i

Formaldehyde Indoor Air Model – Pressed Wood Products, 

Version 2.0 (FIAM-pwp v2.0): 

Model User Manual and Documentation

Exposure Assessment Branch

Economics, Exposure and Technology Division

Office of Pollution Prevention and Toxics

Office of Chemical Safety and Pollution Prevention

U.S. Environmental Protection Agency

1200 Pennsylvania Avenue, N. W.

Washington, D.C. 20460

July 2012
ii

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

This is not an official guidance document and should not be relied upon to determine 

applicable regulatory requirements. This document was prepared to provide exposure 

information for the rulemaking process, and to meet various administrative and legislative 

requirements. Due to the nature of the information available to EPA, the document 

contains various assumptions that may not reflect the regulatory determinations that an 

individual firm or organization would make were it to apply the rule's requirements to its 

specific circumstances. Persons seeking information on specific regulatory requirements 

should consult 40 CFR Part 767, and the preamble for the regulatory action.

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

an endorsement by EPA.
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TABLE OF CONTENTS

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

2. USER INTERFACE................................................................................................................. 3

2.1 Background – Model Versions................................................................................................ 3

2.2 House Screen........................................................................................................................... 4

2.2.1 Inputs and Options.................................................................................................... 4

2.2.2 Basis for Defaults...................................................................................................... 8

2.3 Source Screen ........................................................................................................................ 14

2.3.1 Types of Inputs ....................................................................................................... 14

2.3.2 Adding Default Inputs............................................................................................. 15

2.3.3 Adding Custom Inputs............................................................................................ 16

2.3.3 Basis for Defaults.................................................................................................... 16

2.4 Exposure Screen .................................................................................................................... 21

2.4.1 Inputs and Options for Age of Sources and Formaldehyde Level of Interest ........ 21

2.4.2 Inputs and Options for Exposure Groups................................................................ 22

2.4.3 Inputs and Options for Concentrations Outside the Home ..................................... 23

2.4.4 Basis for Defaults.................................................................................................... 24

2.5 Result Screen......................................................................................................................... 27

2.5.1 Concentration Results............................................................................................. 27

2.5.2 Exposure Results..................................................................................................... 28

2.5.3 Printing Inputs and Results..................................................................................... 28

2.5.4 Archiving Inputs and Results for a Run.................................................................. 28

2.6 Model Access and Additional Features................................................................................. 29

2.6.1 Accessing the IGEMS Website............................................................................... 29

2.6.2 Saving Inputs for Later Access............................................................................... 29

2.6.3 Context-sensitive Help............................................................................................ 29

2.7 Example Applications ........................................................................................................... 29

2.7.1 Example 1 – Modeling with Model Default Values............................................... 29

2.7.2 Example 2 – Modeling a Hypothetical Chamber Test for CARB Compliance...... 30

2.7.3 Example 3 – Demonstrating an Equilibrium Concentration................................... 31

2.7.4 Example 4 – Examining the Incremental Contribution of a Product...................... 32

2.7.5 Example 5 – Varying the Decay Rate (Half Life) .................................................. 33

3. MODEL CALCULATIONS AND ASSUMPTIONS .......................................................... 35

3.1 Generalized Mass-Balance Equation..................................................................................... 36

3.2 Steady-State Model for Initial Formaldehyde Concentration in One Compartment............. 37

3.3 Extension of the Steady-State Model to Two Compartments............................................... 39

3.4 Matthews Model Compared to HBF Model.......................................................................... 40

3.5 Other Potentially Useful Modeling Constructs ..................................................................... 42

3.6 Discussion of Indoor Sinks.................................................................................................... 45

3.7 Decay of Indoor Concentrations............................................................................................ 45

3.8 Default Value for Half Life ................................................................................................... 48

3.9 Temperature and Humidity Adjustments .............................................................................. 51

3.10Exposure Calculations........................................................................................................... 53
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4. EVALUATION OF MODEL ALGORITHMS AND PREDICTIONS............................ 55

4.1 Mathematical Correctness of Model Algorithms.................................................................. 55

4.2 NIST Assessment of Matthews Model Predictions............................................................... 55

4.3 Assessment of FIAM-pwp Predictions – EPA Pilot Study.................................................... 56

4.3.1 Initial Baseline Period............................................................................................. 57

4.3.2 First Loading........................................................................................................... 58

4.3.3 Air-out Period after First Loading .......................................................................... 59

4.3.4 Second Loading ...................................................................................................... 60

4.3.5 Air-out Period after Second Loading...................................................................... 61

4.3.6 Third Loading ......................................................................................................... 62

4.4 FIAM-pwp Predictions for Recent Residential Field Monitoring Studies ............................ 63

Appendix A – Summary of EPA Formaldehyde Pilot Study

Appendix B – Model Sensitivity

Appendix C – Basis for Emission Classes

Appendix D – Historical Testing of PWPs to Derive Slope and Intercept Values

Appendix E – Derivation of Two-zone Steady-state Solution
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LIST OF FIGURES

Figure 2-1. FIAM-pwp House Screen. .............................................................................................4

Figure 2-2. Popup Screen – Create Your Own Structure Type. ......................................................5

Figure 2-3. U.S. Climate Zones. ......................................................................................................5

Figure 2-4. Popup Screen – Create Your Own Climate Zone. ........................................................6

Figure 2-5. FIAM-pwp Source Screen. ..........................................................................................14

Figure 2-6. Source Screen after Adding Default PWP Types .......................................................15

Figure 2-7. Source Screen after Adding Custom Values...............................................................16

Figure 2-8. Exposure Screen..........................................................................................................21

Figure 2-9. Preparing to Add a New Exposure Group...................................................................23

Figure 2-10. Result Screen with First Two Exposure Groups Selected ........................................27

Figure 2-11. Emission-rate Parameters for FIAM-pwp Default PWPs in an Apartment...............32

Figure 2-12. Formaldehyde Concentration Decline with Fast and Slow Decay Periods...............34

Figure 3-1. Representation of Declining Emission Rate by Power-law Function vs. 

Exponential-decay Model.......................................................................................................44

Figure 3-2. Representation of Declining Emission Rate by Power-law Function vs. 

Exponential-decay Model with Periods of Fast and Slow Decay. .........................................44

Figure 4-1. FIAM-pwp House-Screen Inputs Used to Model Study of New California 

Homes.....................................................................................................................................65

Figure 4-2. FIAM-pwp Source-Screen Inputs Used to Model Study of New California 

Homes.....................................................................................................................................66

Figure 4-3. FIAM-pwp Results for Modeling of New California Homes......................................66

Figure 4-4. FIAM-pwp Source-Screen Inputs Used to Model CDC Study of Travel Trailers. .....67

Figure 4-5. FIAM-pwp Results for Modeling of Travel Trailers...................................................67
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LIST OF TABLES

Table 2-1. House Dimensions and Calculated Volume for Five Housing Types........................... 9

Table 2-2. Summer and Winter Indoor Temperatures from RECS 2005, by Climate Zone ........ 10

Table 2-3. Year-round Indoor Temperatures Derived from RECS 2005, by Climate Zone ........ 10

Table 2-4. Modeled Year-round Indoor RH Level, by Climate Zone.......................................... 11

Table 2-5. Alternative Analytical/Regulatory Options for EPA Formaldehyde Rule.................. 17

Table 2-6. Default Slopes and Intercepts for Six Default PWP Types......................................... 18

Table 2-7. Default Exposed Surface Areas (in m2) for Six PWP Types, by Structure Type –

New-home Case .................................................................................................................... 20

Table 2-8. Default Exposed Surface Areas (in m2) for Six PWP Types, by Structure Type –

Renovation Case.................................................................................................................... 20

Table 2-9. Default Annual Hours at Home and Non-home Locations, by Exposure Group........ 24

Table 2-10. Default Concentration Values for Non-home Locations .......................................... 25

Table 2-11. Derivation of Baseline Values for Daycare, School and Work Locations................ 26

Table 2-12. Concentration Values for Selected Non-home Locations, by Emissions 

Scenario ................................................................................................................................. 26

Table 2-13. Run Notes for Example 1.......................................................................................... 30

Table 2-14. Run Notes for Example 2.......................................................................................... 31

Table 2-15. Run Notes for Example 3.......................................................................................... 31

Table 2-16. Run Notes for Example 4.......................................................................................... 33

Table 2-17. Run Notes for Example 5, Part 1 (fast decay period) ............................................... 34

Table 2-18. Run Notes for Example 5, Part 2 (slow decay period).............................................. 34

Table 4-1. Model Inputs (Sources and Sinks) for Initial Baseline Period................................... 57

Table 4-2. Modeling Results for Initial Baseline Period ............................................................. 58

Table 4-3. Model Inputs (Sources and Sinks) for First (Medium) Loading................................ 58

Table 4-4. Modeling Results for First Loading ........................................................................... 59

Table 4-5. Model Inputs for Air-out Period after First Loading ................................................. 59

Table 4-6. Modeling Results for Air-out Period after First Loading........................................... 60

Table 4-7. Model Inputs (Sources and Sinks) for Second (High) Loading................................. 60

Table 4-8. Modeling Results for Second Loading....................................................................... 61

Table 4-9. Model Inputs for Air-out Period after Second Loading ............................................. 61

Table 4-10. Modeling Results for Air-out Period after Second Loading .................................... 62

Table 4-11. Model Inputs (Sources and Sinks) for Third (High) Loading.................................. 62

Table 4-12. Modeling Results for Third Loading........................................................................ 63

Table 4-13. Summary of Recent Formaldehyde Field Monitoring Studies ................................. 64
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1. INTRODUCTION

The model described in this document, the Formaldehyde Indoor Air Model - Pressed Wood 

Products, Version 2.0 (FIAM-pwp v2.0), is intended to assist the user in estimating human 

inhalation exposure to airborne formaldehyde emissions from composite wood products (CWPs1)

installed in new or existing residences, including smaller structures such as camper trailers. The 

model estimates indoor formaldehyde concentrations and associated human inhalation exposures 

for such situations.

FIAM-pwp is an adaptation of a model developed during the 1980s by Dr. Thomas Matthews and 

colleagues2,3at 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 (“steady-state”) indoor concentration calculated by

the model is assumed to decrease over time, as the formaldehyde “reservoirs” in various sources 

(or sinks) are gradually depleted. The decrease over time is assumed to follow a first-order 

exponential process, at a decay rate that corresponds to an assumed half life for the collective 

formaldehyde emissions.

In the mid-1990s, an EPA-sponsored pilot study was undertaken by Versar/GEOMET4to assess 

the contribution of UF-bonded wood products to formaldehyde levels in homes and to evaluate 

current EPA exposure models. Shortly after this study was completed, Versar was tasked by EPA 

to develop a modified version of the Matthews model. 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. The model was not finalized due to 

shifting priorities at the time of its development. 

In 2006, formaldehyde from pressed wood products received national attention when the U.S. 

Federal Emergency Management Agency (FEMA) provided travel trailers and mobile homes for 

habitation by residents of the U.S. gulf coast who were displaced by Hurricane Katrina and 

Hurricane Rita. Some individuals who moved into these temporary homes complained of 

breathing difficulties, nosebleeds, and persistent headaches. In December 2007 and January 

2008, the Centers for Disease Control and Prevention measured formaldehyde levels averaging 

 

1

In this report, the term “composite wood product” (CWP) refers only to hardwood plywood, medium density 

fiberboard, or particleboard whereas “pressed wood product” (PWP) refers to a broader set of wood materials 

including, for example, softwood plywood and oriented strand board.

2

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

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

Formaldehyde Concentration and Possible Corrective Measures. Proceedings of ASHRAE Symposium 

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

3

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

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

4 M. Koontz, H. Rector, D. Cade, C. Wilkes, and L. Niang. 1996. Residential Indoor Air Formaldehyde Testing 

Program: Pilot Study. Report No. IE-2814, prepared by GEOMET Technologies, Inc. for the USEPA Office of 

Pollution Prevention and Toxics under EPA Contract No. 68-D3-0013, Washington, DC.
2

77 parts per billion5,6(ppb; geometric mean) in a random sample of FEMA-supplied occupied 

travel trailers, park models, and mobile homes. In February 2008 U.S. health officials 

announced7that potentially hazardous HCHO levels were found in both travel trailers and 

mobile homes provided by FEMA.

In April 2008 the California Air Resources Board (CARB) formally adopted an Airborne Toxic 

Control Measure8(ATCM) to reduce formaldehyde emissions from CWPs sold, offered for sale, 

supplied, used, or manufactured for sale in California. On July 7, 2010, the Formaldehyde 

Standards for Composite Wood Products Act (FSCWPA) was signed into law. This statute, 

which adds a Title VI to TSCA, establishes formaldehyde emission standards for hardwood 

plywood, particleboard, and medium-density fiberboard that are identical to the CARB ATCM 

Phase II standards9. TSCA Title VI directs EPA to promulgate implementing regulations by

January 1, 2013 that address: sell-through dates for products; stockpiling; third-party testing and 

certification; auditing and reporting of third-party certifiers; recordkeeping; chain of custody;

labeling; enforcement; products made with no-added formaldehyde (NAF) and ultra-low 

emitting formaldehyde (ULEF) resins; laminated products; finished goods; hardboard; and 

products containing de minimis amounts of composite wood products. 

As a result of the activity described above, EPA decided to revise and update, when possible, the 

formaldehyde model developed in the mid-1990s to facilitate an assessment10 of exposures before

and after the EPA formaldehyde rule goes into effect. FIAM-pwp v1.0 was the initial result of 

this effort, with FIAM-pwp v2.0 developed in response to peer review and workgroup comments. 

Version 2.0 adds exposure estimates to the model but indoor sinks have been excluded, for 

reasons discussed later in this document (see Section 3). Section 2 describes the user interface 

for the model and Section 3 describes the mathematical basis, underlying calculations and 

related assumptions. Model evaluation, including comparisons to research-house and fieldmonitoring data, is provided in Section 4. The EPA pilot study mentioned above is summarized 

in Appendix A. Analyses related to model sensitivity are presented in Appendix B. The basis for 

emission classes used on the Source Screen is presented in Appendix C. Historical chamber-test 

data, conducted primarily during the 1980s but extending into the 1990s and providing part of the 

basis for emission-rate parameters used in FIAM-pwp, are described and discussed in Appendix D. 

Appendix E describes the mathematical derivation of the two-zone implementation of the steadystate model. 

 

5 CDC. 2008/2010. Final Report on Formaldehyde Levels in FEMA-Supplied Travel Trailers, Park Models, and 

Mobile Homes. Centers for Disease Control and Prevention. Available at: 

http://www.cdc.gov/nceh/ehhe/trailerstudy/assessment.htm#final. July 2008 (amended December 2010).

6 DHS. 2009. FEMA Response to Formaldehyde in Trailers. OIG-09-83. Department of Homeland Security, 

Office of Inspector General, Washington, DC. Available at:

http://www.dhs.gov/xoig/assets/mgmtrpts/OIGr_09-83_Jun09.pdf.

7

See http://www.msnbc.msn.com/id/23168160/ns/us_news-life/t/cdc-tests-confirm-fema-trailers-are-toxic/.

8

See http://www.arb.ca.gov/toxics/compwood/compwood.htm for recent activity related to this ATCM.

9 Available at http://www.arb.ca.gov/toxics/compwood/compwood.htm.

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

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

1200 Pennsylvania Avenue NW, Washington, DC 20460. July 2012.
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2. USER INTERFACE

2.1 Background – Model Versions

In 2009 an on-line, user-friendly version of the formaldehyde model (FIAM-pwp v1.0) was 

developed along with documentation11 to guide its use. Later that same year a peer review of the 

model was facilitated for EPA by Eastern Research Group, Inc. (ERG). 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 five reviewers in response to the peer-review charge were organized in a report12

prepared for EPA by ERG in October 2009.

Version 2, named FIAM-pwp v2.0 and the subject of this document, was developed to run under 

the Internet Geographical Exposure Modeling System (IGEMS13) platform. Some additions to or 

subtractions from the previous version are related, at least in part, to the peer reviewers’ 

comments. For example, a decision was made to drop certain FIAM-pwp features that provided 

particular challenges or difficulties: (1) inputs for indoor sinks, an area of considerable modeling 

uncertainty; and (2) inputs for emission characteristics of cabinet components, which were very

demanding. In addition, exposure groups are being handled differently in the current version, for 

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

The user interface for the current version includes three input screens (House, Source and 

Exposure Screens) and an output screen (Result Screen) that displays the modeling results; these 

results are continually updated, such that any time the Result Screen is accessed the results 

displayed reflect all current inputs, much like a spreadsheet. First-time users, in particular, are 

advised to go through the input screens in sequence (i.e., House then Source then Exposure), 

examining results either “along the way” or after all inputs have been entered, at the user’s 

discretion.

The House Screen is described in Section 2.2, the Source Screen in Section 2.3, and the 

Exposure Screen in Section 2.4. For each of these screens a description of inputs and options is 

provided first, followed by the basis for model defaults. The Result Screen is described in 

Section 2.5. Instructions for accessing the model via the IGEMS portal are provided in Section

2.6, along with documentation of ancillary model features such as saving runs for later access, 

printing inputs and results for a run, archiving inputs and results in Excel, and accessing the 

context-sensitive help embedded in the model. Example applications are provided in Section 2.7.

 

11 Formaldehyde Indoor Air Model – Pressed Wood Products: Model Documentation. Prepared by Versar, Inc. for 

EPA/OPPT by under Contract No. EP-W-04,035, Work Assignment No. 4-2. August 2009.

12 Peer Review Results for the Formaldehyde Indoor Air Model – for Pressed Wood Products. Prepared by 

Eastern Research Group, Inc for EPA/OPPT under Contract No. EP-W-05-014. October 2009.

13 See http://www.epa.gov/oppt/exposure/pubs/gems.htm.
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2.2 House Screen

This screen (see Figure 2-1) is intended to collect information on the house volume and internal 

conditions (e.g., temperature, airflows) and to provide certain options pertaining to model 

calculations. The inputs here can be divided into three screen portions: (1) documentation of the 

run and selection of structure type and climate zone (upper portion), (2) editable information on 

the number of zones, zone volumes and airflows (middle portion); and (3) indoor conditions and 

formaldehyde decay parameters (lower portion).

Figure 2-1. FIAM-pwp House Screen.

Many of the inputs on this screen are optional, in the sense that default values are provided by

the model. The choice of structure type on the upper portion of the screen provides default values 

for zone volumes and airflow rates, which are displayed on the middle portion of the screen. The 

choice of climate zone on the same line provides default values for indoor temperature and 

humidity, which are displayed on the lower portion of the screen. Other inputs on the lower 

portion (e.g., background concentration) are not linked to any user choice but do have default

values. Although the model can be run using default values, care should be taken in choosing the 

type of structure to be modeled and in choosing appropriate values for inputs such as the 

background concentration, emissions half life, and indoor temperature and relative humidity. 

2.2.1 Inputs and Options

Run Title and Run Notes. Text inputs for these two items are intended to assist the user in 

documenting the purpose of the model run along with certain choices made or options exercised. 

Structure Type and Climate Zone. Both of these items have pick lists from which the user can

choose to obtain default values, which can either be used “as is” or edited. The user also has the 

option of creating additional structure types or climate zones and saving them for later use.

There are five default structure types in FIAM-pwp: single-family (SF) detached homes; SF 

attached homes; apartments; manufactured/mobile homes, and camper trailers. The SF 

detached/attached homes are two-story structures with two zones (upstairs and downstairs) 

whereas the others are single-zone structures.

Upper Portion

Middle Portion

Lower Portion
5

The values associated with FIAM-pwp default structure types are “untouchable” (i.e., they cannot 

be directly edited), but the choices on the pick list include one called Create Your Own Structure 

Type – if this choice is made, then the popup screen shown in Figure 2-2 will appear. At that 

point, the user assigns a structure type name and enters zone descriptions, volumes and airflows, 

and then clicks the Save button. The MAKEACOPY button (see Figure 2-1, above) is used to 

copy and then edit/save any of the default choices. Guidance on entering volume and airflow 

values is provided later in this section, under the heading Zones, Volumes and Air Flows.

Figure 2-2. Popup Screen – Create Your Own Structure Type.

FIAM-pwp uses the U.S. climate zones defined by the Department of Energy (see Figure 2-3) 

There are five climate zones, numbered 1 though 5, with 1 being the coldest and 5 the warmest.

Figure 2-3. U.S. Climate Zones.

Source: U.S. Energy Information Administration (http://www.eia.doe.gov/emeu/cbecs/climzonenew.gif)
6

As with default structure types, the values associated with default climate zones in FIAM-pwp

are “untouchable.” Other values can be assigned and saved via a choice called Create Your Own 

Climate Zone, which is included on the climate-zone pick list – if this choice is made, then the 

popup screen shown in Figure 2-4 will appear. At that point, the user assigns a name, enters the 

desired temperature and humidity values, and clicks the SAVE button. The MAKEACOPY

button on the main screen can be used to copy and then edit/save any of the default choices. 

Regardless of the climate zone chosen, the user can override the associated temperature and 

humidity values by changing them directly in the lower portion of the House Screen.

Figure 2-4. Popup Screen – Create Your Own Climate Zone.

Air Exchange Rate. This item is “grayed out” because no user entry is allowed here; the rate is 

displayed for informational purposes only. The value displayed is calculated by FIAM-pwp using 

volume and airflow information and, thus, can be changed by editing those inputs. 

Zones, Volumes and Air Flows. Houses in FIAM-pwp can have either one or two zones. 

Among the default structure types, single-family detached and attached homes are 

conceptualized as having two separate zones (upstairs and downstairs) whereas manufactured 

homes, apartments and camper trailers are conceptualized as having a single zone. The 

maximum number of zones allowed is two. Thus, for example, to represent a house with three 

stories, at least two of the three stories would need to be combined into one zone. The indooroutdoor airflow rates determine the air exchange rate. For example, for an apartment with a 

volume of 261 m3, the default indoor-outdoor airflow rate of 52.2 m3/h results in an air exchange 

rate of 0.2 ACH (i.e., 52.2 m

3

/h / 262 m

3

= 0.2/h). 

If the user selects Create Your Own Structure Type from the pick list or edits one of the default 

choices after making a copy, then the popup screen previously shown in Figure 2-2 will appear. 

The required information includes the name of the structure type, the description and volume for 

each zone, and airflow rates. For a one-zone house only the first line needs to be completed and 

only two air flows are needed – Flow from Outside and Flow to Outside. These flows must be 

equal to provide an overall flow balance for the house. For a two-zone house six airflows are 

needed – the flow for each zone to and from outdoors plus the flows in both directions between 

the two zones. Individual flow pairs (e.g., flow from and to outside for zone 1) do not necessarily 

need to be balanced, but the total flow out of a zone should match the total flow into it. On the 

main screen, the program updates the total flow per zone as the user provides or edits inputs. If 

the total flows for any zone are not balanced, then the program raises a flag by assigning a red

font to the zone totals. The model still will run in such cases, but the user is advised of the 

possibility of counterintuitive or misleading results.

The default air flows are balanced, that is, the airflow rate from outdoors to indoors is equal to 

that from indoors to outdoors. For two-zone structures – SF detached/attached homes – the house 

volume is equally apportioned between upper/lower stories and identical indoor-outdoor airflow 
7

rates are assumed per story. Further, as an expedience, it is assumed that the airflow rate between 

the two indoor zones is balanced and equal to the indoor-outdoor airflow rate. Thus, for example, 

for a SF detached home with a volume of 811 m

3

or 405.5 m

3

per story, both the indoor-outdoor 

airflow rate per story and the interzonal airflow rate between stories are set at 81.1 m3/h. 

If one of the default two-zone homes is selected but the user prefers to treat the home as a single, 

well-mixed zone, then FIAM-pwp will add the zone volumes and will “collapse” the airflow rates 

to the single-zone case, meaning that the only flow information needed is the airflow rates from 

outdoors to indoors and vice versa. For the single-zone case these two airflow rates should have 

the same value. If the number of zones is changed back to two, then the program will revert to 

the volumes and airflow rates originally associated with these house types. 

Background Concentration. This concentration – the first of eight inputs on the lower portion 

of the House Screen – is intended to reflect the contribution to the indoor formaldehyde 

concentration of both the outdoor concentration and indoor sources other than PWPs (e.g., 

tobacco smoke, permanent press fabrics, paints, cosmetics). The FIAM-pwp default value for the 

background concentration is 7.5 ppb. If a value of zero is chosen for the background 

concentration, then the modeling results will reflect only the contributions of the indoor sources 

selected for the model run via inputs on the Source Screen.

