alternatives to flea collars are flea sprays or flea bombs for use in the house (these can make people sick, though, so use with caution), or pyrethrin dusts for use on your pet. Pyrethrin dusts are not considered toxic, but they can cause rashes and allergic reactions.
Pyrethrin dusts don’t always kill fleas, but work for stunning them. The best way to use a dust is to stand your pet on newspaper while powdering him (you should wear gloves), comb the fleas onto the paper, then burn the whole thing.