EPA’s Response to Peer Review Comments on

the Formaldehyde Indoor Air Model – Pressed 

Wood Products (FIAM-pwp)

U.S. Environmental Protection Agency

Office of Pollution Prevention and Toxics

Exposure Assessment Branch

1200 Pennsylvania Avenue, N.W.

Washington, D.C. 20460

July 2012
ACKNOWLEDGMENT

EPA acknowledges the analytical and draft preparation support of Versar, Inc. of Springfield, 

Virginia in the preparation of this report, provided under Contract No. EP-W-10-005.

Notice

Mention of the names of specific companies, organizations, or entities does not constitute 

an endorsement by EPA.
i

Table of Contents

1 INTRODUCTION...................................................................................................................... 1

1.1 Background..................................................................................................................... 1

1.2 This Report ..................................................................................................................... 2

2 SUMMARY OF PEER REVIEW COMMENTS AND EPA RESPONSES, ORGANIZED 

BY CHARGE QUESTION ....................................................................................................... 3

2.1 Part 1 - Algorithms and Exposure Assumptions......................................................... 3

2.1.1 Comment on the Adequacy of the Mathematical Equations and the Level of Confidence

in Indoor Air Concentrations and Exposure Estimates Provided by the Model.............3

2.1.2 Comment on the Default Scenarios and Assumptions Used by the Model ....................8

2.1.3 Comment on the Data Sources.............................................................................................9

2.1.4 Comment on Whether the Data Sources Provide Unused Information That Would 

Assist in Quantifying Uncertainty or Variability for Subsequent Model Use.................9

2.1.5 Comment on Whether the Model Input Variables and the Estimates Assigned to Them 

Appropriately Reflect Variability ......................................................................................10

2.1.6 Comment on the Adequacy of the Documentation Supporting the Overall Plausibility

of the Exposure Estimates Generated by the Exposure Model.......................................10

2.1.7 Provide Any Other Types of Data to Assess the Reliability of the Overall Exposure 

Model or Specific Model Elements...................................................................................11

2.1.8 General Comments on Algorithms and Exposure Assumptions....................................11

2.2 Part 2 - User Interface ................................................................................................. 12

2.2.1 Provide Comments on the Ease of Use of the FIAM-PWP Model................................12

2.2.2 Comment on the Adequacy of the Model Help Screens.................................................12

2.2.3 General Comments on User Interface ...............................................................................12

2.3 Part 3 – Documentation ............................................................................................... 13

2.3.1 Comment on Whether the References Have Been Accurately Identified.....................13

2.3.2 Provide Comments on the Model Documentation (Does It Present the Model Construct, 

Selected Model Inputs, and Model Results in a Clear, Complete and Useful Manner?)...14

2.3.3 Provide Comments on Whether the Documentation for the FIAM - PWP MakesClear the 

Sources and Anticipated Magnitudes of Variability and Uncertainty in Model Results....15

2.3.4 Comment on Whether the Model Algorithms are Consistent with the Descriptions in 

the Model Documentation..................................................................................................15

2.3.5 General Comments on Documentation.............................................................................15

APPENDIX A – DETAILED REVIEWER COMMENTS AND EPA RESPONSES: 

Part 1 – Algorithms and Exposure Assumptions

2.1.1 Comment on the Adequacy of the Mathematical Equations and the Level of Confidence 

in Indoor Air Concentrations and Exposure Estimates Provided by the Model...........A-1

2.1.2 Comment on the Default Scenarios and Assumptions Used by the Model .............A-11

2.1.3 Comment on the Data Sources......................................................................................A-12
ii

2.1.4 Comment on Whether the Data Sources Provide Unused Information That Would 

Assist in Quantifying Uncertainty or Variability for Subsequent Model Use..........A-13

2.1.5 Comment on Whether the Model Input Variables and the Estimates Assigned to Them 

Appropriately Reflect Variability .................................................................................A-14

2.1.6 Comment on the Adequacy of the Documentation Supporting the Overall Plausibility

of the Exposure Estimates Generated by the Exposure Model..................................A-15

2.1.7 Provide Any Other Types of Data to Assess the Reliability of the Overall Exposure 

Model or Specific Model Elements..............................................................................A-16

2.1.8 General Comments on Algorithms and Exposure Assumptions...............................A-17

APPENDIX B – DETAILED REVIEWER COMMENTS AND EPA RESPONSES: 

Part 2 – User Interface

2.2.1 Provide Comments on the Ease of Use of the FIAM-PWP Model.............................B-1

2.2.2 Comment on the Adequacy of the Model Help Screens..............................................B-2

2.2.3 General Comments on User Interface ............................................................................B-2

APPENDIX C – DETAILED REVIEWER COMMENTS AND EPA RESPONSES: 

Part 3 – Documentation

2.3.1 Comment on Whether the References Have Been Accurately Identified..................C-1

2.3.2 Provide Comments on the Model Documentation (Does It Present the Model Construct, 

Selected Model Inputs, and Model Results in a Clear, Complete and Useful Manner?)....C-2

2.3.3 Provide Comments on Whether the Documentation for the FIAM - PWP Make Clear the 

Sources and Anticipated Magnitudes of Variability and Uncertainty in Model Results....C-4

2.3.4 Comment on Whether the Model Algorithms are Consistent with the Descriptions in 

the Model Documentation...............................................................................................C-5

2.3.5 General Comments on Documentation..........................................................................C-6
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1 INTRODUCTION

1.1 Background

The U.S. Environmental Protection Agency developed a model for predicting the potential 

exposure to formaldehyde associated with use of pressed wood products in new construction, 

new homes, and renovations, the Formaldehyde Indoor Air Model - Pressed Wood Products 

(FIAM-pwp). The FIAM-pwp is an adaptation of a model developed during the 1980s by Dr. 

Thomas Matthews and colleagues1,2at the Oak Ridge National Laboratory. The “Matthews 

model” was designed to estimate the steady-state indoor formaldehyde concentration due to 

emissions from wood products in a single indoor compartment or zone. Product emission rates in 

the model are dependent on the formaldehyde concentration in the vapor phase – as the 

concentration increases the emission rates decrease, other things being equal. The initial (“steadystate”) indoor concentration calculated by the model is assumed to decrease over time, as the 

formaldehyde “reservoirs” in various sources (or sinks) are gradually depleted. The decrease 

over time is assumed to follow a first-order exponential process, at a decay rate that corresponds 

to an assumed half life for the collective formaldehyde emissions. In the mid-1990s, EPA 

conducted an assessment of the contribution of urea formaldehyde (UF)-bonded wood products 

to formaldehyde levels in homes. Evaluations of then-current EPA exposure models were 

conducted and shortly thereafter, a modified version of the Matthews model was developed. The 

primary modifications were (1) estimation of steady-state formaldehyde concentrations for the 

case of a two-zone indoor environment, and (2) incorporation of reversible (re-emitting) indoor 

sinks. The modified model also included the option to be run in a single-zone mode. FIAM-pwp

is characterized as an upper-tier, empirical model wherein the model structure is determined by

the observed relationships among experimental data. It is useful to forecast and describe trends in 

behavior although it is not necessarily mechanistically relevant behavior. The dynamic, timevarying behavior of the state variables in a single run provides deterministic results, rather than a 

set of probabilistic outcomes. 

Early in 2009, FIAM was further revised to include additional features and to make it more userfriendly. Later in the same year, a peer review of the model was facilitated for EPA by Eastern 

Research Group, Inc. (ERG). The mechanism for the peer review was individual letter peer 

reviews conducted following the policy provided in EPA’s Peer Review Handbook 3

rd Edition

(http://www.epa.gov/peerreview/pdfs/peer_review_handbook_2006.pdf). The purpose of the 

peer review was to review and comment on three main aspects of the FIAM-PWP model: the 

soundness of the algorithms and exposure assumptions; the ease of use of the graphical interface; 

and the completeness and clarity of the model documentation. Unedited written comments 

submitted by four peer reviewers in response to the peer review charge were organized in a 

report3prepared for EPA by ERG in October 2009.

 

1

T. Matthews, T. Reed, B. Tromberg, C. Daffron, and A. Hawthorne. 1983. Formaldehyde Emissions from 

Combustion Sources and Solid Formaldehyde Resin Containing Products: Potential Impact on Indoor 

Formaldehyde Concentration and Possible Corrective Measures. Proceedings of ASHRAE Symposium 

Management of Atmospheres in Tightly Enclosed Spaces, Santa Barbara, CA. 

2

T. Matthews, A. Hawthorne, and C Thompson. 1987. Formaldehyde Sorption and Desorption Characteristics of 

Gypsum Wallboard. Environmental Science & Technology 21: 629-634.

3 Eastern Research Group, Inc. Peer Review Results for the Formaldehyde Indoor Air Model – for Pressed Wood 

Products. Prepared for EPA Office of Pollution Prevention and Toxics under Contract No. EP-W-05-014.
2

1.2 This Report

This report provides EPA’s responses to the peer review comments, organized under three main 

headings:

 Part 1 – Algorithms and Exposure Assumptions

 Part 2 – User Interface

 Part 3 – Documentation

The reviewer comments and EPA’s responses are summarized in Section 2 of this report under 

these three main headings. Appendices A, B and C contain detailed extracts of the peer reviewer

comments for Parts 1, 2 and 3, respectively, along with EPA’s responses to the comments.

Since the peer review, the FIAM-pwp model has been revised to produce a streamlined version 

(Version 2.0) that runs under EPA’s Internet Geographical Exposure Modeling System (IGEMS).

Based in part on the reviewer comments, a decision was made to remove certain FIAM features, 

including input screens for indoor sinks, an area of considerable modeling uncertainty, and 

emission characteristics of cabinet components, which had significant input demands for the 

user. In addition, exposure groups are being handled differently in the revised version, for 

consistency with the approach used in EPA’s 2012 formaldehyde exposure assessment4.

 

4

USEPA 2012. Formaldehyde from Composite Wood Products: Exposure Assessment. Draft Final Report. U.S. 

Environmental Protection Agency, Office of Pollution Prevention and Toxics, Exposure Assessment Branch,

1200 Pennsylvania Avenue NW, Washington, DC 20460. July 2012.
3

2 SUMMARY OF PEER REVIEW COMMENTS AND EPA 

RESPONSES, ORGANIZED BY CHARGE QUESTION

2.1 Part 1 - Algorithms and Exposure Assumptions

Comments and responses for this part of the peer review are summarized below, for each of 

seven charge questions. In addition, general comments across all questions are provided for one 

reviewer who chose to provide comments in this manner rather than organizing them under the 

specific charge questions. Detailed extracts of reviewers’ comments and summary EPA responses

related to these charge questions can be found in Appendix A.

2.1.1 Comment on the Adequacy of the Mathematical Equations and the Level of 

Confidence in Indoor Air Concentrations and Exposure Estimates Provided by the 

Model

This particular charge question had the most extensive set of reviewer comments. Responses to 

this question account for ten of the 25 appendix pages listing detailed reviewer comments. Given 

the extensive comments to this question, they are grouped into the following six subheadings:

 Confidence In Model Predictions

 Modeling Of Product Emissions

 Modeling of Emissions Decay

 Modeling of Indoor Sinks

 Modeling of Exposure, Including Product-Specific Exposure

 Miscellaneous Comments

Confidence in Model Predictions. The reviewers generally expressed some confidence in the 

model predictions but also had some reservations, as evidenced by the following verbatim 

comments:

 Reviewer 1 – Based on the data provided and the assessment analysis, the model should 

provide reasonable conservative estimate of the formaldehyde exposure given the relatively 

long half-life time.

 Reviewer 2 – Regarding the level of confidence one might have in the model’s prediction of 

actual indoor concentration and exposure levels, my sense is that they should be considered 

accurate within a factor of 2.

 Reviewer 3 – The mathematical/algorithmic formulation and the computational implementation

of the model under review are adequately explained, with sufficient clarity and level of detail; 

the computational implementation of the model also appears robust and accurate. 

Nevertheless, given the simplifying assumptions in the formulation of the model, and the 

significant variabilities inherent in the data sets that were used to derive parameter values 

for FIAM-pwp, the level of confidence in the indoor air concentration and exposure estimates 

provided by the model, should in general be considered, at most, medium.

 Reviewer 4 – In summary, I concur with the calculation of initial concentration (high 

confidence in result with reliable chamber test data; however, the treatment of sinks and 

average daily concentration for chronic exposure appear to require some revisions to 

improve the quality of the result (low confidence in result).
4

 Reviewer 5 -- The analysis provided in Section 5.3 of the documentation suggests that 

whatever the convolutions involved, the model gives respectable predictions. My reading of 

the tables in that section, which summarize modeling results compared to measured values in 

the EPA pilot study house, is that the model ranged from about 24% low to about 65% high. 

Predictions within that range are quite good, given the uncertainties in input data and

unavoidable modeling assumptions.