Emissions Half Life. This input governs the rate at which indoor formaldehyde concentrations 

decay over time. The FIAM-pwp default value is 1.5 years; with this half life, the emission rate 10 

years after new construction would be about 1% of the initial value. With a half life of 3.0 years it 

would take 20 years (i.e., twice as long) to reach 1% of the initial value.

Temperature and Humidity. Model algorithms to predict initial indoor formaldehyde levels

include temperature and humidity adjustments for the emission rate from PWPs. Default values 

for indoor temperature and relative humidity are assigned based on the user’s choice (or creation) 

of a climate zone. These defaults can be edited directly on the lower portion of the screen, but if 

that is done then these new values would only be associated with the saved run name. 

Alternatively, values can be saved for access in any run by creating a new climate zone or by

copying and editing an existing one (see Structure Type and Climate Zone, earlier in this section). 

Time from ‘Initial Concentration’ and ‘Decay Concentration to'. By default, FIAM-pwp

“decays” initial indoor concentrations that are calculated under a steady-state assumption to time 

points that are 3, 6 and 12 months later than the initial point in time. The input labeled ‘Time from

Initial Conc’ (see Figure 2-1) enables the user to specify an additional point in time (in months) 

over which the model decays the initial concentration values; this input is set to 24 months by

default. The input labeled ‘Decay Concentration to’, with a default of 10 ppb, allows the user to 

specify a concentration for which the model will calculate the time to decay from the initial 

modeled concentration to that value. For a two-zone structure, the model will calculate this decay

time based on the higher of the zone-specific initial modeled concentrations.

Temperature and Humidity Coefficients. The model initially calculates indoor concentrations 

at an indoor temperature of 23 C and an indoor relative humidity of 50 percent, and then adjusts 

the results to the temperature and humidity conditions selected by the user. The temperature and 

humidity coefficients specified here are used by the model to make these adjustments.
8

2.2.2 Basis for Defaults

Structure Type. The five default structure types in FIAM-pwp are the same as those used in the 

EPA’s Formaldehyde from Composite Wood Products: Exposure Assessment dated March 2012. 

Collectively, they account for the major types reported by the American Housing Survey (AHS) 

(http://www.census.gov/hhes/www/housing/ahs/ahs.html) and the Residential Energy Consumption 

Survey (RECS) (http://www.eia.doe.gov/emeu/recs/): single-family (SF) detached homes; SF 

attached homes; apartments; and manufactured/mobile homes or trailers. For the purpose of 

estimating structure volume, a further distinction was made for manufactured homes due to size 

differences – single-wide (SW) versus double-wide (DW). Camper trailers and park models also 

are of interest because, like manufactured homes, these housing types have been used for some 

residents displaced by natural disasters such as Hurricane Katrina. Camper trailers were chosen 

to represent the small-volume temporary housing for displaced residents, as this housing type 

accounted for 80 percent of the smaller-volume structures that were tested as part of FEMA’s 

formaldehyde monitoring in temporary housing14,15.

For SF detached/attached homes, apartments, and manufactured homes, default structure volumes

were calculated based on statistics resulting from the U.S. Department of Energy’s 2005 RECS16, 

using the reported average floor area for the most recently built category (years 2000-2005) and 

assuming a ceiling height of 8.5 feet. For camper trailers, a typical size used for displaced 

hurricane victims was assigned, with an assumed ceiling height of 7 feet. 

The following floor areas and associated dimensions were assumed for the five structure types:

1. SF detached home – 3456 sq ft (36 ft x 48 ft, 2 stories)

2. SF attached home – 2240 sq ft (28 x 40 ft, 2 stories)

3. Apartment – 1120 sq ft (28 ft x 40 ft)

4. Manufactured home – 1216 sq ft (16 ft x 76 ft) for SW; 1680 sq ft (28 ft x 60 ft) for DW

5. Camper trailer – 328 sq ft (8 ft x 35 ft with 4 ft x 12 ft “slide-out”).

The dimensions chosen for SF homes and apartments were consistent with their respective 

average floor areas as reported in the RECS 2005 survey results.

The dimensions listed above are exterior dimensions; for interior volume calculations, 0.5 ft was

subtracted from both the length and width to account for the area occupied by exterior cladding, 

sheathing, studs and wallboard. For SF homes a simplifying assumption was made that the upper 

and lower floors are of identical dimensions. For manufactured homes, a weighted average of 

SW and DW volumes was used; based on statistics from the U.S. Census Bureau’s Manufactured 

Home Survey (MHS) for years 2000-2009 (http://www.census.gov/const/www/mhsindex.html), 

70 percent of the manufactured homes were assumed to be the double-wide type. The resultant 

house volumes are listed, along with the house dimensions, in Table 2-1.

 

14 Centers for Disease Control and Prevention. 2008/2010. Final Report on Formaldehyde Levels in FEMA-Supplied

Travel Trailers, Park Models, and Mobile Homes. July 2008 (amended December 2010).

Available at: http://www.cdc.gov/nceh/ehhe/trailerstudy/assessment.htm#final.

15 Department of Homeland Security. 2009. FEMA Response to Formaldehyde in Trailers. OIG-09-83. Office of 

Inspector General, Washington, DC. Available at:

http://www.dhs.gov/xoig/assets/mgmtrpts/OIGr_09-83_Jun09.pdf.

16 See, for example, http://www.eia.doe.gov/emeu/recs/contents.html.
9

Table 2-1. House Dimensions and Calculated Volume for Five Housing Types

Housing Type Number of 

Stories

Length, 

ft

Width, 

ft

Ceiling 

Height, ft

Volume, 

ft3(m3)

SF Detached 2 48 36 8.5 28,666 (811)

SF Attached 2 40 28 8.5 18,466 (523)

Apartment 1 40 28 8.5 9,233 (261)

Manufactured

 Single-wide

 Double-wide

 Averagea

1

1

1

16

28

--

76

60

--

8.5

8.5

8.5

 9,947 (282)

13,908 (394)

12,720 (360)

Camper Trailer

 Main body

 Slide-out

1

1

35

12

8

4

7

7

2,147 (61)

a Weighted average of single-wide (30 %) and double-wide (70 %); see text.

Climate Zone / Temperature and Humidity. Default indoor temperature-values were determined

for each of five U.S. climate zones using responses to RECS 2005 survey questions about usual 

temperatures maintained in the residence. One of the questions – “At what temperature does your 

household usually keep your home in the winter?” – was asked under three conditions: (1) during 

the day when someone is home; (2) during the day when no one is home; and (3) during sleeping 

hours. The same set of questions also was posed with reference to the summer season.

As described in the exposure assessment report cited in Section 1 of this document, the RECS 

database was analyzed for these questions to estimate the year-round average temperature in 

each climate zone. Table 2-2 summarizes responses to the RECS questions on daytime 

temperature when someone is home and nighttime (sleeping hours) temperature by climate zone. 

The columns labeled “Overall” are weighted averages of the daytime and nighttime average 

temperatures for summer and winter, with weights of 2/3 for daytime and 1/3 for nighttime. The 

overall summer and winter temperatures, in turn, were averaged to derive a year-round average 

temperature, by weighting the season-specific averages by the number of months associated with 

each. The resultant year-round temperature estimates are listed in Table 2-3.
10

Table 2-2. Summer and Winter Indoor Temperatures from RECS 2005, by Climate Zone

U.S.

Climate

Zone

Average Summer Temperature Average Winter Temperature

Day Night Overall Day Night Overall

1 (coldest) 73.4 73.3 73.4 69.1 65.8 68.0

2 73.0 73.0 73.0 69.6 67.6 68.9

3 73.2 73.2 73.2 69.8 68.1 69.3

4 73.9 73.2 73.7 70.9 68.6 70.1

5 (warmest) 75.1 74.8 75.0 72.4 70.6 71.8

Table 2-3. Year-round Indoor Temperatures Derived from RECS 2005, by Climate Zone

U.S.

Climate

Zone

Summer Temperature Winter Temperature

Year-Round

Temperature Average 

Temperature

Number of 

Months

Average 

Temperature

Number of 

Months

1 (coldest) 73.4 3 68.0 9 69.4

2 73.0 4 68.9 8 70.3

3 73.2 4 69.3 8 70.6

4 73.7 5 70.1 7 71.6

5 (warmest) 75.0 7 71.8 5 73.6

The RECS does not ask about humidity levels maintained in the residence. Consequently, a

mass-balance modeling approach was applied to estimate the year-round average indoor relative 

humidity (RH) level for each climate zone. A representative city was chosen for each zone:

 Climate Zone 1 – Milwaukee, WI

 Climate Zone 2 – Chicago, IL

 Climate Zone 3 – New York, NY

 Climate Zone 4 – Charlotte, NC

 Climate Zone 5 – Houston, TX.

Hourly outdoor temperatures for a typical meteorological year17 (TMY) were obtained for each 

city as inputs to a mass-balance model. The modeling approach, described in the formaldehyde 

exposure assessment report cited in Section 1, is consistent with methods described in American 

Society for Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) Standard 160P 

(Criteria for Moisture-Control Design Analysis in Buildings). In the mass-balance model, 

moisture in the indoor air is determined by moisture in the outdoor air, indoor moisture 

generation by the occupants, and moisture removal by a central air-conditioning (AC) system. 

 

17 Available at http://rredc.nrel.gov/solar/old_data/nsrdb/1991-2005/tmy3/.
11

Modeled indoor RH levels for each climate zone are listed in Table 2-4 for two cases – with and 

without an AC unit. The effect of the AC unit is most pronounced in Houston, due primarily to 

the higher outdoor temperature and, hence, the greater AC run time leading to greater moisture 

removal. Because the vast majority of the U.S. housing stock has air conditioning18, the 

estimated RH values with AC were used in assigning default humidity values.

Table 2-4. Modeled Year-round Indoor RH Level, by Climate Zone

Climate Zone RH Level with AC RH Level without AC

1 (coldest) 59.1% 63.8%

2 58.1% 64.7%

3 56.3% 66.4%

4 59.8% 72.6%

5 (warmest) 61.4% 85.3%

Air Exchange Rate and Airflows. The default indoor-outdoor airflows in FIAM-pwp were 

chosen such that the air exchange rate for each of the default structure types is 0.2 air changes 

per hour (ACH). For the exposure assessment report cited in Section 1, the focus is on PWPs in 

new housing; thus, a default value was chosen that lies toward the lower end of the distribution, 

as newer structures tend to be better sealed and have lower air exchange rates. For residences, the 

Exposure Factors Handbook (EFH) suggests 0.45 ACH as a central value and 0.18 as a lowerend value, based on statistics developed by Koontz and Rector19 (1995) from a database of 

perfluorocarbon tracer (PFT) measurement results. The EFH also cites Murray and Burmaster20

(1995), who used the same PFT database to summarize distributions for data subsets defined by

climate region and season; from that analysis, the 10th percentile value ranged from 0.1 to 0.3 ACH 

across climates and seasons, averaging about 0.20 ACH.

In a more recent study by Offerman et al.21 (2008) on ventilation and formaldehyde in new 

California homes, the 25th percentile value for air exchange rate was 0.2 ACH for the two larger 

subsets of homes that were studied. In a study by Maddalena22 (2008) concerning chemical 

 

18 According to the Residential Energy Consumption Survey, 84 percent of U.S. housing units had air-conditioning 

equipment as of 2005 (see http://205.254.135.24/emeu/recs/recs2005/hc2005_tables/detailed_tables2005.html).

19

 M.D. Koontz and H.E. Rector. 1995. Estimation of Distributions for Residential Air Exchange Rates. Report 

prepared for EPA/OPPT under Contract No. 68-D9-0166, Work Assignment No. 3-19. 

20 Burmaster D.M. Murray and D.E. Burmaster. 1995. Residential Air Exchange Rates in the United States: 

Empirical and Estimated Parametric Distribution by Season and Climatic Region. Risk Analysis 15: 459-465.

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

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

Conference, Baltimore, MD.

22 R. Maddalena, M. Russell, D.P. Sullivan, and M.G. Apte. 2008. Aldehyde and Other Volatile Organic Chemical 

Emissions in Four FEMA Temporary Housing Units – Final Report. LBNL-254E, Ernest Orlando Lawrence 

Berkeley National Laboratory, Environmental Energy Technologies Division, Berkeley, CA. Available at:

http://www.cdc.gov/nceh/ehhe/trailerstudy/pdfs/LBNL-254E.pdf.
12

emissions in four FEMA temporary housing units (camper trailers), the measured air exchange 

rate varied from 0.15 to 0.39 ACH across the trailers, averaging 0.25 ACH. Persily et al.23 (2010) 

modeled air infiltration rate distributions for the U.S. housing stock; for single-family homes 

built in 1990 or later, the 25th percentile value was 0.15 ACH and the 50th percentile value was 

0.26 ACH. Given the above statistics, 0.2 ACH was chosen as the value to represent new homes, 

including detached/attached homes, apartments, manufactured homes, and camper trailers. 

For the renovation case that also was examined as part of the formaldehyde exposure assessment, 

a central value such as 0.45 ACH, as suggested in the EFH, may be more appropriate. However, 

the data on which that suggested value is based are somewhat dated and there has been a general 

tendency toward increasing airtightness of U.S. homes over time; for example, the median value 

reported for the Offerman et al. (2008) study was 0.33 ACH. Thus, a value of 0.40 ACH – double

that used for new homes – is suggested for the renovation case.

As noted above, the indoor-outdoor airflow rates determine the air exchange rate. For example, 

for an apartment with a volume of 261 m

3

, the default indoor-outdoor airflow rate of 52.2 m

3

/h

results in an air exchange rate of 0.2 ACH (i.e., 52.2 m

3

/h / 262 m

3

= 0.2/h). The default air flows 

are balanced, that is, the airflow rate from outdoors to indoors is equal to the rate from indoors to 

outdoors. For two-zone structures – SF detached or attached homes – the house volume is 

equally apportioned between the upper and lower stories/floors and identical indoor-outdoor 

airflow rates are assumed for each story. Further, as an expedience, it is assumed that the airflow 

rate between the two indoor zones is balanced and equal to the indoor-outdoor airflow rate. Thus, 

for example, for a SF detached home with a volume of 811 m

3

or 405.5 m

3

per story, both the 

indoor-outdoor airflow rate for each story and the interzonal airflow rate between stories are set 

at 81.1 m3/h. 

Background Concentration. Although no studies have been conducted with the explicit 

purpose of characterizing the background concentration, such a value conceivably can be 

inferred from concentration distributions characterized via field monitoring studies. For example, 

if a field study were to focus on existing (as opposed to new) homes, then those toward the lower 

end of the concentration distribution presumably would be older homes with aged PWP sources 

and, thus, indicative of the background formaldehyde concentration. As noted in Section 3.3 of 

the exposure assessment report cited in Section 1 of this document, the results of field studies, 

when examined in this manner, indicate that an appropriate value for the residential 

formaldehyde background concentration would lie between 5 and 10 ppb. The midpoint of this 

range – 7.5 ppb – was chosen as the FIAM-pwp default value for the background concentration. 

Emissions Half Life. As described in Section 3.8 of this document, suggested values for the 

formaldehyde emissions half life, reflecting its decay indoors over time, range from 1.5 to 3 

years. An estimate of 2.92 years was based on a cross-sectional analysis of homes of varying 

ages, as a proxy for time-series data from one or a few homes. If none of the materials or 

furnishings in such homes were changed by the occupants, then this approach might provide an 

unbiased estimate of the half life. However, as a house ages the occupants will tend to add or 

 

23 A. Persily, A. Musser, and S.J. Emmerich. 2010. Modeled Infiltration Rate Distributions for U.S. Housing. 

Indoor Air 20: 473-485.
13

replace materials and furnishings, some of which may emit formaldehyde. The addition of these 

new sources will result in higher concentration than in the case where no new sources were 

introduced. As a result, the cross-sectional analysis will yield an artificially low estimate of the 

decay rate, which means an artificially high estimate of the half life. Thus, 2.92 years is best 

viewed as an upper-bound estimate. A more likely value, based on chamber studies of collections 

of pressed wood products as they age, is within the range of 1.5 to 2 years. A half life of 1.5 

years was chosen as the FIAM-pwp default value; with this half life, the emission rate 10 years 

after new construction would be about 1% of the initial value.

Temperature and Humidity Coefficients. The default values for these coefficients – 9799 for 

temperature and 0.0175 for humidity – are based on tests by Berge et al.24 involving two 

Norwegian particleboard specimens. These default values were used by HUD in setting the 1984 

standards for particleboard and hardwood plywood paneling, and also have been used in ASTM, 

ANSI and Composite Panel Association (CPA) standards or guidelines. The chamber tests 

underlying these coefficients were conducted at two temperature settings (22 and 28 C) and two 

relative humidity settings (30 and 60 %). 

An alternative set of coefficients – 8930 for temperature and 0.0195 for humidity – derives from 

work by Myers25, who reviewed the extant literature and based his recommended temperature

coefficient on chamber testing of about 40 specimens (particleboard and hardwood plywood 

paneling) from 11 laboratories. The temperature range of these tests was from 20 to 40 C. He

also developed a humidity coefficient, but expressed little confidence in the number; the relative 

humidity in the tests on which the correction factor was based ranged from 20 to 90 percent. 

Thus, his work encompassed more products and wider temperature/humidity ranges than the 

Berge et al. study. The HUD standard was promulgated before Dr. Myers completed his work. 

 

24 A. Berge, B. Mellagaard, P. Hanetho, and E. Ormstad. 1980. Formaldehyde Release from Particleboard –

Evaluation of a Mathematical Model. Holz Als Roh-und Werkstoff 38: 252-255. 

25 G. Myers. 1985. The Effects of Temperature and Humidity on Formaldehyde Emission from UF-bonded Boards:

A Literature Critique. Forest Products Journal 1985: 35:20-31. 
14

2.3 Source Screen

This screen (see Figure 2-5) enables the user to specify the types and amounts of wood products 

used or installed in the house, the zone(s) in which each type is located, and input parameters 

(slope and intercept) that govern the formaldehyde emission rate. Following a description of each 

type of input, guidance is given on choosing default inputs and/or adding custom inputs on this 

screen.

Figure 2-5. FIAM-pwp Source Screen.

2.3.1 Types of Inputs

PWP Type. There are six default PWP types in FIAM-pwp: 

 Oriented strand board (OSB) or softwood plywood (SWPW), typically used as 

underlayment

 Particleboard, often used as the platform to which a countertop laminate is affixed

 Medium density fiberboard (MDF), often used in cabinet drawers and/or shelves

 Coated CWPs (e.g., interior doors that are primed/painted, vinyl-covered molding)

 Hardwood plywood (HWPW), often used for wall paneling and sides/backs of cabinets 

or entertainment centers

 HWPW laminate, with uses similar to HWPW but made by affixing a wood veneer to a 

particleboard, medium-density fiberboard, or veneer-core platform.

A distinction is made between HWPW and HWPW laminate because EPA may regulate these two 

PWP types differently.

Emission Class. Default values for FIAM-pwp emission-rate parameters (slope and intercept) have 

been developed for four emission classes: Baseline; CARB1; CARB2; and NAF. As described 

below, these emission classes correspond to analytical or regulatory options that have been included

by EPA as part of the considerations for a formaldehyde rule.

Slope and Intercept. These two parameters govern the emission rate from PWPs, including 

dependence of the emission rate on the indoor formaldehyde concentration. The slope is a mass 

transfer coefficient that reflects the “backpressure” effect26 of the indoor formaldehyde 

concentration on the emission rate, whereas the intercept reflects the hypothetical emission rate 

when the indoor formaldehyde concentration is zero.

 

26 See, for example, M.A. Jayjock. 1994. Back Pressure Modeling of Indoor Air Concentration from Volatilizing 

Sources, Am. Ind. Hyg. Assoc. J. 55 (3): 230-235.
15

Equilibrium Concentration. The equilibrium concentration (mg/m3) displayed for each source 

is a calculated quantity – intercept (mg/m2-hr) divided by slope (m/hr) – that cannot be edited by 

the user; it is the theoretical maximum concentration that would occur if that were the only 

indoor source, with an arbitrarily large exposed surface area. The equilibrium concentration is 

indicative of the relative strength of emissions from each source. Lower-emitting sources, such 

as oriented strand board (OSB) or softwood plywood (SWPW), can act as net “absorbers” (i.e., 

indoor sinks) in the presence of higher-emitting sources, due to the backpressure effect.

Location (Zone) and Exposed Surface Area. If a two-zone structure is being modeled, then 

sources can be assigned to either or both of the two zones. As discussed later (see Section 2.3.2 

and Section 2.3.3), one input line is required for each source type in each zone. The default 

amounts of exposed surface area for sources in FIAM-pwp are the same as those used in EPA’s 

Formaldehyde from Composite Wood Products: Exposure Assessment dated March 2012. The 

exposed surface area (in m

2

) for a PWP can be viewed as its loading rate (in m

2

/m3) multiplied 

by the structure volume (in m3).

2.3.2 Adding Default Inputs

Each input line or row entered on the Source Screen refers to a specific type of source in a 

specific zone. For any of the five default structure types, the simplest way to add a group of 

sources is using the Add Default PWP Types button – taking this action loads each of the six 

default PWP types in each applicable zone (zone 1 for apartments, manufactured homes and 

trailers; zones 1 and 2 for SF detached/attached structures). Figure 2-6 shows the Source Screen 

after this action has been taken. In loading these defaults, there is a choice of Emission Class 

(baseline, CARB1, CARB2, or NAF) and Case (new-home or renovation). The example shown 

in the figure is for apartments, with baseline emissions and the new-home case. As indicated in 

the figure, user selections are made, or inputs are provided, in the lower portion of the screen; 

these choices/entries are then displayed in the upper portion.

Figure 2-6. Source Screen after Adding Default PWP Types

Display Area

Selection Area
16

Once added, any default can be removed or edited using the Remove/Edit buttons to the right. 

Alternatively, sources can be “turned off” by un-checking the corresponding check-box to the 

left. The column to the left of PWP Type indicates whether a given line or row in the table is a 

FIAM-pwp default (D), revision (R) of a default, or newly created (C). For example, when a 

default source is edited, the character in this column will change from D to R. If the line is later 

changed back to the default values for area, source and intercept (either by re-editing values or 

by using the Restore button), then the character displayed in the corresponding column will 

revert to D.

Sources also can be added, one at a time, using the Add button. In addition to Emission Class 

and Case, choices need to be made for PWP Type and Area. The default area for any 

combination of PWP and zone, displayed to the right, can be changed before clicking the Add 

button; after the button is clicked, the area (or any other value on that line) can be changed using 

the Edit button. In either case, any source for which default values have been changed will be 

marked with an R in the second column. All sources chosen or added can be removed at once 

using the Clear List button. The maximum number of sources allowed by FIAM-pwp is 20.

2.3.3 Adding Custom Inputs

Beyond adding default sources, the user also can add new sources and associated values using 

the Add Custom Values button; this is the only way to add sources if a structure type other than 

one of the defaults has been chosen. Inputs are required for the name of the PWP Type, the 

Emission Class, the Zone (1 or 2), and values for Area, Slope and Intercept. Figure 2-7 shows 

the appearance of the Source Screen after using this button to add one source line.

Figure 2-7. Source Screen after Adding Custom Values

2.3.3 Basis for Defaults

As noted above, the default PWP types available in FIAM-pwp are those used in the EPA 2012

formaldehyde exposure assessment. The four emission classes in FIAM-pwp, which correspond to 

analytical or regulatory options under consideration by EPA for a forthcoming formaldehyde rule, 

are described briefly in Table 2-5. EPA is considering some options beyond those listed, such as 

exclusion of HWPW laminates from the CARB1, CARB2 and NAF options. The basis for these 

classes is described in some detail in Section 4.4.8 of the report on EPA’s formaldehyde exposure 
17

assessment, which is reproduced as Appendix C in this document. In brief, within each emission 

class or regulatory option, a certain percentage of each product type is assumed to be in compliance 

with an existing standard such as CARB1; these assumed percentages vary across the emission 

classes. The procedure for estimating the emission rate for each product type, assuming that it meets 

a given standard, is described below.

Table 2-5. Alternative Analytical/Regulatory Options for EPA Formaldehyde Rule

Option a Description

Baseline Estimated emission levels (year 2013) in the absence of an EPA rule.

CARB1, Including 

Laminated Products

All CWPs, including laminated products, are at emission levels that meet 

CARB bPhase 1 standards. Products that meet CARB Phase 1 standards at 

baseline are assumed to remain at that emission level.

