# 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** 

**Ju ly**  **201 2** 

## 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 Cont**ents 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 I ndoor 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 Quantify

ing 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 Ty pes 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 I t Present the Model Construct, Selected Model I

nputs, and Model Results in a Clea r, Complete and Useful Manner?) ... 14 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.... 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 I ndoor 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 Quantify

ing Uncertainty

or Variability

for Subsequent Model Use.......... A- 13 2.1.5 Comment on Whether the Model I nput 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 Ty pes 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 Comm

ents on the Model Documentation (Does I t 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 t he Documentation for the FIAM - PWP Make Clear the Sources and Anticipated Magnitudes of Variability and Uncertaint

y 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**  **Back ground**  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 colleagues 1 ,2 

### at 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. Inthe 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 str ucture 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 dy namic, 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 3rd 

### 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

f our peer reviewers in response to the peer review charge were organized in a report3 

### prepared for EPA by ERG in October 2009. 1 T. Matthews, T. Reed, B. Tromberg, C. Daffron, and A. Hawthorne. 1983. Formaldehyde Emissions from Com bustion Sources and Solid Formaldehyde Resin Containing Products: Potential Im pact 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 Thom pson. 1987. Formaldehyde Sorption and Desorption Characteristics of Gy psum 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 201 2 . 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 201 2 .

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## 2 **SUMMAR**

**Y OF**  **PEER REVIEW**  **C OMMENTS**  **AND EP**  **A RESPONSES** **, ORGANIZ** **ED BY CHARGE QUESTION**  **2.1 Part 1**  **- Algorithms and Exposure Assumpti** **ons** 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 Leve** **l 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 i** **n 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 signi ficant 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 -- T

he analysis provided in S ection 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 rangedfrom about 24% low to about 65%

high. Predictions within that range are quite good, given the uncertainties in input data and unavoidable modeling assumptions. 

### M**odeling o** **f 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 analy tical measurement of C sat 

### – the saturation concentration of formaldehyde in the air over the emitter of interest

– could provide a potentially

valuable extended methodology f or 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 , exceptfor 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:  T he original Versar

/EPA model (referring to a version developed during the 1980s) presented the linear regression parameters for both the Matthew s model and the Hoetjer model (identified as the HBF model in this early ver sion ), 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

 I f 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. Inaddition, 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 C indoor 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 Matthew 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.  To correctly accountfor net adsorption on these short time scales within the framework of a pseudo
-steady
-state mod el, 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 e

stimation of exposure there appears to be logical error in the defaults. When one invokes the e

xposure d

efaults, 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 f or children which are all less than 10 years in length. This is explained in the documentation and in the online help button; however, Ifound 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:  T he 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. T

he 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 shouldfocus 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’. M

odel output

s 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.  T he 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 r unning 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:  I t 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.  T he uncertainty and variability levels in the available data strongly suggest the need for a distributional/probabili

stic 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 offormaldehyde 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. 

###  T he 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

f rom 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**  Inresponse 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

f ive generic structure ty

pes – 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 i n 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 estimate s 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 M atthews 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 sensitivit y 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 ty pes (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 estimate s 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 study5 

### that 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 Overal** **l Exposure Model or Specific Model Elements**  One reviewer reiterated an earlier comment (see Section 2.1.1) regarding the potential utility of C sat 

### 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 2 007 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 Us** **e of the FIAM -** **PW P 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, f aster 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 web
browser (referring to E

xcel 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:  T he 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 a bove :  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 . T he 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 ofpublications 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,**  **Select ed 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 earl y 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 f ormulation 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.  T he 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 d ocumentation 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 Make s**  **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 R2 

### 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 REVI** **E WER COMMENTS AND** 

**EPA RESPONSES:**  **Part 1**
**– Algorithms and Exposur** **e 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 Re** **sponse** 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. Itis 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/m3) 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 f

rom 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 ( G C C G G rearranged C

C G G

ss

sat ss sat ss ss
    

sat



## C G m max 

## 

### A-3

**2.1. 1**  **Comment on the Adequacy of the**  **Mathematical Equations ... (continued)** 

**Reviewer / Comment Re** **sponse** 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) com puter 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 maj or 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 conj unction with other, more comprehensive, indoor air quality and inhalation exposure systems). Simple im plementations 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 Equati ons**  **... (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

– Itmay 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 perf

ormed 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 af fected 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 maj ority 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 com partments 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 helpf

ul 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. (1 990) 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-com partment (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**  **Commen t 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: Itcan 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. Itis 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. Inconjunction 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

– Itappears 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 1 0 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 Defa ult 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

– Itis 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

– Itis 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**  **Assi st 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 prod ucts 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 Gene rated 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. Itis 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 A ssumptions** 

**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 REVI** **E WER COMMENTS AND**  **EPA RESPONSES:**  **Part 2**
**– User Interface** 

### B- 1 **2.2.1 Provide Comments on the Ease of Use of the FIAM -** **PW P**  **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. Itis 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

– Itwould 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. Itwould 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 Resp** **onse** 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 Jay jock 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

– Itmay 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 Jay jock 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 Mod el 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. Itis 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 bou ndary 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 Documen tation**  **... (continued)**

**Reviewer / Comment Response**  (MODEL CALCULATIONS AND AS SUMPTIONS). 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. Itis 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 t he 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 Make s**  **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