Series Premise Made Short: For reasons that I'm not allowed to describe here (confidentiality), Earth's deciduous and tropical plant life largely dies off. A hardy and oxygen-hungry microbe infests the dying soil and mankind starts to suffer from oxygen deprivation and CO₂ poisoning.

This is projected for year 2062, so tech has improved. What are simple theories for how people can survive on the surface without taking shelter in climate-controlled environments with fancy equipment? Basically, I'm looking for oxygen supplementation that the layman can get a hold of.

And, of course, the level of oxygen deprivation that they can reasonably handle and still survive indefinitely.