Draft 2 (March 2009) – Regulatory Framework Workgroup for MassDEP Indoor
Air Guidance
2.1 When Should the Indoor Air Pathway Be Investigated?
2.1.1 Introduction
Indoor air quality may be affected by numerous sources and activities. Investigation of
the indoor air pathway under the MCP is limited to situations in which the contamination
has entered the indoor air via vapor intrusion from a reportable release of oil and/or
hazardous material to the soil and/or groundwater. Indoor air contamination from
commercial products (cleaners, paints, cosmetics, etc.), occupational chemical use,
tobacco smoke, radon, intrusion of contaminated outdoor air, building materials (such as
asbestos), and mold may pose significant health threats, but these sources are not
regulated under the MCP. While some of the investigation and mitigation guidance in
future sections may be useful in evaluating indoor air quality in these cases, only sources
regulated under the MCP will be addressed in Section 2 of this document.
There has been an increased interest in and concern about vapor intrusion as a pathway
for human exposure to contaminants from groundwater and soil originating from sites of
oil and hazardous material releases. MassDEP developed an Indoor Air SOP (August
2007) for use by its staff, which offers valuable, detailed information regarding this
pathway and has been released for use outside the agency. Most actions described in the
MassDEP Indoor Air SOP start with evidence of a potential indoor air exposure: indoor
air data, elevated contaminant levels in soil, and/or contaminant levels in groundwater
above the GW-2 Standards. Other state and federal guidance listed in the appendix of the
SOP offers “how to” approaches to vapor intrusion, once the decision has been made to
pursue the possibility of human exposure through this pathway. This section addresses
the question of “when” it is necessary to investigate vapor intrusion to indoor air as an
exposure pathway under the Massachusetts Contingency Plan (MCP).
2.1. Relevant MCP Requirements
The MCP includes some specific requirements regarding notification that relate to the
indoor air pathway. 310 CMR 40.0321(1)(d) requires 2-hour notification to MassDEP for
releases that pose an Imminent Hazard. Some sites with indoor air contamination meet
this notification requirement. 310 CMR 40.0313(4) requires 72-hour notification for “a
release to the environment indicated by measurement within the groundwater of equal to
or greater than five milligrams per liter of total volatile organic compounds at any point
located within 30 feet of a school or occupied residential structure, where the
groundwater table is less than 15 feet below the surface of the ground.” 310 CMR
40.0006 defines another 72-hour notification requirement for a Condition of Substantial
Release Migration (SRM), including “releases to the groundwater or to the vadose zone
that have resulted or are within one year likely to result in the discharge of vapors into
school buildings or occupied residential dwellings.” 310 CMR 40.0006 defines Critical
Exposure Pathways (CEP) related to the indoor air pathway as “those routes by which oil
and/or hazardous material(s) released at a disposal site are transported, or are likely to be
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transported, to human receptors via: (a) vapor-phase emissions of measureable
concentrations of oil and/or hazardous materials into the living or working space of a preschool, daycare, school or occupied residential dwelling...” The presence of a CEP
triggers specific mitigation requirements. SRM and CEPs will be discussed in more
detail at 2.2 below.
The reporting and investigation requirements in the MCP triggered by these specific
situations are relatively straightforward. More challenging are the decisions about the
direction and extent of investigation at a site which may lead to obtaining the data for
evaluating the indoor air pathway. Evaluating the indoor air pathway should be
approached in the context of characterizing the nature and extent of contamination at a
site. It would be impossible to list all situations in which the indoor air pathway needs to
be addressed under the MCP. The role of good professional judgment by LSPs and
others cannot be overstated. The evaluation of potential indoor air pathways should be
approached in the same way as other pathways; that is, new site information and data
could indicate the need for further investigation, even if the pathway had previously been
removed from consideration.
Consideration of the indoor air pathway at sites with volatile organic sources may start
from one of many points, such as:

A specific MCP reporting condition

Site use and history

Groundwater data

Soil data

Soil gas data

Indoor air data

Odors or screening data within a building

Presence of Non-Aqueous Phase Liquid
Other relevant factors which may reinforce consideration of an indoor air exposure
pathway based on an initial concern from the elements listed above include:
 Site-specific concerns related to contaminant transport, such as utility corridors or
high hydraulic conductivity
 Building-specific concerns, such as an earthen floor, fieldstone or block wall
foundation, significant cracks, and/or a groundwater sump.