CARB2/Statutory Option,

Including Laminated 

Products

All CWPs, including laminated products, are at emission levels that meet 

CARB Phase 2 standards. Products that meet CARB Phase 2 standards at 

baseline are assumed to remain at that emission level.

NAF c, Including 

Laminated Products

All CWPs, including laminated products, are at emission levels that meet 

NAF standards. Products that meet NAF standards at baseline are assumed 

to remain at that emission level.

a Corresponds to emission class used in FIAM-pwp

b California Air Resources Board

c No added formaldehyde

Slope and Intercept. Appendix D of this document lists emission-rate parameters for various 

types of PWPs that were derived from sets of chamber tests conducted during the 1980s and up 

through the mid-1990s. Although the results from these tests are somewhat dated, an important 

distinguishing feature is that all such tests were conducted at multiple air exchange rates and/or 

product loading ratios, enabling estimation of model parameters – slope and intercept – that 

reflect the dependence of the emission rate on the indoor formaldehyde concentration. 

Equation 3-6 in Section 3.2 of this document is used as a basis for determining the intercept that 

is associated with a product meeting a certain emission standard, under the assumption that the 

air entering the chamber is formaldehyde-free:

b = CSS * (1 + m * Area / Q) * (Q / Area) (Eqn. 2.1)

where:

b = intercept – the emission rate at zero concentration in the air (mg/m2-hr)

CSS = steady-state formaldehyde concentration in the chamber (mg/m3)

m = slope – the mass transfer coefficient (m/hr)

Area = exposed surface area of the product (m2)

Q = airflow rate into / out of the chamber (m3/hr)

= air exchange rate (1/hr) x chamber volume (m3)
18

ASTM E1333-1027 is a commonly used method for demonstrating compliance with formaldehyde

emission standards. The method prescribes an air exchange rate for testing of 0.5 ACH and 

product loading ratios of 0.43 m

2

/m3for PB, 0.95 m

2

/m3for HWPW, and 0.26 m

2

/m3for MDF. 

Using these rates along with an assumed slope (m) for a given PWP type, the corresponding 

intercept can be determined for a hypothetical source that just meets a given emissions standard. 

The assumed slope for each PWP type is the average of the slopes across all sets of emission 

tests for that type (after excluding statistical outliers in certain cases), as listed in Appendix D.

For example, the CARB2 emission standard for MDF is 0.11 ppm or 0.135 mg/m3. An arbitrary 

chamber volume of 100 m

3

, which together with an air exchange rate 0.5/hr yields a value of 50 

m

3

/hr for Q, was used in the calculation. The volume does not matter here, as the exposed 

surface area scales to volume via the loading ratio; with a volume of 100 m

3

the corresponding 

surface area is 26 m2. A slope of 1.06 m/hr is assumed; this value is the average slope from about 

30 sets of chamber tests of different MDF specimens.

Substituting the assumed values given above into Eqn. 2-1, the intercept (b) that corresponds to 

the CARB2 emission standard for MDF is calculated as follows:

b = 0.135 mg/m3* (1 + 1.06 m/hr * 26m2/ 50 m3) * (50 m3/ 26 m2)

= 0.40 mg/m2-hr

Table 2-6 lists the assumed slopes and calculated intercepts for each default PWP type, using the 

procedure described above and assuming a mix of products meeting different standards within 

each emission class, as described in Appendix C.

Table 2-6. Default Slopes and Intercepts for Six Default PWP Types

PWP Type Slope

Intercept by Emission Class:

Baseline CARB1 CARB2 NAF

OSB/SWPW a0.61 0.030 0.030 0.030 0.030

Particleboard 0.70 0.131 0.124 0.122 0.030

MDF 1.06 0.281 0.271 0.269 0.128

Coated CWP 0.52 0.082 0.074 0.071 0.022

HWPW 0.27 0.042 0.025 0.021 0.013

HWPW Laminate 0.27 0.042 0.023 0.021 0.013

a This relatively low-emitting PWP is not subject to any emissions standard.

 

27 ASTM. 2010. Standard Test Method for Determining Formaldehyde Concentrations in Air and Emission Rates 

from Wood Products Using a Large Chamber. Designation E1333-10, American Society for Testing and Materials.
19

Exposed Surface Area. The default amounts of exposed surface area for sources are the same as 

those used in EPA’s 2012 formaldehyde exposure assessment. As described in Section 4.4.6 of 

the EPA exposure assessment report, the following philosophy was adopted in determining 

appropriate loading rates for different types of PWPs in various structure types – technically the 

specific source (cabinets, countertop, paneling, etc.) of each PWP type does not matter, as the 

model treats all sources in the same way; thus, for any loading scenario it is sufficient to 

determine and then sum all exposed surface areas per PWP type. Floor plans were used as a basis 

for developing the loadings. Spreadsheets were developed to document the loadings of PWP 

components in items such as cabinets, doors and trim, and to then sum across like components to 

develop the resultant loadings for each of six PWP types listed in the table above.

The default loading rates differ for the new-home vs. renovation cases. For the renovation case, it 

was assumed that an entire kitchen was renovated, including cabinets, countertop and ancillary 

areas such as a pantry. It was further assumed that the vinyl covering the kitchen floor and 

adjacent hallway floor was replaced with engineered wood. The interior doors, underlayment and 

trim throughout the house were assumed to remain as before the renovation, as were PWPs in 

any areas beyond the kitchen other than the adjacent hallway floor. Table 2-7 lists the

FIAM-pwp default values for exposed surface area (in m

2

), for each of the six PWP types listed 

above in each of the five default structure types, for the case of new construction. Table 2-8 lists 

the same information for the renovation case. 
20

Table 2-7. Default Exposed Surface Areas (in m2) for Six PWP Types, by Structure Type – New-home Case

Structure 

Type

OSB/SWPW Particleboard MDF Coated CWP HWPW HWPW Laminate

Zone Area Zone Area Zone Area Zone Area Zone Area Zone Area

Apartment 1 71.480 1 3.255 1 4.645 1 78.165 1 18.137 1 7.773

Camper 1 15.675 1 1.020 1 2.090 1 13.005 1 29.575 1 12.675

Mobile Home 1 98.478 1 3.906 1 4.366 1 65.786 1 26.610 1 11.404

SF Attached 

Home

1 71.645 1 1.390 1 2.175 1 52.910 1 6.2685 1 2.687

2 71.645 2 5.205 2 4.790 2 57.010 2 15.278 2 6.548

SF Detached 

Home

1 110.965 1 1.080 1 2.615 1 65.495 1 5.474 1 2.346

2 110.965 2 3.095 2 4.355 2 65.620 2 16.464 2 7.056

Table 2-8. Default Exposed Surface Areas (in m2) for Six PWP Types, by Structure Type – Renovation Case

Structure 

Type

OSB/SWPW Particleboard MDF Coated CWP HWPW HWPW Laminate

Zone Area Zone Area Zone Area Zone Area Zone Area Zone Area

Apartment 1 0 1 3.255 1 4.645 1 39.775 1 15.365 1 6.585

Camper 1 0 1 1.020 1 2.090 1 4.470 1 17.017 1 7.293

Mobile Home 1 0 1 3.906 1 4.366 1 23.153 1 34.710 1 14.876

SF Attached 

Home

1 0 1 1.390 1 2.175 1 10.340 1 2.286 1 0.980

2 0 2 5.205 2 4.790 2 33.205 2 18.680 2 8.006

SF Detached 

Home

1 0 1 1.080 1 2.615 1 11.755 1 2.286 1 0.980

2 0 2 3.095 2 4.355 2 31.790 2 27.342 2 11.718
21

2.4 Exposure Screen

This screen (Figure 2-8) enables the user to provide inputs related to the age of indoor sources and 

a formaldehyde level of interest (LOI) that is used in certain exposure calculations as well as 

activity patterns (time spent in different locations or environments) for exposure groups (e.g., 

infants, workers, retires) and formaldehyde concentrations in environments outside the home. As 

with the House Screen, the inputs here can be divided into three screen portions: (1) Age of 

Sources and Formaldehyde LOI (upper portion); (2) Exposure Groups (middle portion); and (3)

Concentrations Outside the Home (lower portion).

Figure 2-8. Exposure Screen

2.4.1 Inputs and Options for Age of Sources and Formaldehyde Level of Interest

Age of Sources. The assumed age of indoor sources at the start of the exposure period (i.e., at the 

time when an individual moves into the modeled structure) affects the modeled indoor 

formaldehyde concentrations to which an individual is exposed – in general, the lower the age of 

sources the higher the indoor concentrations at the start of the exposure period. With a source age 

of zero years – the model default – FIAM-pwp begins exposure calculations with an initial indoor 

concentration in a newly constructed home, which is intended to reflect an equilibrium condition 

shortly (typically within 30 days) after a house has been loaded with PWPs. A source age of zero 

years also would apply to new materials/furnishings added as part of a major remodeling effort. A

model limitation here is that all sources are assumed to be the same age; FIAM-pwp does not allow 

different ages to be specified for different sources.

Upper Portion

Lower Portion

Middle Portion
22

Formaldehyde Level of Interest (LOI). One of the exposure calculations is the percent of time an 

individual is exposed to formaldehyde concentrations at or above some level of interest. By default, 

the LOI is set to 10 ppb; this is easily changeable by entering a different value.

2.4.2 Inputs and Options for Exposure Groups

Default Groups. As shown above in Figure 2-8 (middle portion), FIAM-pwp has six default

exposure groups:

 Infants (0 to < 2 years of age)

 Pre-school/school children (age 2 to < 16 years of age)

 Non-industry workers (age 16 to <64)

 Fabrication-industry workers (age 16 to <64)

 Retirees (age 64 or greater)

 Part-time workers (age 16 to <64)

The choice of age breaks to distinguish infants and pre-school/school children from adults was 

driven by the possible age dependency of cancer slope factors. EPA has developed age-dependent

adjustment factors (ADAFs)28 to address the potential for differential potency associated with 

exposure during early life (below 16 years of age). There is a 10-fold adjustment for ages 0 to <2 

years, a 3-fold adjustment for ages 2 to <16, and no adjustment for ages 16 and older. Workers 

were split into full-time and part-time subgroups; full-time workers were further divided to 

distinguish non-industry workers from wood-industry segments where higher occupational 

formaldehyde exposures can occur. In terms of activity patterns, the retiree group also can be used 

to represent individuals of age 16 to <64 who are unemployed or working at home.

A subset of the exposure groups can be selected for a model run by checking or un-checking the 

check-boxes on the left for each line. The default values for any group can be modified using the 

Edit button to the right; the original/default values can be restored using the Restore button. As 

discussed below, user-defined exposure groups also can be added. The third column from the left 

on each line indicates whether the inputs for each exposure group are default (D) values, revisions 

(R) of defaults, or newly created (C).

The inputs for exposure groups are annual hours spent at each of five locations or environments:

 In zone 1 at home

 In zone 2 at home

 At work, school or daycare

 In a vehicle

 At all other locations

The sum of the default hours at these locations is 8,760, that is, the number of hours in one year; if 

the edited values do not sum to 8,760, then FIAM-pwp displays the sum in red font to indicate that 

corrections are in order. For two-zone structures, Zone 1 is conceptualized as upstairs or the 

sleeping area and Zone 2 is conceptualized as downstairs or the living area. For one-zone 

structures the sleeping and living areas are grouped together in a single zone.

 

28 See, for example, http://www.epa.gov/oswer/riskassessment/sghandbook/chemicals.htm.
23

Custom Groups. User-defined exposure groups can be added via the Add button, in the screen 

area below the listing of the default groups. Figure 2-9 shows illustrative inputs, just before 

adding them. Inputs are the name of the group and the annual hours spent in each of the five 

locations/environments, along with the formaldehyde concentration at the work/school/daycare 

location (see further discussion below, in Section 2.4.3). The hours by location should sum to 

8,760; if not, after the exposure group has been added FIAM-pwp will assign a red font to Total 

Hours to indicate that some adjustments are in order. Once added, newly defined exposure groups 

can be later removed by the user. A maximum of three newly defined exposure groups can be 

added.

For concentrations outside the home, the formaldehyde concentration at the work/school/daycare 

location can vary by exposure group. Thus, when a new exposure group is added the required 

inputs include not only the annual hours at each location but also the concentration at the 

work/school/daycare location.

Figure 2-9. Preparing to Add a New Exposure Group

2.4.3 Inputs and Options for Concentrations Outside the Home

Locations outside of, or away from, the home are included in FIAM-pwp to provide a total or 

“around-the-clock” exposure perspective. The concentration values assigned to these locations are 

displayed on the lower portion of the Exposure Screen (see Figure 2-8 on previous page). The 

default values shown in the figure can be modified by the user. The changes can be made in either 

the ppb or μg/m3column; FIAM-pwp will automatically convert in either direction using the 

relationship 1 ppb = 1.23 μg/m3. The model defaults can be restored at any time using the Load 

Default button at the bottom of the screen.
24

2.4.4 Basis for Defaults

Exposure Groups. The 2009 Exposure Factors Handbook (EFH) update29 (External Review 

Draft) was the primary source of information on activity patterns, that is, how an individual’s time 

is allocated among residential and non-residential locations. Estimates of annual hours at home 

and non-home locations are provided for each exposure group in Table 2-9. The total time spent at 

home was determined from EFH tables that indicate the time spent at home in all rooms 

combined. The time at home in Zone 1 was defined as the sum of time spent in the bedroom or 

bathroom. EFH statistics indicate that infants spend, on the average across the population, one 

hour per day at daycare facilities. Children of school age were assumed to spend 6.5 hours per day 

at school, for 180 days per year. Full-time workers were assumed to spend 8 hours per day at 

work, for 250 days per year. The hours at work or school for part-time workers and retirees were 

based on EFH statistics, as were the hours in a vehicle for each exposure group. The time spent in 

“all other” environments was determined by subtraction from the total time budget of 8,760 hours 

per year.

Table 2-9. Default Annual Hours at Home and Non-home Locations, by Exposure Group.

Exposure Group

Location

Home

Zone 1 a

Home

Zone 2

Work or 

School or 

Daycare

In

Vehicle

All

Other

Total, all 

Locations

Infants 4,958 1,652 365 252 1,533 8,760

Pre-school/School 4,253 1,292 1,170 356 1,689 8,760

Non-industry Workers 3,327 2,032 2,000 590 811 8,760

Fabrication Workers 3,327 2,032 2,000 590 811 8,760

Retirees 3,607 3,538 107 372 1,136 8,760

Part-Time Workers 3,935 2,038 1,000 401 1,386 8,760

a Zone 1 – upstairs (sleeping area); Zone 2 – downstairs (living area).

Concentrations Outside the Home. Unlike the residential concentrations calculated by

FIAM-pwp, which are decayed over time, these concentrations are treated as constant over time. 

The default concentrations (see Table 2-10) assigned to non-home locations, which assume a 

baseline emissions scenario, were derived from several sources. In-vehicle and outdoor/other 

concentrations were based on average values reported from EPA’s air monitoring network (see 

Section 3.1 of the formaldehyde exposure assessment report). The value assigned to vehicles (6 

ppb) is the average reported for the “mobile” land use category; the value assigned to 

outdoors/other (3 ppb) is the average of values reported for “residential” and “commercial” land 

use categories, with consideration also given to outdoor formaldehyde levels measured in 

residential monitoring studies. The value assigned to the fabrication-industry workplace was taken 

from EPA’s occupational exposure assessment for formaldehyde30. The values assigned to 

daycares, schools and non-industry workplaces involved calculations that are described below.

 

29 http://cfpub.epa.gov/ncea/cfm/recordisplay.cfm?deid=209866

30 USEPA. 2011. Draft Final. Assessment of Occupational Exposure to Formaldehyde from Composite Wood Products.

U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics, Chemical Engineering 

Branch, 1200 Pennsylvania Avenue NW, Washington, DC 20460. 6 October 2011.
25

Table 2-10. Default Concentration Values for Non-home Locations

Location

Formaldehyde Concentration

ppb μg/m3

Daycare 9.8 12.0

School 8.7 10.7

Work, Non-Industry 10.0 12.3

Work, Fabrication 199.5 244.9

Vehicle 6.0 7.4

Outdoors/Other 3.0 3.7

In developing the baseline concentrations for daycare, school and non-industry work locations, it 

was assumed that 10 percent31 of the buildings in each category were of recent construction or 

were recently renovated. As with the fabrication-industry workplace, the value for newer buildings 

associated with the non-industry workplace was taken from the EPA’s 2011 Assessment of 

Occupational Exposure to Formaldehyde from Composite Wood Products report referenced 

above. Newer schools were assigned the 95th percentile formaldehyde concentration, thought to 

represent newer or recently renovated buildings, from a relatively large monitoring study32 of 

portable and traditional classrooms in California schools. The value assigned to newer daycare 

facilities is an average of the concentration for non-industry work and averages from field 

studies33,34 of newer homes. For older daycare, school and non-industry buildings, the 25th

percentile for formaldehyde levels measured in the EPA BASE35 study was used. Table 2-11

shows the derivation of the age-weighted baseline concentration values for these building types. 

 

31 The most recent DOE/EIA Commercial Building Energy Consumption Survey (CBECS) indicated that about 5 % of 

office buildings and 8 % of education buildings were of recent construction (i.e., 3 years old or less); see 

http://www.eia.gov/emeu/cbecs/cbecs2003/detailed_tables_2003/detailed_tables_2003.html, Table B-8.

32 CARB. 2004. Environmental Health Conditions in California’s Portable Classrooms. Report to the California 

Legislature by the California Air Resources Board (CARB) and the California Department of Health Services, 

November 2004. Available at: http://www.arb.ca.gov/research/apr/reports/l3006.pdf.

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

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

Conference, Baltimore, MD.

34 A.T Hodgson, A.F. Rudd, D. Beal, and S. Chandra. 2000. Volatile Organic Compound Concentrations and 

Emission Rates in New Manufactured and Site-Built Houses. Indoor Air 10: 178-192.

35 http://www.epa.gov/iaq/base/
26

Table 2-11. Derivation of Baseline Values for Daycare, School and Work Locations

Location

Formaldehyde Concentration, ppb

Older 

Buildings

Newer 

Buildings

Composite aof 

Older & Newer

Daycare 7.0 35.0 9.8

School 7.0 24.0 8.7

Work, Non-Industry 7.0 36.5 10.0

a Composite value = 90% x (value for older buildings) + 10% x (value for newer buildings).

The values for vehicle and outdoor/other concentrations are assumed to be the same for different 

analytical options (baseline, CARB1, etc.) whereas those for the other locations can vary (see 

Table 2-12), because the formaldehyde concentrations in the fabrication workplace, as well as 

newer/renovated daycares, schools and non-industry workplaces, could be affected by a new rule 

for formaldehyde. As with the baseline values, suggested concentration values for other analytical 

options for newer non-industry workplaces were taken from the recent EPA report referenced 

above. For daycare facilities, the percent reductions across analytical options used for fabrication 

and non-industry work were applied. A similar approach was used for schools, except that the 

percent reductions were lowered slightly (i.e., by 10 percent) to account for generally higher 

prevailing air exchange rates in schools, which would tend to temper the effect of formaldehyde 

emission reductions.

Table 2-12. Concentration Values for Selected Non-home Locations, by Emissions Scenario

Location

Formaldehyde Concentration, ppb

Baseline a CARB1 CARB2 NAF

Daycare b9.8 9.5 9.4 8.0

School b8.7 8.1 8.0 7.3

Work, Non-Industry b10.0 9.4 9.3 7.9

Work, Fabrication 199.5 192.0 190.8 93.0

a FIAM-pwp provides baseline values as the defaults for non-home locations.

b

The values assigned to these locations are a weighted average of those for older and newer buildings; see text .

In calculating total exposure for the exposure groups, FIAM-pwp combines the time spent in various

locations with concentrations encountered at those locations. For the work/school/daycare 

location, FIAM-pwp uses the daycare concentration for infants, the school concentration for 

children, and the work concentration for workers and retirees. When a new exposure group is 

added (see Figure 2-9 above), in addition to the time spent at each location the user also needs to 

enter an appropriate value for the concentration at the work/school/daycare location.
27

2.5 Result Screen

This screen (Figure 2-10) lists modeled indoor formaldehyde concentrations (upper portion of the 

screen) together with exposure estimates (lower portion). Further details are provided below.

Figure 2-10. Result Screen with First Two Exposure Groups Selected

2.5.1 Concentration Results

FIAM-pwp lists the concentrations calculated for Zones 1 and 2 under the temperature and humidity

conditions entered by the user; the Zone 2 results are set to the background concentration if the 

user has specified a one-zone structure. Section 3.9 describes the procedures and gives the 

equations that are used to adjust modeled concentrations to the user-specified conditions. 

Concentrations are reported both in volume/volume (ppb) and mass/volume (μg/m3) units.

Initial concentrations – those listed for 0 months under Time from Initial Concentration – reflect 

the condition shortly after a house has been loaded with PWPs. As described in Section 3.7, 

estimates of initial indoor concentrations due to PWPs are decayed exponentially by the model, in 

accordance with a user-specified emissions half life (default of 1.5 years), to develop estimates for 

subsequent points in time. Results are provided for time points that are 3 months, 6 months and 12 

months later than that for the initial concentration values, plus a user-specified point in time (24 

months by default). The user-specified value for length of time over which to decay the initial 

concentration is entered on the House Screen, where the default emissions half life also can be 

changed.

Upper Portion

Lower Portion
28

The results also include the time required for the highest initial indoor concentration to drop below 

a user-specified concentration (an input on the House Screen). The equation used for this 

calculation is given in Section 3.7. By definition, the indoor concentration cannot decay to a value 

that is lower than the background concentration (an input on the House Screen). If the value to 

which the indoor concentration is to be decayed is less than or equal to the background 

concentration, then the calculation result is undefined. In such cases, FIAM-pwp reports 0 months as 

the time to decay.

As described in greater detail in Section 3, the model calculates the initial indoor concentration(s)

under an assumed steady-state condition. In real life, a true steady-state is never achieved, due to 

the dynamics of formaldehyde source emissions and indoor sinks. Formaldehyde sources typically 

do not emit at a constant rate, but rather at a rate that tends to decline over time as the “reservoir” 

of initial free formaldehyde, coupled with that formed through hydrolysis, is gradually depleted. 

Although a true steady-state is never reached, it has been observed in field studies that the 

combined actions of sources and sinks will result in an initial rise in the indoor concentration, 

followed by a “leveling off” period that may be fairly brief and then a longer period of gradual 

decline in concentration. The initial “steady-state” concentrations estimated by FIAM-pwp are 

intended to approximate the indoor concentrations during the leveling-off period, which typically 

would occur within 30 days or less following the installation of PWPs.

2.5.2 Exposure Results

An average daily concentration (ADC) is reported for each zone of the house, or other modeled 

structure, and for each default or added exposure group selected by the user. The percent of time 

during which the concentration in each zone is greater than a user-specified level of interest (LOI) 

is also reported. For each exposure metric, annual averages are reported for the first modeled year 

and for the next ten years thereafter. ADC calculations for exposure groups are based on time spent, 

and concentrations encountered, in the two zones of the house together with times and concentrations 

when the individual is out of the house. Concentrations in the house are calculated by the model and 

decay over time, whereas out-of-house concentrations are input by the user on the Exposure Screen 

and are assumed to be constant over time. Further details on exposure calculations, including 

equations, are provided in Section 3.10.

2.5.3 Printing Inputs and Results

An option at the top of the Result Screen – Generate pdf – enables the user to create a pdf file 

that contains both the inputs and results for a model run. The pdf mimics the appearance of the 

input screens and creates a table listing the same model outputs as shown on the Result Screen.

2.5.4 Archiving Inputs and Results for a Run

Another option at the top of the Result Screen – Download Excel – enables the user to save a file 

in Excel format that contains all inputs and results for the current model run. The saved Excel file 

has three sheets or tabs, named Combined, Inputs and Results. FIAM-pwp creates an Excel file for 

each run name; if multiple runs are made under the same run name and the download button is 

pressed after each run, then FIAM-pwp will append each set of inputs and results to the Excel file. 

