Document ID: chunk:federal_register_of_legislation:F2013C00288:reg:5:p17
Version: federal_register_of_legislation:F2013C00288
Segment Type: reg
Provision Reference: reg 5 (pt 17/18)
Character Range: 3016210–3019161

by residents (HILs A and B) and workers (HIL D).

It is recognised that adopting 0.1 as the set attenuation factor will be very conservative for a number of sites where additional attenuation may occur, for example, sites with deeper sources of vapours or sites with higher building air exchange rates, or where other attenuation mechanisms (including biodegradation) may occur.

The derived soil vapour interim HILs do not address crawl-space buildings or the presence of preferential vapour pathways.
In relation to preferential pathways, most sites comprise subsurface materials with highly variable permeability as well as a number of subsurface utility penetrations. Of particular concern in relation to vapour intrusion are utilities (as illustrated in Figure 6) or geologic features (such as fractures or highly permeable materials) that intersect a vapour source and connect to a building (ITRC 2007: API 2005). In these cases the soil vapour interim HILs may not be adequately protective and a site-specific assessment should be undertaken.
Figure 6. Preferential vapour transport through utility trench (ref: US EPA 2012b, Figure 5)

    5.5.3         Outdoor exposures
The assessment of inhalation exposures associated with outdoor air is of most significance to the derivation of HILs for open space/recreational areas (HIL C). Limited information is available on attenuation factors relevant for outdoor air. The concentrations in outdoor air, which are derived from the migration of vapours from a subsurface source, are expected to be lower than those indoors due to increased dilution, particularly during the daytime.

Review of average radon data suggests that outdoor air concentrations are in the order of 2 to 10 times lower than indoor air concentrations (ECA 1995). Based on this information, the more conservative ratio of indoor air to outdoor air concentrations above a subsurface source of 2 has been considered. This allows for the consideration of outdoor exposures that may occur in areas where less dilution occurs, including sunken sandpits/play areas (and includes cubby houses). It is considered likely that dilution will be higher, however without more sound data that addresses a wide range of outdoor exposures it is not considered appropriate for screening level guidance to be less conservative. Hence, the attenuation factor adopted for the estimation of outdoor air concentrations is 0.05 (half that of the indoor air attenuation factor).

The soil vapour interim HIL C has been calculated on the basis of the equations presented in Appendix B.

As with the estimation of indoor exposures, it is recognised that adopting 0.05 as the soil vapour to outdoor air attenuation factor does not take into account additional attenuation that may occur for sites with deeper sources of vapours, windier environments, or where other attenuation (including biodegradation) may occur. Consideration of these issues should