Meaning
Empirical reduction ratios define the concentration decrease of volatile contaminants as soil gas migrates upward from subsurface sources into indoor air. Calculation of the vapor attenuation factor divides indoor air contaminant concentration by the sub-slab soil gas concentration to quantify site-specific vapor intrusion risks. The scope of this metric applies to human health risk assessments, terminating where building HVAC systems or active sub-slab depressurization alter baseline pressure dynamics.
Migration Ratio
Foundation slab fractures, utility penetrations, and soil permeability variations govern the physical resistance to soil vapor intrusion. Default regulatory attenuation values provide conservative initial screening metrics, but field measurements often reveal lower actual indoor exposures due to natural soil biodegradation. Pilot soil vapor testing established under static soil conditions can understate intrusion rates, whereas full seasonal monitoring reveals expanded risk profiles driven by winter stack effects in heated buildings.
Pathway Attenuation
Soil moisture in unsaturated zones slows vapor diffusion by reducing effective air-filled porosity. Oxygen-rich shallow soils accelerate aerobic degradation of petroleum hydrocarbons before gas reaches floor slabs.
Screening Limit
Measured attenuation values exceeding regulatory threshold ratios trigger immediate indoor air sampling and mitigation planning. Calculated metrics inform environmental engineering decisions regarding soil vapor barrier requirements.
