Overview of TACO Indoor Inhalation Amendments

On May 16, 2013, the Illinois Pollution Control Board added the indoor inhalation exposure route to Illinois EPA’s risk-based cleanup methodology called the Tiered Approach to Corrective Action Objectives, 35 Ill. Adm. Code 742 (TACO). These amendments are effective on July 15, 2013.

The purpose of the amendments is to minimize the exposure of building occupants from volatile chemicals that have the potential to migrate from the soil and groundwater to indoor air. This migration process is called vapor intrusion.

The indoor inhalation pathway will be managed similarly to the existing exposure routes under TACO. It follows the basic framework of TACO’s three tiers, includes calculations for both residential and industrial/commercial remediation objectives, and allows for pathway exclusion, including the use of building control technologies to prevent or minimize human exposures to contamination.

Inserting a new exposure route into TACO required comprehensive changes to the existing regulations. As would be expected, these amendments contain new definitions, equations, parameters, default remediation objectives, and mechanisms for managing the indoor inhalation exposure route.

New Concepts with TACO Indoor Inhalation Amendments

Soil Gas means the air existing in void spaces in the soil between the groundwater table and the ground surface. Contamination existing in soil and groundwater may contribute to soil gas concentrations that can migrate upward through the subsurface and cause an inhalation risk due to contaminant vapors. Soil gas measurements can be taken to determine compliance with the outdoor inhalation exposure route remediation objectives as well as the indoor inhalation exposure route remediation objectives.

Diffusion is the process whereby molecules move from an area of higher concentration to an area of lower concentration. Chemicals migrate via diffusion into buildings through cracks, pores and other openings in the building slab and foundation such as passages for utility lines and sumps. Diffusion is always present at a steady rate of entry.

Advection refers to the movement of soil gas in response to pressure gradients. Advection can be an important mechanism for drawing soil gas and contaminant vapors into or out of (beneath) a building. Heating and cooling systems can create differential pressures inside the building resulting in the movement of soil gas and vapors via advection. Barometric pressure changes or wind may also drive advection. Advection effects are considered within the building zone of influence, defined as the area 5 feet or less, vertically or horizontally, from a building or man-made pathway (existing or potential). Advection is measured by Qsoil, the volumetric flow rate of soil gas into the enclosed space, and equals 83.33 cm3/sec. If Qsoil equals zero, then advection is not being considered in the calculations and all of the requirements for a “diffusion only” investigation apply.


Conceptual diagram of vapor intrusion
into overlying buildings (from U.S. EPA)

Attenuation is the reduction in soil gas concentration as it travels from the subsurface into the indoor air. The Attenuation Factor (α) is the ratio of the concentration in the indoor air to the soil gas concentration and accounts for several processes as contaminants migrate upwards and mix with air inside of the building. As used in the TACO J&E Equations, the Attenuation Factor is chemical, soil and building specific.

Biodegradation refers to the natural degradation of hydrocarbons in the subsurface due to the presence of oxygen (aerobic) and natural microorganisms. Active biodegradation occurs within an unsaturated soil zone that has sufficient oxygen concentration and is clean enough to satisfy the biodegradation model assumption. As contaminants migrate upward through the soil, biodegradation contributes to a decrease in soil vapor concentrations. The J&E model used in the Tier 1 and Tier 2 calculations assumes that biodegradation of soil gas does not occur.

The definition of Man-made Pathways, though not a new concept, has been expanded to include elevator vaults and sumps in regards to the indoor inhalation exposure route. For indoor inhalation, this means utilities or other man-made pathways connected to a building at a point below grade, entering through the foundation or floor of the building. For concrete floors with sumps, the site evaluator may not use Tier 1 or Tier 2 and must either exclude the pathway, meet building control technology requirements, or proceed to Tier 3.

Synopsis of Changes

(Please note: only the Subparts and Tables affected are referenced below.)

Subpart A
Section 742.105 clarifies that an evaluation of the indoor inhalation exposure route addresses the potential of volatile chemicals in soil gas and groundwater to reach human receptors and that this evaluation makes no assurances about the safety or protectiveness of the buildings on or off-site.

Sections 742.110 and 742.115 account for the inclusion of the modified J&E model and the indoor inhalation exposure route, respectively. Section 742.115 has also been amended to establish soil gas as a medium by which both the indoor and outdoor inhalation exposure routes may be evaluated.

Subpart B
Definitions for “Building,” “Building Control Technology,” “Capillary Fringe,” “Qsoil,” “Saturated Zone,” “Soil Gas,” “Soil Vapor Saturation Limit,” “Unconfined Aquifer,” “Volatile Chemicals,” and “Water Table” were added; new incorporations by reference were added; and new Sections 742.222 and 742.227 were added to allow for the use of soil gas data when determining remediation objectives for the indoor and outdoor inhalation exposure routes.

