To limit the probability that excessively noisy fans will lead to occupants disabling or not using fans, which could lead to excessive negative pressure (where supply fans alone are disabled), which could lead to the spillage of carbon monoxide gas from fuel-burning appliances that are susceptible to spillage, which could lead to the acute poisoning or asphyxiation of persons.
To exempt fans with less than 1 m of duct between themselves and unfinished basements, furnace rooms, utility rooms or attics from the sound ratings specified in Table 9.32.3.10.B., on the basis that people do not normally occupy such areas for lengthy periods and would therefore be unlikely to be disturbed by noisy fans.