Classroom Water & Sand Tables: Safety Risks & Prevention

Question
I am hoping you may be able to inform me of any risks that are associated with sand tables/ and water tables in children's classrooms- and how these problems can be avoided?  

Answer
I guess you mean a little tray that kids play in.... I have not seen any problems with transmission of diseases.... I am always concerned about the risk of injury, theoretically, a child can drown in a pan of water, but overall, I don't see any major risks.