To limit the probability that, when an exhaust system is not operating and under conditions of positive stack effect pressure, airborne contaminants (e.g. exhaust fumes from a parking garage) will enter the building, which could lead to negative effects on the air quality of indoor spaces, which could lead to harm to persons.
To exempt certain situations from the application of Sentence 6.3.2.10.(5), where there is little risk of airborne contaminants entering the building.