Other factors such as the sitter's balance and type and density of surrounding floor hazards may be considerably more hazardous to the sitter's health. Especially if floor hazards include: acid, alligators, lava, or landmines.

I heard that the bouncing that people tend to do on those balls is hard on the back. (But I also know that weight lifting is hard on the back, and that just makes your back grow back stronger. (If you do it right.))