There's plenty of information online about the problems associated with Clumping Clay Kitty Litters. There are also some articles saying there is no problem. I think prudence and caution is the rule of the day when it comes to our pets. There are too many alternatives to continue using clay litters.
Recently a friend of mine had a cat who was not able to have bowel movements. After using everything possible recommended by the vet and some herbal laxatives, there was still no results.
In her own words about her cat, Doodle:
It wasn't a hairball but two other issues. The Vet first did a manual evacuation with her finger, then an enema. She did a really good job and Doodle was a trooper. The Vet said a bowlful came out.
There was a pouch that had formed just after the pelvic bone, and stuff got into that and stayed there. She does not know whether or not it will go back to normal but I pray that it will. We have to keep her on that lactulose for now.
Second issue that really shocked me is that there was a 5 or so inch piece that came out that was very clay-like. In other words, it wasn't about to go anywhere. I have always used clay kitty litter. Little Doodle has long hair and likes to be clean, which meant she was always licking the clay dust off of herself!
I figured it out pretty quickly and went straight to the pet store to buy something else. Talking to a couple people who work there, they said they have heard of that before. So now I am trying "Yesterday's News" which is made from old newspapers and not possible to ingest and it doesn't stick to the fur and there is no dust. I've used the pine pellets before, mixed with clay, but they did not like it. I loved it and it gave off a nice smell. It did disintegrate and track on the floor though, which meant it also sticks to fur somewhat.
The Vet told me she got all of it out of her, and she's completely clean. However, as soon as we got home, she went straight to the box and dropped two, inch and a half rock-hard pieces, and just now she went again and and deposited a 3/4 inch soft log. So long as it keeps coming out, we'll be fine.
Choosing Cat Litter can be good for the environment also.