Hermit living down a well. How deep can he safely go?

A holy man lives down a cylindrical well that is 2 metres in diameter. There is no water in the bottom and all the walls are of solid impermeable rock. He is protected from rain by a roof on supports.

He is kept alive by the locals who believe his prayers are necessary for their well-being. They assiduously lower food and water and haul up waste products at regular intervals.

Question

How deep can the well be before his exhalations are sufficient to suffocate him?

Assumptions

There are no toxic gases leaking into the well.

There is no active ventilation to the tunnel. In order to breathe the hermit must rely on diffusion. (or convection - see below)

The well is on the equator so the Sun is directly overhead once per 24 hours. The resulting temperature changes throughout the day and night will presumably cause an exchange of air by convection. (or will they?)

posted over 5 years ago

chasly from UK‭

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