What do you think of a symbol for a corrosive atmosphere?
To keep things simple I think the skull is fine. In the end though if any sort of human survival scale is added what icons are displayed or what info is displayed would be up to SpaceEngineer anyway.
Also it would be interesting to add a flammable icon (for atmospheres with very high concentrations of oxygen), even if the icon would be similar to the high-temperature's one.
This is starting to get into a unique icon for every scenario, the goal should be just a few icons to let someone know whether something is lethal or not and maybe basic info on why.