I think that balloon aliens are an interesting alien body plan. It is something practically never seen on earth and the only lifeforms I’m aware of that do anything similar are kelp that have tiny bladders filled with hydrogen to keep them buoyant in the water. But I was thinking maybe such a body plan would be better adapted for a world with a denser atmosphere with heavier gas.
I have an idea for such a world that is covered in fungal forests and such an atmosphere. There are several creatures that take on a balloon shape in order to float effortlessly through the air. Along with the floaters there are deadly fungal spores, but I wanted another good reason for human explorers to keep their helmets on.
So I need a gas that is heavy but also not very healthy for human lungs.
One idea I had was sulfur hexafluoride that seems perfect based on this video.
If you can float a tinfoil boat on the stuff, you can probably float anything. You don’t even need to try and produce hazardous hydrogen, and going up like the Hindenburg anytime lightning strikes. All you need to do is filter oxygen from the atmosphere, suck down a bladder full and start flapping.
Not to mention it isn’t exactly the best thing to breathe. While many people use it to make their voices sound deeper, it carries the same side effects of helium, first hit makes your voice sound silly, but breathe in too much and not enough oxygen will reach the lungs resulting in eventual death by asphyxiation.
But I wanted to call upon the wisdom of the crowd to make sure I have it right.
So what atmospheric composition would allow balloon animals to float and be toxic to breathe by human explorers?