Hi whoever,
Just to connect a few more dots …
This listing (with a couple of more “honest” reviews) shows a PURGreen certification.
If you go to post #12 here you will see this is a somewhat “bogus” certification (bogus in the sense that there are no standards listed for what they test for) and that the only members of the “alliance” is the Woodbridge Group here. Which of their subsidiaries or “partners” actually makes the foam in any particular mattress is anyone’s guess but at least the headquarters of the Woodbridge Group itself is in Canada.
They have a partnership with Foam Partners with headquarters in the US so perhaps this is the source of the foam.
So based on this … there is a reasonable chance at least that the foam isn’t “poisonous gas” even though it may have some offgassing.
I think that the biggest problem could be the durability issues that come from lower quality or sub standard foam. This may show up in months or years depending on how sensitive you are to foam softening.
It would also be interesting for you to calculate the density of the foam layers based on their weight when you receive the mattress for the benefit of others who are thinking of going in this direction.
Even lower quality materials can provide you with the comfort/pressure relief and support/alignment you need (which is why you see so many initial good reviews of low quality mattresses). If the mattress is suitable for your body type, sleeping positions, and preferences and if foam softening doesn’t lead to you crossing the threshold where you no longer have the comfort and support you need in a couple of years or so (or even longer) … then you probably got what you paid for so the news is not all bad 
Phoenix