 Sensitivity of receptors
 Volatility, toxicity, persistence and/or concentration of the contaminants
Relevant information may be obtained as part of Preliminary or Comprehensive
Response Actions. The requirements for a Phase I Report relevant to this pathway
include: general disposal site information including description of land uses surrounding
the disposal site, on-site buildings, floor and storm drains, subsurface utilities, OHM
storage and disposal structures and areas, disposal site history, nature and extent of
contamination, migration pathways and exposure potential (see 310 CMR 40.0483).
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The Phase II requirements at 310 CMR 40.0835 include Environmental Fate and
Transport of Oil and/or Hazardous Material including “an evaluation of the potential for
groundwater to be a source of vapors of oil and/or hazardous material to indoor air of
occupied structures as described in 310 CMR 40.0900.”
A well-designed, iterative approach to site characterization would indicate consideration
of the indoor air pathway at several points during an investigation. For example, an
initial round of monitoring well installation at a site may not indicate groundwater
contamination near any occupied structures. As the extent of a groundwater contaminant
plume is characterized by further well installation, the location of the contaminated
groundwater may be determined to be beneath an occupied structure. Another example
would be contaminated soil; it may not be considered as a potential source of indoor air
contamination until a review of site history indicates that a location adjacent to or within
a building at the property was used as a hazardous waste storage area.
A common sequence of investigative actions related to the indoor air pathway is to
collect data from groundwater first, followed by soil gas, and finally indoor air sampling.
However, these actions may be taken out of this order; for example, a building tenant
may perform indoor air sampling independent of the MCP process and discover air
quality issues. Soil excavation beneath or near a building (for example, to facilitate
removal of floor drains or a dry well) may trigger concerns about the indoor air pathway,
even before any groundwater monitoring wells are installed.
The decision to evaluate a potential indoor air pathway should be revisited continually
during the characterization of a release of VOCs at a site. This is an iterative process, in
the same way that new data obtained during site characterization is continually evaluated
for new reporting requirements and for determining whether the current Conceptual Site
Model needs revision.
2.1.3
Using the Method 1, GW-2 Standard to Rule out the Vapor Intrusion Pathway
The use of a Method 1 Risk Characterization under the MCP is restricted to sites at which
the contamination is limited to soil and groundwater. The GW-2 Standards were
developed to address the potential for exposure through the indoor air pathway when
contaminants volatilize from groundwater beneath or near buildings. Groundwater is
classified as GW-2 when it is located within 30 feet of an existing or planned building or
structure that is or will be occupied, and the average annual depth to groundwater in that
area is 15 feet or less. The GW-2 Standards were developed based on back-calculations
from the lowest of: (1) the one in one million Excess Lifetime Cancer Risk
concentration, (2) 20% of the Reference Concentration, or (3) the 50% odor recognition
threshold, unless those values were less than the expected indoor air “background”
concentrations. In those cases, the acceptable indoor air concentration defaulted to the
background conditions.
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For some well-characterized sites, comparison of groundwater concentrations to the GW2 Standards may be appropriate as a screening tool and may be sufficient to eliminate
further consideration of the vapor intrusion pathway. Those planning to rely on the GW2 Standards for screening need to evaluate site-specific conditions to support their
decision. Site use is an important consideration when evaluating the potential for an
indoor air pathway. The Department recommends a more comprehensive approach when
considering the potential for indoor air exposures at schools or residences. Regardless of
site use, the presence of the factors listed below would indicate that the indoor air
pathway should be evaluated, even when groundwater classified as GW-2 at the site
meets Method 1 GW-2 Standards.
1) Indoor air contamination identified from indoor air testing and/or odors
potentially attributable to groundwater and/or soil contaminants have been
reported within the structure of concern.
In this situation, the indoor air pathway would need to be evaluated to determine
if the indoor air contamination and/or odors are related to a disposal site. When
indoor air contamination could possibly be attributed to contaminated building
materials or facility operations at or near the structure of concern, the collection of
sub-slab soil gas data would be valuable in determining whether soil or
groundwater contamination near or beneath the building could be contributing to
contaminant concentrations in indoor air. It would be premature to rule out the
vapor intrusion pathway in this situation based solely on the GW-2 standards
without additional lines of evidence.