Modeling of Product Emissions. Three reviewers commented in this area. The first reviewer 

noted that Equation 3-5, which forms the basis for the emission rate modeling engine, appears to 

be a reasonable approximation of reality and is in complete agreement with his previous work on 

“backpressure” modeling. He further suggested that the analytical measurement of Csat – the 

saturation concentration of formaldehyde in the air over the emitter of interest – could provide a 

potentially valuable extended methodology for the evaluation and development of the model’s 

parameters relative to specific emitters. EPA agrees that a well designed analytical study could 

be useful to provide information on the saturated air concentration of formaldehyde and has 

noted it in the model’s documentation as a suggestion for interested researchers. However, such 

a study is not within the scope of this project. The new version of the model includes the 

calculated equilibrium concentration as an intermediate output that is displayed on the Source 

Screen for each source selected or entered by the user.

The second reviewer expressed the following opinion:

There appears to be some confusion between diffusion and convective mass transfer. The model is 

based on convective mass transfer over the surface, which is not based on the Fick’s Law of 

diffusion. However, it is more likely that, except for the initial emission period, the emission rate 

will be controlled by the internal diffusion process as suggested by many studies. So the “slope” 

values derived from the measured data may have included the combined effects of convective 

mass transfer and internal diffusion resistances, resulting in reasonable predictions. At the 

“steady state” (about 30 days from initial loading), the internal diffusion resistance should be the 

dominant resistance for formaldehyde emissions even for particleboard. Discussion of diffusion 

and convective mass transfer, along the lines of the reviewer’s comments, will be added to 

Section 3 of the model documentation, along with related comments from another reviewer who 

referred to a conceptual model published by Mølhave et al. (1995). According to this model, three 

conceptual exponential decay compartments can be attributed to three potential sources of 

formaldehyde emissions:

 Free formaldehyde (short-term compartment);

 Decay of paraformaldehyde and other complex structures (medium-term compartment); and

 Hydrolysis of the resin (long-term compartment).

The third reviewer made several observations and suggestions:

 The original Versar/EPA model (referring to a version developed during the 1980s) 

presented the linear regression parameters for both the Matthews model and the Hoetjer 

model (identified as the HBF model in this early version), as well as the steady-state

equations for both models. It can be shown these are identical models – the difference in 

models is in the regression equation used to derive the parameters, not the theoretical 

construct. Because slightly different results are obtained depending on the regression model 

used and the Hoetjer model is widely cited in the literature, it is recommended that the model 

documentation be revised to include discussion of both approaches. 
5

 If only the Matthews model is retained, then an example calculation should be provided in the 

model documentation showing how the user can start with the HBF model parameters and 

convert these to the equivalent Matthews model. Another option would be to provide the HBF 

equilibrium concentration as supplemental output, to enable an intuitive determination about 

whether a particular product is likely to act as a sink or a source.

 Some pressed wood formaldehyde “emitters” could theoretically act as sinks under certain 

conditions based on the properties of wood and the Matthews model (e.g., products with a 

very low equilibrium concentration or, equivalently, low Matthews model intercept). 

Therefore, there may be some usefulness in presenting the emission rates by product in the 

results, with a negative emission rate indicating a net sink effect.

Adding the Hoetjer model is beyond the scope of the current EPA effort but discussion of the 

two approaches, as well as the example calculation suggested, has been included in the model 

documentation. In addition, as noted above, the equilibrium concentration for each emitter has 

been added as an intermediate model output; this output is intended to assist model users in 

identifying, for example, a relatively weak emitter that may act as a sink or “net absorber” in the

presence of other, stronger emitters. An example has been added to the model documentation to 

demonstrate how the user can identify a source that is acting as a sink in the presence of other 

sources, by running the model with and without the weak source (if a “source” is acting as a sink, 

then the modeled concentration when that source is included along with the stronger emitters will 

be lower than when the source is excluded). A new model feature, enabling users to check or 

uncheck sources for inclusion in or exclusion from a model run, makes this procedure very

straightforward to apply.

Modeling of Emissions Decay. Three reviewers commented on this area. The first reviewer 

noted that the approach of exponentially decaying the indoor concentration is unconventional, 

but practical considering the many uncertainties involved in real-life situations, most notably

choosing the right half-life. He supported the idea of relying on measured field data to estimate 

the decay of formaldehyde concentration and to be conservative in the estimation, as he believes 

the model to be. The second reviewer expressed the opinion that the value of the information that 

has been incorporated in FIAM-pwp would be greatly enhanced if its formulation explicitly

considered decaying emission rates and provided appropriate formulas for implementing them 

(either within FIAM-pwp or in conjunction with other, more comprehensive, indoor air quality

and inhalation exposure systems). The third reviewer suggested that basic research on the decay

profile of individual products should be encouraged in the model documentation (rather than 

exclusively relying on the historical data from the 1980s). The reviewer suggested that, at a 

minimum, the model should be revised to allow consideration of periods of rapid and slow decay

in the emission rate as observed by several researchers.

In response to the comment, discussion has been added to the model documentation regarding

how rapid and slow emissions decay could be handled via a series of related model runs. The 

suggestion regarding basic research on the decay profile of individual products has been 

incorporated in Section 3 of the revised model documentation. At this time, EPA is not revising 

FIAM-pwp to decay emission rates rather than concentrations, or to explicitly allow periods of 

rapid and slow emissions decay within a single model run, because (1) the underlying 

mathematics for decaying emission rates rather than concentrations are relatively complex, and 

(2) data to support user choices for emission parameters governing periods of rapid and slow 

decay, as well as the point in time when a transition between the two periods occurs, are 

relatively sparse. In addition, the model as designed achieves the goal of supporting EPA 

exposure assessments.
6

Modeling of Indoor Sinks. One reviewer commented extensively on indoor sinks, expressing

some concerns with how sinks are handled in the model, including the following specific comments:

 The treatment of indoor sinks in the current EPA model is problematic because absorption 

and desorption of formaldehyde from indoor sinks is a transient process over relatively short 

time scales not well suited to a steady-state or pseudo-steady-state model construct.

 It appears that the guidance for the sink intercept given on EPA model guidance page 3-7 

was developed by setting intercept b = ka x Cindoor assuming dM/dt = 0. When there is a 

linear relationship between accumulated mass and indoor air concentration at equilibrium 

conditions, dM/dt must equal zero. Therefore, deviations of the intercept from the calculated 

value would imply a sustained loss of equilibrium, violating the assumption used in assigning 

the sink slope and intercept in the Matthews equation. Furthermore, the guidance provided 

for selection of the equilibrium intercept for indoor sinks appears to treat this parameter as 

calibration or adjustment factor rather than as a predictive input.

 To correctly account for net adsorption on these short time scales within the framework of a 

pseudo-steady-state model, a non-zero value of dM/dt must be calculated using Equation 3-

13 and set equal to the Matthews emission rate equation to derive the intercept. This value of 

dM/dt will vary as a function of time. Such an approach would appreciably increase the level 

of complexity needed to assign the sink term because indoor concentration and mass storage 

vary as a function of time. If an estimate of the indoor concentration profile is available as a 

function of time, it should be possible to determine M(t) and dM(t)/dt as a function of time by 

numerical integration or a finite difference model. However, at 30 days, dM/dt may be 

sufficiently small that a reasonable default recommendation in the model would be to neglect 

the effect of sinks.

EPA has carefully reviewed the reviewer comments relating to sinks, such as those provided above, and 

based on its review has removed the indoor-sinks module from the revised FIAM-pwp model. The 

supporting data needed to establish a mathematical model of sink effects for each material, let alone a 

combination of materials, are not readily available and development of such data or conduct of related 

research is beyond the scope of this project. Users of the model can incorporate sinks within the indoorsource module, recognizing the limitations of that approach. (The mathematical treatment and input 

requirements for sinks, when they were included in the model, were identical to that used for sources.)

As the science regarding sinks advances, EPA could consider adding sinks to the model in the future.

Modeling of Exposure, Including Product-Specific Exposure. Two reviewers provided 

several comments in this area. The first reviewer noted situations for which the model either is 

inconsistent or provides erroneous results:

 Relative to the estimation of exposure there appears to be logical error in the defaults. When 

one invokes the exposure defaults, the 5 age groups for children in the model all show 10 

years for the exposure period. Of course, this is impossible for any of the age groups for 

children which are all less than 10 years in length. This is explained in the documentation 

and in the online help button; however, I found it somewhat confusing. My suggestion is that 

the default shown in the program for any age group should be the maximum time a person 

can spend in that age group.

 The program allowed me to put in a single day for the duration of the exposure but the results 

indicate that a calculation error occurs for this input. The calculated ADC is above the initial 

concentration and the time percentages come out to be >100%. If this error is not easy or 

practical to fix then explanation should be included advising the minimum time for duration 

of exposure.
7

These potential concerns were addressed by handling exposure groups and exposure durations 

differently in the revised FIAM-pwp. The model now has six default exposure groups covering 

different age ranges, and the model results include year-by-year ADC calculations for each 

exposure group, for the first eleven years of the model run, such that the user can select the ADC 

for the time frame of interest.

The second reviewer commented extensively on exposure estimation in general and productspecific exposure in particular:

 The usefulness of the model could be strengthened by discussing the incremental contribution 

of added products because regulations, if any, are likely to be developed on a productspecific basis. The model documentation and results emphasize the aggregate contribution of 

all products (and background sources), and running the model with and without a product of 

interest can produce counterintuitive results. The model documentation should focus on both 

the determination of incremental exposure attributable to a specific product and total 

exposure for a loaded home. 

 Assessment of acute/chronic exposure is associated with different uncertainties and 

conceptual models. In the acute setting, the back-pressure effect may attenuate the 

contribution of a new product such that the total acute indoor exposure concentration is not 

affected. However, in a chronic or long- term timeframe, it is reasonable to expect that the 

majority of available formaldehyde and precursors ultimately will partition from the product 

into the indoor space. My recommendation is that a conceptual framework be 

developed/documented to ensure that users take these factors into consideration when 

investigating acute versus chronic timeframes. For the acute timeframe, it is important to 

consider the total collection of assembled indoor products. For chronic exposure, a more 

accurate characterization of dose attributable to a single product is likely obtained by 

considering the decay profile of that product in the absence of other products; the attenuation 

due to the backpressure effect is temporary, and eventually, a source that is initially 

attenuated will begin to emit formaldehyde as the indoor-air concentration decreases. 

 For acute exposure, an option could be provided to identify ‘baseline products’ and ‘added 

products’. Model outputs of exposure and product-specific emission rates would be 

presented with and without the added products. I believe this would greatly assist users of the 

model in intuitively understanding the backpressure effect, as well as assist specific 

manufacturers, regulators and other interested parties in understanding the incremental 

effect of specific products on indoor air levels.

 The incremental chronic exposure attributable to a specific product or collection thereof 

should be assessed, rather than the aggregate cumulative exposure for all indoor products. 

Because the typical situation will reflect products of various ages, the chronic dose 

attributable to a specific product seems to be a more relevant metric than the total indoor 

concentration over time assuming an initial loading.

 Under the current model guidance, the true increased cancer risk attributable to installation 

of a specific product would not be reliably determined. Initially, the backpressure effect will 

suppress a portion of formaldehyde emissions, such that if the user were to compare 

cumulative dose with and without the product the lifetime dose for that product will be 

underestimated. If the equilibrium concentration (i.e., the theoretical concentration in the 

absence of ventilation) of a product is less than the pre-existing steady-state concentration, 

then running the model with and without the product would result in the physically 

impossible conclusion of negative incremental cancer risk because the product would initially 

act as a net sink.
8

EPA has modified the model to address this comment; an example has been added to the model 

documentation to demonstrate how the user can identify a source that is acting as a sink in the 

presence of other sources, by running the model with and without the weak emitter. The model 

now has considerable flexibility in the source inputs, allowing the users to check or un-check 

sources such that individual products or groups thereof can be examined from both short- and 

long-term perspectives.

Miscellaneous Comments. Reviewers had several additional comments or suggestions that did 

not fall under any of the above categories:

 It is suggested that the modelers consider a modeling program based on The Advanced 

Continuous Simulation Language (ACSL) computer language. ACSL easily handles 

simultaneous differential equations while providing parameter estimates from data via 

optimization and an intuitive graphical interface.

 The uncertainty and variability levels in the available data strongly suggest the need for a 

distributional/probabilistic model formulation that explicitly employs available data to 

develop ranges rather than point estimates of concentrations and exposures.

 One concern is the interaction between ventilation rate and decay rate. Given long-term 

testing data, the total reservoir of formaldehyde and formaldehyde precursors for a given 

ventilation condition could be calculated. Therefore, in the future when decay characteristics 

are better understood, it should be possible to adjust for ventilation rate and account for total 

mass of formaldehyde generated over the lifetime of the product.

 The model should provide sufficient ancillary information to understand the sensitive 

parameters and identify other potential areas for testing.

