The flea and tick collar first has to be activated. Pulling the flea collar releases and loosens the powdered chemicals---usually tetrachlorvinphos (TCVP) and propoxur---that provide the pest protection. Once the collar is activated, it must be used immediately.
The flea and tick collar is applied to the animal's neck and adjusted to fit snugly (but not tight enough to apply too much pressure). The collar brushes up against the dog's or cat's neck and releases the chemicals onto the fur and then the skin. The chemical gets distributed and lies within the hairs. The collar continues to release the as long as the animal wears it, usually up to six months. To maintain protection, a fresh collar must be applied periodically.
Fleas & Ticks
When a flea or tick jumps onto the animal, it must crawl through the hairs to reach the skin to bite. It comes in direct contact with the chemicals in the fur, which penetrate and kill it, similar to the effect of household bug spray on an insect.
Collars can be effective in controlling fleas and ticks, but it's possible the pet's fur will not be completely saturated with powder, allowing some pests to live in areas of the animal's body. Many pet owners now prefer a solution that targets the skin rather than the fur (like FRONTLINE and Revolution). A flea collar also loses effectiveness when wet.
Veterinarians have had some concerns that the chemicals in flea and tick collars could be dangerous for pets and children (see "Chemical Flea Collars Threaten Pets and Kids, as Well as Pests" and "Poisons on Pets" under "References" below). Studies have shown the chemicals in collars made by major pet products companies can damage nerves and possibly cause cancer if used over a long period of time. The study also cites potential risk to kids, who pet the animal, get the chemicals on their hands and then place their hands in their mouths.