Microbiological contaminants include viruses, bacteria, fungal spores, protozoa, algae, animal dander, and animal and insect excreta. They often live and reproduce in microclimates, such as damp basements and bathrooms. Heat exchanger coils and condensate pans, refrigerator condensate and defrost pans and ductwork often provide microclimates if not installed and maintained properly. Humidifiers that don't heat the water can be a source of microbiological contamination.
It is important to keep relative humidity below or at 70% to prevent materials in the building from absorbing water and supporting the growth of fungi. Condensation can cause humidity levels near cool surfaces to rise above 70%. In order for microbiological organisms to have a negative impact on the occupants of a building, there needs to be an environment favourable to the organism, and the occupants must come into contact with the contaminant.