With regard to the programming language, Version 2.0 of the FIAM-pwp has changed 

substantially from the peer-reviewed version and has been incorporated into an existing webbased platform, IGEMS. The new web-based version has a much quicker response time, 

particularly when users are providing inputs. The sensitivity/uncertainty discussion in the FIAMpwp v2.0 documentation has been bolstered, including an appendix containing the sensitivity

analysis that was performed as part of EPA’s 2012 formaldehyde exposure assessment for 

illustration purposes. EPA acknowledges that Monte Carlo simulation, or a similar probabilistic 

tool, would be useful given the many combinations and permutations that an assessment of 

formaldehyde from pressed wood products can entail. However, such an enhancement is not 

within the current scope of this project.

2.1.2 Comment on the Default Scenarios and Assumptions Used by the Model

The following specific suggestions were provided by reviewers in response to this charge question:

 The modelers should consider using and including the seminal study by Murray and 

Burmaster (1995) as an important reference for potential use with this model. 

 First, it should be made clear that many of the default emissions parameters in the model are 

based on historical testing and may or may not be representative of current products. Second, 

clear guidance should be provided to end users (e.g. importers, manufacturers, non-profit 

organizations or regulators) about the data and steps required to assess the HCHO 

incremental exposure attributable to the introduction of a specific product into the home.
9

EPA agrees with these comments and has revised the FIAM-pwp documentation accordingly.

The Murray and Burmaster article, referenced as one of the sources considered in choosing an air 

exchange rate for the recent formaldehyde exposure assessment, has been cited in the model 

documentation. Some discussion relating to historical testing, as well as an example relating to 

incremental exposure, has been added to the model documentation, keeping in mind that an

assessment of incremental exposure is not a primary intent of the model. Additional reviewer 

comments regarding the historical data underlying default emissions parameters are included 

under the next charge question, relating to data sources for the model. 

2.1.3 Comment on the Data Sources

In response to this charge question, one reviewer reiterated his comment regarding the utility of 

the Murray and Burmaster data in choosing a residential air exchange rate. Two of the reviewers 

commented that the data underlying emissions parameters are dated and, thus, not necessarily 

reflective of current or recent manufacturing practices. One reviewer commented on the airflow 

rates and underlying data for the default house selections labeled as Conventional 1 and 

Conventional 2. These houses are no longer included as default selections in the model; they 

have been replaced by five generic structure types – single-family attached and detached homes, 

apartments, mobile/manufactured homes, and camper trailers – as detailed in the revised FIAMpwp (v2.0) User’s Manual and Documentation report. The same reviewer commented that use of 

the Langmuir isotherm for wallboard as a sink should be further explained or justified. The 

Agency has modified the model; sinks are no longer explicitly included due to a number of 

uncertainties, such as those noted in the peer reviewer comments under the first charge question 

above (see Section 2.1.1). 

2.1.4 Comment on Whether the Data Sources Provide Unused Information That Would 

Assist in Quantifying Uncertainty or Variability for Subsequent Model Use

The reviewers had several suggestions in response to this charge question:

 Although the data sources are incomplete for quantifying the uncertainty, it is possible to 

conduct an analysis using error propagation theory with assumptions on the uncertainties in 

the raw data.

 The Murray/Burmaster database on air exchange rates in U.S. residences could assist in 

gauging the variability of this parameter. Also, I believe the current data used in the model 

on emission sources discloses a level of uncertainty and variability that could also be mined 

by a person who is knowledgeable about these data and their origins.

 The variability in the available data could be used to derive distributional estimates of 

emission rates that would support probabilistic (and population based) indoor concentration 

and corresponding exposure estimates with explicit characterization of uncertainties and 

variabilities.

 The uncertainty and sensitivity analysis presented in Matthews et al. 1985 could be discussed.

EPA acknowledges that probabilistic tools would be useful to address the many possible 

combinations/permutations of inputs that an assessment of formaldehyde from pressed wood 

products could entail. However, as noted previously, such an enhancement is not within the 

current scope of this project. The uncertainty and sensitivity analysis presented by Matthews et al. 

has been summarized as part of an appendix on model sensitivity that has been added to the 

revised model documentation.
10

2.1.5 Comment on Whether the Model Input Variables and the Estimates Assigned to 

Them Appropriately Reflect Variability

The reviewers had several suggestions for addressing variability, ranging from use of high- and 

low-end model inputs (e.g., emissions strength) to a distributional/probabilistic approach. The 

Agency has incorporated additional text addressing these comments in the model documentation. 

For example, the model sensitivity analysis that was described in EPA’s 2012 formaldehyde 

exposure assessment report has been included in an appendix to the revised model documentation.

Other reviewer comments or suggestions related to uncertainty or variability are as follows:

 The single exponential equation for decay is consistent with the historical cross-sectional 

residential data but inconsistent with studies that have investigated single products or 

collection of products over time. The true area under the curve for dose given a long enough 

exposure time is likely better represented by the two- or three- compartment exponential 

models than the single exponential model because these models better represent the decay in 

the reservoir of formaldehyde precursors in a specific product or collection of products.

 EPA should perform a sensitivity analysis to determine whether measurement of the mass 

transfer coefficient is a data need or data gap when newly acquired chamber test data are 

entered.

 The documentation does not provide a credible method for selecting the indoor sink 

parameters.

 The model screens and documentation should make it clear that Conventional 1 represents a 

single unoccupied test home. The analysis used to derive Conventional 2 should be described.

 The degree to which the historical data in Table 2-2 reflects domestic and imported 

production is not known.

EPA agrees that the first two suggestions, regarding formaldehyde decay and the mass transfer 

coefficient, are of interest; however, conducting additional research and analysis is beyond the 

scope of the current effort, and the model as designed meets the Agency’s needs for exposure 

assessment. The next two comments/suggestions have been addressed by (1) removing indoor 

sinks in the revised model, due to a number of uncertainties that may lead to misleading results if 

sinks were included in model runs (see Section 2.1.1), and (2) replacing the houses denoted as 

Conventional 1 and Conventional 2 with five generic structure types (see Section 2.1.3). EPA 

has addressed the final comment by noting this uncertainty in the model documentation.

2.1.6 Comment on the Adequacy of the Documentation Supporting the Overall 

Plausibility of the Exposure Estimates Generated by the Exposure Model

Several reviewers suggested that model estimates could be compared with measurement results 

from prior field studies to better evaluate their plausibility. Some other reviewer comments or 

suggestions related to the plausibility of exposure estimates are as follows:

 Comparison of model estimates with relevant observational data sets for indoor-outdoor 

relationships and of personal level (e.g. from studies such as RIOPA – see, e.g., the Liu et al., 

2006, reference in the Addendum of this review) would be valuable in assessing, at least 

qualitatively, the plausibility and relevance of the exposure estimates derived from FIAM-pwp.

 EPA has not shown how use of an overall residential half-life is to be used to derive product 

specific dose. Running the model with and without the product of interest will not result in 

meaningful conclusions as discussed previously. Accordingly, there is great uncertainty in 

product specific doses under the current model framework.
11

 EPA has not considered whether the historical 2.92-year cross-sectional half-life is 

representative of the decay profile of modern products or the potential variability in the 

formaldehyde reservoirs and decay between products.

EPA has addressed the comments on half life by the inclusion of additional text in the model’s 

documentation. With regard to the last comment, EPA has considered the possibility that a value 

of 2.92 years for the half life may be too high; the default value for the half life in the revised 

version (v2.0) of FIAM-pwp is 1.5 years. A section has been added to the revised documentation 

on using the model to estimate the average concentration in a recent California field monitoring 

study5that focused on newly constructed residences, for comparison with the concentration that 

that was measured in the study.

2.1.7 Provide Any Other Types of Data to Assess the Reliability of the Overall Exposure 

Model or Specific Model Elements

One reviewer reiterated an earlier comment (see Section 2.1.1) regarding the potential utility of 

Csat as a critical emitter variable. A second reviewer reiterated an earlier comment concerning a 

framework for assessing product-specific, long-term exposure (again see Section 2.1.1). EPA has 

added text to the model documentation in response to these comments. A third reviewer noted 

that there are various studies and related data sets, both from the U.S. and abroad, that have 

appeared in recent years and could potentially be used for evaluation of specific FIAM-pwp

elements; one such data set has been used for that purpose, as noted above. The reviewer also 

provided an addendum containing a number of literature references pertaining to such studies 

and data sets. The references have been evaluated and the applicable ones have been

incorporated in the revised model documentation.

2.1.8 General Comments on Algorithms and Exposure Assumptions

One reviewer provided general comments rather than replying to the specific areas above:

 Seems like a convoluted way to model emissions: The dependent variable of primary interest

– the HCHO concentration – is inherent in the in the prediction algorithm; i.e., it is also used 

as an independent variable. Why not just use the intercept values and simplify the model?

 That said, the analysis provided in section 5.3 of the documentation suggests that whatever 

the convolutions involved, the model gives respectable predictions.

 Regarding the selection of a decay constant (or half-time) for predicting the rate at which the 

HCHO concentration will decrease, I suspect the default choice of k = 0.23 yr

-1

(or t1/2 = 3 

years) might be improved by having different values for different time periods.

 One significant shortcoming of this model, and many indoor models that assume complete 

mixing within compartments, is that it doesn’t deal with “intimate” sources such as toys, 

cribs, bedding materials, and clothing. I think the most straightforward approach to such 

sources is to assume a smaller compartment or zone around such sources for the purpose of 

predicting concentrations.

Other reviewers had no difficulty with the dependent variable being inherent in the prediction 

algorithm. EPA agrees that allowing for different decay values for different time periods is a 

 

5

F. J. Offermann, J. Robertson, D. Springer, S. Brennan, and T. Woo. 2008. Window Usage, Ventilation, and 

Formaldehyde Concentrations in New California Homes: Summer Field Sessions. ASHRAE IAQ 2007 

Conference, Baltimore, MD.
12

reasonable suggestion; however, the modeling framework needed to accomplish this is not trivial 

and would require a resource-intensive analysis to supply appropriate values. As noted in Section

2.1.1 (under the heading Modeling of Emissions Decay), discussion has been added to the model 

documentation regarding how rapid and slow emissions decay could be handled via a series of 

related model runs. The FIAM-pwp enables the user to model intimate sources using a two-zone 

modeling approach whereby one of the zones is used to represent the “virtual air space” 

surrounding an individual exposed to an intimate source.

2.2 Part 2 - User Interface

Comments and responses for this part of the peer review are summarized below, for each of two 

charge questions along with general comments across all questions for one reviewer. Detailed 

comments and responses related to these charge questions can be found in Appendix B.

2.2.1 Provide Comments on the Ease of Use of the FIAM-PWP Model

The reviewers generally felt that the user interface was relatively clean, straightforward and easy 

to use. Several reviewers also commented that the online version is very slow whereas the local 

version, although faster, requires adjusting local computer settings in ways that may be 

challenging for some systems and users. These latter comments, as well as some comments 

regarding specific aspects of the interface, were addressed by developing an online version of 

FIAM-pwp that is run under IGEMS with a redesigned, faster responding user interface. Some 

specific reviewer comments/suggestions that have been addressed in the revised user interface 

and/or model documentation are as follows:

 It would be useful to provide some sort of guide for users to step through the modeling 

process (such as step 1, 2, 3, ....)

 Maybe some “warnings” regarding the level of uncertainties in the calculations could be 

incorporated, as reminders, in the data-entry screens.

 Replacing this field (referring to air change rate) with plain text would make clear that this 

value cannot be edited.

 The codes in the model do not match Table 2-2 in all instances.

 Consider writing a routine to allow the information to be printed directly from the webbrowser (referring to Excel output that is an option with the local version of the current 

model).

2.2.2 Comment on the Adequacy of the Model Help Screens

The reviewers generally felt that the help screens are intuitive and adequate in content. There 

were some comments related to facets of the “audio help” that did not appear to work, but this 

model feature will not be included in the version (v2.0) that will be running under IGEMS. One 

specific reviewer suggestion regarding help screens that has been addressed is as follows:

 The help screens could be enhanced by including some statements regarding the level of 

precision expected for some of the entries.

2.2.3 General Comments on User Interface

One reviewer provided general comments rather than replying to the specific areas above:

 More careful attention to significant figures is needed.
13

 On the Sinks Screen, it seems counter-intuitive that the B value is positive rather than

negative.

The first comment has been addressed in the revised version of the model. The second comment 

is no longer applicable because sinks are being removed in the revised version, due to a number 

of uncertainties that may lead to misleading results if sinks were included in model runs (see 

Section 2.1.1).

2.3 Part 3 – Documentation

Comments and responses for this part of the peer review are summarized below, for each of four 

charge questions along with general comments across all questions for one reviewer. Detailed 

comments and responses related to these charge questions can be found in Appendix C.

2.3.1 Comment on Whether the References Have Been Accurately Identified

The reviewers had several suggestions for additional references; in one case a reviewer provided a 

long list of possible references. The following are specific reviewer suggestions for references:

 The fourth reference (the CARB Final Regulation Order...) should be identified as available 

at: http://www.arb.ca.gov/regact/2007/compwood07/fro-final.pdf. The 8th reference (D. 

Hare...) can be found at, and should be identified as available at: 

http://www.ecobind.com/research/Evaluating_the_Contribution.pdf.

