What are risks of wearing cat flea collars on ankles?

They are not . Effective on humans and the chemical they contain can make you ill.
Flea Collar Risks. What is the reasoning for wearing them in the first place? Is this because of a job involving being around flea carriers? I would not suggest the wearing of these at all because they contain chemicals that are known to be toxic to the nervous systems of children and chemicals that carry possible cancer risks.