[wgehring]

Hello, I have a very specific question on toxic gases (nothing sinister, I assure you). I am writing a science fiction book (trying, anyway), and I want to make it as chemically accurate as I can with my limited knowledge.

My question is this: Of the gases that are toxic to humans, is there one that could exist in an oxygen-based atmosphere in large enough quantities to remain toxic, and in such a way that it could stay prevalent in that atmosphere indefinitely (or at least for a very very long time)? The tricky part I think is that native plants and animals have to be able to exist in that atmosphere. Some fictional license can be taken with that, but I would like to ground it in fact if I can. NOTE: The gas does not have to be naturally occurring. My idea is that the atmosphere was "poisoned," so it can be man-made. Another thing, it needs to be colorless in the concentration needed to be lethal.

I was looking at phosgene as a possibility, but I don't know enough about chemistry to know whether or not this fits the bill.

So, is there an evil gas out there that I can use, or am I going to have to rethink my plot here?

Thanks in advance for the help.

[billnotgatez]

wouldn't any gas toxic to humans be toxic to most mammals and likely toxic to all animals except maybe insects?

[wgehring]

I guess it's also a question of theoretical biology then. Like I said, I can always take a little creative license with that. Maybe the plants and animals on this other planet have a biological makeup different from humans which makes them susceptible to different toxins than humans (the same way weed-killer won't kill a rat).

My real question is, can an artificially produced toxic gas exist in an oxygen-based atmosphere for a prolonged period of time (a few thousand years) without breaking down or being absorbed by plants and things? And if so, what colorless gas could fit these parameters?

My little plot element here is that a certain species on the planet was basically annihilated by chemical warfare, to such an extent that the atmosphere was saturated with this toxin (at least enough to be harmful still). I know that sort of scale is pretty ridiculous, but don't worry about that, it's fiction. I just want to know what gas (if any) I could use that would be chemically accurate.

[orgopete]

I don't know about the success of your plot, but the easiest thing that can be kind of practical is to build up CO2 and deplete oxygen. It would be easy to imagine that plants can begin to produce CO2 and not produce oxygen (they already do, its called night). Because plants can utilize CO2 to produce oxygen, it is feasible to presume that plants could consume lower levels of oxygen than animals. Therefore, low oxygen becomes a toxic atmosphere. As oxygen is depleted into carbon dioxide, water will become more acidic from carbonic acid. This could also be used to argue for it being the toxic gas.

[billnotgatez]

the atmosphere could become high in CO2 for a while.
then the plants could make it back into more oxygen
thus the animals die and the plants stay alive
single cell organisms (animal) might still survive and provide the balance needed between plants and animals.

some theories on the earths creation have the planet with mostly CO2 and as the plants evolve the atomosphere gets more oxygen.