I have already taken into concern renewable sources and several other factors.
Oxygen easily gets into this underground system via diffusion and likewise CO2 easily goes out. Plants also draw CO2 from the atmosphere, facilitating diffusion of CO2 from high concentration(underground) to low concentration(atmosphere) Oxygen goes the opposite way again via diffusion. And this keeps happening. The only way these people get hypoxic underground besides lung and heart issues is if tunnels collapse on them. This tunnel collapse leads to injury and in some cases hypoxia.
But is this plausible that plants on the surface draw enough CO2 that diffusion is enough to get CO2 out and O2 in? Will there be some point underground where CO2 just builds and builds(a poison zone if you want to put it that way)?
Basically I am asking if there is some point where machinery(small if needed) or some other process besides photosynthesis is needed to facilitate gas exchange between atmosphere and underground Could making a chamber for plants with lots of light underground get rid of this poison zone(or if not, move it down further)?
Now here is some planet data that I know for sure:
Radius: 2 earth radii
Mass: 4 earth masses
Gravity: 1G(earth gravity)
This may or may not be needed to answer this question. I think it might be needed in order to know the depth at which this poison zone would be assuming a constant atmospheric CO2.