It took some trial and error to track down the cause, but a friend of a friend who worked in a lab with the right equipment was kind enough to analyze an air sample and immediately spotted the high percentage. They also offered two remedies:

Nitrogen gas tank and ventilator. Dilutes the air, time-limited to the capacity of the tank.           (new)

Oxygen scrubber face mask. Long-lasting, but makes breathing very difficult.           (new)