Plot hole? Venusville and no air

Quade, Benny and Molina all run thru Venusville into a secret door to get to Kuato in the interior caverns. With the air cutoff in Venusville, they all fall on the ground gasping for air. Yet Kuatos secret cavern has plenty of air and is defined as a "separate sector". Why wouldn't all the Mutants in Venusville just follow the same path to more air?