My first thought was that the sulfur hexafluoride might be toxic, but according to Wikipedia the gas is colorless, odorless, non-toxic and non-flammable. The gas is actually dense enough to even float some light woods. To give you some reference; air is 784 times less dense than water compared to the 6 times of sulfur hexafluoride, and water is about 2.5 times less dense than dirt in case you were wondering.