The opening shows men releasing chlorine gas into the sewers and the gas rising up into houses through the drain pipes. Soil and graywater lines in houses all have traps which prevent the sewer gases (methane) from rising into the houses. Those traps would prevent chlorine gas coming into the houses as well.
Without those traps, no modern house would be habitable due to the foul odors from the sewers if there was nothing to prevent gases from the sewers entering buildings connected to the sewers.
Without those traps, no modern house would be habitable due to the foul odors from the sewers if there was nothing to prevent gases from the sewers entering buildings connected to the sewers.