It can cause serious or fatal respiratory disease - silicosis. Here's a quote for the CDC website:
http://www.cdc.gov/niosh/92-102.html
Abrasive blasting with sands containing crystalline silica can cause serious or fatal respiratory disease.
The National Institute for Occupational Safety and Health (NIOSH) requests assistance in preventing silicosis and deaths in workers exposed to respirable crystalline silica during sandblasting. Sandblasters, exposed coworkers, and their employers urgently need information about the respiratory hazards associated with sandblasting. Your assistance in this effort will help prevent silicosis and death, a national goal for health promotion and disease prevention stated in Healthy People 2000 [DHHS 1990].
The Alert describes 99 cases of silicosis from exposure to crystalline silica during sandblasting. Of the 99 workers reported, 14 have already died from the disease, and the remaining 85 may die eventually from silicosis or its complications. NIOSH requests that editors of trade journals, safety and health officials, labor unions, and employers bring the recommendations in this Alert to the attention of all workers who are at risk.