 Any and all the references, particularly from the EPA, GEOMET, Versar or industry groups,

should be identified by their web links, if available, and if not available on the web, should be 

considered for posting online with a link.

 The two additional references (Murray and Burmaster on residential ventilation rate 

database and Jayjock on “Backpressure” modeling) should be considered for inclusion. 

 For more information regarding formaldehyde emissions from pressed wood products, model 

users could be referred to a U.S. EPA (1996) ORD report – Sources and Factors Affecting 

Indoor Emissions from Engineered Wood Products.

 It may be helpful to provide citations to studies that have summarized recent measured 

formaldehyde indoor air concentrations in new and existing structures. Examples include 

Hodgson and Levin 2003 and Hodgson et al. 2000.

 Groah’s white papers and reports have been invaluable in my past work on formaldehyde. 

However, I recommend that these references be replaced with peer-reviewed publications, 

conference proceedings, or governmental citations. Examples of publications addressing the 

long-term decay of formaldehyde emissions from consumer products include: Mølhave et al. 

1995; Brown 1999; Zinn et al. 1990; Myer and Hermanns 1985; and CARB 2007.

EPA has made all references used for development of this model available in the model 

documentation. Additionally, some of the suggested references have been added to the 

revised model documentation.
14

2.3.2 Provide Comments on the Model Documentation (Does It Present the Model Construct, 

Selected Model Inputs, and Model Results in a Clear, Complete and Useful Manner?)

The reviewers had a variety of specific suggestions/comments relating to model documentation:

 The model is not truly a steady-state one since the emission source decays over time. It is a 

“quasi-steady state” model in which the concentration at a given calculation point was 

treated as “constant” for the purpose of simplifying the calculation without losing too much 

accuracy.

 Most data provided on formaldehyde emissions are from early studies.

 Validity of the exponential decay model is questionable. Recent data have shown that the 

power-law model would be more suitable; secondary emissions due to hydrolysis are also 

possible.

 The Fick’s law is for diffusion, while here the process is governed by convective mass 

transfer across the boundary layer.

 The definition of vapor phase concentration and bulk phase concentration is unclear. I see no 

need to define a vapor phase concentration since, by default, we are always dealing with the 

vapor concentration in the air or in the material in this model.

 For me it would be more logical to change the order of Chapters 2 and 3.

 The use of “D” on page 3-4 to be unnecessarily complex.

 The main issue with the documentation is the justification of the formulation of certain 

elements of the model per se (e.g., the assumption of constant rather than decreasing 

emission rates, etc.) rather than of potential alternatives.

 The treatment of sinks is not transparent and confusing because the rate of storage changes 

with time but the model used is a steady-state model. The approach used appears to be more 

an attempt to modify model results to reflect the measured concentration than a rigorous 

handling of sinks.

 Some stakeholders may not be familiar with the linear regression protocol required to 

estimate the parameters for the Matthews and Hoetjer formaldehyde mass transfer models. 

The documentation could be revised to show an example linear regression for the data from 

Koontz et al. (1996).

 The documentation should explain the relationship between the Matthews and Hoetjer 

equation. The Hoetjer equation or equivalent mass transfer equation is frequently cited in the 

literature and some stakeholders may not be familiar with the Matthews equation.

 Consider including in an appendix the analysis of the EPA Pilot Study data used to determine 

the mass transfer coefficient and intercept (p. 5-3).

EPA has addressed most of these comments in Section 3.0 of the revised model documentation. 

As noted previously, indoor sinks have been removed in the revised version, due to uncertainties 

such as the one noted above that may lead to misleading results if sinks were included in model 

runs. Discussion in this regard has been included in Section 3.0 of the revised documentation.
15

2.3.3 Provide Comments on Whether the Documentation for the FIAM - PWP Makes

Clear the Sources and Anticipated Magnitudes of Variability and Uncertainty in 

Model Results

Several reviewers indicated that sources of uncertainty were addressed well in the model 

documentation. One reviewer felt that a table explicitly listing various sources of uncertainty and 

variability associated with specific processes and parameters of the model, along with a factor or 

range specifying the approximate magnitude of each uncertainty/variability, would be extremely 

useful. Ideally, the reviewer would like to see a probabilistic “re-formulation” of the model, with 

explicit discussion in the documentation of the appropriate distributions of values for the various 

inputs. Another reviewer suggested a simple Monte Carlo analysis as an appendix to provide 

some guidance on whether refinement of a particular input parameter is likely to change the 

conclusion. EPA agrees that more information on how the each input parameter can affect the 

results would be useful; for this reason, the sensitivity analysis that is part of EPA’s 2012 

formaldehyde exposure assessment report has been added to the model documentation. The same 

reviewer also suggested incorporating a mass transfer coefficient sensitivity analysis into the 

model or model documentation, along with a discussion of the uncertainty and sensitivity

analysis presented in Matthews et al. 1985. As noted previously in this document (see Section 

2.1.4), a summary of the Matthews analysis also has been added to the model documentation.

2.3.4 Comment on Whether the Model Algorithms are Consistent with the Descriptions in 

the Model Documentation

Reviewers generally agreed that the model algorithms are consistent with their descriptions in 

the model documentation. One reviewer verified the accuracy of model calculations by

comparing results with those obtained from a different indoor-air model with the ability to 

handle backpressure effects. The same reviewer noted that the slope in the Matthews equation is 

not consistently defined, referring to pages 2-11 and 3-4 of the model documentation, and he also 

noted that the model documentation is not clear regarding whether the background concentration 

is included in the ADC and LADC calculations. Another reviewer stated a preference for R

2

values rather than the correlation coefficients included in Table 2-2. EPA has added clarifying 

text to the documentation to address the points made in the referenced pages and table. 

2.3.5 General Comments on Documentation

One reviewer provided general comments rather than replying to the specific areas above:

 I found the documentation to be generally clear and quite useful. 

 The slope should not be described as “...in the form of a first-order sink representation.”

 The phrase “...flows of formaldehyde mass into and out of a fixed volume must be equal...” is 

not correct. In fact, equation 3.1 contradicts that.

 While the term “half-life” has a certain value because of its understandability, the normal 

way to determine it is from the decay constant.

 One area that could be significantly improved is the mathematical introduction to the model.

EPA has revised the above-referenced text to include more clarity and transparency in the 

revised model/documentation, except for the statement relating to the first-order sink 

representation, which is no longer applicable because sinks have been removed from the model.
APPENDIX A – DETAILED REVIEWER COMMENTS AND

EPA RESPONSES: Part 1 – Algorithms and Exposure Assumptions
A-1

2.1.1 Comment on the Adequacy of the Mathematical Equations and the Level of 

Confidence in Indoor Air Concentrations and Exposure Estimates Provided by the Model

Reviewer / Comment Response

Zhang – The mathematical equations are adequately explained except that 

there appears to be some confusion between diffusion and convective mass 

transfer. The model is based on convective mass transfer over the surface, 

which is not based on the Fick’s Law of diffusion. The air velocity over the 

surface would have significant effect on the “mass transfer coefficient”, and 

hence the model parameter, “slope”. The values of the “slope” appear to be 

small (maximum of 3.0 in the case of painted dry wall). This is likely that the 

most data are from small scale chamber tests where air velocity over the 

material surface is small compared to the typical airflow conditions in 

ventilated rooms. Therefore, the model may underestimate the emission rates 

if the emission is indeed controlled by the convective mass transfer process. 

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – However, it is more likely that except for the initial emission period, 

the emission rate will be controlled by the internal diffusion process as 

suggested by many studies including those cited in the report for pressed 

wood products. So the “slope” values derived from the measured data may 

have included the combined effects of convective mass transfer and internal 

diffusion resistances, resulting reasonable predictions. At the “steady state” 

(about 30 days from initial chamber test), the internal diffusion resistance 

should be the dominant resistance for formaldehyde emissions even for 

particleboard. To understand and proof this point, a more detailed numerical 

simulation can be made for homogeneous sources as assumed in the study 

(i.e., there were free formaldehyde uniformly distributed in the material).

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – After establishing the steady state initial concentration based on 

Mathew’s model, the concentration in a zone that has a higher concentration 

is assumed to decay exponentially over time, given a half life time as the 

input parameter. This approach is unconventional, but practical considering 

the many uncertainties involved in real situation. The challenge would be to 

choose the right the half-life time.

No response/action 

required

Zhang – Formaldehyde may not be distributed uniformly inside the material. 

They may be more concentrated in resin spots and less in the wood 

particles/chips. In addition, hydrolysis is possible for additional 

formaldehyde generation within the material, resulting longer and higher 

emission rates in the long term. This is still a research topic and no reliable 

model has been developed yet. Therefore, I support the idea of relying on 

measured field data to estimate the decay of formaldehyde concentration, 

and be conservative in the estimation, as intended by the model.

No response/action 

required

Zhang – Based on the data provided and the assessment analysis, the model 

should provide reasonable conservative estimate of the formaldehyde 

exposure given the relatively long half-life time.

No response/action 

required
A-2

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

Jayjock – Regarding the appropriateness of the algorithms and assumptions, 

Matthew’s basic approach appears to be based on solid first principles of 

conservation of mass and the validity of diffusion via Fick’s Law. Equation 

3-2 appears to be relatively simple, self-evident and quite appropriate.

No response/action 

required

Jayjock – Similarly, equation 3-5 forms the basis for the emission rate 

modeling engine. It is well explained and appears to me to be a reasonable 

approximation of reality. This equation is in complete agreement with 

previous work that I have done on “backpressure” modeling.

No response/action 

required

Jayjock – Equation 3-5 is essentially identical 

to an algorithm that I have often used for 

backpressure modeling (shown to the right):

Csat = the saturation concentration (mg/m

3

) of 

formaldehyde in the air over the emitter of 

interest. That is the concentration that would 

be expected in a volume (with high surface 

area/volume ratio) in which the emitting walls 

were made up entirely of the emitter of interest and there is zero ventilation.

Note the equivalence of this last equation to 3-5 above

and the identity of m (shown t the right):

Clearly, the relatively straightforward analytical 

measurement of Csat could provide a potentially valuable 

extended methodology for the evaluation and development of the model’s 

parameters relative to specific emitters.

EPA agrees that Csat is a 

useful interpretation aid, 

but measurements such as 

those suggested here are

outside of the scope of 

this project; the 

suggestion as been noted 

in Section 3 of the model 

documentation

Jayjock – I believe that the treatment of indoor sinks as developed by and 

explained in the model represent a real step forward in indoor modeling 

science. I would suggest that the author of the documentation in this area 

expand the explanation to include some example data and treatment that 

illustrates the estimation of parameters for the Langmuir model. That is, the 

last paragraph on page 3-7 supplies the estimated values but my sense is that 

the explanation would be significantly more lucid if the actual procedure and 

values used in the fitting exercise were demonstrated and included.

After consideration of all 

peer review comments 

received, indoor sinks 

have been removed from 

the model because of 

modeling uncertainties 

(see Section 2.1.1)

Jayjock – Regarding the level of confidence one might have in the model’s 

prediction of actual indoor concentration and exposure levels, my sense is 

that they should be considered accurate within a factor of 2.

No response/action 

required

Jayjock – Relative to the estimation of exposure there appears to be logical 

error in the defaults. When one invokes the Exposure Defaults within the 

model, the 5 age groups for children in the model all show 10 years for the 

exposure period. Of course, this is impossible for any of the age groups for 

children which are all less than 10 years in length. This is explained in the 

documentation and in the online help button; however, I found it somewhat 

confusing. My suggestion is that the default shown in the program for any 

age group should be the maximum time a person can spend in that age group.

The model now has six 

default exposure groups 

covering different age 

ranges, and the model 

results include year-byyear ADC calculations 

for each group

Jayjock – It is not difficult to imagine relatively high doses of CH2O 

potentially causing biological damage over a period of a few days. As such, 

the estimation exposure in the Results Screen of Average Daily 

Concentration (ADC) and Average Daily Dose (ADD) could be of particular 

significance to the toxicologist in assessing the overall risk from exposure to 

indoor formaldehyde. Depending on the specific details of the toxicokinetics 

The model now has six 

default exposure groups 

covering different age 

ranges, and the model 

results include year-byyear ADC calculations

max

max

max

( )

:

(1 )

C G

C

G

G

rearranged

C

C

G G

ss

sat

ss

sat

ss

ss

  

 

Csat

G

m

max


A-3

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

of formaldehyde the toxicologist may be interested in understanding the 

exposure to humans on the worst (first) day of exposure. The program 

allowed me to put in a single day (0.00274 years) for the duration of the 

exposure but the results indicate that a calculation error occurs for this input.

The calculated ADC is above the initial concentration and the time 

percentages come out to be >100%. If this error is not easy or practical to fix 

then an explanation should be included that advises the minimum time for 

duration of exposure.