The Inputs sheet has one column for each input and creates one Excel row for each model run. The 

Results sheet has columns for each output and creates 11 rows for each model run, because ADC 

results are provided for Zone 1, Zone 2, and exposure groups for each of 11 years (year 0-1, year 

1-2, etc.). FIAM-pwp concentration results, which relate to single points in time, are duplicated 

across the 11 rows. Similarly, the Combined sheet containing inputs and results has 11 rows and 

duplicates both the inputs and the modeled concentrations across the 11 rows that are generated 

for each run.
29

2.6 Model Access and Additional Features

2.6.1 Accessing the IGEMS Website

The IGEMS website that houses FIAM-pwp can be accessed at https://ofmpub.epa.gov/igems-jsp/.

After the Privacy Act Notification has been acknowledged, a user can log in if he/she has an 

account (otherwise set up a new account). After logging in, a user can access FIAM-pwp by

selecting it benath Formaldehyde Indoor Air Model from the choices to the left.

2.6.2 Saving Inputs for Later Access

The first time FIAM-pwp is used, the Run FIAM button at the bottom will not be available until 

inputs have been provided or choices have been made on the House Screen, Source Screen and 

Exposure Screen. When the run button is pressed, FIAM-pwp will prompt for a Run File Name.

Up to 20 characters can be used in naming the file. After the SAVE button is pressed, FIAM-pwp

will show the Result Screen. A new run file can be created, or a current one saved, at any time –

creating, opening and saving run files works much like similar functions in Microsoft Windows. 

Once a run file has been named and saved, it will be included on the list that can be accessed by

pressing the Open File button. If the SAVE button is pressed using a file name that already exists, 

FIAM-pwp will ask whether the existing run file should be overwritten. 

2.6.3 Context-sensitive Help

FIAM-pwp provides context-sensitive help via a series of buttons with a “?” icon. When the icon 

is pressed a dialog box appears with help specific to the input or option with which it is associated. 

Pressing the Close button returns the user to the input or output screen. There is also an Overview

button for each of the input screens that; when this button is pressed FIAM-pwp displays a brief 

description of the purpose of the screen and the types of inputs required. All of the help dialogs 

provided by FIAM-pwp can be saved by pressing the Save FIAM Help button that is accessed on 

the Result Screen.

2.7 Example Applications

Six examples are provided in this section to illustrate certain FIAM-pwp features and options as well as 

the insights that can be obtained from making model runs, particularly a series of related runs.

2.7.1 Example 1 – Modeling with Model Default Values

This example illustrates how FIAM-pwp results can be obtained quickly when using default values 

for items such as type of structure, climate zone, indoor sources, and exposure groups. As 

described in previous sections, the default values reflect decisions or choices made in the EPA 

2012 formaldehyde exposure assessment (see footnote 10 in Section 1). The example further 

illustrates how minor changes from the defaults can be made to view complementary results.

The key choices for this example are indicated in the run notes provided in Table 2-13 for Part 1. 

First, on the House Screen, Apartment (a one-zone structure) was selected as the structure type 

and Zone 5 was selected as the climate zone. Second, default PWP types were loaded on the 

source screen with the choices of baseline emissions and new construction (as opposed to 

renovation). Third, default exposure groups were chosen on the Exposure Screen. Model defaults 

were used for all other inputs not mentioned above. The modeling results indicated an initial 

indoor formaldehyde concentration of 78.6 ppb.
30

Table 2-13. Run Notes for Example 1

Screen Notes

Part 1

House Select Apartment and Climate Zone 5; defaults otherwise

Source Select "Add Default PWP Types" with Emission Class = baseline, Case = 

new-home (six sources will appear, all checked)

Exposure Select default exposure groups (no action needed; all checked by default)

Result Initial concentration – 78.6 ppb (97.1 μg/m3) for Zone 1, N/A for Zone 2

Part 2

House Keep Apartment and Climate Zone 5

Source Select "Add Default PWP Types" with Emission Class = CARB2, Case = 

new-home (Note: clear previous list before adding new sources)

Result Initial concentration – 68.5 ppb (84.6 μg/m3) for Zone 1, N/A for Zone 2

Part 3

House Keep Apartment and Climate Zone 5; change air exchange rate to 0.4 ACH

by changing flows to and from outside from 52.26 to 104.52 m3/h

Source Select default PWP types – Emission Class = baseline, Case = renovation

(Note: clear previous list before adding new sources)

Result Initial concentration – 49.0 ppb (60.5 μg/m3) for Zone 1, N/A for Zone 2

Part 4

House Select SF Detached; keep Climate Zone 5

Source Program will issue a warning due to change in structure type; click OK.

Select default PWP types – Emission Class = baseline, Case = new-home

(Note: clear previous list before adding new sources; 12 sources will appear)

Result Initial concentration – 57.1 ppb (70.5 μg/m3) for Zone 1, 

59.9 ppb (74.0 μg/m3) for Zone 2

Next (see Part 2), the emission class was changed to CARB2, which has lower emissions than 

baseline. All other inputs were kept the same as for Part 1. The modeled initial concentration in 

this case was 68.5 ppb. Then (Part 3) the emission class was changed back to baseline while 

choosing the loading rates for renovation instead of new construction and changing the air 

exchange rate from 0.2 to 0.4 ACH, by doubling the airflow rates to/from outdoors; the modeled 

initial concentration in this case was 49.0 ppb. Last (Part 4), baseline emissions and new 

construction were chosen while changing the structure type to SF detached (a two-zone structure). 

The initial modeling results in this case were 57.1 ppb for Zone 1 (sleeping area) and 59.9 ppb for 

Zone 2 (living area).

2.7.2 Example 2 – Modeling a Hypothetical Chamber Test for CARB Compliance

This example is drawn from the 2012 EPA exposure assessment report previously cited in Section 1.

ASTM E1333-10, Standard Test Method for Determining Formaldehyde Concentrations in Air 

and Emission Rates from Wood Products Using a Large Chamber (ASTM 2010), is commonly 

used to demonstrate compliance with formaldehyde emission standards. The method prescribes an 

air exchange rate for testing of 0.5/hr and product loading ratios of 0.43 m

2

/m3for PB, 0.95 m

2

/m3for 

HWPW, and 0.26 m

2

/m3for MDF. Using these rates and an assumed slope, the intercept can be 

determined for a hypothetical source that just meets a given emissions standard. The CARB2 
31

emission standard for MDF is 0.11 ppm (110 ppb) or 0.135 mg/m3. Per the 2012 EPA exposure 

assessment, a MDF specimen complying with the standard would have an intercept value of 0.40 

mg/m2-hr, assuming a slope of 1.06 m/hr based on historical chamber tests of MDF specimens.

FIAM-pwp was used to model a hypothetical chamber test with an MDF specimen that just meets 

the standard (see notes for this example in Table 2-14). An assumed air exchange rate of 0.5/hr

and chamber volume of 100 m3equates to an airflow rate of 50 m

3

/hr into and out of the chamber. 

The volume assumed does not matter, as the exposed surface area scales to the volume via the 

loading ratio – with a volume of 100 m

3

the corresponding MDF surface area is 26 m

2

. The 

assumed slope was 1.06 m/hr, the average value from ~ 30 historical tests of various MDF 

specimens (see Appendix D of this report). The modeling result – an initial concentration of 108.5 

ppb – confirms compliance of the hypothetical MDF specimen with the standard (110 ppb). 

Table 2-14. Run Notes for Example 2

Screen Notes

House Use “Add Your Own Structure Type” to create new type named Chamber

Zone 1 (Chamber) – volume = 100.0 m

3

; flow to/from outside = 50.0 m3/hr

Check box next to “Check here if one-zone case”

Create climate zone named Standard Conditions, 23 °C and 50 % RH

Change background concentration to 0.0 ppb

Source Add custom values for PWP type MDF, Emission Class Meet CARB2 110 ppb

Area = 26.0 m2, Slope = 1.06 m/hr; Intercept = 0.4 mg/m2-hr

Exposure No exposure group selected (uncheck all)

Result Initial concentration = 108.5 ppb (134.1 μg/m3)

2.7.3 Example 3 – Demonstrating an Equilibrium Concentration

The default MDF source in FIAM-pwp has a baseline emission rate characterized by an intercept 

of 0.28122 mg/m2-hr and a slope of 1.06 m/hr. The formaldehyde equilibrium concentration, 

obtained by dividing the intercept by the slope, is 0.2653 mg/m3or 265.3 μg/m3. Attainment of an 

equilibrium concentration can be demonstrated via modeling. FIAM-pwp was run with a 

hypothetical chamber volume of 100 m

3

and an airflow rate of 50 m

3

/hr, as in the previous 

example. An arbitrarily large exposed surface area of 100,000 m

3 was assumed. The modeled 

concentration in the chamber (265.2 μg/m3; see run notes in Table 2-15) is consistent with the 

calculated value for the equilibrium concentration.

Table 2-15. Run Notes for Example 3

Screen Notes

House Use created structure type named Chamber

Zone 1 (Chamber) – volume = 100.0 m3; flow to/from outside = 50.0 m3/hr

Check box next to “Check here if one-zone case”

Use created climate zone named Standard Conditions, 23 °C and 50 % RH

Change background concentration to 0.0 ppb

Source Add PWP type MDF, Emission Class = baseline, Case = new-home

Area = 100,000 m2, Slope = 1.06 m/hr; Intercept = 0.28122 mg/m2-hr

Exposure No exposure group selected (uncheck all)

Result Initial concentration = 214.6 ppb (265.2 μg/m3)
32

2.7.4 Example 4 – Examining the Incremental Contribution of a Product

As described in Section 2.1, a prior online version of FIAM-pwp (v1.0) was subjected to a peer 

review. In commenting on the backpressure effect, one of the reviewers noted the following:

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

models. In the acute setting, the backpressure 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 or 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. 

To illustrate some of the reviewer’s points, the incremental contributions of illustrative CWPs

were examined by modeling them both as a single source and in the presence of other CWPs. As 

shown in Figure 2-11, FIAM -pwp has loadings of six default PWP types for apartments; these were 

used in a series of model runs with an apartment as the structure types (see run notes in Table 2-16). 

For these runs, the background concentration was set to 0.0 ppb.

Figure 2-11. Emission-rate Parameters for FIAM-pwp Default PWPs in an Apartment.

For the first run, only MDF was selected (by checking the corresponding check-box), resulting in a 

modeled initial concentration of 23.4 ppb. Next, the model was run checking all sources except MDF, 

with a resultant modeled initial concentration of 70.7 ppb. The modeled initial concentration when 

all sources, including MDF, were selected was 77.3 ppb – less than the sum of results from the 

first two runs and, thus, illustrating the suppression of emissions in the presence of other sources. 

A similar series of runs was made with OSB/SWPW as the focus. The modeled concentration for 

OSB/SWPW only was 22.9 ppb – similar to that for MDF, but with an exposed surface area about 

15 times as large. The modeled concentration with all sources except OSB/SWPW (88.3) ppb) 

was higher than that obtained with all sources, including OSB/SWPW (77.3 ppb). This outcome 

illustrates that OSB/SWPW acts as a “net absorber” in the presence of the other default PWP types 

used in the example. The relative emissions strength of each PWP type is apparent from its 

equilibrium concentration, which for MDF is more than five times that of OSB/SWPW

(equilibrium concentrations are listed in Figure 2-11).
33

Table 2-16. Run Notes for Example 4

Screen Notes

Part 1

House Select Apartment and Climate Zone 5; background = 0; defaults otherwise

Source Select default PWP types – Emission Class = baseline, Case = new-home

Check only the MDF type

Exposure No exposure group selected (uncheck all)

Result Initial concentration – 23.4 ppb (28.9 μg/m3)

Part 2

House Keep Apartment, Climate Zone 5, and background = 0

Source Keep default PWP types – Emission Class = baseline, Case = new-home

Check all PWP types except MDF

Result Initial concentration – 70.7 ppb (87.4 μg/m3)

Part 3

House Keep Apartment, Climate Zone 5, and background = 0

Source Keep default PWP types – Emission Class = baseline, Case = new-home

Check all PWP types, including MDF

Result Initial concentration – 77.3 ppb (95.5 μg/m3)

Part 4

House Keep Apartment, Climate Zone 5, and background = 0

Source Keep default PWP types – Emission Class = baseline, Case = new-home

Check only the OSB/SWPW type

Result Initial concentration – 22.9 ppb (28.3 μg/m3)

Part 5

House Keep Apartment and Climate Zone 5

Source Keep default PWP types – Emission Class = baseline, Case = new-home

Check all PWP types except OSB/SWPW

Result Initial concentration – 88.3 ppb (109.0 μg/m3)

2.7.5 Example 5 – Varying the Decay Rate (Half Life)

As discussed later in this report (see Section 3.5), several peer reviewers suggested multiple or 

changing decay rates to represent, for example, “fast” (earlier) and “slow” (later) periods of 

exponentially declining formaldehyde emissions. Such an undertaking, although a “stretch” of 

FIAM-pwp’s capabilities, is possible provided that adequate care is taken. The key to varying the 

decay rate is having a means to predict the initial concentration for the point in time at which this 

rate is changed.

For this example it was assumed that the fast decay could be represented by a half life of one year 

and the slow decay by a half life of three years. The fast-decay period was assumed to last one 

year, followed by a slow-decay period of nine years. The resultant indoor-concentration time 

series over the 10 years, calculated externally from the model, is shown in Figure 2-12 together 

with the default background concentration (7.5 ppb) that was assumed for the example. The 

change in the decay rate can be seen as the “kink” in the time series at an elapsed time of one year.
34

Figure 2-12. Formaldehyde Concentration Decline with Fast and Slow Decay Periods.

Inputs for the example were simplified by assuming a single emitter – MDF. For the first run (Part 1;

see Table 2-17), a half life of 1.0 years was assumed together with an MDF exposed surface area 

of 18.5 m

2

; the modeled initial concentration was 63.7 ppb and the modeled concentration after 

one year was 35.6 ppb. For the second run (Part 2; see Table 2-18), the assumed half life was 

changed to 3.0 years. For this run it also was necessary to adjust the source inputs such that the 

modeled initial concentration would be 35.6 ppb. Through trial and error it was determined that 

this outcome could be obtained by changing the intercept for the MDF emission rate from the 

default value of 0.28122 to 0.1455 mg/m2-hr. The modeled initial concentration in this case was 

35.6 ppb and the modeled concentration after 108 months (9 years, or 10 years cumulative) was 

11.0 ppb. The collective results from the two runs are consistent with the time series in Figure 2-12.

Table 2-17. Run Notes for Example 5, Part 1 (fast decay period)

Screen Notes

House Select Apartment as the structure type

Use created climate zone named Standard Conditions, 23 °C and 50 % RH

Half life = 1.0 years

Source MDF/baseline/new-home with surface area = 18.35 m2and default values for 

slope/intercept (add only this type with default surface area; edit surface area)

Result Initial concentration = 63.7 ppb (78.7 μg/m3)

Concentration at 12 months = 35.6 ppb (44.0 μg/m3)

Table 2-18. Run Notes for Example 5, Part 2 (slow decay period)

Screen Notes

House Change half life to 3.0 years

Change time from initial concentration to 108 months (to get to 10 years)

Source Change intercept for MDF to 0.1455 mg/m

2

-hr (by editing)

Result Initial concentration = 35.6 ppb (44.0 μg/m3) 

Concentration at 108 months = 11.0 ppb (13.6 μg/m3) 

Fast Decay

Slow Decay
35

3. MODEL CALCULATIONS AND ASSUMPTIONS

This section describes the technical basis for the calculation routines in the software, which consist 

of two major components. The first of these predicts the initial “steady-state” formaldehyde 

concentration(s) in one or two zones of the house due to the combined actions of sources and sinks 

in a newly constructed home. The primary input for this component is an empirical emission-rate 

algorithm (i.e., estimated initial emission rate as a function of formaldehyde concentration and 

other environmental variables) for various types of pressed-wood products. For more information 

regarding formaldehyde emissions from pressed-wood products, the reader is referred to an EPA report.

36

The second component of the model is a decay function that gradually reduces the estimated initial 

steady-state concentration(s) due to indoor sources and sinks over a specified period of time. The 

concentrations are reduced in accordance with an exponential decay function, using a decay rate 

that is determined from a user input for the formaldehyde emissions half life. With this function, 

the concentrations at any point in the “life” of a home can be predicted, on the assumption that the 

materials in the house do not change over time and the air exchange rate, indoor temperature and 

indoor humidity also remain constant.

The first model component is based on the fundamental principle of conservation of mass in an 

indoor environment, as described in Section 3.1. A model (see Section 3.2) was developed during 

the 1980s to predict the formaldehyde concentration in a single indoor compartment under an 

assumed steady-state condition, taking into account the dependence of formaldehyde emission 

rates on the prevailing indoor concentration. This model has been extended (see Section 3.3) to 

predict formaldehyde concentrations in two indoor compartments under the same assumption. 

Whether a one-chamber or two-chamber model is used in the calculations depends on the type of 

structure selected by the model user (see Section 2.2).

An alternative formulation of the model, with an embedded calculation of the equilibrium 

concentration, is described and discussed in Section 3.4. Other potentially useful modeling 

constructs are presented and discussed in Section 3.5. Although the model, as currently 

formulated, technically has the capability to represent indoor sinks, this model feature has been 

dropped due to several uncertainties together with the lack of supporting data. Some discussion of 

this topic is provided in Section 3.6.

Section 3.7 describes the basis for the equation used to decay the initial steady-state 

concentration(s) over time and provides examples to illustrate calculations for concentrations at 

any point in time and for the time to reach a pre-defined concentration such as 10 ppb. Section 3.8

documents the basis for selection of a default value for the formaldehyde concentration half life, 

which is needed to decay the concentration over time. Section 3.9 describes the equations used to 

adjust indoor formaldehyde concentrations calculated by the model, under baseline indoor 

conditions of 23  C and 50 % relative humidity, to the user’s inputs for temperature and humidity

conditions. Section 3.10 describes the exposure calculations used in the model.

 

36 USEPA. 1996. Sources and Factors Affecting Indoor Emissions from Engineered Wood Products: Summary and 

Evaluation of Current Literature. EPA-600/R-96-067, NTIS PB96-183876, June 1996. Project Summary is 

available at: http://nepis.epa.gov/Adobe/PDF/P1000I7X.PDF.
36

3.1 Generalized Mass-Balance Equation

The formaldehyde concentration in an indoor environment is dependent on two types of factors: 

(1) those that increase the concentration, such as emissions from formaldehyde sources, and (2) 

those that decrease the concentration, such as dilution with outdoor air assumed to be lower in 

concentration than that indoors. Although the two types of factors would appear to be distinct 

entities, they actually are interdependent because of formaldehyde’s general behavior – emission 

rates from pressed-wood products are dependent on the concentration in the indoor volume, and 

the formaldehyde concentration, in turn, is dependent on these emission rates as well as removal 

mechanisms.

The principle of conservation of mass in an indoor environment is fundamental to the 

formaldehyde modeling process. Simply stated, this principle means that the rate of change over 

time in the formaldehyde mass indoors is determined by the rates of generation and removal. This 

principle can be expressed mathematically for an indoor compartment or zone as follows:

     

 

     

in Zone i

num sinks

p

i p

in Zone i

num sources

k

i k

j i

j

ij i

j i

j

i i i ji j

i

i Q C Q C Q C Q C E S

dt

dC V

1

,

1

0 0 0 ,

(Eqn. 3-1)

where:

V = indoor volume (m3)

C = formaldehyde concentration (mg/m3)

t = time (hr)

i, j = indoor compartments (0 signifies the outdoors)

Q = airflow rate between indoor zones or between indoor zone and 

outdoors (m3/hr)

Ei,k = emission rate for formaldehyde source k in zone i (mg/hr)

Si,p = sorption rate for formaldehyde absorbent p in zone i (mg/hr).

Model results are presented in both volume/volume units (ppb) and mass/volume units (μg/m3). 

Conversion between these unit conventions is performed using the ideal gas law, assuming 

constant pressure (1 atmosphere) and the user-specified temperature.
37

3.2 Steady-State Model for Initial Formaldehyde Concentration in One Compartment

A model developed in the 1980s by Matthews and colleagues was designed to estimate the steadystate formaldehyde concentration resulting from emission sources in a single indoor compartment. 

The following discussion is in large part excerpted from Matthews et al.37 The underlying model 

theory and its derivation are described in the same document and in a paper by Matthews et al.

38

At steady state, the formaldehyde (CH2O) concentration in a single compartment can be expressed 

as follows:

[CH2O]SS = [CH2O]out + CH2OER/(F * ACH * VOL) (Eqn. 3-2)

where:

[CH2O]SS = steady-state formaldehyde concentration inside the compartment

(mg/m3)

[CH2O]out = formaldehyde concentration outside the compartment (mg/m3), 

assumed to be constant over time

CH2OER = emission rate of a formaldehyde source inside the compartment 

(mg/h)

F = fraction of air coming into the compartment that mixes within 

the volume (i.e., the mixing factor)

ACH = air exchange rate with outdoors for the compartment (hr-1)

VOL = volume of the compartment (m3).

Assuming that F is equal to unity and using Q (airflow rate into and out of the compartment, in 

m

3

/hr) to denote the product of ACH * VOL, Equation 3-2 becomes:

[CH2O]SS = [CH2O]out + CH2OER/Q (Eqn. 3-3)

Application of the model as expressed in Equation 3-3 is simplified by assuming that all 

parameters in the equation remain constant (at steady state) and that there are no permanent losses 

of formaldehyde due to irreversible sorption to sinks.

Emissions from pressed-wood sources are area-dependent in that the magnitude of the emission is 

a direct function of the surface area (Area) of the source in the compartment. The equivalent of 

Equation 3-3 for an area-dependent source is:

[CH2O]SS = [CH2O]out + (CH2OER * Area)/Q (Eqn. 3-4)

with CH2OER in units of mg/m

2

-hr and Area in m2.

 

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

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

Formaldehyde Concentration and Possible Corrective Measures. In Proceedings of ASHRAE Symposium 

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

38 T. Matthews, A. Hawthorne, C. Daffron, T. Reed, and M. Corey. 1983b. Formaldehyde Release from Wood 

Products. In Proceedings of the 17th International Particleboard Symposium, Washington State University, 

Pullman, WA.
38

The formaldehyde emission rate, CH2OER, is formulated consistent with a one-dimensional 

representation of Fick’s Law39, describing the emission rate as the release rate when the 

formaldehyde air concentration is zero, reduced by the product of the mass transfer coefficient and 

the vapor-phase concentration. This formulation can be expressed as follows:

CH2OER = -m * [CH2O]V + b (Eqn. 3-5)

where:

m = the mass transfer coefficient (m/hr)

[CH2O]V = the CH2O concentration in the vapor phase (mg/m3)

b = a constant; the emission rate at zero CH2O concentration in 

the air (mg/m2-hr).

As noted by Matthews, Hawthorne and Thompson (see footnote on this page):

Thus, the modeled CH2O emission rates maximize at zero CH2O concentration and decrease 

linearly to a net emission rate of zero at some equilibrium CH2O concentration. If the CH2O 

concentration is raised beyond the equilibrium level with the addition of new CH2O sources, 

extrapolation of the model to higher CH2O concentrations indicates that sorption of the CH2O by 

the original ‘emitter’ is expected to occur. Experimental results of individual and paired emitters in 

small-scale environmental chambers by Pickrel et al.40 and Godish et al.41are consistent with this 

theory. Paired source contributions of pressed-wood products, insulation, carpeting and UFFI 

typically yielded CH2O concentrations less than the sum of the concentrations for the individual 

emitters. In addition, Godish found that the CH2O emission strength of many weaker emitters in 

paired source combinations was enhanced following brief exposure to a stronger emitter. This 

suggests a temporary sorptive mechanism for CH2O followed by reemission of CH2O at reduced 

CH2O concentrations.

The vapor-phase concentration, [CH2O]V, by definition is equal to [CH2O]SS at steady state; 

therefore, substitution of [CH2O]SS and Equation 3-5 into Equation 3-4 results in the following 

expression:

 

 

Q

m Area

CH O

Q

b Area

CH O

out

SS *

1

*

2

2





 (Eqn. 3-6)

In the case of multiple pressed-wood sources, Equation 3-4 becomes:

[CH2O]SS = [CH2O]out + (CH2OERk* Areak) /Q (Eqn. 3-7)

where the subscript k refers to the k

th source. By extension, Equation 3-6 then becomes:

 

 

 

 

Q

m Area

CH O

Q

b Area

CH O

k

k k

out

k

k k

SS 









*

1

*

2

2

(Eqn. 3-8)

 

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

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

40 J. Pickrel, L. Griffis, B. Mokier, G. Kanapilly, and C. Hobbs. 1984. Formaldehyde Release Rate Coefficients from 

Selected Consumer Products. Environmental Science & Technology 18: 682-686.