Subpart C
“Outdoor” was inserted before “Inhalation Exposure Route” in Section 742.310; Section 742.312 was added that states when the indoor inhalation exposure route may be excluded from consideration. As part of Section 742.312, the exposure route may be excluded by use of a building control technology that meets the requirements of Subpart L.

In both Sections 742.310 and 742.312, a new option was added for exposure route exclusion called demonstration of active biodegradation. This exclusion option is available to sites contaminated by benzene, toluene, ethylbenzene and total xylenes.

Subpart E
Sections 742.505 and 742.510 were modified to explain how to use the Tier 1 outdoor inhalation remediation objectives for soil gas in Appendix B, Table G. Section 742.505 was also expanded to explain how to use the Tier 1 indoor inhalation remediation objectives for soil gas and groundwater in Appendix B, Tables H and I. Section 742.515 was created to define how compliance shall be determined when using Appendix B, Tables H and I.

Subpart F
A condition was added that the calculated Tier 2 indoor or outdoor inhalation remediation objective for soil gas cannot exceed the soil vapor saturation limit; it was stipulated that individuals who opt for diffusion only mode of contaminant transport must calculate remediation objectives for both soil gas and groundwater; and it was specified that, if a contaminant has both carcinogenic and noncarcinogenic effects for any applicable exposure route or receptor, the remediation objectives shall be calculated for each effect and the lower remediation objective shall apply.

Subpart G
IRIS was replaced with OSWER Directive 9285.7-53 as the alternate source for toxicological-specific information; Section 742.700(g) was changed to exclude the construction worker population from the indoor inhalation exposure route; Section 742.712 was added to provide the SSL soil gas equation for the outdoor inhalation exposure route; Section 742.717 was added to require use of the modified J&E model and to explain how the J&E equations for soil gas data are to be applied for the indoor inhalation exposure route; and under Sections 742.717(i), it is described when the soil vapor saturation limit is to be used as the soil gas remediation objective for the indoor inhalation exposure route.

Subpart H
Section 742.805(e) was added to require individuals to use Section 742.812 when developing Tier 2 groundwater remediation objectives for the indoor inhalation exposure route, and Section 742.812 was added to explain how the J&E equations are to be used for developing groundwater remediation objectives. Section 742.812 sets the default thickness of the capillary fringe at 37.5 cm and describes when the solubility limit is to be used as the groundwater remediation objective.

Subpart I
Section 742.935(a) was added to allow exposure route exclusion (as an alternative to Section 742.312) for the indoor inhalation exposure route; Section 742.935(b) was added to allow the use of building control technologies (as an alternative to those described in Subpart L) as a means to prevent or mitigate human exposures under the indoor inhalation exposure route; Section 742.935(c) was added to allow the use of soil gas data (as an alternative to Section 742.227) to establish remediation objectives for the indoor inhalation exposure route; Section 742.935(d) was added to allow the use of soil data to establish remediation objectives for the indoor inhalation exposure route; Section 742.935(e) was added to allow the use of groundwater data (as an alternative to Sections 742.805 and 742.812); and language was adjusted elsewhere in this Subpart to account for this new Section.

Subpart J
New Section 742.1000(a)(7) requires the use of institutional controls whenever indoor inhalation remediation objectives are based on a diffusion only mode of contaminant transport. Section 742.1000(a)(8) requires the use of institutional controls whenever indoor inhalation remediation objectives are based on a building control technology. Section 742.1000(a)(9) requires the use of institutional controls whenever remediation objectives are developed using the J&E model for any Tier (to assure validity of the full concrete floor assumption). New Section 742.1015(j) states that a groundwater ordinance may not be used to exclude the indoor inhalation exposure route. This is because an ordinance restricting the source of drinking water would be incapable of protecting the enclosed air space of a building from the migration of contaminants in the groundwater.

Subpart L
This Subpart contains the requirements for four types of building control technologies: sub-slab depressurization systems, sub-membrane depressurization systems, membrane barrier systems, and vented raised floors.

Appendix A, Table A
A column was added to distinguish between the outdoor inhalation exposure route and the soil component of the groundwater ingestion exposure route. The difference in values is based on the fraction of organic carbon content (foc).

Appendix A, Tables E and F
New chemicals were added based on the recent amendments to the Groundwater Quality Standards (35 Ill. Adm. Code 620). The entire table has been alphabetized by target organ. For these tables, all similar-acting chemicals were updated to avoid any confusion that may have been caused by updating parameter values for volatile chemicals only.