Jayjock – Relative to alternate calculations methodologies it is suggested that 

the modelers consider a modeling program based on The Advanced 

Continuous Simulation Language (ACSL) computer language. ACSL easily 

handles simultaneous differential equations while providing parameter 

estimates from data via optimization and an intuitive graphical interface.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Georgopoulos – The mathematical/algorithmic formulation and the 

computational implementation of the model under review are adequately 

explained, with sufficient clarity and level of detail; the computational 

implementation of the model also appears robust and accurate. Nevertheless, 

given the simplifying assumptions (discussed further in the following 

paragraphs) in the formulation of the model, and the significant variabilities 

inherent in the data sets that were used to derive parameter values for FIAMpwp, the level of confidence in the indoor air concentration and exposure 

estimates provided by the model, should in general be considered, at most, 

medium. In this reviewer’s opinion, the uncertainty and variability levels in 

the available data strongly suggest the need for a distributional/probabilistic 

model formulation that explicitly employs available data to develop ranges 

rather than point estimates of concentrations and exposures.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Georgopoulos – Considering the (current) model formulation, FIAM-pwp 

essentially consists of three components, i.e. (a) estimation of formaldehyde 

emission rates from various types of pressed wood products (based on data 

from various field and chamber studies), (b) estimation of indoor 

mixing/dispersion through simple one-zone and two-zone models that 

incorporate sink and back-pressure effects, and (c) simple inhalation 

exposure calculations that employ average intakes for adults and of children 

in different age groups. In this reviewer’s opinion, a major drawback in the 

formulation of the FIAM-pwp is that components (a) and (b) are not 

“separated” in a way that would allow, in a straightforward manner, usage of 

the formaldehyde emission rates with other available indoor air quality and 

inhalation exposure models. The value of the information that has been 

incorporated in FIAM-pwp would be greatly enhanced if its formulation 

explicitly considered decaying (e.g. exponentially decreasing) emission rates 

and provided appropriate formulas for implementing them (either within 

FIAM-pwp or in conjunction with other, more comprehensive, indoor air 

quality and inhalation exposure systems). Simple implementations of 

exponentially decaying source with one-zone and two-zone indoor air 

models are well established.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments
A-4

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

Unice – In summary, I concur with the calculation of initial concentration 

(high confidence in result with reliable chamber test data), however, the 

treatment of sinks and average daily concentration for chronic exposure 

appear to require some revisions to improve the quality of the result (low 

confidence in result). I believe the usefulness of the model could be 

strengthened by discussing the incremental contribution of added products 

because regulations, if any, are likely to be developed on a product-specific 

basis (e.g.. MDF vs. industrial panel). Currently, the model documentation 

and model results emphasize the aggregate contribution of all products (and 

background sources), and running the model with and without a product of 

interest can produce counterintuitive results as described below. Finally, the 

relationship between the HBF and Matthews model should be made clear in 

the model documentation. Please see my detailed comments below.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – The model documentation should focus on both the determination of 

incremental exposure attributable to the introduction of a specific product 

and the total exposure for loaded home. Some users will be interested in 

understanding the total formaldehyde concentration, but other users, such as 

trade associations, importers and manufactures, will be most interested in 

understanding the potential risk associated with a specific product or 

collection of products.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – Currently, the model does not provide a framework for answering 

questions about the effect of the introduction of new product into a preexisting home, particularly for chronic exposure. I am concerned that the 

model does not provide guidance on the specific testing (chamber or 

alternative testing) that a manufacturer or other interested parties should 

perform to refine the estimate of chronic exposure.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – It may be prudent for EPA to consider a technical workshop prior to 

finalizing the model with the purpose of developing consensus 

recommendations for the model documentation regarding a specific set of 

tests that could be performed to determine product specific (e.g. MDF, wallpaneling, industrial panel, etc.) ADC, LADC ADD, and LADD. In

conjunction with this, a strategy could be developed for framing LADC and 

LADD in a manner that is consistent with product specific exposures rather 

than overall residential half-life.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – One of the challenging aspects of reviewing this model and 

providing cohesive recommendations is that the current assessment of acute 

and chronic exposure is associated with different uncertainties and 

conceptual models. In the acute setting, the back-pressure effect may 

attenuate the contribution of a new product such that the total acute indoor 

exposure concentration is not affected by the added product. However, in a 

chronic, or long term, timeframe (e.g. 10 to 20 years), it is reasonable to 

expect that the majority of available formaldehyde and formaldehyde 

precursors will ultimately partition from the product into the indoor space. 

My recommendation is that a conceptual framework be developed in the 

model documentation to ensure that users take these factors into 

consideration when investigating acute versus chronic timeframes. For the 

acute timeframe, it is important to consider the total collection of assembled 

indoor products, or alternatively, the background concentration attributable 

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text; some 

references also have been 

added
A-5

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

to the existing products. For chronic exposure, a more accurate 

characterization of dose attributable to a single product is likely obtained by 

considering the decay profile of that product that would be true in the 

absence of other products. The reason for this is that the attenuation that is 

observed due to the backpressure effect is temporary, and eventually, a 

source that is initially attenuated will begin to emit formaldehyde as the 

overall indoor air concentration level decreases. The model documentation 

should refer to the types of decay studies that could be done to understand 

the long term formaldehyde dose attributable to a single product or collection 

of related products. Examples of these studies include (Zinn et al. (1990), 

Mølhave et al. (1995), Myer and Hermanns 1985 and Brown 1999). 

Unice – For acute exposure, the most straightforward approach is to 

determine a baseline background indoor air concentration and determine 

whether the introduction of a specific product (e.g. wall paneling at a 

specified loading rate) or collection of products (e.g. kitchen cabinets) will 

increase the indoor air concentration to a level exceeding a threshold of 

concern. Under this approach, an option would be provided to the user to 

identify the ‘baseline products’ (or ‘indoor background from existing 

products’) and ‘added products’. In the model, output exposure and product 

specific emission rates would be presented with and without the added 

products. I believe this would greatly assist users of the model in intuitively 

understanding the backpressure effect, as well as assist specific 

manufacturers, regulators and other interested parties in understanding the 

incremental effect of specific products on indoor air levels. In the absence of 

this modification, the user could simply run the model with and without the 

source in question and simply compare the result and this approach was used 

in the model documentation in the evaluation of the addition of sinks. 

However, I believe that inclusion of this feature explicitly would greatly 

assist users unfamiliar with mass transfer theory and the interaction between 

formaldehyde sources. This feature would be particularly useful if the model 

were configured to output the emission rate and percent contribution to total 

emissions of the ‘baseline’ sources with and without the ‘added’ source.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text, including 

an example application

Unice – In my view, one of the urgent federal policy questions regarding 

pressed wood products is whether the absence of federal regulations with 

performance based standards (with the exception of manufactured homes) 

has resulted an increased prevalence of imported products failing to meet the 

established voluntary industry standards or with potential emission rates 

higher than domestic production. I believe the slight reconfiguration of the 

model I am recommending would be extremely useful for explicitly 

addressing questions of this nature.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text, including 

an example application

Unice – In contrast to the current model guidance and my recommendation 

for acute exposure, I believe the incremental chronic exposure attributable to 

a specific product (e.g., MDF panels) or defined collection of similar 

products (e.g., kitchen cabinets) should be assessed, rather than the aggregate 

cumulative exposure for all indoor products (i.e., the cross-sectional 

approach). Because the typical situation will reflect products of various ages, 

the chronic dose attributable to a specific product seems to be a more 

relevant metric than the total indoor concentration over time assuming an 

initial loading.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text
A-6

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

Unice – Evaluation of the decay curve for a single product or class of 

products in isolation is likely to be more useful for understanding chronic 

risk and informing policy decisions about managing chronic risk than the 

suggested cross-sectional decay half-life approach. In addition, new 

construction represents a minor fraction of total residences, so the case where 

a new home is loaded with new products is the exception rather than the 

typical case for most of the population. In short, the emissions half-life is 

currently considered a ‘house characteristic’, whereas I believe it would be 

more appropriate to frame this in terms of a ‘product-specific’ characteristic.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – Because the majority of the available reservoir is likely to be 

depleted over the lifetime of the product, incremental risk from separate 

products or collection of products can be added to derive total risk or an 

acceptable risk range by type of product (e.g., MDF) could be established. 

This incremental approach may slightly over-estimate true risk depending on 

the assumed tenure of the occupants, but this can be somewhat accounted for 

by assuming a finite emission lifetime of an individual product (e.g., 10 

years) or by selecting a target background indoor concentration higher than 

ambient air levels corresponding to the decay concentration where emissions 

from the source under consideration would be considered de minimus.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – Under the current model guidance, the true increased cancer risk 

attributable to installation of a specific product such as wall-paneling in an 

existing home to repair damaged plaster or kitchen cabinets installed during 

remodeling would not be reliably determined. Initially, the backpressure 

effect will suppress a portion of the formaldehyde emissions, such that if the 

user were to compare cumulative dose with and without the product based on 

current model guidance, the lifetime dose for that product will be 

underestimated. If the equilibrium concentration (i.e., the theoretical 

concentration in the absence of ventilation) of a product such as kitchen 

cabinets is less than the pre-existing steady state concentration, running the 

model with and without kitchen cabinets would result in the physically 

impossible conclusion of negative incremental cancer risk because the 

cabinets would initially act as a net sink.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – The assessment should be based on a conceptual model for decay in 

formaldehyde emissions over time and basic research on the decay profile of 

individual products should be encouraged in the model documentation 

(rather than exclusively relying on the historical data from the 1980’s). At a 

minimum the model should be revised to allow consideration of periods of 

rapid and slow decay in the emission rate as observed, for example, in Zinn 

et al. 1990, Mølhave et al. 1995, Myer and Hermanns 1985 and Brown 1999.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – One conceptual model for assessing chronic exposure for 

formaldehyde releasing products was published by Mølhave et al. (1995). In

this study, a selection of furniture and fitments supplied by a retailer was 

placed into a large scale chamber and measurements were made over an 8-

week period. Conceptually, three exponential decay compartments were 

attributed to three potential sources of formaldehyde emissions, including:

• free formaldehyde (short-term compartment);

• decay of paraformaldehyde and other complex structures (medium-term 

compartment); and

• hydrolysis of the resin (long-term compartment).

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text, as well 

as discussion regarding 

how the model could be 

run with “fast” and 

“slow” decay periods
A-7

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

Using an exponential peeling procedure, the authors determined relative 

sources strengths and half-lives for each of these three compartments. A twocompartment (slow and fast decay) analysis using an EPA model also 

adequately represented the transient data. The corresponding accumulated 

dose was compared to the California Proposition 65 safe harbor level to 

determine the incremental contribution of a specific set of products (i.e.,

living room furnished with IKEA furniture). This product-specific testing

may be helpful in refining the estimate of LADD and LADC and the 

approach to understanding whether compliance with Proposition 65 was 

achieved was reasonable. I strongly recommend that the model 

documentation be modified to make clear how this type of testing could be

incorporated into the model. Currently, the half-life parameter pertains to a 

structure-based decay rate rather than a product-based decay rate.

Unice – For the default case, the decay rate presented in Zinn et al. (1990) 

seems more appropriate than the 2.92 year half-life derived from crosssectional sectional data in the original Versar model. Zinn et al. (1990) 

estimated an apparent half-life of 216 days using approximately one year of 

large chamber data for particle board and a natural logarithm linear 

regression statistical analysis. The half-life reported in Zinn et al. (1990) is 

not directly comparable to that adopted in the original and current Versar 

model because Zinn et al. used the relationship HCHO = F + G x ln(time) 

with regression constants F and G whereas the EPA half-life was based on a 

traditional exponential decay model. In lieu of the original logarithmic 

equation, the more generic two-compartment (i.e., fast and slow decay) 

exponential decay model could be fit the Zinn et al. data.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text, as well 

as an discussion regarding

how the model could be 

run with “fast” and 

“slow” decay periods

Unice – Finally, one concern is the interaction between ventilation rate and 

decay rate. Given long-term testing data, the total reservoir of formaldehyde 

and formaldehyde precursors for a given ventilation condition could be 

calculated. Therefore, in the future when decay characteristics are better 

understood, it should be possible to adjust for ventilation rate and account for 

total mass of formaldehyde generated over the lifetime of the product.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – The original Versar model presented the linear regression 

parameters both the Matthew’ model and the Hoetjer model (identified as 

HBF model in 1986 Versar model), as well as the steady equations for both 

models. These two models are both mass transfer based approaches, 

however, the mass transfer coefficient obtained by linear regression is not 

identical. This can be seen in the documentation for the 1986 Versar model, 

where the correlation coefficient is typically higher for the Hoetjer than for 

the Matthews model. Because slightly different results are obtained 

depending on the regression model used and the Hoetjer model is widely 

cited in the literature, it is recommended that the model documentation be 

revised to include discussion of both approaches.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – The regression model for the HBF mass transfer relationship is 

given in Myers and Nagaoka (1981). The linear regression models are:

where: 

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text
A-8

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

K = -m = mass transfer coefficient (m/h)

CS = steady state concentration (mg/m3)

N = air change rate (1/hr)

L = loading rate (m2/m3)

Ceq = concentration at zero ventilation rate (N=0)

b = emission rate at zero formaldehyde concentration

The simplified steady state models are:

It can be shown these are identical models 

and that Ceq= b/-m. Therefore, the difference 

in models is in the regression equation used 

to derive the parameters, not the theoretical construct. The equilibrium 

concentration is the vapor phase concentration at the surface of the board and 

the bulk phase concentration that would be true in the absence of ventilation. 

