Is Naphthalene crystals in my ceiling dangerous to my health.?

Can anyone please tell me the risks of having m amounts of Naphthalene crystals tered throughout my ceiling, even sitting on exhaust fan covers in the kitchen and bathroom. The owner has just done this hoping to deter possoms but now my house stinks like moth. Is it toxic to inhale? Does it react to heat(heater in winter)? Is it a fire risk? Thanks