After the captured Englishmen had been there a few hours they began to die, and toward the end of the night 123 had died, and the rest came out more dead than alive, and ailing. At first the air had been good in the cave; but when the captives had inhaled all the good air, and no fresh air came in, it became bad, just like what was in the well, and they died.

Why does the good air become bad when many people come together?

Because, when people breathe, they take in good air and breathe out bad air.