The additive that is being sold to make plastic float is a very common filler that is used in a lot of different industries because it is extremely cheap and fairly easy to get. Some of the products you have in your home have it in them (potting soil, ceiling tiles, etc.). I currently work as a chemical engineer at a manufacturing plant that uses it daily. The danger with it is getting it in your eyes and lungs. If you get it in your eyes, DON"T RUB THEM. You will scratch your eyes and do a lot of damage. Also, do not inhale it. It can embed in your lungs and cause issues. Since the material is very light, it will form dust clouds when working with it in large quantities. When working with this material you should always be wearing safety goggles (that are sealed to your face so it cannot get in your eyes), and a dust mask (we use the white disposable dust masks that you can buy at any hardware store). I have personally gotten it in my eyes and have always been able to wash it out with lots of water. I have also inhaled it in small quantities and ended up coughing for an hour or two. Like many things in life, if you take the proper precautions and treat it with respect, you shouldn't have any issues.
Eric