The Matthews parameter b is a combination of the mass transfer coefficient 

K and equilibrium concentration Ceq. The mass transfer coefficient is 

determined by the properties of the board or object surface, whereas the 

equilibrium concentration is determined by the formaldehyde generating 

properties of the panel and varies with temperature, humidity and age (Berge 

et al. 1980). These concepts are less transparently elucidated by the 

Matthews emissions equation. The easiest way to incorporate a discussion of 

the two approaches would be to show how m, b, Ceq and K are calculated 

for the kitchen cabinet data presented in Koontz et al. (1996) using both the 

HBF and Matthews regression equations (in an Appendix). The purpose of 

this appendix would be to provide clear guidance on the type of chamber 

tests (i.e. at least three N or L combinations for the linear regression) that 

would be needed to determine the parameters that are input into the model.

Unice – My preference would be to allow input the parameters from either 

approach as was the case in the original model. Although the mass transfer 

basis is identical for each model, the parameters of the Hoetjer model are 

more intuitive. For example, based on the equilibrium concentration, it is 

easy to see that the worst case concentration in the absence of ventilation 

(N=0) is 0.19 mg/m3. It is easy to conceptualize that this is approximately 

the level that would be expected in a confined space like the inside of a 

closed kitchen cabinet. The Matthews model parameters are less intuitive. 

The maximum emission rate which can be derived directly from b is only 

true in the absence of formaldehyde in the bulk air, or at the beginning of the 

exposure period. However, the Ceq of the HBF model can be easily 

calculated. The concentration in the absence of ventilation (and background 

sources) is b/-m or Ceq= b/-m.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – Having made the recommendation to allow for both approaches, I 

recognize that is likely not possible due to time and financial constraints –

and given the essential identical model construct perhaps of minimal benefit. 

My recommendation if only the Matthews model is retained in the model is 

to provide an example calculation in the model documentation showing how 

the user can start with the HBF linear regression or the HBF model 

parameters and convert the parameters to the equivalent Matthews model, or 

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text
A-9

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

Unice – The caveat should be included that this conversion does not account 

for the slightly different result that is obtained using the HBF versus the 

Matthews linear regression models. Another option would be to provide the 

HBF equilibrium concentration as supplemental output. Knowledge about 

the equilibrium concentration allows an intuitive determination about 

whether a particular product is likely to act as a sink or a source based on an 

estimate of the anticipated indoor air concentration. In conjunction with this 

conversion equation, a statement should be included indicating that for 

plausible ranges of the N/L ratio, selection of a particular linear regression 

model is not expected to appreciably affect the results. In conjunction with 

this conversion equation, a statement should be included indicating that for 

plausible ranges of the N/L ratio, the selection of a particular linear 

regression model is not expected to appreciably affect the results.

EPA has added the 

equilibrium concentration 

as an intermediate output 

(on the Source Screen) 

and has revised the 

documentation and/or 

help screen text

Unice – The model used to represent sinks accounts for reversible sinks. 

Prior research using unpainted drywall showed that this sink caused only a 

minor permanent loss of formaldehyde and that the wallboard serves as a 

buffer to sudden changes in indoor air concentrations (Matthews et al, 1987). 

However, the treatment of indoor sinks in the current EPA model is 

problematic because absorption and desorption of formaldehyde from indoor 

sinks is a transient process over relatively short time scales not well suited to 

a steady-state or pseudo steady state model construct.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties (see Section 

2.1.1)

Unice – It appears that the guidance for the sink intercept given on EPA 

model guidance page 3-7 was developed by setting intercept b = ka x 

Cindoor assuming dM/dt = 0. When there is a linear relationship between 

accumulated mass and indoor air concentration at equilibrium conditions, 

dM/dt must equal zero. Therefore, deviations of the intercept from the 

calculated value would imply a sustained loss of equilibrium, violating the 

assumption used in assigning the sink slope and intercept in the Mathew’s 

equation. Furthermore, the guidance provided for selection of the 

equilibrium intercept for indoor sinks appears to treat this parameter as 

calibration or adjustment factor rather than as a predictive input.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties

Unice – For wallboard, the surface area is high in magnitude, therefore minor 

arbitrary decreases in the intercept from the values recommend in the model 

documentation will yield a noticeable apparent sink effect. However, there 

does not appear to be justification for assigning these deviations.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties

Unice – The short term data from Loading 1 at the test house clearly show 

that the initial sink effect from wallboard can be appreciable. To correctly 

account for net adsorption on these short time scales within the framework of 

a pseudo steady state model, a non-zero value of dM/dt must be calculated 

using Equation 3-13 and set equal to the Matthews emission rate equation to 

derive the intercept. This value of dM/dt will vary as a function of time. Such 

an approach would appreciably increase the level of complexity needed to 

assign the sink term because indoor concentration and mass storage vary as a 

function of time. If an estimate of the indoor concentration profile is 

available as a function of time (e.g. for the test home), it should be possible 

to determine M(t) and dM(t)/dt as a function of time by numerical integration 

or finite difference model. However, at 30 days, dM/dt may be sufficiently 

small that a reasonable default recommendation in the model would be to 

neglect the effect of sinks.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties
A-10

2.1.1 Comment on the Adequacy of the Mathematical Equations ... (continued)

Reviewer / Comment Response

Unice – The treatment of sinks in the EPA formaldehyde model reflects a 

reduction in sophistication from that which is available in the prior 

EPA/Versar MCCEM model.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties

Unice – As shown in Table 2, the presence of the carpet sink has a de 

minimus effect on the net emission rate in Zone 1 given a source generating 

a steady state formaldehyde concentration of 10 ppb. Approximately 7 or 8 

hours after introduction of the source, the rate of increase in storage in the 

sink is negligible and therefore there is no net effect attributable to the 

presence of this sink.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties

Unice – From this example, it should clear that when the Matthews slope and 

intercept are assigned in accordance with the proposed guidance, dM/dt is 

zero and there should be no net effect of the sink on the emission rate. Any 

apparent effect is caused by minor deviations in the intercept or rounding 

errors, which do not appear to have a theoretical basis.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties

Unice – Some pressed wood formaldehyde “emitters” could theoretically act 

as sinks under certain conditions based on the properties of wood and the 

Matthews model (e.g. products with very low equilibrium concentration or, 

equivalently, low Matthews model intercept). For example, low emitting 

products with unreacted urea can act as scavengers (Meyer and Hermanns 

1985). This could potentially be an interesting question for situations like 

manufactured home where a product with excessive free formaldehyde levels 

could ‘contaminate’ otherwise low emitting products. Therefore, there may 

be some usefulness in presenting the emission rates by product in the results, 

with negative emission rate indicating a net sink effect. Because a steady 

state model is used, one concern about implicit sinks, especially as more low 

emitting products become available on the market, is that it is difficult to 

account for whether or not the storage capacity of the implicit sink has been 

exceeded over the time frame of interest. This could be a minor potential 

concern in some situations, such as when few high emitting products are 

paired with several very low emitting products. Presentation of the emission 

rate as output would provide a starting point for understanding whether 

implicit sinks are present.

EPA has added the 

equilibrium concentration 

as an intermediate output 

(on the Source Screen) 

and has revised the 

documentation and/or 

help screen text

Unice – To provide an extreme example, the Mathew’s mass transfer 

parameters for kitchen cabinets were paired with a range of assumed 

background concentrations. This example illustrates two key points. First, 

the use of the HBF model equilibrium concentration provides a more 

intuitive way to anticipate the results than the Matthews intercept b. Second, 

the output in the model of the emission rate attributed to each of the 

individual components models could potentially provide useful diagnostic 

information to the user or modeler. As wood panel resin technologies 

improve and more ‘green resins’ become available, the potential exists for 

low observed equilibrium concentrations. Depending on the properties of the 

panel, these products may or may not act as reversible and/or irreversible 

sinks. I believe the model should provide sufficient ancillary information to 

understand the sensitive parameters and identify other potential areas for 

testing.

EPA has added the 

equilibrium concentration 

as an intermediate output 

(on the Source Screen) 

and has revised the 

documentation and/or 

help screen text
A-11

2.1.2 Comment on the Default Scenarios and Assumptions Used by the Model

Reviewer / Comment Response

Jayjock – My suggestion is that the modelers should consider using and 

including the seminal study by Murray and Burmaster as an important 

reference for potential use with this model (ref: Murray and Burmaster: 

Residential Air Exchange Rates in the United States: Empirical and 

Estimated Parametric Distributions by Season and Climatic Region, Risk 

Analysis, Vol 15, No. 4, 1995). Of particular value are data from different 

geographical sectors and seasons.

EPA has added the 

suggested reference to the 

revised documentation 

Unice – It is not clear how the manufacturer of a specific product (e.g. MDF 

panel) could do a study with their specific product to refine the crosssectional residential half-life. Therefore, there is not clear guidance regarding 

how to assess the degree to which some parameters are conservative. I am

concerned that too much emphasis is placed on the large database of 

historical emissions parameters provided in the model (Table 2-2) and 

historical decay rate evaluations (Section 3.6). I believe the model 

formulation and model documentation should be more forward looking in 

two respects. First, it should be made clear that many of the default 

emissions parameters in the model are based on historical testing and may or 

may not be representative of current products. Second, clear guidance should 

be provided to end users (e.g. importers, manufacturers, non-profit 

organizations or regulators) about the data and steps required to assess the 

HCHO incremental exposure attributable to the introduction of a specific 

product into the home.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text, keeping 

in mind that incremental 

exposure is not the main 

intent of the model
A-12

2.1.3 Comment on the Data Sources

Reviewer / Comment Response

Jayjock – Note the above comment on residential air exchange rate and a 

referenced source of good information on this important parameter. I also do 

not know of other data sources on the emission of formaldehyde from wood.

EPA has added the 

suggested reference to the 

revised documentation

Georgopoulos – The data sources for FIAM-pwp appear reasonable. 

However, since most of them appear be to rather old, one may question

whether they are still the most relevant data sources in relation to currently 

available/used pressed wood products (given changes in manufacturing 

procedures as well as in changes in relative levels of domestically 

manufactured versus imported pressed wood products). The answer may in 

fact be yes, but it should be explicitly stated and supported.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Unice – It is unknown the degree to which the data in Table 2-2 reflect 

manufacturing practices in 2009, and the potential for low or high bias exists. 

Users of the model should be encouraged to collect and use data for products 

currently in the supply chain. This could be accomplished by deemphasizing 

the data in Table 2-2 (perhaps by moving to Appendix), by requesting 

voluntary submission of chamber data from industry prior to release of the 

model, or by having a mechanism whereby the database in Table 2-2 is 

routinely updated as new information becomes available.

EPA has moved Table 2-2 

to an appendix and has 

added clarifying text to 

the revised documentation 

and/or help screen text

Unice – Airflow between the basement and outside was low, which may or 

may not be representative for other structures. For example, if a lower 

efficiency furnace is located in the basement, make-up air for combustion 

could induce substantial airflow in the winter. To the casual user, it will not 

be immediately obvious that Conventional 1 represents the data for one test 

house under limited conditions. Therefore, I recommend that this home be 

explicitly labeled in the model as the pilot home to indicate that this data is 

for the home from the EPA pilot study.

EPA has modified the 

house selections used in 

the model

Unice – More details regarding the analysis of PFT data used to derive the 

airflow rates Conventional 2 should be provided.

EPA has modified the 

house selections used in 

the model

Unice – The use of Langmuir isotherm for wallboard should be further 

explained, as this was found to not be valid in the original report for the pilot 

study (Koontz et al. 1996).

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties (see Section 

2.1.1)
A-13

2.1.4 Comment on Whether the Data Sources Provide Unused Information That Would 

Assist in Quantifying Uncertainty or Variability for Subsequent Model Use

Reviewer / Comment Response

Zhang – Although the data sources are incomplete for quantifying the 

uncertainty, it is possible to conduct an uncertainty analysis using error 

propagation theory with assumptions on the uncertainties in the raw data.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Jayjock – The Murray/Burmaster database on air exchange rates in U.S. 

residences could assist in gauging the variability of this parameter. Also, I 

believe the current data used in the model on emission sources discloses a 

level of uncertainty and variability that could also be mined by a person who 

is knowledgeable about these data and their origins.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Georgopoulos – Yes: the variability in the available data could be used to 

derive distributional estimates of emission rates that would support 

probabilistic (and population based) indoor concentration and corresponding 

exposure estimates with explicit characterization of uncertainties and 

variabilities.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – The approach to wallboard sinks used in the current EPA model 

should be consistent with the wallboard sink equations and data presented in 

Matthews et al. (1987). The use of Langmuir isotherm should be further 

explained, as this was found to not be valid in the original report (Koontz et 

al. 1996).

