Hello I built an underground room measuring 2,5m x 3m x 2,3 just below the ground, the walls are about 20cm thick in solid concrete, the ceiling has iron structure and semi hollow bricks in the concrete, the whole room is revested in ceramic and porcelain tiles inside, the entrance is a 80cm x 80cm opening on the floor and it´s "sealed" with a wooden door. Its a flotation tank (among other names), its basically a concrete with ceramic tiles tub measuring 2,3m x 1,2m x 40cm. It´s used in sessions of 1 or 2 hours (I was thinking about longer sessions) and it should be isolated from light and sound, but Im afraid of the quantity of O2 and CO2. The room is not perfectly sealed, it has 6 "pipes" in the walls that carries the electrical wires (the pipes are long and they make many curves. I read somewhere that CO2 from breathing accumulating in the room is more urgent than the lack of oxygen itself, that CO2 is heavier than the air, so does it accumulate from the floor up? Because if it does how should I remove it if it´s already an underground room? I cant use collers because the sessions have to be made in complete silence; I thought about making 1 or 2 holes on the door with pipes making curves inside de door, will this add enought O2 to offset the breathing (by the way the person using is completely relaxed), and will it remove the CO2 since supposedly it´s on the floor because it´s heavier than the O2? Its in Brazil so the weather is tropical most of the year. The water in the tank is 24 hours heated at about 37 celsius, it has about 700 liters of water with about 400kg of epsom salt (magnesium sulfate). Does this water affect anything in the ventilation matter? Sorry for the long text, there is no easy way to explain without giving details. Any idea is appreciated.