2) The structure of concern has an earthen floor, fieldstone or block wall foundation,
significant cracks, and/or a groundwater sump.
These conditions could allow an unusually direct connection between the interior
of the structure and the soil gas and/or groundwater contamination beneath the
structure. They are not consistent with the assumptions used in the Johnson and
Ettinger model used to calculate GW-2 Standards, and so would indicate the need
for evaluating the indoor air pathway at levels in groundwater below the GW-2
standards.
3) Non-Aqueous Phase Liquid (LNAPL or DNAPL) is present or is likely to be
present within 30 feet (horizontally) of the potentially impacted structure.
Again, these conditions are not consistent with the assumptions used in the
Johnson and Ettinger model used to calculate some GW-2 Standards, and so
would indicate the need for evaluating the indoor air pathway even if
concentrations in groundwater are less than the GW-2 standards.
4) VOC contamination is present in soil or soil gas beneath or near a structure of
concern.
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A contaminant source such as a dry well, leaking floor drain or spill location,
could allow contaminant volatilization beneath or near a structure of concern
without contamination of the underlying aquifer. Evidence of contamination
could come from screening results or analytical data from soil or soil gas
samples. The MassDEP Policy #WSC-02-411 Implementation of the MADEP
VPH/EPH Approach lists soil gas screening levels for evaluating indoor air
impacts from petroleum contamination (see Tables 4-9 and 4-10). Pursuant to
310 CMR 40.0942(1)(d), "If one or more Volatile Organic Compounds is present
in vadose zone soil adjacent to an occupied structure (e.g., within six feet,
measured horizontally from the wall of the structure, and within ten feet,
measured vertically from the basement floor or foundation slab) then the soil has
the potential to result in significant indoor air concentrations of OHM and Method
1 alone cannot be used to characterize the risk at the disposal site." However, the
presence of contaminated soil or soil gas at greater distances from an occupied
structure does not necessarily indicate the lack of an indoor air pathway. In this
situation, additional characterization would be advised, in order to determine the
nature and extent of the contamination and determine whether there is a
substantial likelihood that an indoor air exposure pathway is complete.
5) Contaminant concentrations in groundwater not classified as GW-2 are present at
levels high enough to allow vapor intrusion, even though the location of the
contamination is beyond the regulatory criteria defining the GW-2 groundwater
category, such as just beyond 30 feet from an occupied building or just slightly
deeper than an average annual depth to groundwater of 15 feet.
Data from existing sites has shown that indoor air may be affected at sites with
depths to groundwater greater than the 15-foot GW-2 limit, when contaminant
concentrations in groundwater are high. It would be prudent to obtain additional
lines of evidence in these cases before ruling out the potential for the vapor
intrusion pathway.
New Section - March 2009
2.2 MCP Notification Requirements Related to Indoor Air
In certain situations, the presence of indoor air contamination may trigger a separate
reportable condition under the MCP. These conditions may include both two-hour and
72-hour notification requirements, as outlined below.
Two Hour Notification – Imminent Hazards
“Any release of any oil and/or hazardous material, in any quantity or concentration, that
poses or could pose an Imminent Hazard, as described in 310 CMR 40.0321 and
40.0950” is listed at 310 CMR 40.0311(7) as a criterion for a release which requires
notification within two hours. The MCP at 40.0006 defines an “Imminent Hazard” as a
“hazard which would pose a significant risk of harm to health, safety, public welfare or
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the environment if it were present for even a short period of time, as further described in
310 CMR.40.0950.” 310 CMR 40.0321 lists specific releases deemed to pose an
Imminent Hazard to health, safety, public welfare and/or the environment. Releases
related to indoor air conditions include: 40.0321(1)(a) - a release to the environment
which results in the presence of oil and/or hazardous material vapors within buildings,
structures, or underground utility conduits at a concentration equal to or greater than 10%
of the Lower Explosive Limit and 40.0321(1)(d) - a release to the environmental of oil
and/or hazardous material which poses a significant risk to human health when present
for even a short period of time, as specified in 310 CMR 40.0950.
Releases which could pose an Imminent Hazard to human health are listed at 310 CMR
40.0321(2). The release listed at 40.0321(2)(c) is “a release to the environment for which
estimated long-term risk levels associated with current exposures are greater than ten
times the Cumulative Receptor Risk Limits in 310 CMR 40.0993(6). Past exposures may
be included in such evaluations to the extent that it is reasonable to quantify those
exposures.”
