This is a rather old article, but it does bring up a good question about flea collars,
Have you ever wondered what the residue on the collar was when you opened the package? How exposed will I be if I or my family handle this Flea collar?
Flea Collars May Expose Children To InsecticidesWhen children pet the family dog, are they exposed to flea collar insecticides present in the animal's coat? That's the subject of a novel study by Mississippi researchers described here today at the national meeting of the American Chemical Society, the world's largest scientific society. The study takes particular note of flea collars with organophosphate insecticides, which also are commonly used on garden and farm plants