From OEHHA:

The Office of Environmental Health Hazard Assessment (OEHHA) within the California Environmental Protection Agency is adding titanium dioxide (airborne, unbound particles of respirable size) to the list of chemicals known to the State of California to cause cancer for purposes of the Safe Drinking Water and Toxic Enforcement Act of 1986 (Proposition 65 ).  The listing does not cover titanium dioxide when it remains bound within a product matrix.  The listing of titanium dioxide (airborne, unbound particles of respirable size) is effective September 2, 2011.

For more information or the current Proposition 65 list see the above link.