Measuring Indoor Infiltration Flux at -4Pa Under-Pressurization

摘要

The feasibility of designing and operating a volatile emission flux chamber that exerts a negative pressure over the test area was examined and developed. The availability of such an approach would address a current need in health risk assessment practice for situations where potential vapor intrusion into buildings is being evaluated prior to a buildings construction, or an existing building evaluation requires infiltration flux data under negative pressure conditions. Traditional volatile emission flux chambers have been widely used to estimate rates of toxic chemical flux into indoor air for buildings situated on top of contaminated soil and/or groundwater. The traditional USEPA flux chamber design is intended to measure diffusive flux rates under conditions of ambient pressure. Some leading mathematical models used for indoor air inhalation risk assessment include, in addition to the diffusive term, a pressure driven (advective) component to the overall flux rate. This is intended to account for potential increases in flux with building underpressurizations (the so-called "chimney effect"). The discrepancy between the conceptual underpinnings of the traditional flux chamber and the leading mathematical models for vapor intrusion makes direct comparisons of bare soil flux rates (measured outdoors) to the mathematical models results for indoor flux somewhat incongruent. A significant challenge posed by the design objective to exert a negative pressure over the test area was the need to finely modulate the induced vacuum. The very small pressure differential that is representative of typical building underpressurization with respect to ambient conditions is difficult to both measure and control in the field. Air balancing with the aid of a micro-pressure sensor is one approach. Another is to place a containment around the flux chamber and maintain underpressurization conditions that pass-through the chamber to the point of infiltration. The approach provides a means to measure the advective flux associated with the induced indoor pressure while using the recommended USEPA surface emission isolation flux chamber. This approach will provide useful data for evaluating the affect of underpressurization for comprehensive health risk assessment without using overly conservative mathematical models.