From Health magazine
Whats in kittys box: It could be chemicals that affect lungs and may cause cancer.

Anyone who has changed a litter box is familiar with that cough-inducing dust cloud. It likely contains low levels of crystalline silica, a carcinogen. (Check the bag or box before you pour it into Fluffys litter box.) “If the warning says to go to the ER if you swallow, its safe to assume its really toxic,” Natural Resources Defense Council scientist Jennifer Sass, PhD, says. Replace with greener versions made from corn, wheat, alfalfa, cedar, and even pine—all of which work well. You can find natural litters at major pet stores like Petco. To give the natural variety an odor-eating boost, mix in a little baking soda, Sass says. And be sure to keep boxes in ventilated spots such as a screened-in porch.