A Human Health Risk Characterization for Imminent Hazard Evaluations related to vapor
intrusion would be conducted using Method 3, as described in 310 CMR 40.0993. It
would focus on actual or likely exposures to human under current site conditions, for a
five year period of time, unless circumstances indicate that a shorter time period is
appropriate. As described in 310 CMR 40.955(2), the calculated Excess Lifetime Cancer
Risk is compared to the one-in-100,000. For non-cancer risks, the Hazard Index is
compared to (a) one for oil or hazardous materials that have the potential to cause serious
effects (including but not limited to lethal, developmental, or neurological effects)
following short-term exposures, for example lead or cyanide and (b) ten for all other oil
or hazardous materials.
72 Hour Notification
310 CMR 40.0313(4) requires 72-hour notification for “a release to the environment
indicated by measurement within the groundwater of equal to or greater than five
milligrams per liter of total volatile organic compounds at any point located within 30
feet of a school or occupied residential structure, where the groundwater table is less than
15 feet below the surface of the ground.”
310 CMR 40.0313(5) requires 72-hour notification for a Condition of SRM, where such
condition is associated with a release for which notification otherwise is or has at any
time in the past been required in accordance with 310 CMR 40.0300. 310 CMR 40.0006
defines a Condition of Substantial Release Migration (SRM), including “releases to the
groundwater or to the vadose zone that have resulted or are within one year likely to
result in the discharge of vapors into school buildings or occupied residential dwellings.”
There is a requirement to report a Condition of SRM only when there is evidence
associating it with a release that is otherwise reportable. This is especially important
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considering the number of other potential sources of indoor air contamination, such as
consumer and personal care products.
Notification Requirements Related to Indoor Air
Two-Hour Notification for Imminent
72-Hour Notification
Hazards
Release to the environment resulting in
Release to the environment indicated by
OHM in structures at a concentration equal measurement with the groundwater of > 5
to or greater than 10% of the LEL
mg/L of total VOCs at any point located
within 30 feet of a school or occupied
residential structure where the groundwater
table is less than 15 feet below the surface
of the ground.
Release to the environment of OHM which Condition of SRM – releases to the
poses a significant risk to human health
groundwater or to the vadose zone that
when present for even a short period of
have resulted or are within one year likely
time
to result in the discharge of vapors into
school building or occupied residential
Release to the environment for which
dwellings.
estimated long-term risk levels associated
with current exposures are greater than ten
times the Cumulative Receptor Risk Limits
in 310 CMR 40.0993(6)
2.3 Critical Exposure Pathways
2.3.1 Identification of a Critical Exposure Pathway (CEP)
2.3.1.1. The CEP Concept
The CEP concept in the MCP identifies sensitive school and residential receptors and
requires that actions be taken to address their exposures to contamination from vapor
intrusion and drinking water. This is a conservative approach aimed at protecting
sensitive populations in homes, schools and daycare facilities where the frequency of
exposure is likely high. The CEP approach expedites consideration of cost-effective
measures to eliminate, reduce or prevent exposures. The requirement for action in the
case of a CEP is not based on risk, but rather the presence of measureable OHM,
regardless of the results of the risk characterization. This approach compensates for
uncertainties in chemical toxicity information and risk characterization. It also provides
an extra level of protection for sensitive individuals, such as infants, children, pregnant
women, and those who are ill or have compromised immune systems. The goal of the
CEP requirements in the MCP is to take actions, where feasible, to break the indoor air
pathway as soon as it is identified.
2.3.1.2 Defining CEP
As defined at 310 CMR 40.0006, Critical Exposure Pathways mean those routes by
which oil and hazardous material(s) released at a disposal site are transported, or are
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likely to be transported, to human receptors via (a) vapor-phase emissions of measurable
concentrations of OHMs into the living or working space of a pre-school, daycare, school
or occupied residential dwelling; or (b) ingestion, dermal absorption or inhalation of
measurable concentrations of OHMs from drinking water supply wells located at and
servicing a pre-school, daycare school or occupied residential dwelling. A CEP requires
notification only when it is associated with a release that is otherwise reportable. For
example, measurable concentrations of OHMs in the air of an occupied residential
dwelling do not constitute a CEP if they are related solely to chemicals used in pursuing a
hobby at the residence.
