Citation:

Zimmerman, J., A. Williams, B. Schumacher, C. Lutes, R. Warrier, B. Cosky, B. Thompson, C. Holton, AND K. Bronstein. Impact of Multiple HVAC Systems on Indoor Air VOC and Radon Concentrations from Vapor Intrusion During Seasonal Usage. ATMOSPHERE. MDPI, Basel, Switzerland, 16(4):378, (2025). https://doi.org/10.3390/atmos16040378

Impact/Purpose:

This paper examines seasonal and short-term temporal variability of volatile organic compounds (VOCs) measured in indoor air and subslab soil gas (SSSG) samples collected at six commercial buildings in Fairbanks, Alaska. Passive and active VOC measurements were collected weekly for 15 months between December 2020 and April 2022. Various indicators and tracers (I&T), including indoor radon, differential pressure, temperature, and other environmental factors, were also collected. Results from this study help clarify the factors that influence temporal trends in VOC concentrations across multiple sampling zones in commercial buildings in a subarctic climate zone and thus inform vapor intrusion sampling strategies.Indoor air VOC concentrations exhibited less variability at buildings near the VOCs source (near the point of original release). In contrast, the buildings more distant from the source exhibited a more variable seasonal distribution of indoor air VOC concentrations. The dominant factors for the buildings not over the source area are a peak in soil temperature, usually in late August or early September, combined with initiation of the building heating systems in late fall. The combination of these factors creates a stack effect within the buildings that increases the transport of contaminated soil vapor into them. The dominant factor for buildings over the source (nonprofit and tailor and tuxedo shop) is seasonal summer high pattern with little to no effect from HVAC usage. The stack effect mechanisms, not associated with the effect of an increase in building air exchange due to HVAC systems during seasonal usage, had more influence on long-term temporal trends in VOC concentrations. Therefore, the HVAC systems and operations in the buildings across the site have little effect on long-term temporal trends in indoor air VOC concentrations. With the clearer understanding of the relationship of I&T and their resulting effect upon the stack effect and vapor intrusion into buildings near and distant from the VOC source in this geologic and climatic zone, we can improve the timing of our assessment methods for VI in similar zones, resulting in the improvement of the protection of human health in indoor environments. The findings of this paper will be of interest to EPA Program Office, EPA Regional Partners, the general public and local communities.

Description:

Subsurface contamination can migrate upward into overlying buildings, exposing the buildings’ inhabitants to contaminants that can cause detrimental health effects. This phenomenon is known as vapor intrusion (VI). When evaluating a building for VI, one must understand that seasonal and short-term variability are significant factors in determining the reasonable maximum exposure (RME) to the occupants. RME is a semi-quantitative term that refers to the lower portion of the high end of the exposure distribution—conceptually, above the 90th percentile exposure but less than the 98th percentile exposure. Samples were collected between December 2020 and April 2022 at six non-residential commercial buildings in Fairbanks, Alaska. The types of samples collected included indoor air (IA); outdoor air; subslab soil gas; soil gas; indoor radon; differential pressure; indoor and outdoor temperature; heating, ventilation, and air conditioning (HVAC) parameters; and other environmental factors. The buildings in close proximity to the volatile organic compound (VOC) source/release points presented less variability in indoor air concentrations of trichloroethylene (TCE) and tetrachloroethylene (PCE) compared to the buildings farther down gradient in the contaminated groundwater plume. The VOC data pattern for the source area buildings shows an outdoor air temperature-dominated behavior for indoor air concentrations in the summer season. HVAC system operations had less influence on long-term indoor air concentration trends than environmental factors, which is supported by similar indoor air concentration patterns independent of location within the plume. The use of soil temperature and indoor/outdoor temperatures as indicators and tracers (I&Ts) across the plume as predictors of the sampling period could produce a good estimation of the RME for the building occupants. These results, which show the use of soil temperature and indoor/outdoor temperatures as I&Ts, will help advance investigative methods for evaluation of VI in similar settings and thereby improve the protection of human health in indoor environments.

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