Appendix A, Table J
This is a new table identifying the 59 TACO volatile chemicals that are considered contaminants of concern for the indoor inhalation exposure route.

Appendix A, Table K
This is a new table identifying the soil vapor saturation limits for volatile chemicals.

Appendix B, Table G
This is a new table containing the Tier 1 soil gas remediation objectives for volatile chemicals under the outdoor inhalation exposure route. Residential, industrial/commercial, and construction worker values are given.

Appendix B, Table H
This is a new table containing the Tier 1 soil gas and groundwater remediation objectives for volatile chemicals under the indoor inhalation exposure route for the diffusion and advection mode of contaminant transport. In this scenario, the remediation objectives are calculated using a Qsoil parameter value of 83.33 cm3/sec. Residential and industrial/commercial values are given. Remediation objectives are not provided for the construction worker population since this receptor group is not at risk from indoor inhalation. The exposure duration for indoor construction in almost all cases is less than the exposure duration for the residents or commercial workers, so protection of these two receptors will ensure protection of the construction worker during the period of indoor construction. Individuals using remediation objectives from Appendix B, Table H must use institutional controls in accordance with Subpart J.

Appendix B, Table I
This is a new table containing the Tier 1 soil gas and groundwater remediation objectives for volatile chemicals under the indoor inhalation exposure route for the diffusion only mode of contaminant transport. In this scenario, the remediation objectives are calculated using a Qsoil parameter value of 0.0 cm3/sec. Residential and industrial/commercial values are given. Remediation objectives are not provided for the construction worker population since this receptor group is not at risk from indoor inhalation (see the preceding paragraph for further explanation). Individuals using remediation objectives from Appendix B, Table I must use institutional controls in accordance with Subpart J.

Appendix C, Table A
Equation S30 was added to calculate soil gas remediation objectives for the outdoor inhalation exposure route.

Appendix C, Table B
SSL parameters were added to correspond with Equation S30.

Appendix C, Table E
The default values were updated to be consistent with current scientific literature. These changes have been made for all chemicals (not just volatile chemicals) so as to minimize confusion and avoid complicating the footnotes. A new column of default values was inserted for Dimensionless Henry’s Law Constant at 13˚ C that are used in the J&E equations, and three new footnotes were added. All values are now expressed in scientific notation.

Appendix C, Table F
New methods were added for determining the following physical soil parameters: total soil porosity, air-filled soil porosity, and water-filled soil porosity.

Appendix C, Table L
This is a new table containing the J&E equations used in calculating remediation objectives for the indoor inhalation exposure route.

Appendix C, Table M
This is a new table containing the J&E parameters used in calculating remediation objectives for the indoor inhalation exposure route.

How the Illinois indoor inhalation regulations differ from vapor intrusion requirements in other states

  1. No default attenuation factors

    TACO does not contain a table of default attenuation factors. To develop Tier 1 and Tier 2 remediation objectives, Illinois EPA uses a modified Johnson and Ettinger model with chemical and soil-specific inputs to calculate unique attenuation factors. See Section 742.717.

  2. No exclusion distances

    Illinois EPA’s approach is site-wide, not building-specific. TACO requires evaluation of the entire site (as defined by each regulatory program) to make sure any potential future buildings are safe from vapor intrusion. This means that site evaluations are not defined by existing buildings and a specified perimeter distance around them (25 or 100 feet, for example).

  3. No indoor air measurements in Tiers 1 and 2

    Indoor air samples are highly susceptible to bias from occupant sources (smoking, dry cleaning, household chemical use and storage, etc.). Sample collection is also invasive, requiring site evaluators to obtain access to indoor space. For these reasons, TACO does not contain a table of indoor air remediation objectives and the use of indoor air data to demonstrate compliance is limited to a Tier 3 evaluation. See Section 742.935(a).

  4. Allows pathway exclusion for benzene, toluene, ethylbenzene and total xylenes (BTEX) based on a demonstration of active biodegradation

    Illinois EPA added this specific option under pathway exclusion because recent research (validated by environmental regulators nationally) suggests that active biodegradation of petroleum constituents is an effective way to address the indoor inhalation pathway.

How sampling data apply to TACO’s tiered approach 

Media sampledTACO framework
Soil gasPathway exclusion, Tiers 1 and 2
GroundwaterPathway exclusion, Tiers 1 and 2
SoilTier 3
Subslab soil gasTier 3
Indoor airTier 3

This fact sheet is for general information only and is not intended to replace, interpret, or modify laws, rules, or regulations.

June 2013