41 T. Godish and B. Kanyar. 1985. Formaldehyde Source Interaction Studies. Forest Products Journal 35: 13-17.
39

3.3 Extension of the Steady-State Model to Two Compartments

The formulation for the single-compartment case is adapted for use with a two-compartment case, 

by solving appropriate mass-balance equations. The mass-balance equation for each compartment, 

neglecting sinks (see Section 3.6), is as follows:



    

in Zone i

num sources

k

i ji j i i ij i i k

i

i Q C Q C Q C Q C E

dt

dC V

1

0 0 0 ,

(Eqn. 3-9)

where:

Vi = volume of compartment i (m3)

Ci = concentration in compartment i (mg/m3)

t = time (hrs)

i,j = indoor compartments (0 signifies the outdoors)

Qij = flow rate from compartment i into compartment j (m3/hr)

Ei,k = emission rate for formaldehyde source k in zone i (mg/hr)

It is acknowledged here that there are formaldehyde sources other than pressed-wood products in 

the indoor environment; however, such emitters (e.g., tobacco smoke, permanent press fabrics, 

paints, cosmetics) typically are comparatively weak. Together, the outdoor concentration and these 

weaker emitters contribute to what can be described as a "background" concentration. As an 

expedience, for purposes of this model, the background concentration is treated as if all such 

contributions can be represented by the outdoor concentration. Studies of background 

concentrations indicate that the concentrations indoors are generally offset from outdoors due to 

these other sources by approximately 5 ppm (see Section 2.2.2). Although the emission rate of 

these other formaldehyde sources can be impacted by the presence of new sources such as PWPs, 

this impact is expected to minimal. Therefore, treating these other sources as an outdoor 

contribution provides a meaningful construct for isolating the impact of the new sources, by

providing a constant background offset.

With this representation, equation 3-9 can be alternately expressed as follows:



    

in Zone i

num sources

k

i B ji j i i ij i i k

i

i Q C Q C Q C Q C E

dt

dC V

1

0 0 ,

(Eqn. 3-10)

where:

CB = background concentration, which is defined as the outdoor 

concentration plus the contribution from indoor sources other than 

pressed-wood products (assumed to be constant over time).

Considering the source equation as described in Equation 3-5 for multiple pressed-wood products 

in either zone, the sum of these emission sources in each compartment can be expressed as:

      

k k

i k i k i i k i k

k

Ei k CH OERi k Areai,k m , Area , C b , Area ,

'

, 2 , * * * * (Eqn. 3-11)

where:

CH2OERi,k = unit emission rate of source k located in compartment i

(mg/m2-hr)
40

Areai,k = area of source k located in compartment i (m2)

mi,k = mass transfer coefficient of source k located in 

compartment i (m/hr)

bi,k = emission rate at zero CH2O air concentration for source 

k located in compartment i (mg/hr)

Ci = CH2O air concentration in compartment i (mg/m2).

Considering the combined Equations 3-10 and 3-11 for each compartment, and solving 

simultaneously for the steady-state condition (see Appendix A), the following equations are

obtained:

   

  10 20 10 21 10 2 12 20 12 2 1 20 1 21 1 2

01 20 01 21 01 2 21 02 20 21 2 1 21 2

1, Q Q Q Q Q z Q Q Q z z Q z Q z z

Q Q Q Q Q z Q Q C Q Q z y Q y

C

B

SS

      

      

 (Eqn. 3-12)

   

  20 10 20 12 20 1 21 10 21 1 2 10 2 12 2 1

02 10 02 12 02 1 12 01 10 12 1 2 12 1

2, Q Q Q Q Q z Q Q Q z z Q z Q z z

Q Q Q Q Q z Q Q C Q Q z y Q y

C

B

SS

      

      

 (Eqn. 3-13)

where:

SS = indicates the steady-state solution

z1 = k

in Zone

num sources

k

mk Area



1

1

 for the k sources located in compartment 1

y1 = k

in Zone

num sources

k

bk Area



1

1

 for the k sources located in compartment 1.

z2 = k

in Zone

num sources

k

mk Area



2

1

 for the k sources located in compartment 2

y2 = k

in Zone

num sources

k

bk Area



2

1

 for the k sources located in compartment 2.

3.4 Matthews Model Compared to HBF Model

An alternative to the Matthews model is the HBF model (name coined by Myers42), a model 

developed by Hoetjer43 for which mathematically identical versions have been developed by Berge 

et al.44 and by Fujii et al.45 The underlying theory and derivation of the model have been 

summarized by Myers.

 

42 G Myers. 1984. Effect of Ventilation Rate and Board Loading on Formaldehyde Concentration: A Critical Review 

of rhe Literature. Forest Products Journal 34: 59-68.

43 J Hoetjer. 1978. Introduction to a Theoretical Model for the Splitting of Formaldehyde from a Composite Board. 

Methanol Chemie Netherland, June 8.

44 A Berge, B Milligaard, P Haneto, and E Ormstad. 190. Formaldehyde release from Particleboard – Evaluation of a 

Mathematical Model. Holz als Roh-Und Werkstaff 38:251-255.

45 S Fujii, T Suzecki, and S Koyagaehiro. 1973. Study on Liberated Formaldehyde as Renewal for JIS Particleboard. 

Kenjal Shiken Joho 9: 10-14.
41

The HBF model formulation for the steady-state concentration for a single source is as follows:

CSS = CEQ / (1 + N / kL) (Eqn. 3-14)

where:

CSS = the steady-state formaldehyde concentration (mg/m3)

CEQ = the formaldehyde equilibrium concentration (mg/m3)

N = the air exchange rate (1/hr)

k = mass transfer coefficient (analogous to m in Matthews model) (m/hr)

L = product loading ratio (m2/m3)

It can be shown that the Matthews and HBF formulations are identical models and that CEQ = b/m.

The only difference between the two models is in the regression equations that are used to derive 

the parameters, not their theoretical constructs.

The Matthews formulation, under the assumption that the outdoor concentration is zero, can be 

expressed as follows by substituting Equation 3-5 into Equation 3-4:

CSS = (-m * CSS + b) * Area / Q (Eqn. 3-15)

where CSS is as defined above and m, b, Area and Q are as defined previously in Section 3.2. 

Substituting ACH * VOL for Q, Equation 3-15 can be expressed as:

CSS = (-m * CSS + b) * Area / (ACH * VOL) (Eqn. 3-16)

Equation 3-16 can be expressed using the same terms as in Equation 3-14, by setting N = ACH and 

by setting L = (Area / VOL):

CSS = (-m * CSS + b) / (N/L) (Eqn. 3-17)

Equation 3-17 can be solved for CSS as follows:

  (Eqn. 3-18)

Setting the Matthews (Equation 3-18) and HBF (Equation 3-14) formulations equal to each other 

yields the following equality:

(Eqn. 3-19)

Equation 3-19 can be solved for CEQ as follows:

(Eqn. 3-20)

Therefore, setting CEQ = b/m leads to identical model formulations.
42

3.5 Other Potentially Useful Modeling Constructs

One of the peer reviewers commented on the potential utility of the saturation concentration (Csat) 

of formaldehyde in air over an emitter of interest, as follows:

Equation 3-5 is reproduced below with slightly different notation along with an equivalent equation 

from my previous work:

max Gss  m(Css )  G (Equation 3-5 with different notation)

Gss = steady state emission rate (mg/m2-hr). Equivalent to CH2OER’

Gmax = the maximum emission rate which occurs at zero backpressure; that is, at zero 

concentration of formaldehyde in the air (mg/m2-hr). Equivalent to b.

m = the mass transfer coefficient (m/hr)

Css = vapor-phase concentration of formaldehyde at steady state (mg/m3). Equivalent to 

[Ch2O]v

Equation 3-5 above is essentially identical to an algorithm that I have often used for backpressure 

modeling, shown below:

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 Equation 3-5 above and the identity of m:

Csat

G

m

max 

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. 

Several peer reviewers suggested possible alternatives to the exponential-decay model used in 

FIAM-pwp to represent declining formaldehyde emissions over time. One suggestion was that a 

power-law model may be more suitable and another suggestion was the use of an expression with 

multiple or changing decay rates to represent, for example, “fast” (earlier) and “slow” (later) 

periods of exponentially declining emissions. Brown46 fitted a double-exponential model to results 

from chamber experiments, noting that “the high but decreasing emission rates in the first 3 months 

were consistent with free formaldehyde and formaldehyde produced from easily hydrolysed 

chemical bonds” and that “longer-term, near-constant emissions may be due to ongoing hydrolysis 

of resins used in the wood panels.” One peer reviewer noted a conceptual model47 with three 

exponential-decay compartments, 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).

 

46 Brown, S.K. 1999. Chamber Assessment of Formaldehyde and VOC Emissions from Wood-Based Panels. Indoor 

Air 9: 209–215.

47 L. Mølhave, L. Dueholm, and L.K. Jenson. 1995. Assessment of Exposures and Health Risks Related to 

Formaldehyde Emissions from Furniture: A Case Study. Indoor Air 5: 104-119.

max

max

max

( )

:

(1 )

C G

C

G

G

rearranged

C

C

G G

ss

sat

ss

sat

ss

ss

  

 
43

A single-exponential model for declining formaldehyde emissions over time has the following form:

ER(t) = ER(0) e

-kt

Where:

ER(t) is the emission rate at time t (e.g., elapsed hours);

ER(0) is the initial emission rate (t = 0); and

k is the rate constant governing the exponential decline in the emission rate.

The power-law function takes the following form:

ER(t) = at -k

Where a is a constant and k is an exponential index, dimensionless.

Although the power-law function is undefined when t is zero, an initial emission rate can be “set”

by assigning an arbitrarily small value to t (the rate is highly sensitive to the value chosen for t).

The plots in Figure 3-1 illustrate the declining behavior of the power-law function, with a set to 

50 and k set to 0.25, in comparison to the single-exponential model, with ER(0) set to 90 and k set 

to either 0.1 or 0.5. The power-law function generally has a sharper curvature than the exponential 

model. This sharper curvature can be matched to an extent with the exponential model, but only by 

choosing a more rapidly declining emission rate (e.g., with k set to 0.5 as in the figure). The shape 

of the time-related decline obtained using the power-law function can be nearly matched by the 

exponential-decay model, however, when it is modified to have an initial period of faster decline 

followed by a second period of slower decline48, as illustrated in Figure 3-2. If, for example, the 

conceptual model above with three potential sources of formaldehyde emission is a representative 

construct, then the approach of using multiple exponentials as a semi-empirical approximation 

may be preferable (recognizing the associated need for estimating multiple parameters) because 

first-order processes are exponential in form. An example application of FIAM-pwp with a varying 

decay rate is provided in Section 2.7.5.

The above constructs, although potentially useful, currently share one limitation – little existing 

data to provide a basis for assigning appropriate parameter values. They should be kept in mind, 

however, for future research efforts and modeling applications. By comparison, as described in 

Section 3.8, there has been a fair amount of research on the formaldehyde emissions half life, 

providing some basis for choosing a value for the rate of decline for the simpler single-exponential

decay model.

 

48 The use of changing values for the exponential decay can be viewed as a simplified or constrained version of the 

double-exponential model; the full, or unconstrained, model would use both exponentials in parallel rather than in 

series. The current configuration of FIAM-pwp does not allow use of the full double-exponential model, due in 

part to uncertainty regarding appropriate parameter values to assign to the rate constants for faster and slower 

decline. Faster and slower exponentials can be applied sequentially, as done here, with FIAM-pwp by making a 

series of two related model runs, as illustrated in Section 2.7.5. 
44

Figure 3-1. Representation of Declining Emission Rate

by Power-law Function vs. Exponential-decay Model.

Figure 3-2. Representation of Declining Emission Rate by Power-law Function

vs. Exponential-decay Model with Periods of Fast and Slow Decay.
45

3.6 Discussion of Indoor Sinks

In concept, the treatment of indoor sinks in the calculation module could be accomplished within 

the model framework by recognizing that the emission term, as given by Equation 3-5, is 

comprised of two components. The first component (-m*[CH2O]v) is in the same form as a firstorder sink representation. The second component (b) is a constant, representing a constant 

emission rate. By setting b equal to zero and m to a nonzero value, this “source” representation 

actually reflects the behavior of a first-order sink. Alternatively, setting m to zero and b to a 

nonzero value represents a product as a constant emitter.

Recognizing these behaviors, the source term could be used to represent a simplistic sink by

assigning a large enough value to m to result in a net flux of formaldehyde into a material that acts 

as a sink. However, this simplistic approach does not allow a fully realistic representation of the 

re-emission from sinks that has been observed in laboratory and field studies. In the model, the 

concentration in a compartment is assumed to decline from an initial “steady-state” concentration 

in accordance with a user-specified half-life. This model representation tends to diminish the 

effect of the sink over time, much like the emission rates for materials that act as sources. In

reality, sinks for formaldehyde tend to accumulate mass when air concentrations are high, and to 

re-emit this mass as the air concentration decreases. The accumulation rate in the sink at any point 

in time is a function of the air concentration and the mass residing in the sink at that point, as well 

as other sink characteristics.

Because the complex relationships such as those described above cannot be fully captured by 

using the current modeling framework to represent sinks, it is recommended that users do not 

attempt to model sinks using FIAM-pwp. Ignoring the potential effects of indoor sinks will tend to 

result in a more conservative estimate, that is, higher modeled concentrations. The magnitude of

prediction error with exclusion of indoor sinks is believed to be relatively small, on the order of 10 %

for most formaldehyde modeling situations.

3.7 Decay of Indoor Concentrations

The initial (steady-state) indoor formaldehyde concentration, as calculated by the model described 

above, is assumed to decrease over time as the reservoir of formaldehyde in various sources and 

sinks is gradually depleted. The decrease in the initial value over time is assumed to follow a firstorder exponential process, expressed as follows:

 

-kt

t B i B C  C  C  C e (Eqn. 3-21)

where:

Ct = formaldehyde concentration indoors (ppb) at time t (in years), due to 

the combination of background concentration and emissions from 

PWPs (Note: Ct = Ci at t = 0)

CB = background concentration (ppb), which is defined as the outdoor 

concentration plus the contribution from indoor sources other than 

pressed-wood products (assumed constant)

Ci = initial concentration indoors (ppb)

k = first-order rate constant for the exponential decline (years-1).
46

Note that, in Equation 3-21, only the contribution from the new PWPs (i.e., Ci - CB) is decayed 

because the background contribution is assumed to be constant, as discussed in Section 3.3. The 

value of k is determined from the user-specified half life for the indoor formaldehyde 

concentration, assumed to be dominated by an exponential decay in formaldehyde source strength. 

Solving for t results in the following equation:

k

C C

C C

t

t B

i B



















ln

(Eqn. 3-22)

Solving for k results in the following equation:

t

C C

C C

k

t B

i B



















ln

(Eqn. 3-23)

Neglecting for the moment the background contribution to the indoor concentration (i.e., assuming 

that the background concentration is zero), Equation 3-23 reduces to:

t

C

C

k

t

i















ln

(Eqn. 3-24)

By definition, Ct is equal to half of Ci when the half-life is reached:

 0.5

i

T

C

C

(Eqn. 3-25)

where:

T = half-life (in years).

By substitution, Equation 3-24 becomes:

 

T

k

ln 2

 (Eqn. 3-26)

Substituting the default half life of 1.5 years (see Section 2.2) for T in the above equation gives:

k = 0.462 years-1

Indoor concentrations at later points in time (i.e., subsequent to the initial, steady-state values for 

each zone) are calculated by decaying only the portion of the indoor concentration that is 

attributable to indoor pressed-wood products. For example, to obtain the indoor concentration 3 

months (0.25 years) after the initial value, with an assumed half-life of 1.5 years, Equation 3-21

would apply:

C(0.25 yr) = CB + (Ci - CB) e

-0.462*0.25 (Eqn. 3-27)
47

If Ci were 50 ppb and the background concentration were zero, then the indoor concentration three

months later would be 44.5 ppb. With the default background concentration of 7.5 ppb (see 

Section 2.2), the indoor concentration would be 57.5 ppb initially and 52 ppb after three months.

In either case, the portion of the initial indoor concentration due to indoor pressed-wood products 

(50 ppb) would be reduced by 5.5 ppb over three months with a half life of 1.5 years.

The time required for the initial indoor concentration to decay to a user-specified concentration 

(e.g., 10 ppb, the default value) can be determined from the first-order exponential process using 

Equation 3-22 as follows, given an initial indoor concentration greater than the specified 

concentration and assuming a background concentration of zero:

t = ln (Ci / 10) / k (Eqn. 3-28)

Continuing with the above example, where k is 0.462 years-1and Ci is 50 ppb, by substitution into 

Equation 3-28 we get:

t = ln (50 / 10) / 0.462 (Eqn. 3-29)

for which the value of t is 3.48 years, or 41.8 months.

The model uses the higher of the two initial indoor concentrations, when a two-zone model is 

selected, for the calculation. Because the user is allowed to specify a background concentration 

other than zero, which is assumed to be constant over time, Equation 3-22 can be used to account 

for the background contribution while decaying only the component due to indoor sources and 

sinks: 

k

C

C C

t

B

i B



















10

ln

(Eqn. 3-30)

With the default background concentration of 7.5 ppb and an initial concentration of 57.5 ppb, the 

value for t would be 6.48 years. The result from Equation 3-30 is multiplied by 12 and reported in 

months. If the highest initial indoor concentration is lower than the background concentration, then 

zero months is reported as the time required for the indoor concentration to decay to background.
48

3.8 Default Value for Half Life

Ideally, an appropriate half-life value for representing the long-term decay of formaldehyde 

emissions and concentrations in residences would be determined by taking measurements in a 

house soon after it was loaded with pressed-wood products and at periodic points in time over the 

next several years. Continuing with this ideal situation, the products would not change throughout 

the measurement period and factors such as indoor temperature and humidity, as well as the 

indoor-outdoor air exchange rate, would change only minimally in response to outdoor weather 

conditions and occupant activities indoors. Further, the experiment would be repeated in several 

houses. The data for each house would be analyzed to fit a nonlinear (e.g., exponential) decay 

function to the time-varying indoor concentrations, and the resultant estimates would be compared 

across houses (or the data would be pooled across houses) to determine an appropriate central 

value for the concentration half life.

The ideal data set has not been collected, however, most likely because the complexities and costs 

of multiple measurements over time, coupled with the need to restrict certain occupant activities 

over several years (or to operate a research house to gain experimental control), would be 

prohibitive. There are two practical alternatives to the ideal experimental situation, both of which 

have been attempted: (1) collect or assemble point-in-time measurements from houses of varying 

ages, as a proxy for the time-related decay in one or a series of houses; and (2) conduct chamber 

tests at periodic points in time for pressed-wood products, either individually or in combination.

Either alternative has significant limitations. For example, cross-sectional data from many houses 

are, at best, a crude proxy for the time-related behavior of a single house, or of a limited set of 

well-characterized houses – the analysis is confounded by varying, and generally unknown, 

construction features, occupant activities and weather conditions at the times of measurement. 

Chamber tests, on the other hand, have the advantage of careful experimental control. However, 

for multiple tests over time, the material to be tested typically would not be kept in the chambercontrolled environment throughout the testing period, but rather during only the relatively brief 

occasions when measurements were to be taken. Further, most such tests have been conducted 

only for a single product or material. The time-related rate of decline depends on the formaldehyde 

concentration; the rate of decline is more rapid for a single product because other products are not 

present to increase the formaldehyde concentration and thereby retard its rate of formaldehyde 

release. Limited tests of product/material ensembles have been conducted, but none have included 

the full array of sources and sinks that typically would be present in an occupied house. With these 

limitations in mind, some previous investigations into the half life of formaldehyde emissions or 

concentrations are summarized below.

As summarized in a Versar 1988 report49, estimates of decay half lives for formaldehyde 

concentrations in residences range from 2 to 44 years, with most estimates below 5 years. These 

estimates have been based largely on pooled cross-sectional data across a variety of residences, as 

a proxy for time-series data from one or a few houses. It is noteworthy that the higher estimates 

(20 to 44 years) are from data sets involving houses aged from new to 50 years, whereas the lower 

estimates (2 to 4 years) are from data sets where the houses generally were 10 years old at most. If 

none of the materials or furnishings in a house were to be changed, and if the occupants were to 

 

49 Versar. 1988. Formaldehyde Exposure in Residential Settings: Sources, Levels, and Effectiveness of Control 

Options. Update (September 30, 1988) of 1986 report by Versar, Inc. for the USEPA Office of Toxic Substances, 

prepared under Contact No. 68-02-3968.
49

avoid using consumer products that emit formaldehyde, then the indoor concentration could be 

expected to eventually decay to some background level, similar to that if no sources were present. 

As a house ages, however, occupants will tend to add or replace materials and furnishings, some 

of which may emit formaldehyde. Such changes, then, will introduce new sources, thereby raising 

the formaldehyde concentration relative to the no-change case and artificially inflating the half-life 

estimate.

The above conjecture is partly supported by limited studies involving repeat measurements at 

different points in time from one or two houses, for which the half-life estimate ranged from 2.0 to 

2.6 years. A careful analysis by Versar of a cross-sectional data set for 396 manufactured homes, 

as described in the 1988 report referenced on the previous page, resulted in a half-life estimate of 

2.92 years. This estimate is believed to be an upper bound for the true half life in a house that has 

some significant formaldehyde sources from the outset of its construction and occupancy history.

Chamber studies to investigate the decay of formaldehyde have focused primarily on individual 

sources and, not surprisingly, have produced shorter half-life estimates than most residential 

studies. As summarized in the same (1988) Versar report, the estimated half lives from chamber 

studies have ranged from 0.2 to 2.58 years. One noteworthy investigation, conducted by Matthews 

et al.50, involved a combination of sources (particleboard, hardwood plywood paneling and MDF) 

under controlled environmental conditions (23  C and 50 % relative humidity). The “slow” decay

study, with an air exchange rate of 0.4 air changes per hour (ACH) in the chamber, produced a 

half-life estimate of 1.52 years. The “fast” decay study was conducted with relatively high air 

exchange rates designed to keep the chamber formaldehyde concentration below 0.1 ppm. The 

lower background concentration was expected to increase the emission rates and, thus, shorten the 

decay periods relative to the “slow” decay study. Resultant half-life estimates were 0.76, 0.62 and 

1.08 years for particleboard, paneling and MDF, respectively. 

Of the two decay studies by Matthews et al., the “slow” study is believed to be more indicative of 

the formaldehyde decay behavior in residences. However, the authors added the caution that care 

should be taken in interpreting the study results. For example, the duration of the study may have 

been inadequate to accurately reflect the emission decay period for the collection of sources. In 

addition, the data may reflect a relatively rapid decay of emissions from a few of the strongest 

emitting sources, as opposed to the entire collection of products, whereas in real life one might 

typically expect to find a combination of relatively strong and weak emitters, or of sources and sinks.

In another decay study for an individual source, Zinn et al.51 estimated an apparent half-life of 216 

days (0.6 years) using approximately one year of large chamber data for particleboard and a natural 

logarithm linear regression statistical analysis. As noted by one of the peer reviewers:

The half-life reported in Zinn et al. (1990) is not directly comparable to that adopted in the original 

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

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

 

50 T. Matthews, T. Reed, B. Tromberg., K. Fung, C. Thompson, J. Simpson, and A. Hawthorne. 1985. Modeling and 

Testing of Formaldehyde Emission Characteristics of Pressed-Wood Products: Report XVIII to the U.S. 

Consumer Product Safety Commission. ORNL/TM-9867, Oak Ridge National Laboratory, Oak Ridge, TN.

51 Zinn, T.W., Cline, D. and Lehmann, W.F. 1990. Long-term Study of Formaldehyde Emission Decay from 

Particleboard. Forest Products Journal 40(6): 15-18.
50

decay model. The Zinn et al. logarithmic equation was used by CARB52 to calculate the emissions 

inventory presented in the technical basis for the formaldehyde airborne toxic control measure. In 

lieu of the original logarithmic equation, the more generic two-compartment (i.e., fast and slow 

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

For the various reasons noted in the above review and discussion, there is considerable uncertainty 

as to the most appropriate value to represent the central tendency for the decay rate of 

formaldehyde, in a residence where construction features, house contents and occupant activities 

remain relatively stable over time. The cross-sectional data from residential studies are believed to 

yield estimates erring on the high side (i.e., longer estimated than actual half-life), and the 

temporal data from chamber studies are believed to yield estimates erring in the opposite direction. 

