There are many litters that can be harmful to your cat as well as the environment.
Unfriendly cat litters are:
Clay Litter: clay litter is strip-mined and this causes environmental damage during extraction. The dust that flies thru the air after kitty tries to cover up her mess is bad for your cat's lungs. This dust can cause asthma and respiratory concerns.
Clumping Litter: Clumping litter contains sodium bentonite and this is mined just like the above clay litter. It is damaging to our environment. There is some speculation that this type of litter can get into your cat's lungs or intestines and cause damage.
Scented Litter: The fragrances in scented litter come from several undisclosed combinations of chemicals. Which can contain phthalates. Phthalates are hormone disruptors. Fragrances can trigger allergies in pets and humans.
Healthier Greener Litter
Cat litter made with wheat, corn, ground corn cobs, alfalfa pettets and wheat. These are all good choices.
There are pine litters and cedar litters these can be good choices as well. There is some research indicating that some cats maybe allergic. If you use this type and your cat sneezes everytime he uses the litter box, you may want to swith to another type.
Some brands you may like are:
Naturals Corn Cat Litter I think they sell it exclusively at Petco
Worlds Best Cat Litter is made from corn and 100% natural
Swheat Scoop Natural cat litter
Feline Pine (remember just to be mindful if kitty sneezes after use he/she may have some allergy to pine)