Apparently, most types of play sand contain crystalline silica and asbestos tremolite. The silica is derived from quartz stone and is a known carcinogen.
OSHA (Occupational Health and Safety Administration) says this about crystalline silica, "Silica, Crystalline: Silicosis is a disabling, nonreversible and sometimes fatal lung disease caused by overexposure to respirable crystalline silica. More than one million U.S. workers are exposed to crystalline silica, and each year more than 250 die from silicosis. There is no cure for the disease, but it is 100 percent preventable if employers, workers, and health professionals work together to reduce exposures." Apparently, the small pieces of silica can be inhaled and trapped in lung tissue. More info here: http://www.non-toxickids.net/2008/05/safer-play-sand.html