The collective evidence described above indicates that an appropriate half-life value probably lies 

between 1.5 and 3.0 years. The value of 2.92 years from the cross-sectional analysis described 

above will tend to err on the conservative side. That is, assuming that the initial concentration in a 

house is predicted accurately, the 2.92-year value likely will cause the modeled concentration to 

decay at a slower rate than might be observed in a stable residential setting.

A 2005 paper by Groah53 focused on the topic of decay in formaldehyde concentrations over time. 

The author notes that “newer sources of formaldehyde are continually being brought into homes as 

they age.” Examples would include the installation of new kitchen cabinets when remodeling or 

the addition of new furnishings such as furniture or shelving that may contain pressed-wood 

products. To the extent that such situations exert an upward bias on half-life estimates that are 

based on an analysis of homes of different ages, it could be argued that the default half-life value 

of 2.92 years is too conservative, as mentioned above. Two studies were noted by Groah that 

involved chamber testing of a collection of pressed-wood products as they aged. One (by

Matthews et al., cited on the previous page) estimated the emissions half-life to be 1.52 years and 

the other (Groah and Gramp54) indicated a half-life of 2.10 years. Based on these studies, a halflife on the order of 1.5 to 2.0 years is suggested should the user wish to apply a less conservative 

value. 

 

52 California Air Resources Board (CARB). 2007. Initial Statement of Reasons for Proposed Rulemaking, Proposed 

Airborne Toxic Control Measure to Reduce Formaldehyde Emissions from Composite Wood Products. Available 

at: http://www.arb.ca.gov/regact/2007/compwood07/fro-final.pdf..

53 W. Groah. 2005. Decay or the Decrease in Formaldehyde Concentrations or Emissions over Time and UFbonded Wood Panel Products. Report prepared for the Composite Panel Association, Formaldehyde Council Inc., 

Hardwood Plywood and Veneer Association, and the Kitchen Cabinet Manufacturers Association.

54 W. Groah and G. Gramp. 1988. An Estimate of Home Occupant Exposures to Formaldehyde Gas from Plywood 

and Reconstituted Wood Products Bonded with Formaldehyde Based Adhesives. CPA/HPVA 1988 Exposure and 

Assessment Package Report, Hardwood Plywood and Veneer Association, Reston, VA. Available at:

http://www.ecobind.com/research/Decay_or_Decrease_in_Emissions.pdf.
51

3.9 Temperature and Humidity Adjustments

As noted in Section 2 and in the introduction to this section, the model initially estimates the 

formaldehyde concentration(s) in a structure under indoor conditions of 23  C temperature and 50 

% relative humidity – these are consistent with the conditions under which most chamber tests 

have been conducted, in accordance with ASTM standard E 133355, for estimation of emission 

rates or concentrations under an assumed steady-state condition. The model then adjusts the initial 

concentration estimates under “standard conditions” to the user’s inputs for temperature and 

humidity. 

The temperature adjustment used in the model is of the form:

   R TB TU

T C C e

1 1

0



 (Eqn. 3-31)

where:

CT = Temperature-adjusted formaldehyde concentration 

C0 = Formaldehyde concentration at the baseline temperature 

R = Temperature coefficient for adjustment

TU = User-specified temperature,  K

TB = Baseline temperature, 296  K (23  C).

The humidity adjustment, applied to the temperature-adjusted concentration, is of the form:

  B U

T

TH A H H

C

C

 



1

 (Eqn. 3-32)

where:

CTH = Temperature- and humidity-adjusted formaldehyde concentration

CT = Temperature-adjusted formaldehyde concentration 

A = Humidity coefficient for adjustment

HU = User-specified relative humidity, %

HB = Baseline relative humidity, 50 %.

Combining equations 3-31 and 3-32, the temperature- and humidity-adjusted concentration can be 

expressed as follows:

CTH  C0KTH (Eqn. 3-33)

where KTH is the combined temperature and humidity adjustment factor:

  

  B U

R T T

TH A H H

e

K

B U

 





1

1 1

(Eqn. 3-34)

 

55 ASTM. 1996a. Standard Test Method for Determining Formaldehyde Concentrations in Air and Emission Rates 

from Wood Products Using a Large Chamber. Designation E 1333-96 (Reapproved 2010), American Society for 

Testing and Materials, Philadelphia, PA.
52

Thus, the adjustments require temperature and humidity coefficients, for which the model provides 

two alternative sets of values. The first set of coefficients – the model default -- was calculated by

Berge et al.56 for two Norwegian particleboard specimens. These coefficients were used by HUD 

in setting the 1984 standards for particleboard and hardwood plywood paneling, and also have 

been used in ASTM, ANSI and Composite Panel Association (CPA) standards or guidelines. 

Chamber tests underlying these coefficients were conducted at two temperatures (22 and 28 C) 

and at two relative humidities (30 and 60 %). The coefficients reported by Berge et al. were 9799 

for temperature (assumes an Arrhenius type of model) and 0.0175 for humidity (assumes a linear 

model).

The second set of coefficients was reported by Myers57, who reviewed the extant literature and 

developed his recommended temperature coefficient based on chamber testing of about 40 

specimens (particleboard and hardwood plywood paneling) from 11 laboratories. The temperature 

range of the product tests was from 20 to 40 C. Dr. Myers also developed a humidity coefficient, 

but expressed little confidence in the number; the relative humidities in the tests on which the 

correction factor was based ranged from 20 to 90 percent. Thus, his work encompassed more 

products and wider temperature/humidity ranges than the Berge et al. study. It is worth noting that 

the HUD standard was promulgated before Dr. Myers completed his work. His reported correction 

coefficients were 8930 for temperature and 0.0195 for relative humidity, using the same model 

assumptions as Berge et al.

The temperature and humidity adjustments modify the emission rate from pressed-wood products 

and, therefore, the portion of the indoor concentration due to indoor sources and sinks. That is, the 

temperature and humidity adjustment is applied only to the portion of the steady-state 

concentration contributed by pressed-wood products. The adjustment applied to the two-zone 

concentrations calculated using Equations 3-12 and 3-13 takes the following form:

  C SS adjusted  C SS  CB KTH  CB

*

1, , 1,

(Eqn. 3-35)

  C SS adjusted  C SS  CB KTH  CB

*

2, , 2,

(Eqn. 3-36)

Similarly, the adjustment applied to the one-zone concentration calculated using Equation 3-8 

takes the following form:

     CH O SS adjusted  CH O SS  CB KTH  CB*

2 , 2

(Eqn. 3-37)

 

56 A. Berge, B. Mellagaard, P. Hanetho, and E. Ormstad. 1980. Formaldehyde Release from Particleboard –

Evaluation of a Mathematical Model. Holz Als Roh-und Werkstoff 38: 252-255.

57 G. Myers. 1985. The Effects of Temperature and Humidity on Formaldehyde Emission from UF- bonded Boards: 

A Literature Critique. Forest Products Journal 1985: 35:20-31. 
53

3.10 Exposure Calculations

As noted in Section 2.5, the model calculates the following exposure metrics for each zone of the 

house or other modeled structure:

 Average daily concentration (ADC)

 Percent of time concentration is greater than the user-specified level of interest (LOI).

The ADC in each zone is calculated in yearly increments, by integrating Equation 3-21. The value 

of this integral, between two times (t1 and t2) that are exactly one year apart, is as follows:

ADC = ((Ci - CB)/-k)*e-k*t2 + CB*t2 - ((Ci - CB)/-k)*e-k*t1- CB*t1 (Eqn. 3-38)

 where:

Ci = initial concentration indoors (ppb)

CB = background concentration indoors (ppb)

k = first-order rate constant for the exponential decline (years-1)

t1 = starting time (years)

t2 = ending time (years).

For a new house, the values of t1 and t2 for the first year are 0 years and 1 year, respectively. The t1

and t2 values are 1 and 2 years for the second year, 2 and 3 years for the third year, and so on. If, 

for example, the starting concentration (i.e., at t1 = 0 years) were 57.5 ppb, the background 

concentration were 7.5 ppb, and the value of k were 0.462 years-1(half life of 1.5 years), then the 

ending concentration would be 39 ppb per Equation 3-21 and the value of the integral for the first 

year would be 47.5 ppb per Equation 3-38.

The percent of time during which the concentration in a modeled zone is greater than the LOI also 

is calculated in yearly increments, as follows:

% Time =100 * (ln(Ci-CB) - ln(LOI-CB)) / k – t2) (Eqn. 3-39)

where:

% Time = percent of time concentration is greater than LOI

LOI = level of interest (ppb).

Continuing with the example where Ci = 57.5 ppb, CB = 7.5 ppb, and t1 = 0 (i.e., new house), and 

with a value of 10 ppb for the LOI, the calculation result for each of the first six years (i.e., for t2

ranging from 1 to 6) is equal to 100%. For year 7 (t2 = 7) the calculation result is 48.4%; 

thereafter, the calculation result is negative, which the model resets to a value of 0%.

The ADC for any exposure group is calculated, in yearly increments, as a weighted average of

concentrations encountered in each zone of the house or other modeled structure and 

concentrations encountered in non-home locations: 

ADC = (HZ1*ADCZ1 + HZ2*ADCZ2 + HWSD*CWSD + HV*CV+ HO*CO) / 8760 (Eqn. 3-40)

where:

H = hours spent per location

ADC = average daily concentration in Zone 1 or Zone 2 (ppb)

Z1 refers to Zone 1

Z2 refers to Zone 2
54

C = concentration at location away from home (ppb)

WSD refers to work, school or daycare location

V refers to in-vehicle location

O refers to other/outdoor location

The ADC values per zone decrease with each passing year, per Equation 3-38, whereas 

concentrations in locations outside or away from the home or modeled structure are assumed to be 

constant across the years. The sum of hours across the various locations (Z1, Z2, WSD, V and O) 

must equal 8760 for the calculation result to be valid. For a one-zone modeled structure, Z2 is set 

equal to Z1 for the calculation.
55

4. EVALUATION OF MODEL ALGORITHMS AND PREDICTIONS

The evaluation of the model has been performed from two perspectives: (1) independent checks on 

the correctness of model algorithms; and (2) comparison of model predictions with measurements 

in real-world situations.

4.1 Mathematical Correctness of Model Algorithms

An Excel spreadsheet has been developed as a tool for assessing the mathematical correctness of 

algorithms used in the model. The Excel tool includes individual sheets for inputs corresponding 

to model input screens – House, Source, and Exposure – together with a separate sheet that 

displays the calculation results (predicted HCHO concentrations). Input areas within each screen 

are set up similarly to those in the model, with the exception that no drop-down selections are 

provided for sources, sinks or cabinets. Instead, input areas (cells) are designated for direct entry 

of Slope, B and Area values. Results are provided for both the one-zone and two-zone 

implementation of the model, in concentration units of mg/m3and ppm. 

Results in the Excel tool were compared with those reported by the model for a variety of cases 

representing different combinations/values for background concentrations, sources, and exposure 

groups, for both one-zone and two-zone implementations. This exercise led to discovery of minor 

errors in an earlier version in the handling of ppm vs. mg/m3units for the background 

concentration (now ppb and μg/m3units) and in the temperature/humidity adjustment for indoor 

formaldehyde concentrations. Other than those discrepancies, the results agreed exactly in all 

cases. The errors discovered in the model’s calculation routine have been corrected. 

4.2 NIST Assessment of Matthews Model Predictions

An effort was undertaken by researchers58 at the National Institute of Standards and Technology

(NIST) to validate the Matthews model on which this model is based. Particleboard underlayment, 

hardwood-plywood paneling and medium-density fiberboard (MDF) products initially were 

characterized in chambers by measuring their HCHO surface emission rates over a range of 

formaldehyde concentrations, air exchange rates and two combinations of temperature and relative 

humidity. The products then were installed in a two-room prototype house in different 

combinations, and equilibrium HCHO concentrations were monitored in the house.

The following findings are excerpted directly from the authors’ abstract:

Particleboard underlayment and mdf, but not paneling, behaved as the emission model predicted 

over a large concentration range, under both sets of temperature and relative humidity. Good 

agreement was also obtained between measured formaldehyde concentrations and those predicted 

by a mass-balance indoor air quality model.

The mass-balance model referenced here is the Matthews model. The following excerpt, taken 

directly from the authors’ conclusions, provides some further insights on model performance:

In developing its models, ORNL assumed that there is relatively free formaldehyde diffusion within 

the "bulk phase" of pressed-wood products. Formaldehyde emission results from the difference in 

concentration between this "bulk phase" and the atmosphere, in accordance with Fick's first 

 

58 S. Silberstein, R. Grot, K. Ishiguro, and J. Mulligan. 1988. Validation of Models for Predicting Formaldehyde 

Concentrations in Residences due to Pressed-Wood Products. JAPCA 38: 1403-1411.
56

diffusion law. This assumption is apparently fairly descriptive of particleboard and mdf, but not of 

hardwood plywood paneling. It is hypothesized that internal diffusion is important in formaldehyde 

emission by pressed-wood products, and layering of solid thin sheets of plywood obstructs diffusion 

to a greater extent than do chips and pieces in the other pressed-wood products. In other words, 

formaldehyde emission may not be as diffusion-limited in plywood as it is in other pressed-wood 

products. This would explain why formaldehyde emission by plywood appeared relatively 

insensitive to temperature and RH used in the present study. Higher temperature and RH increase 

resin degradation to formaldehyde and increase the formaldehyde diffusion rate. Apparently, the 

small changes of temperature and RH used in the present study did not increase the "bulk phase" 

formaldehyde concentration or the diffusion rate sufficiently to overcome the obstructing layers of 

the relatively low-emitting plywood used in this study.

It is noteworthy here that particleboard and MDF generally are much higher HCHO emitters than 

plywood or hardwood plywood paneling. Thus, the NIST validation effort indicated accurate 

model predictions for the major HCHO emitters among pressed-wood products.

4.3 Assessment of FIAM-pwp Predictions – EPA Pilot Study

An assessment undertaken by the author of this document involved comparison of model 

predictions with concentrations measured in the EPA pilot study house under various conditions, 

including before and after loading the house with pressed-wood products. The assessment was 

conducted by using known/measured factors such as zone volumes, airflow rates, indoor 

temperature and humidity, and source/sink loading areas together with estimates of emission 

model parameters for each type of source or sink. Model inputs were developed for six cases (time 

periods within the pilot study) at the conventional house – initial baseline, first loading, air-out 

after first loading, second loading, air-out after second loading, and third loading. Inputs supplied 

for model input screens – House, Sources and Sinks – are described in a general manner below. 

Further details specific to each of the six cases are given in the subsections that follow.

The house screen requires inputs for the type of house, the zone volumes and airflow rates, the 

background formaldehyde concentration, and the indoor temperature and humidity conditions. The 

house used for the pilot study – Conventional 1 – was selected. Airflow measurements with PFTs 

on 14 different occasions during the pilot study averaged 0.59 ACH and varied over a relatively 

narrow range of 0.5 to 0.7 ACH. The default airflow rates provided with the model for this 

conventional house, which correspond to an air exchange rate of 0.59 ACH, were used for all 

model runs. 

Outdoor formaldehyde concentration measurements during the pilot study varied over the narrow 

range of 0.001 to 0.003 ppm, averaging about 0.002 ppm. The average of 0.002 ppm was used for 

this input rather than case-specific values. The model calculates formaldehyde levels in each zone 

of the house under a base set of temperature/humidity conditions (23 C, 50 %) and a usersupplied set of conditions. The user-supplied conditions were chosen to represent the average of 

the values measured upstairs for each case at the conventional house. The major formaldehyde 

sources all were located upstairs. The default temperature and humidity coefficients provided with 

the model, for adjustment of modeled formaldehyde concentrations from the base conditions to the 

user-supplied conditions, were used for all model runs.

Model parameters for sources – Slope and B – have been estimated previously from large-chamber 

emission tests conducted by industry or the USEPA, and are supplied with the model. The major 
57

sources installed in the conventional house – underlayment, kitchen/bathroom cabinets, interior 

doors, and hardwood plywood – were chamber tested under three different air exchange rates as 

part of the pilot study. Source loading areas for the model runs are described later as they pertain 

to each modeled case.

There are two major types of potential sinks in conventional house 1 – carpet/padding and painted 

wallboard. Sinks can be represented in the Matthews model using the same parameters as those for 

sources. The Langmuir isotherm model described in the formaldehyde pilot study report (p. 6-44) 

was used to develop parameter estimates for sinks, for use in the Matthews model.

The modeling approach, rationale and assumptions for each of the six modeled cases are described 

below. The results of this evaluation exercise collectively indicate that the modified (two-zone) 

Matthews model can predict measured values quite well for the conventional house, but errs 

somewhat on the conservative side (i.e., it tends to “over-predict”). The greatest area of

uncertainty in applying the model appears to be in choosing appropriate values for indoor sinks. 

One additional area of uncertainty stems from the fact that humidity levels were not measured 

downstairs; as noted previously, upstairs values for temperature and humidity were used in 

constructing the model inputs.

4.3.1 Initial Baseline Period

Model inputs for this period are summarized in Table 4-1. The only source present in the house 

during the initial baseline period was the plywood subfloor, and the only sink was painted 

wallboard (carpet was not yet installed). The area for plywood was taken as the product of the 

upstairs volume (304 m

3

) and an assumed loading ratio of 0.43 m

2

/m3, or 131 m

2

. For wallboard 

the loading ratios for walls (0.95 m

2

/m3) and ceilings (0.43 m

2

/m3) were used to estimate a total 

area of 420 m

2

. Model source/sink parameters (intercept and slope) were taken from the model 

defaults for softwood plywood. An intercept value of 0.04 mg/m2-h was chosen for wallboard 

after running the model with plywood only and obtaining an indoor formaldehyde concentration in 

the vicinity of 10 ppb.

Table 4-1. Model Inputs (Sources and Sinks) for Initial Baseline Period

Source/Sink Slope (m/h) Intercept (mg/m2-h) Area (m2)

Softwood plywood -0.61 0.03 131

Painted wallboard -3.00 0.04 420

Modeling results for the initial baseline are shown in Table 4-2. With plywood as the only source, 

and without including wallboard as a sink, the modeled values for the conditions under which 

measurements were taken (23.98  C, 23.6 % RH) were 8 ppb upstairs and 7 ppb downstairs, close 

to the measured values of 9.5 and 9.1 ppb for the respective zones. Addition of wallboard as a sink 

had no appreciable effect on the modeled values. It is plausible that, at this point in time, the 

wallboard had reached a state of equilibrium and, thus, acted as neither a net source nor a net sink. 

When the intercept parameter for wallboard was arbitrarily halved (to 0.02 mg/m2-h), to cause it to 

behave as net sink, the modeled values were 5.0 ppb upstairs and 4.4 ppb downstairs under the 

conditions when measurements were taken.
58

Table 4-2. Modeling Results for Initial Baseline Period

Source/Sink Inputs

Results (ppb) for Base/Measured Conditions

23  C, 50 % RH 23.98  C, 23.6 % RH

Upstairs Downstairs Upstairs Downstairs

Softwood plywood as a source 10.3 9.0 7.8 6.9

Add wallboard as a sink 10.7 9.4 8.2 7.2

Measured values -- -- 9.5* 9.1

*The measured value is an average across 3 monitoring sites (living room, kitchen and bedroom).

4.3.2 First Loading

The modeled sources for the first (medium) loading included underlayment, cabinets, interior 

doors and countertop (Table 4-3). Plywood was not included as a source because it was covered 

by underlayment or by floor tile. Loading areas for the sources were taken from Section 3.5 of the 

pilot-study report, and slopes and intercepts were taken from the pilot-study values provided with 

the model for underlayment, doors and cabinets. For countertop, industrial particleboard was 

selected to obtain slope/intercept parameters (average values were used). The intercepts chosen for 

carpeting and wallboard were predicated on an indoor-air concentration near 40 ppb. 

Table 4-3. Model Inputs (Sources and Sinks) for First (Medium) Loading

Source/Sink Slope (m/h) Intercept (mg/m2-h) Area (m2)

Underlayment -1.27 0.28 46

Cabinets -0.48 0.08 59

Interior doors -0.50 0.08 35

Countertop -0.70 0.30 4

Carpeting -1.30 0.065 94

Painted wallboard -3.00 0.15 420

The modeling results for this case (Table 4-4) are compared with measured values taken at run 4 

(28 days after loading), by which time the indoor-air concentrations appeared to have “leveled off” 

(see Figure A-1 in Appendix A). The addition of carpeting lowered the modeled concentrations 

slightly, and further addition of wallboard lowered the modeled values by about 10 percent 

compared to estimates with no sinks. The modeled concentrations for measured conditions, with 

carpeting and wallboard included, were 5-10 ppb higher than measured values under those 

conditions.
59

Table 4-4. Modeling Results for First Loading

Source/Sink Inputs

Results (ppb) for Base/Measured Conditions

23  C, 50 % RH 23.65  C, 50.1 % RH

Upstairs Downstairs Upstairs Downstairs

Underlayment, cabinets, doors and 

countertop as sources

47 40 51 43

Add carpeting as a sink 46 39 49 42

Add wallboard as a sink 42 36 45 39

Measured values (run 3, day 28) -- -- 41 28

4.3.3 Air-out Period after First Loading

For this air-out period, plywood again was the only source and wallboard was the only sink. In this 

case, however, it was necessary to account for the fact that the wallboard had adsorbed significant 

mass during the first loading. Some of this mass would be lost during the air-out period, which 

lasted 7 days, but the amount lost was uncertain. Consequently, the intercept for the first loading 

(see Table 4-5) was used as an upper bound when developing inputs and half this value was used 

as a lower bound.

Table 4-5. Model Inputs for Air-out Period after First Loading

Source/Sink Slope (m/h) Intercept (mg/m2-h) Area (m2)

Softwood plywood -0.61 0.03 131

Painted wallboard -3.00 0.15/0.075* 420

*Two runs were made, one with 0.15 as the intercept and one with 0.075.

As shown in Table 4-6, the modeling results with the alternative intercept values for wallboard 

surround the measured values. The model estimates with the smaller intercept value, to account for 

the likely possibility that the wallboard had released a fair amount of mass during the air-out 

period, are closer to the measurement results.
60

Table 4-6. Modeling Results for Air-out Period after First Loading

Source/Sink Inputs 

Results (ppb) for Base/Measured Conditions

23  C, 50 % RH 23.44  C, 53.4 % RH

Upstairs Downstairs Upstairs Downstairs

Softwood plywood as a source 10.3 9.0 11.5 10.0

Add wallboard as a re-emitting sink 

(intercept = 0.15) 34 29 38 33

Change wallboard intercept to 0.075 18 16 20 18

Measured values -- -- 23 26

4.3.4 Second Loading

For the second (high) loading, the sources (Table 4-7) were similar to those for the first loading, 

but with a larger area for underlayment and with the addition of hardwood plywood paneling as 

another source. The intercept values chosen for carpet and for wallboard were based on 

concentration estimates obtained from running the model without any sinks.

Table 4-7. Model Inputs (Sources and Sinks) for Second (High) Loading

Source/Sink Slope (m/h) Intercept (mg/m2-h) Area (m2)

Underlayment -1.27 0.28 94

Cabinets -0.48 0.08 59

Interior doors -0.50 0.08 35

Countertop -0.70 0.30 4

Paneling -0.98 0.16 36

Carpeting -1.30 0.095 94

Painted wallboard -3.00 0.225 420

Modeled values (Table 4-8) were compared to measurements taken at run 2 (day 12), by which 

time it appeared that the indoor concentrations had started to “level off” (see Figure A-1 in 

Appendix A). This earlier time of leveling off, relative to that for the first loading, was likely due 

to the wallboard sink being substantially “pre-loaded” with formaldehyde mass as a result of the 

first loading. The modeling results without sinks were 70-75 ppb upstairs and 60-65 ppb 

downstairs. The addition of carpeting and wallboard as sinks lowered the modeled values by about 

10 percent. The modeled values with sources and sinks were about 10 ppb higher than those 

measured upstairs (differences similar to those for the first loading), and about 15 ppb higher than 

the measured downstairs values.
61

Table 4-8. Modeling Results for Second Loading

Source/Sink Inputs 

Results (ppb) for Base/Measured Conditions

23  C, 50 % RH 23.48  C, 50.1 % RH

Upstairs Downstairs Upstairs Downstairs

Underlayment, cabinets, doors, 

countertop and paneling as sources

70 60 74 63

Add carpeting as a sink 68 58 72 61

Add wallboard as a sink 63 54 67 57

Measured values (run 2, day 12) -- -- 56 40

4.3.5 Air-out Period after Second Loading

For the air-out period after the second loading, the only sources and sinks again were plywood and 

wallboard (Table 4-9). As with the first air-out period, it was necessary to account for the fact that 

the wallboard had adsorbed mass during the second loading and then had lost some of this mass 

during the air-out period. To accommodate uncertainty in the amount of mass in the wallboard 

near the beginning of this air-out period, an intercept close to that for the second loading was used 

as an upper bound, and half that value was used as a lower bound. 

