It seems to me there are only a few ways to approach this:
* Develop an immunity (pro-active)
* Develop a filter (counter-active)
* Develop an antidote/medicine (re-active)
The problem is that we need more details (from you, the author) about what this chemical is. What it is nearly exclusively determines the "right" approach and determines how their experimentation goes.
For example, if the "chemical" is made up of primarily thick particles that're just omnipresent in the air, then water over a piece of clothing might make a good filter. Then all they need to do is (1) realize this and (2) make a filter for a hut or something and have a fan blowing, creating a positive-pressure safe cabin.
If it's a chemical which slowly enters the bloodstream, slowly causing damage until it just gets too far... then it's probably already too late for them.
But you should:
* Think about what you want the solution's form to be (see above list)
* Find chemicals which might plausibly be in the atmosphere that would cause this
* Perhaps re-ask the question (or edit your current one), including more information about the technology they have on their ship (i.e. 225
do they have a science lab? Or is it JUST a lander, like in "The 100")
Added details after the chemical composition is added:
From what I'm reading about phosgene and LPS is that their symptoms are likely to go unnoticed by the landing party for a while. The CO is more likely to be noticed from the effects more immediately (well, I guess this depends on the density).
As <PERSON> mentioned, it is suspicious that the advance scanner didn't detect it - especially given the CO.
I think your chemical composition (and, more generally, the focus on the chemical pollutant) may be complicating things for you. You may want to find one thing which:
* A "standard" batch of scans might not detect --or-- may be pushed away from the lander causing it to not detect it
* Gives at least some [noticeable] hint as to what it may be
Things to explore:
* Stuff in the air (like chemicals) (which you've already done)
* Stuff that's just present (like radiation)
* Stuff that's on stuff (like a powdered chemical on food sources or the ground)
* Stuff that's in stuff (like a mutation in a plant that used to be safe but is now poisonous)
* Stuff that you haven't developed an immunity to (like a flu which your immune system has never been exposed to before)
* Stuff that lives in people (i.e. parasites)
Parasites could give you an interesting angle: you could simultaneously explain why it wasn't detected by the scanner AND why it's affecting people at different rates by saying that there aren't that many of them per acre. 225