I'm going to go out on a limb, just to be contrarian, and say that no amount of perfluorodecalin is poisonous. You can immerse yourself in it, fill your lungs with it, fill your entire digestive tract with it, and replace a significant amount of your blood with it. It is completely inert to biology, and is readily permeable to atmospheric gases.

[0] https://en.wikipedia.org/wiki/Perfluorodecalin