I remember unwrapping my cat's flea collars as a child. They were made of thick, rubbery plastic that had a distinctly powdery texture. But do these curious and seemingly innocuous necklaces put millions of children in harm's way?

Flea collars cover an animal's fur with a layer of pesticide, which kills fleas by disrupting the function of the insects' nervous system. Some collars contain tetrachlorvinphos (TCVP) and propoxur, chemicals that are toxic to the nervous system and carcinogenic. The EPA maintains that the amount of pesticide residue on your pet's fur is not harmful, but NRDC asserts that it is.

In a small study conducted by NRDC, 50 percent of dogs wearing TCVP flea collars had enough residue on their fur to pose a danger. That means a toddler who had two hours of exposure each day to the pet could be exposed to chemical levels exceeding EPA standards.

"Kids explore their environment in a hand-to-mouth way," says NRDC senior scientist Miriam Rotkin-Ellman. "So we think products such as these are unsafe." NRDC is petitioning that products containing these chemicals be taken off the market. To find out if your flea-prevention product is safe, visit greenpaws.org.