Actually, it was a CO2 problem
They had balanced the Oxygen demand, but were losing respirable carbon due to the concrete in the structure absorbing CO2 as part of its chemical fixation process. Paradoxically, burning coal indoors would have helped - but then they would have had an Oxygen problem.
Life is NOT easy...
Do some research before opening your mouth. Strangely, this website is full of people who know a little of what they talk. If you also knew a little, you would be a wise man...