Distinguishing the Contribution of Vapor Intrusion and Indoor Sources of Volatile Organic Chemicals to Indoor Air Using Building Pressure Cycling

Oral Presentation

Prepared by H. Dawson, W. Wertz, T. McAlary, T. Gabris
Geosyntec Consultants, Inc., 1220 19th St. NW, Washington, District of Columbia, 20036, United States

Contact Information: hdawson@geosyntec.com; 202-370-4351


The impact of Vapor intrusion (VI) on indoor air is challenging to assess using conventional monitoring approaches because of temporal variability in volatile organic compound (VOC) concentrations arising from VI and off-gassing of VOCs from background sources, which lead to uncertainty in identifying a Reasonable Maximum Exposure (RME) for the occupants of a building. Regulatory agencies are responding to this uncertainty by asking for at least two rounds of sampling to reduce the possibility of failing to identify unacceptable risks and recommending a building survey to identify and remove indoor sources before sampling. However, intensive sampling of indoor air concentrations at VI sites have demonstrated that limited numbers of samples are inadequate for defining exposure concentrations due to VI in indoor air and experience has demonstrated that surveys often miss background sources and some background sources cannot be removed.

This paper presents a test method, Building Pressure Cycling (BPC), that can quantitatively distinguish the contribution of background sources and VI to indoor air concentrations in a short time frame (e.g., over a weekend). The method measures VOC emissions (mass loading [M/T]) into a building through VI under depressurized building conditions, which promotes VI, and background emissions within a building under pressurized building conditions, which inhibits VI. The concentration measured while the building is positively pressurized represents the contribution of background sources to indoor air. The difference between the concentrations measured when the building is depressurized versus pressurized represents the contribution of VI to indoor air and is a measure of the RME due to VI alone. Test results obtained at two commercial buildings studied as part of a SERDP ESTCP research project ER-201503 over several seasons exhibited considerably less temporal variability (< 60% seasonal variability vs 10X to 100X) than conventional indoor air sampling methods.