Having tried many many types of litter over the years, it didn't occur to me that any of them might be a health hazard to either cats or humans until a little while ago. I was more concerned with finding one that didn't cost a fortune and was acceptable to my cat.
We settled on Bob Martin crystals, he seemed happy with that one, and as the crystals are a whitish colour it made it easier to check for problems, like blood in urine. Had tried the cheaper clay ones and found they stuck together in the tray and stuck so fast to the actual tray that it was extremely difficult to scrape the resulting mess off. This was the litter made with bentonite clay.
I have since discovered the following:
Clay litter is harmful to kittens and cats. A cat will lick itself clean, paws etc. after a visit to the tray, the clay clings to the paws and will be ingested by the cat. After a short time it will build up in the intestines and will not pass through. This can kill kittens and cats. Google will give more information on death from this stuff. It contains silica which is really bad for humans if inhaled.
Silicon crystals (Bob Martin, Waitrose and others) also contain silica, inhaling the dust can aggravate human respiratory systems and cause permanent respiratory problems in cats. A cat will obviously raise dust when digging in the litter.
I have discovered this after using it for almost 6 years, too late to heal the damage that may have been done to my cat who does now have a breathing problem, however I will not be buying it again. Will be changing the trays tonight to Cats Best.
Our other cat has Benevo litter which is bio degradable and flushable, it is fine and scatters around but is soft on the paws and will not harm her. Okoplus Cats best is also a good alternative.
I would suggest reading more about this subject, look on the web, and PLEASE try to find a safe litter for your pet.