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.