[Clauses (a) and (b)] To limit the probability of the operation of principal ventilation fans when outdoor air supply and furnace circulation fans are not operating, which could lead to depressurization of the dwelling unit, which could lead to the spillage of combustion products from fuel-burning appliances that are susceptible to spillage.
This is to limit the probability of the entry of carbon monoxide gas into living space, which could lead to the acute poisoning or asphyxiation of persons.