Table 4-9. Model Inputs for Air-out Period after Second Loading

Source/Sink Slope (m/h) Intercept (mg/m2-h) Area (m2)

Softwood plywood -0.61 0.03 131

Painted wallboard -3.00 0.19/0.095* 420

*Two runs were made, one with 0.19 as the intercept and one with 0.095.

As shown in Table 4-10, the model results with alternative intercept values for wallboard 

surround the measured values, and the model estimates with the smaller intercept value are closer 

to the measurement results. This pattern is the same as that observed for the comparison of 

modeling and measurement results for the first air-out period.
62

Table 4-10. Modeling Results for Air-out Period after Second Loading

Source/Sink Inputs 

Results (ppb) for Base/Measured Conditions

23  C, 50 % RH 22.83  C, 49.6 % RH

Upstairs Downstairs Upstairs Downstairs

Softwood plywood as a source 10.3 9.0 10.0 8.7

Add wallboard as a re-emitting sink 

(intercept = 0.19) 43 36 41 35

Change wallboard intercept to 0.095 22 19 22 19

Measured values -- -- 27 25

4.3.6 Third Loading

For this repeat of the high loading, the model inputs for sources and sinks (Table 4-11) were the 

same as those used for the second loading, including the intercepts for carpeting and wallboard. 

Table 4-11. Model Inputs (Sources and Sinks) for Third (High) Loading

Source/Sink Slope (m/h) Intercept (mg/m2-h) Area (m2)

Underlayment -1.27 0.28 94

Cabinets -0.48 0.08 59

Interior doors -0.50 0.08 35

Countertop -0.70 0.30 4

Paneling -0.98 0.16 36

Carpeting -1.30 0.095 94

Painted wallboard -3.00 0.225 420

The measurement results for this loading indicated that concentrations had “leveled off” (see 

Figure A-1 in Appendix A) by day 7; measured values for run 1 (taken on day 7) therefore were 

used as a basis for comparison with modeling results. The modeled values were about 10 ppb 

higher than measured upstairs but about 25 ppb higher downstairs (Table 4-12). One possible 

explanation for the greater difference downstairs is that the humidity level, which was not 

measured downstairs, may have been lower than the value used in the model run.
63

Table 4-12. Modeling Results for Third Loading

Source/Sink Inputs 

Results (ppb) for Base/Measured Conditions

23  C, 50 % RH 22.63  C, 64 % RH

Upstairs Downstairs Upstairs Downstairs

Underlayment, cabinets, doors, 

countertop and paneling as sources 70 60 89 76

Add carpeting as a sink 68 58 86 74

Add wallboard as a sink 63 54 80 69

Measured values (run 1, day 7) -- -- 72 43

The results of this assessment collectively indicate that model predictions typically agreed with 

measurement results within ± 25 percent. 

4.4 FIAM-pwp Predictions for Recent Residential Field Monitoring Studies59

Recent (i.e., since year 2000) residential formaldehyde field studies of note are summarized in 

Table 4-13. The first two studies – Offerman et al. (2008) and Hodgson et al. (2000) – both were 

conducted on new homes, with the Offerman study covering 100+ new single-family homes in 

California and the Hodgson study covering fewer structures in the eastern and southeastern 

regions of the U.S. but including both manufactured and site-built homes. The average (geometric 

mean) formaldehyde concentration indoors was between 30 and 35 ppb for both studies, with 

maximum values of 117 and 58 ppb reported by the two studies, respectively.

Studies of existing homes in urban U.S. areas by Liu et al. (2006) and Sax et al. (2006) measured 

lower average (median) formaldehyde concentrations, on the order of 10-15 ppb, and lower 

maximum values as well, with the highest reported maximum (45 ppb) in a Los Angeles home 

during the winter. Two Canadian studies (Gilbert et al 2006; Gilbert et al 2005) had nearly 

identical results; in each case the average (geometric mean) indoor formaldehyde concentration 

was around 25 ppb and the highest measured concentration was slightly above 70 ppb. 

Some other residential environments of interest have been monitored on occasion. For example, 

during December/January 2007-2008 the CDC measured formaldehyde levels in 500+ structures 

of the types (travel trailers, park models and mobile homes) used by FEMA for displaced 

Hurricane Katrina/Rita victims (CDC 2008/2010; DHS 2009). The average (geometric mean) 

formaldehyde concentration in these structures was 77 ppb (81 ppb in travel trailers) and the 

maximum measured concentration was 590 ppb. Under an interagency agreement with the CDC, 

the Lawrence Berkeley National Laboratory (LBNL; Maddalena et al. 2008) studied four FEMA 

temporary housing units (camper trailers) to assess indoor emissions of aldehydes and other 

VOCs. Measured formaldehyde levels in these structures ranged from 269 to 753 ppb.

 

59 References cited here are listed below Table 4-13; portions of this section have been extracted from the following source:

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

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

1200 Pennsylvania Avenue NW, Washington, DC 20460. July 2012.
64

Table 4-13. Summary of Recent Formaldehyde Field Monitoring Studies

Study/Reference Description (Location and 

Number/Types of Structures)

Formaldehyde Concentration, ppb

Central Value Range

Offerman et al. 

2008 108 new single-family homes in CA 31.1 (median) 3.8 to 116.7

Hodgson et al. 

2000

New homes in eastern/SE U.S.:

4 new manufactured homes

7 new site-built homes

Geometric Means:

34

36

21 to 47

14 to 58

Liu et al. 2006 234 homes in Los Angeles County, CA; 

Elizabeth, NJ; and Houston, TX 16.3 (median) 10.2 to 26.4

(5th - 95th percentiles)

Sax et al. 2006

Inner-city homes:

NY City (46) – winter (W), summer (S)

Los Angeles (41) – Winter (W), fall (F)

Medians:

10 W, 15 S

15 W, 12 F

4-18 W, 5-41 S

6-45 W, 6-26 F

Gilbert et al. 2006 96 homes in Quebec City, Canada 24.0 (geo. mean) 7.8 to 73.2

Gilbert et al. 2005 59 homes in Prince Edward Island, 

Canada 27.0 (geo. mean) 4.5 to 71.1

Hodgson et al. 

2004 4 new relocatable classrooms 8 (average indooroutdoor level)

4 to 12

(indoor-outdoor)

CDC 2008/2010

519 structures:

Travel trailers (360)

Park models (90)

Mobile homes (69)

77 (geo. mean)

81 (geo. mean)

44 (geo. mean)

57(geo. mean)

3 to 590

3 to 590

3 to 170

11 to 320

Maddalena et al. 

2008 4 FEMA camper trailers 463 (average) 269 to 753

References Cited in Table 4-13

CDC. 2008/2010. Final Report on Formaldehyde Levels in FEMA-Supplied Travel Trailers, Park Models, and Mobile 

Homes. Centers for Disease Control and Prevention. Available at:

http://www.cdc.gov/nceh/ehhe/trailerstudy/assessment.htm#final. July 2008 (amended December 2010).

N.L. Gilbert, M. Guay, J.D. Miller, S. Judek, C.C. Chan, and R.E. Dales. 2005. Levels and Determinants of 

Formaldehyde, Acetaldehyde, and Acrolein in Residential Indoor Air in Prince Edward Island, Canada. 

Environmental Research 99(1): 11-17.

N.L. Gilbert, D. Gauvin, M. Guay, M.E. Héroux, G. Dupuis, M Legris, C.C. Chan, R.N. Dietz, and B. Lévesque. 

2006. Housing Characteristics and Indoor Concentrations of Nitrogen Dioxide and Formaldehyde in Quebec City, 

Canada. Environmental Research 102(1): 1-8.

A.T Hodgson, A.F. Rudd, D. Beal, and S. Chandra. 2000. Volatile Organic Compound Concentrations and Emission 

Rates in New Manufactured and Site-Built Houses. Indoor Air 10: 178-192.

A.T. Hodgson, D. G. Shendell, W.J. Fisk, and M.G. Apte. 2004. Comparison of Predicted and Derived Measures of 

Volatile Organic Compounds inside Four New Relocatable Classrooms. Indoor Air 14 (Supplement 8): 135-144.

W. Liu, J. Zhang, L. Zhang, B.J. Turpin, C.P. Weisel, M.T. Morandi, T.H. Stock, S. Colome, and L.R. Korn. 2006. 

Estimating Contributions of Indoor and Outdoor Sources to Indoor Carbonyl Concentrations in Three Urban Areas of 

the United States. Atmospheric Environment 40: 2202-2214.

R. Maddalena, M. Russell, D.P. Sullivan, and M.G. Apte. 2008. Aldehyde and Other Volatile Organic Chemical 

Emissions in Four FEMA Temporary Housing Units – Final Report. LBNL-254E, Ernest Orlando Lawrence Berkeley 

National Laboratory, Environmental Energy Technologies Division, Berkeley, CA.

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

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

Baltimore, MD.

S.N. Sax, D.H. Bennett, S.N. Chillrud, P.L. Kinney, and J.D. Spengler. 2004. Differences in Source Emission Rates of 

Volatile Organic Compounds in Inner-City Residences of New York City and Los Angeles. Journal of Exposure 

Analysis and Environmental Epidemiology 14 (Supplement 1): S95-S109.
65

To further assess the predictive capabilities of FIAM-pwp, the model was used to predict the 

average or central-value formaldehyde concentration measured in two of the above field studies –

the Offerman et al. (2008) study of new California homes and the CDC (2008/2010) study of 

recently constructed travel trailers. In both cases, the predictions were made relying primarily on 

model defaults, occasionally supplemented in cases where ancillary study measurements 

suggested that a model input other than the default value might be more appropriate.

As the title of the Offerman et al. (2008) reference indicates, the reported field measurements were 

conducted in new California homes during the summer season. Neither the average age nor the age 

distribution of the study homes was reported; they are simply described as “new.” The average 

temperature and humidity in the homes during monitoring similarly was not reported. The paper 

does note, however, that these were single-family detached homes, most of which were further 

described as having no mechanical outdoor air systems and no nighttime ventilation cooling 

systems. The median air exchange rate in this subset of homes was reported as 0.33 ACH by

Offerman el al. (2008).

Given the above descriptions, the inputs/choices shown below in Figure 4-1 were made on the 

House Screen. Among those choices/inputs, the following are changes to model default values:

 “Standard Conditions” for temperature (23 °C) and relative humidity (50 %)

 Single-family detached home conceptualized as a single zone, with airflows edited to 

yield an air exchange rate of 0.33 ACH.

Figure 4-1. FIAM-pwp House-Screen Inputs Used to Model Study of New California Homes.

For the Source Screen (see Figure 4-2), model defaults were used for all PWPs. The only change 

made was the reassignment of sources associated with Zone 2 to be located in Zone 1, consistent 

with the single-zone conceptualization of the house as noted above.
66

Figure 4-2. FIAM-pwp Source-Screen Inputs Used to Model Study of New California Homes.

As shown in Figure 4-3, results of the model run for this case included a predicted concentration 

of 39.2 ppb (48.4 μg/m3) for these homes following construction. By comparison, the authors of 

the paper reported a median concentration of 31.1 ppb (38.3 μg/m3; neither the arithmetic nor the 

geometric mean was reported) for the subset of new homes modeled here. As noted above, the age 

of the monitored houses was not reported in the paper, other than their being “new.” If the average 

house age was six months, for example, then the model prediction of 32.6 ppb (40.3) μg/m3 would 

apply; this prediction agrees closely with the reported median value. The model prediction for 

these homes at an age of 12 months was 27.4 ppb (33.9 μg/m3.

Figure 4-3. FIAM-pwp Results for Modeling of New California Homes.
67

Travel trailers accounted for nearly 70 % of the structures that were monitored by CDC (2008/2010).

The structure type selected on the House Screen was camper trailer using model defaults, including an 

air exchange rate of 0.2 ACH, with the single exception of temperate and humidity, for which the same 

values were used as for the single-family detached case. For the Source Screen (see Figure 4-4), 

model defaults were used for all PWPs.

Figure 4-4. FIAM-pwp Source-Screen Inputs Used to Model CDC Study of Travel Trailers.

The model prediction of 78.3 ppb (see Figure 4-5) agrees closely with the geometric mean (81 ppb)

reported by CDC. However, from the information given in the CDC report, the trailers were 

approximately one to two years old when monitored. Thus, the predicted value of 52.1 ppb at an 

age of 12 months would be more appropriate. The 12-month predicted value is about a third lower 

than the geometric mean of the field measurements. One possible reason for the difference is that 

some of the trailers monitored by CDC may have included import products (e.g., for wall paneling 

or cabinets sides/back); the maximum monitored concentration of 590 ppb among the CDC travel 

trailers supports this speculation. If, for example, the intercept for HWPW were doubled to 

account for this possibility, then the predicted concentration at 12 months would be 66.2 ppb.

Figure 4-5. FIAM-pwp Results for Modeling of Travel Trailers.

The modeling results indicate that FIAM works reasonably well for its primary intended purpose –

prediction of concentrations in new residential structures shortly after their construction.
Appendix A

Summary of EPA Formaldehyde Pilot Study60

 

60 Portions of this appendix have been extracted or adapted from the following sources:

D. Hare, R. Margosian, W. Groah, S. Abel, G. Schweer, and M. Koontz. 1996. Evaluating the Contribution of UFbonded Building Materials to Indoor Formaldehyde Levels in a Newly Constructed House. In Proceedings of the 

30th International Particleboard/Composite Materials Symposium, Washington State University, Pullman, WA, 

pp. 93-108. Available at: http://www.ecobind.com/research/Evaluating_the_Contribution.pdf.

M. Koontz, H. Rector, D. Cade, C. Wilkes, and L. Niang. 1996. Residential Indoor Air Formaldehyde Testing 

Program: Pilot Study. Report No. IE-2814, prepared by GEOMET Technologies, Inc. for the USEPA Office of

Pollution Prevention and Toxics under EPA Contract No. 68-D3-0013.
A-1

A.1 STUDY OBJECTIVES AND DESIGN

At a public meeting in January 1993, EPA representatives outlined a plan for a proposed 

formaldehyde (HCHO) study that was to be conducted in two phases – a main study and a pilot 

study. The main study was to involve testing using passive monitors in 108 newly constructed 

conventional and manufactured houses located throughout the United States. A smaller pilot study 

was to be conducted first to evaluate the feasibility and logistical considerations of the 

experimental design for the larger main study.

In September 1994 the National Particleboard Association (NPA) signed a Cooperative Research 

and Development Agreement (CRDA) with EPA to fund a pilot study in newly constructed 

conventional and manufactured houses. The general purpose of the pilot study was to evaluate 

methods used to measure the contribution of UF-bonded building materials to indoor HCHO 

concentrations in newly constructed conventional and manufactured houses.

The specific objectives of the pilot study were to:

 Test logistical considerations relevant to carrying out experimental procedures for the 

testing program in a single conventionally-built single-family house and in several 

manufactured houses.

 Demonstrate that experimental variables or conditions likely to affect formaldehyde

concentrations in new houses (i.e., UF-bonded wood product emission characteristics

and loading rates, temperatures and indoor air exchange rates) can be controlled, 

individually and jointly varied, held sufficiently constant, and that the response can be 

measured to a specified precision.

 Demonstrate that test results can be obtained across a range of different experimental

conditions, similar to that which can be present in new houses.

 Estimate the extent of variability of the experimental results and the variation with

changes in experimental conditions.

 Determine how to account for, or to eliminate or minimize, residual formaldehyde

carryover between test runs in the conventional house due to effects of inherent sinks.

 Evaluate the ability to control and vary the air exchange rate of houses using an 

adjustable mechanical air handling system.

The study was guided by a Quality Assurance Project Plan (QAPP) with a detailed description of 

the experimental design and variables, monitoring protocols, and quality assurance/control 

procedures that would be employed. According to the QAPP, the pilot study would include one 

unoccupied, conventionally built, single-family house and four unoccupied manufactured houses. 

Four manufactured houses were specified in the pilot study because it was not practical to install 

and remove products in a manufactured house. The conventional house was to be a two-story 

house on a slab or crawlspace. The manufactured-house portion of the pilot study eventually was 

dropped due to resource limitations.
A-2

The QAPP also specified that the following variables would be evaluated in the pilot study:

 Product emission characteristics: Only “medium” emitting products were to be used. 

Emissions from particleboard and from plywood wall paneling, as measured by ASTM 

method E 1333 (cited previously in this document) were to be between 0.12 and 0.14 

ppm. Because there were no established protocols for measuring formaldehyde 

emissions from kitchen/bathroom cabinets, commercially available cabinets 

constructed with “medium” emitting materials (melamine-wrapped particleboard) were 

to be used.

 Product loading rate: Two different loading combinations of products – Medium and 

High (defined later) – were to be installed in the house.

 Environmental conditions in the house:

- Temperature of 75 °F

- Ventilation rate of 0.5 air changes per hour (ACH)

Relative humidity in the house, while not a controlled variable, was targeted at 50 percent. Air 

leakage was to be determined through blower-door tests, with efforts to make the house as airtight 

as possible. Ventilation rates were to be controlled by a mechanical heat recovery ventilator 

(HRV) attached to the heating and air conditioning (HAC) system. Air exchange rates were to be 

measured by two tracer gas methods – constant release of perfluorocarbon tracers (PFTs, EPA 

Method IP-4A) and periodic injection/decay of sulfur hexafluoride (SF6, EPA Method IP-4B). All 

USEPA methods cited on this page have been compiled in a compendium61.

Four product loadings were to be used – two Medium and two High. Prior to loading the products 

in the house, a baseline value for the house without any experimental UF-bonded wood products 

was to be determined. The products then would be installed in the house and indoor formaldehyde 

concentration measurements taken over approximately a 30-day period. At the completion of 30-

day period, the products were to be removed from the house. The house would be allowed to “air 

out” for a period of time until a new equilibrium level (baseline) was reached, after which the next 

set of products would be installed. The order of the loading configurations in the house was to be 

randomized, with the single constraint that Loadings 1 and 2 would be different (i.e., one High and 

one Medium).

Indoor and outdoor formaldehyde concentrations (24-hour time-integrated values) were to be 

measured by EPA method IP-6A (Solid Adsorbent Cartridge) 7, 12, 28, and 33 days after products 

were installed in the house. If necessary, additional testing could take place to confirm any trends 

or unexpected results. Readings on days 12 and 33 were considered statistical replicates of 

readings on days 7 and 28, respectively. Indoor formaldehyde concentrations were to be measured 

in the kitchen, living room, upstairs bedroom, basement, and outdoors. One duplicate sample was 

to be collected on each sampling occasion, at a location to be varied systematically.

Product emission tests were to be conducted in a large chamber in accordance with ASTM E 1333, 

at three different air exchange rates. Because there were no established protocols for largechamber testing of doors and cabinets, the following loading rates were be used:

 

61 USEPA. 1990. Compendium of Methods for the Determination of Air Pollutants in Indoor Air. Report No. 

EPA/600/4-90/010, Atmospheric Research and Assessment Laboratory, USEPA, Research Triangle Park, NC. 
A-3

 Doors – five doors, at a total loading rate of 0.125 ft2/ft3

 Kitchen cabinets/countertops – one base and one wall cabinet with doors closed, at a 

loading rate of 0.133 ft2/ft3. A section of countertop was to be placed on the base 

cabinet during testing.

The above loading rates were similar to those established by the Department of Housing and 

Urban Development (HUD) for large-chamber testing of particleboard and industrial panels.

For sink-effect testing of painted gypsum wallboard and carpet/padding, products were to be 

placed in a small clean environmental chamber. A known concentration of formaldehyde gas was 

to be injected into the chamber. By accurately measuring time-varying concentrations in the 

chamber, it could be determined whether the material was absorbing formaldehyde gas and then 

re-emitting it after the source was turned off.

A.2 STUDY METHODS

Securing a house builder or owner who was willing to participate in the study proved to be more 

difficult and costly than anticipated, due mainly to the constraint that the house could not be 

occupied during the test period. A homeowner was located in Centreville, MD, who was building 

a rental house and was willing to lease it for as long as needed. The house was a conventionally 

built, 1326 ft2, two-story Cape Cod style with a full basement (volume of 168 m3). The QAPP had 

called for a house with crawl space, but efforts to find such a house in the study area were 

unsuccessful. The total volume of the finished living space was 10,746 ft3(304 m

3

). The first floor 

of the house (712 ft2floor area) had 5931 ft3(168 m

3

) of living space that consisted of a living 

room, kitchen, bathroom/laundry room, and bedroom. The second floor of the house (614 ft2floor 

area) had 4815 ft3(136 m

3

) of living space that consisted of two bedrooms and a bathroom. The 

door from the first-floor finished living space to the unfinished basement was sealed shut during 

the entire project. The house contained no furniture or draperies. 

Industry representatives were responsible for procuring all pressed-wood products that would be 

used in the pilot study. Sufficient quantities of 5/8” particleboard underlayment, 3/4” industrial 

particleboard for countertops, 1/4” hardwood plywood wall paneling (3-ply birch face, tropical 

hardwood back and core with 7 cut grooves along the length of each panel to simulate randomwidth lumber planking), interior partition doors, and kitchen and bathroom cabinets were obtained 

from member companies or participating associations, or were purchased at local building supply

centers. Emission characteristics of the particleboard and plywood wall paneling used in the study 

were determined prior to selecting the materials for testing. 

Initial emission characteristics of the products used in the study (see Table A-1) were determined 

by ASTM E 1333 at 0.5 ACH. All products were stored in a controlled-access warehouse until 

they were installed in the house. During storage, products were wrapped in 6-mil plastic to 

minimize formaldehyde off-gassing. It was discovered during the study that temperature control in 

the warehouse was marginal and that temperatures ranged up to 85 ° F.

Products were installed in the house according to manufacturers’ recommended installation 

instructions, and conventional practices were followed. The particleboard underlayment was 

installed with screws for easy removal. The kitchen and bath cabinets were screwed tightly against 

the wall. The plywood wall paneling was nailed to the wall to minimize damage to the gypsum 

wallboard.
A-4

Table A-1. Initial Emission Characteristics of Products Used in the Pilot Study

Product Emissions, ppm Loading Rate, ft2/ft3

Particleboard Underlayment 0.144 0.130

Plywood Wall Paneling 0.114 0.290

Cabinets 0.053 0.133

Interior Doors 0.052 0.125

Loading areas for pressed-wood products are shown in Table A-2. For the Medium loading 

scenario, particleboard underlayment was installed on the first floor, along with interior doors on 

both floors and a full set of kitchen/bathroom cabinets and countertops. For the High loading 

scenario, particleboard underlayment was installed on both the first and second floors. In addition, 

twelve 4’ X 8’ sheets of plywood wall paneling were installed. To establish a relatively uniform 

loading and to avoid using partial panels, four full-size sheets were installed in an upstairs 

bedroom, in the downstairs bedroom, and in the living room. Interior doors and a full set of 

kitchen and bathroom cabinets and countertops (same number as for Medium loading) also were

installed. The total exposed surface area for cabinets was calculated to be 631 ft2(59 m

2

). 

Particleboard underlayment was not installed in the kitchen and bathroom areas of the house, as 

those areas were covered by vinyl sheet goods that had been permanently installed during final 

preparation of the house before field testing was initiated.

Table A-2. Loading Areas for Pressed-wood Products at the Conventional House

Component

Loading Area, ft2

1st Floor 2nd Floor

Medium 

Loading

High

Loading

Medium

Loading

High

Loading

Underlayment 496.2 496.2 --- 519.3

Paneling --- 256.0 --- 128.0

Doors 203.6 169.6

Countertop 37.9 5.65

Blower-door tests indicated that the house had numerous leaks in the building shell and between 

the first floor and basement. Air leakage sites were identified (service penetrations between the 

basement and first floor and along the base plate) and sealed with caulk and foam. Numerous leaks 

also were found in the forced-air HAC distribution system. Sealing procedures reduced the 

leakage rate by more than 50 percent, from ~ 10 ACH at 50 Pa (corresponding to ~ 0.5 ACH under 

moderate environmental conditions) to ~ 4 ACH at 50 Pa.

