No confined space should ever be assumed safe without instruments to check air quality. Many a people have died from entering vaults, tunnels, caves etc that had unforseen reasons for the oxygen to be displaced/consumed. And usually you won't even realize your dying.
oOFrostByteOo t1_j87mkwt wrote
Reply to Will a 400m long tube have breathable oxygen inside? by SnekkyGlekky
No confined space should ever be assumed safe without instruments to check air quality. Many a people have died from entering vaults, tunnels, caves etc that had unforseen reasons for the oxygen to be displaced/consumed. And usually you won't even realize your dying.