According to the San Francisco Chronicle, the synthetic turf contains polycyclic aromatic hydrocarbons, or PAHs, which could cause cancer. We're not sure about artificial grass of the sort people are putting in their lawns. Though it is also made of plastic, lead chromate (also found in artificial Christmas trees and strings of lights) seems to be a bigger concern there; tests done by the lawn industry, of course, have found no threat.
It's a funny thing: grass, especially clipped short and green, is no more natural or native out in California than artificial turf. The fertilizers and pesticides required to keep grass lush and green pollute the water. Which is worse? It's hard to say.
So what this seems to come down to is a taste critique. People who prefer a more naturalistic aesthetic want "real" grass, while those who are more focused on economy and utility go for the synthetic. Where do you come down? Would you put a fake grass lawn in your back yard to save water?
infographic via the San Francisco Chronicle