Chemicals causing musty smells are mostly produced by Actinomycetes, Streptomyces, Cyanobacteria and other bacteria, fungi and algae. Examples of compounds responsible for musty odor are Geosmin (muddy, resembling damp soil, produced by Streptomyces), trichloroanisole (TCA, smelling like damp cellar, usually produced from chlorinated phenolic compounds by Aspergillus, Penicillium, Actinomycetes, Botrytis cinerea, Rhizobium, or Streptomyces), tribromoanisole (TBA, produced from brominated phenolic compounds by the same bacteria), dimethylmethoxypyrazine (obnoxious musty odor that might be caused by Pseudomonas and Actinobacteria), 2-methylisobomeol (earthy, peat-like), 3-methylbut-2-ene-1-thiol, cis-3-octenal, nonanal, decanal.
Indoor mildew odor in old houses has been shown to be associated with allergic symptoms, asthma, chronic bronchitis, vision and sleep issues.