What can I add to an oxygen/nitrogen atmosphere to make it unpalatable or poisonous to humans, yet stable and breathable to local creatures?
I have a roughly Earth-like planet in a solar system some distance from ours. The atmosphere of this planet is largely similar to that of Earth, but particularly, has a somewhat higher oxygen content; currently, I'm thinking about 26-28% oxygen (O2(g)) and 65-68% nitrogen (N2(g)), compared to Earth's 21% oxygen and 78% nitrogen. The surface temperature of the planet varies between about 230 K (-43°C) and 320 K (+47°C) depending on location and time of year.
Compared to Earth's atmosphere, I would like to add something to the planet's atmosphere that has the following properties:
- Concentration: a few percent by mass
- At least extremely foul-smelling, ideally poisonous, to humans as we know them; doesn't need to be foul-smelling if poisonous; the higher the toxicity, the better, but beggars can't be choosers so I'll take what I can get
- Does not pose a problem for local creatures that have evolved in such an atmosphere (this is an absolute requirement)
- Reasonably evenly distributed among the other atmospheric gases (I don't want it to form a separate layer at some altitude where, if you can just avoid that, everywhere else is basically fine)
- Can be maintained over time in a high-oxygen environment, possibly with geological replenishment (for example through volcanic eruptions) or through insolation breaking up some other gas that does get replenished on an on-going basis
- Doesn't react with the rest of the atmosphere at the first opportunity
I haven't really decided all that much about the planet's geography or geology, so if some aspect of that makes it easier or for that matter harder to pull this off, I'm open to input.
I was thinking about methane, but that massively fails at least the requirement of non-reactivity in a high-oxygen environment.
Hence two very closely related questions:
- What gases might be good options to satisfy all the criteria above?
- If all of the criteria aren't satisfiable using a single gas added to the atmosphere, then what is the closest I can reasonably get?