How much energy would it take to fractionally distill air? Ie. Liquify all the air in your house, boil off just the gasses you want, and pipe that back?
Then you could choose for example to have 23% oxygen instead of 20% for a little more alertness, 0 ppm CO2 for a fresh feeling, and get rid of all the trace offgasses, even those that wouldn't be absorbed by activated charcoal.