Table 2.

Sewer main to indoor air attenuation factors measured using perfluorocarbon tracer compounds.

Source location	Duplex location	Attenuation factor
Sewer manhole upstream	422 Basement	0.004
	422 Main floor	0.006
	420 Basement	< 0.001
	420 Main floor	< 0.001
Sewer manhole downstream	422 Basement	0.015
	422 Main floor	0.018
	420 Basement	0.010
	420 Main floor	0.021

The tracer compounds released into the sewer manholes were detected at the sewer lateral sample points at concentrations higher than those detected inside the duplex demonstrating migration through the sewer lateral. Tracer concentrations detected in the sewer lateral sample points indicated similar magnitude of transport into the sewer lateral from the upstream manhole and the downstream manhole (see Supplemental Material, Tracer Study Report). It is not clear why higher amounts of tracer from the downstream manhole were detected inside the duplex. In addition, the tracer compound released directly into the sewer lateral inside the basement was not detected within the air of the duplex at any of the four sampling locations. This indicates limited leakage from the sewer lateral lines running through the basement of the duplex. Instead, the results suggest that the sewer lateral leaks below the duplex foundation and that sewer vapors then enter the duplex through the foundation.