When mold tainted air is inhaled, the microscopic spores travel deep into the lungs. The bronchi get progressively smaller as they lead to the alveoli, and the spores get caught. This blockage makes it difficult for air to pass and alveolar gas exchange to occur.

The spores are antigens that invoke an acute immune response. This immune attack results in short term bronchispasms and smooth muscle constriction, followed by long term inflammation and disruption to the flow of air.