A humid indoor environment encourages the growth of molds (fungi) and house dust mites. Certain genetically susceptible people can often develop allergic reactions to these. Bronchial asthma, allergic rhinitis, allergic conjunctivitis, allergic fungal sinusitis, and occasionally hives are usually the consequence of allergic sensitization to molds and/or house dust mite droppings.
Fungal/mold enzymes can induce inflammatory changes in the airway. However, unlike an allergic reaction, the inflammation only lasts during the actual exposure. Molds may irritate the mucous membrane in susceptible individuals.