The Department’s Bureau of Waste Site Cleanup uses a broad definition of "living or
working space" for the purpose of notifying MassDEP of a Condition of Substantial
Migration via the vapor intrusion pathway. For the purpose of this notification
requirement, BWSC considers any basement with head room to conduct any living or
working activities to apply, regardless of whether the basement is currently being used as
living or working space. Crawlspaces would not apply to this definition of living or
working space.
The intent of a broad definition for notification purposes is to have clear notification
criteria to report a CEP (i.e., if it is detected in the basement, report to DEP within 72
hrs), and to not have the subjective decision of whether the space is livable be associated
with notification. Those situations where a basement is rarely used or visited should be
dealt with in the feasibility component of the CEP.
2.3.2 Feasibility Evaluation
Pursuant to 310 CMR 40.0414, IRAs are presumed to require the elimination, mitigation
or prevention of the indoor air pathway. This is a rebuttable presumption, based on a
feasibility evaluation. The statutory feasibility criteria include whether a technology
exists, whether costs outweigh the benefits, whether expertise is available and whether a
disposal location is available. There are a range of effective remedial actions available to
address the vapor intrusion pathway, including the installation of sub-slab
depressurization systems (SSDS), structural mitigation, and building ventilation
modifications. Individuals with the needed expertise are also available and most
remedial actions to address a CEP would not involve the need for off-site OHM disposal.
So, the cost-benefit analysis will likely drive the feasibility evaluation for prevention,
elimination and/or mitigation of CEPs.
Documentation for a CEP feasibility evaluation should include: description of the critical
exposure pathway as it relates to the Conceptual Site Model, a list of measures evaluated
to prevent, eliminate or mitigate the CEP, estimated costs of measures and an explanation
of how the costs were determined, and a description of the basis for determining the
measures feasible or infeasible.
In the Department’s experience with vapor intrusion mitigation, sub-slab depressurization
systems (SSDS) have been cost-effective, being both reasonable in cost and effective at
reducing the concentrations of contaminants in indoor air. Site-specific explanations
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should be provided in the case of cost estimates outside the range usually associated with
typical mitigation systems and when typically effective technologies are rejected as
infeasible.
There may be situations in which an owner-occupant may decline cost-effective measures
that could be taken to address CEP conditions where the concentrations do not pose a
Significant Risk. In this case, the owner’s decision drives the feasibility evaluation by
determining that the benefits of the mitigation do not justify the costs. This situation
should be documented in writing.
2.3.3 Actions taken to address CEP
Immediate Response Actions (IRAs) are presumed to require the elimination and/or
mitigation of CEPs, pursuant to 310 CMR 40.0414(3). IRAs are also presumed to require
the prevention and/or mitigation of CEPs, pursuant to 310 CMR 40.414(4). A typical
action taken to eliminate or mitigate a CEP would be the installation of a sub-slab
depressurization system to address measurable OHM in indoor air. This would be done
as an Immediate Response Action, with the required submittals of an IRA Plan, and IRA
Status and Remedial Monitoring Reports. (Insert a flow chart with the timing
requirements for these submittals)
The Department has not typically required responsible parties to prevent CEPs.
However, the prevention of CEPs is a logical and cost-effective practice many parties
implement at several stages during the MCP process. Consider the example of a building
formerly housing a dry cleaning business, where PCE has been found in groundwater and
in soil near floor drains and a dumpster. The groundwater contaminant plume has
resulted in measurable PCE concentrations in the indoor air of one downgradient
residence, triggering a CEP condition. A SSDS is installed to mitigate the vapor
intrusion. An adjacent residence does not contain measurable concentrations of OHM in
indoor air, despite soil gas data indicating the presence of PCE. While no CEP exists at
the second residence, it would be prudent and proactive to fix cracks and close sumps in
the basement floor to prevent the CEP there. Another example of CEP prevention would
be expedited elimination of source areas, such as the removal of contaminated soil or
implementation of in-situ treatment. The Department recommends the prevention of
CEPs through timely and efficient implementation of source removal actions and
contaminant reduction technologies. Source control performed early in the process will
likely reduce exposures and overall costs.