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties (see Section 

2.1.1)

Unice – The uncertainty and sensitivity analysis presented in Matthews et al. 

1985 could be discussed.

The Matthews analysis 

has been added to an 

appendix in the revised 

documentation
A-14

2.1.5 Comment on Whether the Model Input Variables and the Estimates Assigned to 

Them Appropriately Reflect Variability

Reviewer / Comment Response

Jayjock – A user could choose to use inputted highest emission values (no 

sinks) which maximize the concentration and exposure within the range of 

his or her knowledge of the scenario to obtain a high end prediction. This 

could be followed by a run in which he or she would use inputs that were on 

the low end of the range (and include sinks) that again reflected the 

modeler’s knowledge or confidence in the situation. The comparison of 

outputs would provide some quantitative level of variability extant within the 

system.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Jayjock – Another possibility would be to take the data in Table 2-2 and 

determine which data sets might be reasonably combined as presumably 

coming from the same population. Parameters from these data sets could be 

analyzed for the statistical variability and the parameters of the emission 

equations could be represented as best-fit probabilistic distribution functions.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Georgopoulos – As mentioned in the answers to previous questions, a 

distributional/probabilistic approach, for defining model input variables and 

deriving concentration and exposure estimates, would, in this reviewer’s 

opinion, be more appropriate in relation to this model.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – The single exponential equation for decay is consistent with the 

historical cross sectional residential data but inconsistent with studies that 

have investigated single products or collection of products over time (Zinn et 

al. 1990, Mølhave et al. 1995, Myer and Hermanns 1985 and Brown 1999). 

The true area under the curve for dose given a long enough exposure time is 

likely better represented by the two- or three- compartment exponential 

models than the single exponential model because these models better 

represent the decay in the reservoir of formaldehyde precursors in a specific 

product or collection of products.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text, as well 

as discussion regarding

how the model could be 

run with “fast” and 

“slow” decay periods

Unice – The mass transfer coefficient is not routinely measured in most 

chamber testing. EPA should perform a sensitivity analysis to determine 

whether measurement of the mass transfer coefficient is a data need or data 

gap when newly acquired chamber test data is entered into the model.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – The model documentation does not provide a credible method for 

selecting the indoor sink parameters.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties (see Section 

2.1.1)

Unice – The model screen and documentation should make clear the 

Conventional 1 represents a single unoccupied test home. The analysis used 

to derive Conventional 2 should be described.

EPA has modified the 

house selections used in 

the model

Unice – The degree to which the historical data in Table 2-2 reflects 

domestic and imported production is not known.

EPA has moved Table 2-2 

to an appendix and has 

added clarifying text to 

the revised documentation 

and/or help screen text
A-15

2.1.6 Comment on the Adequacy of the Documentation Supporting the Overall Plausibility 

of the Exposure Estimates Generated by the Exposure Model

Reviewer / Comment Response

Jayjock – My sense is that the estimates have been clearly shown to be 

plausible relative to results of comparing the modeled estimates to the 

measured data in the Pilot House study. Even through all of the practical 

difficulties, which were very well explained in the documentation, this study 

appears to provide adequate evidence that the model is certainly capable of 

providing credible estimates.

No response/action 

required

Jayjock – Perhaps a more practical test of the model’s accuracy and 

reliability might come from its comparison to measured data in some 

realistic scenarios as viewed and implemented by or from the perspective of 

a non-expert user.

EPA has added this type 

of comparison to the 

evaluation of model 

results in Section 4.0

Georgopoulos – Ideally, the exposure estimates that are derived from FIAMpwp should be characterized within a total exposure context that would take 

into account influences of numerous other indoor and outdoor sources and 

sinks as well as of indoor and outdoor gas and heterogeneous phase 

chemistry of formaldehyde (that may result in net production or removal in a 

given microenvironment).

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Georgopoulos – Comparison of model estimates with relevant observational 

data sets for indoor-outdoor relationships and of personal level (e.g. from 

studies such as RIOPA – see, e.g., the Liu et al., 2006, reference in the 

Addendum of this review) would be valuable in assessing, at least 

qualitatively, the plausibility and relevance of the exposure estimates derived 

from FIAM-pwp.

EPA has added this type 

of comparison to the 

evaluation of model 

results in Section 4.0

Unice – The EPA pilot study, while very useful, is limited to one structure. I 

recommend pulling together a collection of representative baseline loadings 

and comparing the model results to published concentrations of 

formaldehyde in new and existing structures.

EPA has added this type 

of comparison to the 

evaluation of model 

results in Section 4.0

Unice – Average Daily Concentration/Dose EPA has not shown how use of 

an overall residential half-life is to be used to derive product-specific dose. 

Running the model with and without the product of interest will not result in 

meaningful conclusions as discussed previously. Accordingly, there is great 

uncertainty in product-specific doses under the current model framework.

EPA has added an 

example application and 

clarifying text to the 

revised documentation 

and/or help screen text

Unice – EPA has not considered whether the historical 2.92 year crosssectional half-life is representative of the decay profile of modern products 

or the potential variability in the formaldehyde reservoirs and decay between 

products.

EPA has revised the 

default value for the half 

life to 1.5 years and has 

added clarifying text to 

the revised documentation 

and/or help screen text
A-16

2.1.7 Provide Any Other Types of Data to Assess the Reliability of the Overall Exposure 

Model or Specific Model Elements

Reviewer / Comment Response

Jayjock – Please see the discussion in 1.1 above in which an additional 

critical emitter variable – Csat – is explicated using the basic emission rate 

algorithm: Eqn 3-5. Data on this variable could be directly measured in a 

straightforward manner. This could occur by first constructing a box of the 

emitting material with an internal volume sufficient to provide a good limit 

of analytical detection. The box should be allowed to equilibrate such that 

the internal concentration is no longer increasing. It is suggested that this 

would occur within a week but could be confirm experimentally. After this 

period, the exact internal air volume of the box should be sampled quickly. 

The measured concentration represents 69% of the amount of formaldehyde 

in the box if good mixing occurred during relatively rapid sampling. (Note, 

having a small fan in the box will hasten equilibrium and assure good mixing

during sampling.)

EPA agrees that Csat is a 

useful interpretation aid, 

but measurements such as 

those suggested here are

outside of the scope of 

this project

Georgopoulos – There are various studies and related data sets, both from the 

US and abroad, that have appeared in recent years and could - potentially -

be used for evaluation of specific FIAM-pwp elements. The Addendum to 

this review contains a selection of literature references pertaining to such 

studies and data sets.

EPA has added this type 

of comparison to the 

evaluation of model 

results in Section 4.0 ad 

selected references have 

been added

Unice – As mentioned previously, I strongly believe a consensus framework 

must be in place to assess product-specific long-term average exposure. I 

believe the current lumped, or whole-residence based, approach will create 

confusion and greatly complicate risk management decisions if the end point 

of concern is based on chronic exposure.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments
A-17

2.1.8 General Comments on Algorithms and Exposure Assumptions

Reviewer / Comment Response

Tucker – Seems like a convoluted way to model emissions: The dependent 

variable of primary interest – the HCHO concentration – is inherent in the in 

the prediction algorithm; i.e., it is also used as an independent variable.

Nearly all the data appear to have been taken under conditions where the 

HCHO concentration was well above 0.01 ppm, which value is suggested by 

the model as a concentration of concern. So the practical value of the slope 

in the emission rate equation is not evident unless one is going to model 

situations that are clearly unhealthy. Why not just use the intercept values 

and simplify the model?

EPA has carefully 

considered this 

comment but has not 

modified the emissions 

algorithms

Tucker – That said, the analysis provided in section 5.3 of the documentation 

suggests that whatever the convolutions involved, the model gives 

respectable predictions. My reading of the tables in that section, which 

summarize modeling results compared to measured values in the EPA pilot 

study house, is that the model ranged from about 24% low to about 65% 

high. Predictions within that range are quite good, given the uncertainties in 

input data and unavoidable modeling assumptions.

No response/action 

required

Tucker – Regarding the selection of a decay constant (or half-time if you 

prefer) for predicting the rate at which the HCHO concentration will 

decrease, I suspect the default choice of k = 0.23 yr-1 (or t1/2 = 3 years) 

might be improved by having different values for different time periods. For 

example, the k value for the period 0-3 months may be higher than the one 

for 3-12 months, which may be higher than for 12-24 months, etc.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text, as well 

discussion regarding how 

the model could be run 

with “fast” and “slow” 

decay periods

Tucker – One significant shortcoming of this model, and many indoor 

models that assume complete mixing within compartments, is that it doesn’t 

deal with what I refer to as “intimate” sources. Examples are toys, cribs, 

bedding materials, clothing, and flooring materials for children and bedding 

and clothing for elderly people. I think the most straightforward approach to 

such sources is to assume a smaller compartment or zone around such 

sources for the purpose of predicting concentrations.

The model can be 

configured to address 

intimate sources, by

using a “virtual air

space” around an 

individual exposed to 

an intimate source as 

one of the model zones
APPENDIX B – DETAILED REVIEWER COMMENTS AND 

EPA RESPONSES: Part 2 – User Interface
B-1

2.2.1 Provide Comments on the Ease of Use of the FIAM-PWP Model

Reviewer / Comment Response

Zhang – The interface design is easy to use in general. No response/action required

Zhang – The online version is very, very slow The new version of the 

model runs on-line under 

the IGEMs platform

Zhang – Berge and Myer - temperature effects. It is better to use one only or 

combine it to provide a default number since users do not know which one to 

use.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – It would be useful to provide some sort of guide for users to step 

through the modeling process (such as step 1, 2, 3, ....)

EPA has added example 

applications to the revised 

documentation

Jayjock – I found the user interface is remarkably clean and straightforward. No response/action 

required

Georgopoulos – Local use of the program in its current form would still 

require setting it up so it is accessed by the browser – this requires adjusting 

local computer settings in ways that may be challenging for some systems –

and users. Nevertheless, the web-based version is sufficiently “user-friendly” 

and the data entry screens are clear.

The new version of the 

model runs on-line under 

the IGEMs platform

Georgopoulos – Maybe some “warnings” regarding the level of uncertainties 

in the calculations could be incorporated, as reminders, in the data-entry 

screens.

EPA has added clarifying 

text to the introductory 

screen for the model

Unice – I chose to use the local version of the software which performed 

reasonably. I found the interface to be more cumbersome than similar models 

like MCCEM, IAWX, EFAST or WPEM, but this is mostly attributable to 

the use of a web-based platform and not a fault of the developer.

The new version of the 

model runs on-line under 

the IGEMs platform

Unice – Replacing this field (referring to air change rate) with plain text 

would make clear that this value cannot be edited.

This field has been 

“grayed out” to indicate 

that it cannot be directly 

edited

Unice – When a source is modified, the user should be required to rename 

the source and a separate library of user entered sources should be 

maintained. Currently, it is impossible to know whether someone overrode 

the default values or the values presented are the default values.

No longer applicable –

library of sources has 

been changed

Unice – Note also that the codes in the model do not match Table 2-2 in all 

instances.

The library of sources in 

the model has been 

changed

Unice – When a screen is printed in standard portrait view, the edge is cut 

off. It would extremely helpful to have a ‘Print All’ button that would 

generate a single page for printing with all the inputs and outputs suitable for 

printing in portrait view.

The new version of the 

model runs on-line under 

the IGEMs platform

Unice – Please consider writing a routine to allow the same information to be 

printed directly from the web-browser. (referring to Excel output)

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Unice – In some instances the next box is out of order. (referring to Tab key) The new version of the 

model runs on-line under 

the IGEMs platform
B-2

2.2.2 Comment on the Adequacy of the Model Help Screens

Reviewer / Comment Response

Jayjock – In general, I found the content of the help screen to be very good 

and, as expected, reflective of the documentation.

No response/action 

required

Jayjock – My suggestion is to increase the length of the lines in the help 

window to fill up the space and also increase the font size to use up this 

additional utilized area.

The new version of the 

model runs on-line under 

the IGEMs platform

Jayjock – I could not get the “audio help” to work. The new version of the 

model runs on-line under 

the IGEMs platform

Georgopoulos – The model-help screens appear generally adequate; they 

could be enhanced by including some statements regarding the level of 

precision expected for some of the entries.

EPA has added clarifying 

text to introductory 

screen for the model

Unice – The model help screens are intuitive and adequate. The audio 

prompts generally worked well, although some were missing failed to load.

The new version of the 

model runs on-line under 

the IGEMs platform

2.2.3 General Comments on User Interface

Reviewer / Comment Response

Tucker – I had little trouble with the user interface. No response/action 

required

Tucker – More careful attention to significant figures is needed. FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Tucker – On the Sinks Screen, it seems counter-intuitive that the B value is 

positive rather than negative

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties (see Section 

2.1.1)
APPENDIX C – DETAILED REVIEWER COMMENTS AND

EPA RESPONSES: Part 3 – Documentation
C-1

2.3.1 Comment on Whether the References Have Been Accurately Identified

Reviewer / Comment Response

Jayjock – The fourth reference (the CARB Final Regulation Order...) should 

be identified as available online at: 

http://www.arb.ca.gov/regact/2007/compwood07/fro-final.pdf (last accessed 

October 9, 2009). 

