Answer:
Carbon dioxide levels in their blood did not reach a high level to be able to process the Oxygen.It takes a high Carbon dioxide level and low blood pH to feel like you are suffocating. The people in the pit didn’t realize they were suffocating due to these factors.
In the real sense, they were suffocating as there was no oxygen in the pit but their carbon dioxide and pH chemoreceptors indicated that everything was alright.
I would expect their pH levels to be normal.