2.3.4. Ending Mitigation with a CEP
Mitigation systems installed to address a condition of CEP would typically run until the
CEP conditions have been eliminated. At this point, an Immediate Response Action
Completion Report would be submitted and the response action would proceed through
the other actions required under the MCP.
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In the case where measurable levels of OHM persist in the living or working space of a
pre-school, daycare, school or occupied residential building, the mitigation would
continue as an IRA. If an IRAC cannot be submitted, the mitigation system would run
through Phase II and Phase III of the MCP. The feasibility criteria listed in 310 CMR
40.0860 include evaluating the feasibility of: (1) implementing a Permanent Solution, (2)
reducing the concentration of OHM to levels that achieve or approach background; (3)
reduction the concentration of OHM to levels at or below applicable soil UCLs and (4)
evaluating the feasibility of eliminating, preventing, or mitigating Critical Exposure
Pathway(s).
If, at the conclusion of the Phase III Feasibility Evaluation, the selected remedial action
alternative allows the mitigation system operation to be terminated, mitigation under the
CEP condition would end. Specific conditions must be met in order to discontinue
operation of a mitigation system installed to address a CEP under the MCP. All Phase II
and Phase III requirements must be completed. Monitoring for contaminant level
rebound after system shut-down must support the conclusion that a condition of No
Significant Risk exists without operation of the mitigation system. (See the Assessment
and Mitigation Sections of this guidance for details about recommended monitoring
frequency and evaluation.) Finally, the Phase III feasibility evaluation must support the
conclusion that continued operation of the CEP mitigation system is not feasible, because
the costs of continued system operation outweigh the benefits. The overall requirements
for Response Action Outcomes (RAO) listed at 310 CMR 40.1003 would also apply.
Sources of OHM must be eliminated or controlled. Pursuant to the Response Action
Performance Standards applicable to RAOs, there should be consideration of response
actions to reduce the overall mass and volume of OHM.
Operation of a mitigation system may continue to reduce indoor air contamination
concentrations towards levels which would exist in the absence of the disposal site.
Affected parties who are not PRPs (such as tenants or residents in affected properties not
owner or controlled by the PRP) may choose to continue operation of vapor intrusion
mitigation systems outside of the MCP process, especially if it would mitigate other
health threats such as radon intrusion. It is expected that the physical components of
mitigation systems installed to address CEPs would be left in place by the PRP if the
property owner or building occupants indicate their intent to continue operation of a
system to reduce contamination below the levels of No Significant Risk to achieve or
approach background levels.
See diagrams below.
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Life Cycle of a CEP
* Action presumed,
unless measures are
not feasible and no
IH exists or is likely
to exist
Condition Notify Conduct
*
IRA
No IRA
Action
IRAC
No
Action
not
feasible
not
feasible
Phase III
Feasibility
Evaluation
Feasibility
Evaluation
IRA cont’d
no
feasible
** without continued
operation of an Active
Remedial System
IRA
Action
feasible
Condition**
addressed?
yes
IRAC
Action
Phase IV
/V/ROS
Related to a
Release that
is
Reportable?
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Measurable OHM in Indoor Air
Are OHM levels in indoor air
above Threshold Values?
Yes
No
Are OHM levels related to a
Release that is Reportable?
No
May choose to perform
risk reduction outside of
the MCP process
Yes
Was OHM in air measured in a
living or working space of a
pre-school, daycare, school or
occupied residential building?
Continue to characterize
site and assess risk
through the MCP
process.
No
Yes
CEP Notification, Conduct
IRA
No
Is elimination, mitigation or
prevention of the indoor air
pathway feasible?
Yes
No
Eliminate,
mitigate or
prevent
indoor air
pathway as
IRA
Immediate
Response
Action
Completion
Report
No
Submit IRA
Status and
Remedial
Monitoring
Reports
Phase II Risk CharacterizationDo site conditions including the
indoor air pathway pose No
Significant Risk?
Yes
IRA Condition
addressed
without
continued
operation of an
Active Remedial
System?
No
Phase III
Feasibility
Evaluation
No
Response
Actions
Phase
IV/V/ROS
Yes
Achieve Response
Action Outcome
CEP is addressed as an IRA until an IRAC has been achieved (if
possible) AND the feasible remedial action alternative selected upon
conclusion of the Phase III Feasibility Evaluation allows response actions
to be terminated.
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