As pointed out in Section 5.11 of ASHRAE Standard 62-1989 "Relative humidity in habitable
spaces should be preferably maintained between 30 and 60%, to minimise growth of allergenic or
pathogenic organisms." [1]. Section 5.12 adds that "If the relative humidity in occupied spaces and
low-velocity ducts and plenums exceeds 70%, fungal contamination (for example, mold, mildew,
etc.) can occur." [1] While this language has not been interpreted to indicate that supply duct
humidity must be kept below 70% to meet the requirements of the standard, it is a strong
recommendation [2]. In contrast, normal comfort conditions for humans require temperature of
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approximately 22 to 25°C, with relative humidity between 40 and 60% [1]. However, dew-point
air supply, of which the relative humidity can exceed 95%, is quite extensively used in
air-conditioning systems based on economic considerations. Relative humidity control in the air
supply for air-conditioning is therefore important for public health.