IEA Radon_Reporter_April 2025

THE RADON REPORTER | 15 VAPOR INTRUSION chlorinated VOCs is unchanged since 2007, but there has been increasing recognition that petroleum hydrocarbons differ from chlorinated VOCs in their potential for VI. There generally were no separate screening distances for petroleum hydrocarbons in 2007, but the typical screening value for petroleum hydrocarbons has largely decreased from 100 ft. in 2012 to 30 ft. in 2018. Since 2018, more states have adopted these values. For residential, single-family buildings, the states have largely settled upon attenuation factors proposed by the USEPA: i.e., 0.001 for groundwater, 1 for crawl spaces and 0.03 for shallow soil gas. Those attenuation factors can be used to predict indoor air concentrations from a given level of subsurface contamination. The attenuation factors for groundwater and for crawl spaces have largely remained unchanged over the last 15+ years. The default attenuation factors for shallow soil gas were often 0.1 to 0.02 in 2007 and 2012 but largely were changed to 0.03 in the 2018 and current surveys. States continue to vary as to what types of screening values to use, with little consensus on whether to include bulk soil data and whether to have different values for deep versus shallow soil gas. The number of chemicals included in published screening value tables varies from state to state, from fewer than 10 to more than 300. This variability has remained large from year to year. The screening levels generally are not tied to a conceptual site model and this limitation can make it difficult to define what future sampling may be needed. The highest and lowest specific screening values used for groundwater, shallow soil gas, and indoor air and the State where each maximum or minimum is published are shown in Table 1 for three compounds of interest. There are significant differences from state to state as to what level of subsurface impacts will trigger further investigations. Over time, however, there has been reduction in the observed ranges for trichloroethene (TCE) and tetrachloroethene (PCE). As states continue to update their screening levels, the size of the range in values should tend to decrease. A number of states addressed short-term exposures to TCE between 2012 and 2018, but since that time there has been relatively little action on the topic. The underlying science related to developmental health effects continues to be questioned and some organizations have moved away from basing risk management decision making policy on that research. Mitigation is also an evolving topic with states increasing mitigation content year over year. For example, the number of states referencing differential pressure targets increased from 5 in 2018 to at least 16 today. Mitigation is discussed in more detail below. VI continues to be an evolving practice and there continues to be relatively little consistency from state to state with regards to exactly what should be considered a significant VI pathway and how the VI pathway is addressed. For interested parties responsible for addressing sites in multiple jurisdictions, these variations in VI guidance and regulation pose challenges in providing a consistent approach. Mitigation State VI guidance tends to be detailed and at times prescriptive about how to screen a site, but often has little or no information about VI mitigation. Overall, a little more than half of the jurisdictions mention VI mitigation in their guidance document or as part of their overall site remediation program. If details are provided, common mitigation considerations include targeted differential pressure, vapor membrane thickness, design submission requirements and emission controls. A summary of VI mitigation information provided by states is included in Table 2. There are now 16 states that recommend a targeted differential pressure for active sub-slab depressurization systems. The target values range widely, from a recommendation to ‘demonstrate the presence of a negative pressure field’ (Alaska, Indiana, Michigan) to 10 pascals (Pa) (California, DTSC). Varied differential pressure targets may reflect states’ objectives to balance between system energy consumption and system effectiveness. At least nine states use or make accommodations for targeting a lower differential pressure of 1 Pa (0.004 inches of water) which would minimize system energy usage while still maintaining a measurable vacuum under the slab. Details about vapor membranes also vary. Eleven states specify a recommended thickness of a vapor membrane, ranging between 3 and 100 mils, with most states (9 out of 11) having at least part of their accepted range falling between 30 and 60 mils. These recommendations may no longer reflect current vendor recommendations for barriers, as membrane thickness may not equate with effectiveness. Some states, like Georgia, note that membrane thickness may be more important for constructability than for VOC diffusion. At least 15 states record other factors that are important to successful implementation of membranes either instead of or in addition to stating a membrane thickness. These factors include specific VOC resistivity (e.g., documented membrane resistance to TCE), seam and