# EPA On-line Tools for Site Assessment Calculation

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## Johnson and Ettinger Attenuation Factor

The attenuation factor, α, is a proportionality constant relating indoor air concentrations to soil or groundwater concentrations:

C_{indoor air} = α_{SG} x C_{soil gas} or C_{indoor air} = α_{GW} x C_{groundwater} x H

This attenuation of vapor phase concentrations from the subsurface to indoor air can be computed by the simplified Johnson and Ettinger (1991) relationship:

A larger α indicates less attenuation and a smaller value indicates more attenuation. For example, at an α = 0.001 a subsurface concentration of 1000 [μg/m^{3}] will attenuate to an indoor air concentration of 1 [μg/m^{3}]. At an α = 0.1, the same subsurface concentration of 1000 [μg/m^{3}] will only attenuate to an indoor air concentration of 100 [μg/m^{3}]. Note that different attenuation factors will be produced for soil gas and groundwater contamination sources (because of different D_{T}
^{eff} values as explained below).

**Building Terms:**

- A
_{B}is the area of the enclosed space that is in contact with soil [m^{2}]. This includes both floor and wall-space that contacts the subsurface. - Q
_{B}is the building ventilation rate [m^{3}hr^{-1}], which is usually estimated to be the product of the enclosed-space volume (V_{B}[m^{3}]) and the indoor air exchange rate with outdoor air (E_{B}[hr^{-1}]). - L
_{T}is the distance (depth) from the bottom of the foundation to the vapor source (for soil gas contamination) or water table (for groundwater contamination) [m]. - η is the fraction of the enclosed space surface area open to vapor intrusion [unitless].

**Subsurface Terms:**

- Q
_{soil}is the pressure-driven soil gas flow rate into the building [L/min]. - D
_{T}^{eff}is the effective diffusion coefficient through the unsaturated zone (for soil gas contamination) or through both the capillary and unsaturated zones (for groundwater contamination). This term incorporates soil and compound-specific parameters and is computed using the Millington approximation. - D
_{crack}^{eff}is the effective diffusion coefficient through the foundation cracks. It is assumed that the foundation cracks are dry, therefore D_{crack}^{eff}is set equal to D_{T}^{eff}.

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