Biological contaminants were described in Chapter 5 in the context of illness syndromes and disease risks associated with exposure to airborne organisms and products of biological origin. Discussion of airborne bacteria and viruses was in the context of their role in causing infectious disease and potentially contributing to other problems; mites, insects and animal allergens, in the context of causing chronic allergic rhinitis and asthma. Because mold is such a significant indoor environment concern, this chapter is devoted to moldrelated health concerns and risk factors for mold infestation.