Similarly, the 8th reference (D. Hare...) can be found and should be 

referenced at: http://www.ecobind.com/research/Evaluating_the_Contribution.pdf

(last accessed October 9, 2009).

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Jayjock – Indeed, any and all the reference particularly from the EPA, 

GEOMET, Versar or industry groups should be identified by their web links, 

if available, and if not available on the web, should be considered for posting 

online with a link.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Jayjock – The two additional references indicated above (Murray and 

Burmaster on residential ventilation rate database and Jayjock on 

“Backpressure” modeling) should be considered for inclusion. 

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Georgopoulos – The Addendum to this review presents a selection of 

references. This reviewer would recommend consideration of at least some 

of these references in future revisions of FIAM-pwp and its documentation.

Selected references 

among those suggested 

have been added to the 

revised documentation

Unice – The credit for the picture of the kitchen cabinets should be provided. No longer applicable – a 

new picture is being used

that requires no credit

Unice –For more information regarding formaldehyde emissions from 

pressed wood products, model users could be referred to U.S EPA (1996) 

ORD report – Sources and Factors Affecting Indoor Emissions from 

Engineered Wood Products.

The reference has been 

added to the revised 

model documentation

Unice – A basis should be provided for the recommend range of 1 to 10 ppb 

(referring to background concentration) on page 2-4. Similarly, it should be 

explained why the concentration 10 ppb is ‘hard coded’ into the model in the 

percent of time > 0.01 ppm output.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Unice – It may be helpful to provide citations to studies that have 

summarized recent measured formaldehyde indoor air concentrations in new 

and existing structures. Examples include Hodgson and Levin 2003 and 

Hodgson et al. 2000. One of the concerns that these publications raise is that 

the Matthews parameter database including in the model is primarily for 

historical products, however, more recent indoor air data shows that 

emissions have declined.

EPA has moved Table 2-2 

to an appendix and has 

added clarifying text to 

the revised documentation 

and/or help screen text, 

including some new 

references

Unice – Groah’s white papers and reports have been invaluable to me in my 

past consulting work on formaldehyde. However, I recommend that these 

references (essentially a white paper and report prepared on behalf of 

HPVA) be replaced with peer-reviewed publications, conference 

proceedings, or governmental citations. Examples of publications addressing 

the long term decay of formaldehyde emissions from consumer products 

include: Mølhave et al. 1995; Brown 1999; Zinn et al. 1990 (this study was 

relied upon by CARB); Myer and Hermanns 1985; and CARB 2007 – see 

Appendix B. One key point to be derived from these studies is that a single 

exponential equation does not describe the decay in emissions adequately.

EPA prefers to retain the 

Groah reference and has 

added discussion 

regarding how the model 

could be run with “fast” 

and “slow” decay periods; 

selected references also 

have been added

Unice – I recommend citing Jayjock 1994 in reference to the backpressure 

effect first introduced on page 2-16 of the documentation.

EPA has added the 

suggested reference
C-2

2.3.2 Provide Comments on the Model Documentation (Does It Present the Model Construct, 

Selected Model Inputs, and Model Results in a Clear, Complete and Useful Manner?)

Reviewer / Comment Response

Zhang – The “steady-state assumption” has been mentioned too many times 

while describing the user-interfaces. This is not necessary and it distracts 

users/readers from focusing the input parameters. Also, the mode is not truly 

a steady-state one since the emission source decays over time. It is a “quasisteady state” model in which the concentration at a given calculation point 

was treated as “constant” for the purpose of simplifying the calculation 

without losing too much accuracy. An additional reference may be cited to 

justify such treatment (Zhang et al. 1997).

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – The “flow balance” should be changed to “mass flow rate balance” 

to be accurate.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – I am concerned that most data provided on formaldehyde emissions 

are from early studies.

EPA has moved Table 2-2 

to an appendix and has 

added clarifying text to 

the revised documentation 

Zhang – Validity of the exponential decay model is questionable. Recent 

data have shown that the power-law model would be more suitable, and 

secondary emissions as a result of hydrolysis are also possible. Limitations 

of the exponential decay model should be pointed out.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – Table 2.4: Why the “medium” and “low” emitting ensembles have 

the same slope and intercept for “Shelves”?

The Cabinet Screen has 

been removed from the 

model due to excessive 

input demands; cabinets 

can be modeled using the 

Source Screen

Zhang – The Fick’s law is for diffusion, while here the process is governed 

by convective mass transfer across the boundary layer. Air velocity and 

turbulence over the surface hence affect the mass transfer coefficient, instead 

of simply the diffusion coefficient in Fick’s law.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – The definition of vapor phase concentration and bulk phase 

concentration is unclear. In Eqn. 3-5, [CH2O]V should simply be the 

concentration in the bulk air (outside the air boundary layer over the 

surface). I see no need to define a vapor phase concentration since by 

default, we are always dealing with the vapor concentration in the air or in 

the material in this model. This naturally then leads to the assumption that 

the concentration in the bulk air outside the boundary layer is the same as the 

steady-state concentration in the compartment.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Zhang – Page 4-2: “Product loading rate – do you mean “loading ratio”? 

Same in Table 4-1.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Jayjock – As mentioned above, I see the clarity and consistency of 

explanation as a real strength of this documentation.

No response/action 

required

Jayjock – A personal preference for improvement involves the ordering of 

Chapters 2 and 3. As I was beginning to read the information in Chapter 2 

(USER INTERFACE) I found myself compelled to go and read Chapter 3 

EPA prefers to keep the 

original ordering of 

Chapters 2 and 3
C-3

2.3.2 Provide Comments on the Model Documentation ... (continued)

Reviewer / Comment Response

(MODEL CALCULATIONS AND ASSUMPTIONS). For me it would be 

more logical to change the order of these chapters.

Jayjock – In another relatively minor area I found the use of “D” on page 3-4 

to be unnecessarily complex.

EPA has removed the “D” 

term from all equations

Georgopoulos – The documentation for FIAM-pwp is quite thorough in 

presenting the model formulation and implementation as well as the data 

sources that were used to develop parameter estimates for this 

implementation.

No response/action 

required

Georgopoulos – However, the main issue with the documentation is the 

justification of the formulation of certain elements of the model per se (e.g.,

the assumption of constant rather than decreasing emission rates, etc.) rather 

than of potential alternatives.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – Consider explaining in the model documentation why the 

concentration time points of 3, 6 and 12 months were selected for output.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Unice – As described in more detail earlier, the treatment of sinks is not 

transparent and confusing because the rate of storage changes with time but 

the model used is a steady state model. The approach used appears to be 

more an attempt to modify the model results to reflect the measured 

concentration than a rigorous handling of sinks. The approach to wallboard 

sinks used in the current EPA model should be consistent with the wallboard 

sink equations and data presented in Matthews et al. (1987).

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties (see Section 

2.1.1)

Unice – Some stakeholders that may choose to use the new EPA 

formaldehyde model may not be familiar with the linear regression protocol 

required to estimate the parameters for either the Matthews and Hoetjer 

formaldehyde mass transfer models. It is recommended that documentation 

be revised to show an example linear regression for the recent data Koontz et 

al. (1996).

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Unice – As described previously, the documentation should explain the 

relationship between the Matthews and Hoetjer equation. The Hoetjer 

equation or equivalent mass transfer equation is frequently cited in the 

literature and some stakeholders may not be familiar with the Matthews 

equation.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Unice – The documentation should note that it is assumed that the number of 

occupied adsorption sites are much less than the available sites, reducing the 

isotherm equation to a simplified linear form.

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties

Unice – Consider presenting the analysis of PFT measurements and raw data 

in an appendix for Conventional 2 (p. 2-7). Is this data similar to data 

available in MCCEM? What is the basis of 0.5 ACH for manufactured 

home?

EPA has modified the 

house selections used in 

the model

Unice – Consider including in an appendix the analysis of the EPA Pilot 

Study data used to determine the mass transfer coefficient and intercept (p. 

5-3).

EPA has added this type 

of analysis to an appendix 

that contains historical 

chamber test data 
C-4

2.3.3 Provide Comments on Whether the Documentation for the FIAM - PWP Makes Clear the 

Sources and Anticipated Magnitudes of Variability and Uncertainty in Model Results

Reviewer / Comment Response

Zhang – There are good description of the possible sources of variability in 

the model prediction. However, it would be helpful if a more quantitative 

analysis is conducted on the uncertainties of the model coefficients and how 

they would propagate to the predicted results.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Jayjock – I believe that the author does a particularly good job of indicating 

the sources of relative uncertainty within the model itself and within the 

basic data-sets and relationships that feed the tool; however, I would have 

liked to see a level of quantitative uncertainty analysis such as described 

above and herein. Specifically, the author should consider presenting a range 

of outputs caused by a reasonably anticipated range of input variables within 

all the particular categories.

EPA has added an 

appendix on model 

sensitivity/uncertainty

Georgopoulos – Though extensive information on the data that were used for 

the model is included, in this reviewer’s opinion, the types and the extents of 

variability and uncertainty associated with various model elements are not 

fully revealed in the documentation. An ideal solution would be a 

probabilistic “re-formulation” of the model, with explicit discussion in the 

documentation of the appropriate distributions of values for the various 

inputs. As an alternative, even with a deterministic formulation of the model, 

a table listing explicitly various sources of uncertainty and variability 

associated with specific processes and parameters of the model, along with a 

factor or range specifying the approximate magnitude of each 

uncertainty/variability, would be extremely useful.

The model as designed 

achieves the goal of 

supporting EPA 

exposure assessments

Unice – Consider incorporating a mass transfer coefficient sensitivity 

analysis into the model or model documentation. The uncertainty and 

sensitivity analysis presented in Matthews et al. 1985 could be discussed.

EPA has added an 

appendix on model 

sensitivity/uncertainty

Unice – Consider providing a database of homes similar to MCCEM 

database to provide a larger range for interzonal airflows.

EPA has modified the 

house selections used in 

the model; there are now 

five generic structure 

types available

Unice – Consider including a simple Monte Carlo analysis as an appendix to 

provide some guidance on whether refinement of a particular input parameter 

is likely to change the conclusion.

EPA has added an 

appendix on model 

sensitivity/uncertainty
C-5

2.3.4 Comment on Whether the Model Algorithms are Consistent with the Descriptions in 

the Model Documentation

Reviewer / Comment Response

Jayjock – Indeed, from what I can determine from studying this work the 

model algorithms are for the most part consistent with their descriptions in 

the model documentation. I did detect either an error in sign or the omission 

of an explanation in the correlation coefficients presented in Table 2-2.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Jayjock – I would also prefer to see this “fit relationship” described as the 

correlation coefficient squared (R2) because this value represents the 

percentage of the variance in the model described by the algorithm.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Georgopoulos – The model algorithms appear consistent with model 

documentation; issues with the algorithms were addressed in the answers to 

questions of Part 1.

No response/action 

required

Unice – The model equations provided in the documentation were reviewed. 

To further verify the model calculations were implemented as intended using

an alternate approach, an example loading was run to steady state using 

IAQX Source Type 31. The model results were successfully reproduced 

using this independent approach.

No response/action 

required

Unice – The slope in the Matthews equation is not consistently defined. For 

example on p 2-11, ER = mC+b, but on page 3-4, ER = -mC+b.

FIAM has been revised 

accordingly, including 

documentation and/or 

help screen text

Unice – One area that is particularly confusing is the model output of 

average daily concentration (ADC) and lifetime average daily concentration 

(LADC). From the model documentation it is not clear whether the 

background concentration is included in the ADC and LADC.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text
C-6

2.3.5 General Comments on Documentation

Reviewer / Comment Response

Tucker – I found the documentation to be generally clear and quite useful. 

Users should have little trouble following it.

No response/action 

required

Tucker – Page 2-4, 4th paragraph: you should not describe the slope as “...in 

the form of a first-order sink representation...”

Indoor sinks have been 

removed from the model 

because of modeling 

uncertainties (see Section 

2.1.1)

Tucker – Page 3-2, 2nd paragraph: the phrase “...flows of formaldehyde 

mass into and out of a fixed volume must be equal...” is not correct. In fact, 

equation 3.1 contradicts that.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Tucker – Page 3-4, first line: shouldn’t “independent” be “dependent”? EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Tucker – While the term “half-life” has a certain value because of its 

understandability, the normal way to determine it is from the decay constant, 

which these days is a simple matter with spreadsheets like Excel, where k 

can be determined easily from the “trendline” equation for a first-order 

exponential decay of the concentration versus time data.

EPA has added clarifying 

text to the revised 

documentation and/or 

help screen text

Tucker – One area that I feel could be significantly improved is the 

mathematical introduction to the model. I have sketched out an alternative 

approach that I believe would be helpful to the general user, and have 

attached it. I don’t necessarily suggest this as a modification to FIAM, but as 

a general way to introduce mass balance concepts.

EPA has carefully 

considered this comment

but has not modified 

the mathematical 

introduction