To limit the probability that the full negative pressure in the return air plenum will not be available to draw in outdoor air, which could lead to the inadequate replacement of indoor air with outdoor air, which could lead to the excessive depressurization of dwelling units by exhaust fans, 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.