Initial study plans did not include measurement of formaldehyde concentrations in the basement, 

because it was thought that the combination of house sealing procedures and a closed interior door 

at the top of the stairwell to the basement would minimize air communication with the remainder 

of the house. However, during tracer gas studies to characterize the air exchange rate for the house 

it became obvious that tracer injected into the living area was being transferred to the basement.

Consequently, it was decided to include formaldehyde measurements in the basement. In addition, 
A-5

PFT sources and samplers were configured so that the average airflow rate between the basement 

and living area could be quantified during each sampling event.

Following initial baseline testing, products for Loading 1 (Medium) were installed and indoor 

concentration levels were measured in accordance with the protocols outlined in the QAPP. It 

became evident during this time that there was difficulty in controlling relative humidity levels in 

the house, which reached over 70 percent. Humidity levels were lowered to the desired range by

adding additional dehumidification equipment in the house. Because high humidity causes higher

formaldehyde emissions, the house was declared to be in an “upset condition” during this time and 

it was decided that Loading-1 products should be left in the house until conditions stabilized. The 

final readings for Loading 1 were taken 78 days after product loading.

It also was determined during this time that resource constraints would not permit the full 

implementation of the loading schedule established in the study plan, which was modified for 

conduct of only three loadings in the house – one Medium and two High loadings. In addition, the 

air-out period between successive loadings was fixed at seven days instead of waiting for

formaldehyde levels to reach equilibrium.

Following the removal of Loading-1 products from the house and a seven-day airing-out period, 

Loading-2 (High) products were installed. Formaldehyde concentrations were measured on days 7, 

12, 28, and 33. Following removal of Loading-2 products from the house and a seven-day airingout period, Loading-3 (High) products were installed. Formaldehyde concentrations again were 

measured on days 7, 12, 28, and 33 after loading. Following removal of Loading 3 products, the 

house was returned to the homeowner. 

A limited amount of sink-effect testing was conducted as part of the pilot study. From results of 

the laboratory sink-test results and results from Loadings 1, 2 and 3, it appeared that the gypsum 

wall board was acting as a significant sink. Because the exposed surface area of the wallboard was 

greater than all of the UF-bonded products installed in the house, it was considered critical to fully 

characterize the sink effect behavior of painted gypsum wallboard. It also was considered 

important to determine whether the carpet/padding used in the house was acting as a sink or had 

any barrier effect on formaldehyde emissions from the particleboard underlayment. 

The original protocol for “barrier testing” was modified to evaluate the floor system with both

particleboard underlayment and carpet/padding, instead of just the carpet/padding. A sample of 

particleboard underlayment was placed in a stainless-steel pan and inserted in a small chamber. 

Formaldehyde concentrations in the chamber were measured continuously with a real-time 

(Interscan) analyzer. As a separate test, a second sample of particleboard with carpet/padding 

directly above it was placed in a stainless-steel pan and inserted in the chamber. Differences in the 

concentration profiles for the two tests would reflect the barrier effect of the carpet/padding used 

in the pilot study. To minimize the inherent variability within a piece of particleboard, specimens 

used for these tests were prescreened via small-chamber emission tests62– the two pieces with the 

closest emission rates were used for the tests.

 

62 ASTM. 1996. Standard Test Method for Determining Formaldehyde Concentrations in Air from Wood Products 

Using a Small-Scale Chamber. Designation D 6007-02 (Reapproved 2008), American Society for Testing and 

Materials, Philadelphia, PA.
A-6

A.3 STUDY RESULTS

The results of formaldehyde, temperature, humidity and air-exchange measurements during the 

formal “runs” associated with each loading, as well as their respective baseline periods, are 

compiled in Table A-3. Formaldehyde concentrations measured in the upstairs rooms generally 

were similar to one another and higher than those measured in the basement. The highest 

concentration measured in the house – 76 ppb – was in the kitchen during Loading 3 (High). The 

monitoring results collectively indicate that, for all three loadings, indoor HCHO concentrations 

had peaked by the time of measurements taken 33 days after product loading. The formaldehyde 

results also indicate that monitored levels for Loadings 2 and 3 generally were consistent with one 

another and that these “high” loadings had higher HCHO levels (but also a higher baseline) than 

the “medium” loading (Loading 1).

Although measured air exchange rates generally were in the target range of 0.4 to 0.6 ACH, the 

PFT estimates consistently were higher than those based on SF6. One possible reason for the 

discrepancy is the inherent difference between the two measurement technologies. The SF6

method involves periodic injection and mixing of the tracer through the HAC system, followed by

real-time analysis as the concentration declines. The PFT method relies on a near-constant release 

of tracers from multiple locations near the perimeter of the house, with time-integrated sampling 

over a 24-hour period. 

Table A-3. Summary of Monitoring Results for Loadings 1, 2, and 3

Runa Formaldehyde Concentration, ppb Temperature, F RH, % Air Exchange, ACH

LR KIT BR BMT AMB LR KIT BR BMT AMB LR 2ND AMB SF6 PFT

Loading 1 (Medium)

Baseline 9.1 9.8 9.5 9.1 1.3 75.7 75.8 74.0 63.4 49.0 22.8 24.4 55.2 -- 0.50

1 27.5 29.9 20.1 -- 1.0 73.8 73.4 73.2 68.5 56.9 43.2 41.0 60.2 0.44 0.54

2 29.6 31.3 29.9 26.5 7.0 73.8 73.6 73.4 66.7 52.4 46.2 43.2 76.8 0.44 0.54

3 41.7 43.9 36.3 28.2 2.1 75.1 75.0 74.9 71.1 69.6 63.1 60.2 93.9 0.40 0.54

4 36.8 39.4 31.1 -- 1.7 75.5 74.8 73.4 73.1 66.0 50.4 49.8 60.6 0.40 0.57

Loading 2 (High)

Baseline 23.0 23.7 22.1 25.7 2.8 75.2 73.1 74.3 77.0 84.4 54.5 52.3 80.3 0.39 --

1 60.1 60.8 60.5 39.7 2.9 75.3 73.5 74.5 76.2 81.5 57.1 55.5 88.2 0.38 0.61

2 55.8 58.1 54.7 40.0 2.3 75.1 73.4 74.3 75.4 80.1 50.9 49.3 74.1 0.38 0.71

3 46.7 48.5 53.3 38.0 1.4 74.4 73.9 75.6 75.7 75.4 50.7 43.8 71.2 0.36 0.59

4 39.0 40.8 48.1 30.7 0.9 73.4 73.2 73.2 73.6 63.1 51.6 43.2 71.6 0.39 0.60

Loading 3 (High)

Baseline 26.0 25.8 27.7 25.0 <1.0 73.3 72.8 73.3 73.3 61.4 54.9 44.3 85.0 0.41 --

1 71.9 76.1 65.6 42.6 1.1 75.0 73.2 72.5 73.6 76.1 62.7 65.3 95.1 0.39 0.69

2 53.8 55.2 54.7 29.9 1.1 72.3 72.4 72.0 70.7 60.3 49.7 47.1 86.7 0.36 0.64

3 43.2 46.2 39.5 20.3 1.1 71.7 71.8 69.5 66.6 49.1 43.8 43.7 87.2 0.42 0.62

4 42.3 40.4 44.1 22.8 1.4 71.7 71.9 70.7 66.9 55.4 46.1 44.9 88.8 0.35 0.53

Legend: a Run 1 occurred 7 days after product loading;

LR = living room; KIT = kitchen; Run 2 occurred 12 days after product loading;

BR = bedroom; BMT = basement; Run 3 occurred 28 days after product loading;

AMB = ambient; 2ND = second floor. Run 4 occurred 33 days after product loading.
A-7

Upstairs measurements (average of kitchen, living room and bedroom locations) for the three 

loadings are plotted in Figure A-1 against elapsed time after UF-bonded wood products were 

installed (day “zero” indicates baseline measurements). The curvature of the lines is due to the 

spline fit that was applied and is not meant to imply that values between measurement points can 

be readily interpolated. The figure illustrates the general consistency across Loadings 2 and 3 and 

their differences from Loading 1. Although concentrations for Loading 1 are similar to those for 

Loadings 2 and 3 toward the end of their respective test periods, indoor humidity levels were 

relatively high near the end of Loading 1.

Figure A-1. Upstairs Formaldehyde Concentrations over Time for Three Loadings.

Differences in baseline levels and in the general shape of the time series for Loading 1 versus 

Loadings 2 and 3 can be explained partly by the wallboard sink effect. The wallboard sink was 

largely “empty” at the outset of Loading 1 but was largely “pre-loaded” at the outset of Loadings 2 

and 3, as reflected in the higher baseline values for the later loadings. The time series for Loadings 

2 and 3 have a shape consistent with that of an exponentially declining emitter, as would be 

expected when the dominant sink (wallboard) has been pre-loaded and becomes a “net zero 

emitter,” such that the declining rate of emissions from aging wood products drives the 

concentration profile. For Loading 1, however, this profile is dampened by the relatively large 

mass being absorbed by the wallboard sink in an “unloaded” state. Some of the difference in 

baseline values for Loading 1 versus Loadings 2 and 3 also may be due to the relatively low 

humidity level at the time when baseline measurements were taken for Loading 1.

0

10

20

30

40

50

60

70

80

0 5 10 15 20 25 30 35

F

orm

aldeh

y

de C

o

ncentratio

n, p

p

b

Time from Loading, days

Loading 1

Loading 2

Loading 3
A-8

The key measurement parameter for the study, formaldehyde by the DNPH method, was in control 

across all measurement periods. QA spikes indicated that recoveries were consistently in the range 

of 90-110 percent, well within the accuracy goal of ±20 percent. Results of duplicate DNPH 

samples indicated a combined sampling and analytical precision of ±10 percent or better, again 

well within the corresponding data quality objective. The standard deviation across the high 

loadings, for measurements at the same location and at the same elapsed time since loading of UFbonded wood products, was not much larger than that for duplicate measurements (i.e., at the same 

time and location within any given loading). This finding points to the repeatability of highloading results for the conventional house, after the sinks were pre-loaded as a result of the 

preceding medium loading.

Emission rates for UF-bonded wood products that were installed in the pilot study house, based on 

large-chamber concentrations that were adjusted to standard conditions of 77  F and 50% RH per 

ASTM E 1333, are listed in Table A-4. Each product was tested at target air exchange rates of 

0.25, 0.5 and 1.0 ACH. Two patterns that were evident for all products are noteworthy. First, as 

the air exchange rate is increased, the emission rate also increases, but at a rate that is less than 

proportional to the increase in air exchange. This trend is indicative of a concentration 

“backpressure” effect, as applied in the Matthews model. Second, for each product the emission 

rate was lower for products used for later loadings (products were tested near the time of each 

loading). These results indicate that some emissions decay likely occurred during storage, even 

though products were wrapped in plastic. The elevated temperature in the warehouse may have 

accelerated the decay process somewhat.

Table A-4. Computed Emission Rates for UF-Bonded Products

Loading

Emission Rate, μg/m2-h

Underlayment Paneling Cabinets Doors

At Target ACH of 0.25a

1

2

3

104.9 (0.25 ACH)

99.2 (0.25 ACH)

90.4 (024 ACH)

Not Used

42.9 (0.25 ACH)

32.9 (0.25 ACH)

45.6 (0.17 ACH)

45.2 (0.24 ACH)

45.7 (0.25 ACH)

52.0 (0.24 ACH)

Not Tested

49.9 (0.25 ACH)

At Target ACH of 0.50

1

2

3

178.3 (0.50 ACH)

162.6 (0.50 ACH)

149.9 (0.51 ACH)

Not Used

72.3 (0.50 ACH)

56.8 (0.50 ACH)

77.1 (0.51 ACH)

69.9 (0.50 ACH)

68.5 (0.50 ACH)

74.2 (0.50 ACH)

Not Tested

68.4 (0.51 ACH)

At Target ACH of 1.0

1

2

3

221.9 (1.01 ACH)

200.6 (0.99 ACH)

190.2 (1.01 ACH)

Not Used

104.9 (0.99 ACH)

77.0 (1.01 ACH)

82.8 (1.00 ACH)

73.4 (0.99 ACH)

74.9 (1.01 ACH)

79.9 (1.00 ACH)

Not Tested

74.2 (1.00 ACH)

a Actual air exchange rates during each test are given in parentheses.
A-9

Other products used in the house were assessed for formaldehyde emission characteristics. For the 

paint used on the wallboard and interior doors at the study house, the manufacturer’s data 

indicated that the wallboard paint contained 0.001% formaldehyde by weight and the door paint

contained 0.007% formaldehyde. A carpet/padding sample was sent to EPA’s Air and Energy 

Engineering Research Laboratory (AEERL) in Research Triangle Park for small-chamber testing. 

Results of the chamber tests, conducted at 0.5-0.55 ACH, 74  F and 55-60 % RH, indicated an 

emission rate of 1.4 μg/m2-h for the carpet and the same rate for the padding (after subtracting the 

chamber background) at an elapsed time of 24 hours following insertion in the chamber, or an 

emission rate of 2.8 μg/m2-h for these two constituents combined. The combined emission rate is 

one to two orders of magnitude lower than that for UF-bonded wood products.

Small-chamber tests were conducted to evaluate sink characteristics of the carpet and padding. 

Two tests were conducted under identical air exchange rates of 1.5 ACH, but with different 

formaldehyde concentrations in the input stream because the rate of adsorption to the sink was

suspected to be a function of the air concentration. In both tests, a 7.1-inch square piece of carpet 

and padding was placed in a tight-fitting aluminum pan with edges equal to the depth of the carpet 

and padding. The pan containing the carpet and pad was then placed horizontally on the chamber 

floor. The chamber was sourced with a formaldehyde stream of 0.2 ppm during the first test and 

0.12 ppm during the second. Comparison of the theoretical (concentrations that would be expected 

in the absence of sinks) and measured concentrations indicated that formaldehyde was adsorbed 

by the carpet, and the trend in the data appeared to reflect a first-order removal process. 

Chamber tests for gypsum wallboard were conducted to evaluate its sink characteristics. As with 

the carpet tests, these two tests were conducted under identical conditions with the exception of 

the formaldehyde feed-stream concentration, to evaluate the effect of concentration on the rate of 

mass transfer to and from the wallboard. For each test, two 10.5-inch square pieces of painted 

wallboard were securely fastened back-to-back and edge sealed with sodium silicate, leaving a 

total of 220.5 square inches of exposed wallboard surface area. The wallboard was then placed in 

the center of chamber in a vertical position, supported by stainless steel wire. The chamber was 

sourced with a formaldehyde stream of 0.2 ppm during the first test and 0.12 ppm for the second, 

at 1.5 ACH. The resulting data from these tests indicated that the sink effect was more pronounced 

for wallboard than for the carpet/padding ensemble. The concentration data were fit to a Langmuir 

isotherm model to estimate adsorption and desorption rate constants for the two types of sinks.

Tests to evaluate the behavior of underlayment both with and without a carpet/padding barrier 

were conducted under identical chamber conditions using a tight-fitting aluminum pan with edges 

equal to the depth of the underlayment, carpet and padding. In the first test, a 7.1-inch square piece 

of underlayment was placed in the pan with nothing above it. In the second test, a 7.1-inch square 

piece of underlayment was placed in the pan with an equally sized piece of padding and carpet 

placed above it. For both tests, the pan was then placed horizontally on the chamber floor. The 

resulting data indicated that the underlayment was releasing formaldehyde. While it was tempting

to conclude from the results that the carpet and padding had no effect on the emission of 

formaldehyde, the carpet/padding sink test described above indicated otherwise. 
A-10

A.4 STUDY CONCLUSIONS

The following conclusions, primarily related to the objectives given in Section A.1, were reached 

as a result of the pilot study:

 Some logistical difficulties were encountered that prevented the study from being 

completed as outlined in the QAPP. Among the greatest difficulties were locating a 

study house and maintaining humidity levels in the house within the prescribed range. 

Humidity was not a primary experimental variable for the study, but was considered to 

be an important covariate that should be tightly controlled. Logistical aspects such as 

acquiring, storing and accessing the materials associated with house loading went 

remarkably smoothly, due in part to the high level of cooperation from various industry

representatives and their close working relationship with the study team. 

 Little difficulty was encountered in controlling most experimental variables at the 

study house. The primary variables – product loading rate, emission rate, temperature, 

and air exchange – all were generally kept within their respective target ranges. Some 

emissions decay was evident for wood products stored for successively later loadings. 

 It was demonstrated that different test results could be obtained across different 

experimental conditions and that sufficient precision in measurement response could be 

obtained. Differences across the experimental conditions (medium versus high loading) 

could be distinguished, although these differences were partially dampened by the 

substantial sink effect of the gypsum wallboard. Appropriate precision was obtained 

largely because of the consistently high resolution, accuracy and precision for the 

sampling method (solid adsorbent cartridge) with a 24-hour sampling duration.

 The study data were marginally sufficient for estimating the extent of variability of 

results and the variation with changes in experimental conditions. The limited number 

of loadings, coupled with the unique circumstances of the first loading (little mass in 

the indoor sinks at the outset), made it difficult to properly estimate the variance 

component associated with repeats of the same loading configuration. However, it was 

demonstrated that the variance across repeats of the high loading, for a specific 

sampling location at the same elapsed time relative to loading of the house, was only 

marginally higher than the variance for duplicate formaldehyde samples.

 Formaldehyde carryover between successive tests at the conventional house, due to 

inherent sinks, was not trivial. The sink effect, due largely to adsorption of 

formaldehyde by painted gypsum wallboard, was not eliminated by the prescribed 

“airing out” of the house. The monitoring results, however, did suggest that baseline 

formaldehyde values were reasonably reproducible once the house had been loaded 

with UF-bonded wood products.

 The ability to control and vary the air exchange rate of the conventional house using a 

heat recovery ventilator (HRV) was demonstrated. This ability was demonstrated for 

two of the three target air exchange rates (0.2 and 0.5 ACH) planned for the full study; 

the third target (1.2 ACH) could not be addressed because the HRV lacked sufficient 

capacity. Achieving the lower target of 0.2 ACH was made possible by sealing 

procedures that substantially reduced the air leakage of the house and, thus, the 

sensitivity of its air exchange rate to weather conditions.
Appendix B

Model Sensitivity
B-1

B.1 ANALYSIS BY MATTHEWS

Prior work by Matthews and colleagues at Oak Ridge National Laboratory during the mid-1980s 

forms the primary basis for FIAM-pwp, as noted in Section 1 of the main document. The 

researchers generated a series of reports documenting their work for the U.S Consumer Product 

Safety Commission. One of these reports63 included a sensitivity analysis for formaldehyde 

concentration and emission-rate models; the concentration model was very similar to that used for 

FIAM-pwp, including temperature and humidity adjustments. The measure of sensitivity was the 

fractional change in model output per unit change in each coefficient; for example, a sensitivity of 

0.1 would mean that a 10% change in the model is expected with a 100% change in the 

coefficient.

The results of the sensitivity analysis depended on both the values of the model coefficients 

determined for a given pressed-wood product data set and the environmental conditions that are 

substituted into the model. Results of the analysis indicated that the concentration model was most 

sensitive to values for the coefficients representing the temperature and relative humidity (RH) 

dependence, regardless of product type. The highest value for the sensitivity measure, for MDF, 

was 0.45 for temperature; similar, but somewhat lower, sensitivity values were found for 

particleboard and paneling. The highest sensitivity value for RH was 0.40, for paneling.

B.2 ANALYSIS BY VERSAR64

Versar performed sensitivity analysis on a prior, mathematically identical, version of FIAM-pwp to 

guide the choice of factors to be varied and the relative number of variations for each as part of an 

earlier set of model runs. This analysis was conducted for two cases available in the prior version 

of FIAM-pwp – a conventional home (i.e., the conventionally built single-family detached home

used in the EPA formaldehyde pilot study; see Appendix A) and a manufactured home. The 

conventional home, with a volume of 471 cubic meters, is conceptualized as having two zones 

whereas the manufactured home is a single-zone structure with a volume of 222 cubic meters. 

As summarized in Table B-1 for the conventional home, ten factors (input parameters) were 

systematically varied for the sensitivity analysis. The background concentration and average 

concentration encountered when an individual is away from home both were set to zero for this 

analysis, so that modeled concentrations and exposures would reflect only the impact of PWPs in 

the house. The source and sink loading areas in the table represent one of the actual loading 

conditions that were used in the pilot study. 

Sensitivity was examined by varying each input parameter – one at a time – by 20 percent in each 

direction (i.e., 20 % lower and 20 % higher) from the base condition shown in Table B-1. Five 

model outputs were examined for sensitivity to the ten factors: (1) initial concentration in zone 1 

(upstairs); (2) concentration 24 months later in zone 1; (3) average daily concentration (ADC); (4) 

average daily dose (ADD65); and (5) % of time the concentration is ≥ 0.01 ppm (10 ppb). Values 

of these outputs for the base condition are listed in Table B-2.

 

63 T. Matthews, T. Reed, B. Tromberg., K. Fung, C. Thompson, J. Simpson, and A. Hawthorne. 1985. Modeling and 

Testing of Formaldehyde Emission Characteristics of Pressed-Wood Products: Report XVIII to the U.S. 

Consumer Product Safety Commission. ORNL/TM-9867, Oak Ridge National Laboratory, Oak Ridge, TN.

64 Extracted/adapted from Formaldehyde from Composite Wood Products: Exposure Assessment. Draft Final 

Report. U.S. Environmental Protection Agency, Office of Pollution Prevention and Toxics, Exposure 

Assessment Branch,1200 Pennsylvania Avenue NW, Washington, DC 20460. July 2012.

65 The model output no longer includes ADD values.
B-2

Table B-1. Input Values for Sensitivity Analysis – Base Condition for Conventional Home

Input Parameter(s) Base Value(s)

Air Exchange Rate (ACH) 0.59

Emissions Half Life (years) 2.92

Temperature (°C) 23

Relative Humidity (%) 50

Source Loading Areas

a

(m2) 46, 59, 35, 4

Sink Loading Areas

a

(m2) 94, 420

House Age at Move-in (years) 5

Years Lived in House 5

Hours in House on Weekday, Weekend Day 15, 18

Inhalation Rate (m3/day) 13.3

a

Sources are underlayment, cabinets, interior doors, and 

countertop; sinks are carpet/padding and painted wallboard.

Table B-2. Model Results for Base Condition for the Conventional Home

Initial Conc in 

Zone 1, 

mg/m3

Conc 24 Months 

Later in Zone 1, 

mg/m3

Average Daily 

Concentration, 

mg/m3

Average Daily 

Dose, 

mg/kg-day

% of Time 

Concentration 

≥ 0.01 ppm

0.0514 0.0320 0.0061 0.0011 13.7

Changes in each model output associated with the change in each input, in each direction, were 

averaged and then expressed as a percent change from the result for the base condition. The 

sensitivity results are summarized in Figure B-1 as the percent change for each model output 

associated with a 20 % change in each input. Certain inputs, by definition, do not affect certain 

model outputs. For example, the emissions half life has no effect on the initial concentration and 

the inhalation rate affects only the average daily dose. Based on this analysis, the most sensitive 

input parameters, in order of relative sensitivity (most sensitive first), are indoor temperature, 

emissions half life, house age, and indoor humidity.
B-3

Figure B-1. Relative Model Sensitivity for the Conventional Home.

For the manufactured home (Table B-3), the same ten factors were systematically varied for the 

sensitivity analysis. As with the conventional home, the background concentration and average 

concentration encountered when an individual is away from home both were set to zero for the 

analysis. A broader set of PWPs, with generally higher loading rates, was used for this case. As a 

result, the modeled initial concentration was about three times as high as the conventional home 

case (Table B-4). Due to the higher concentration, a higher threshold (0.02 ppm) was used to 

examine the relative frequency of higher concentrations. As with the conventional home, the most 

sensitive input parameters for this case are indoor temperature, emissions half life, house age, and 

indoor humidity (Figure B-2).
B-4

Table B-3. Input Values for Sensitivity Analysis – Base Condition for Manufactured Home

Input Parameter(s) Base Value(s)

Air Exchange Rate (ACH) 0.50

Emissions Half Life (years) 2.92

Temperature (°C) 23

Relative Humidity (%) 50

Source Loading Areas

a

(m2) 95, 53, 26, 3, 43, 36

Sink Loading Areas

a

(m2) 72, 158

House Age at Move-in (years) 5

Years Lived in House 5

Hours in House on Weekday, Weekend Day 15, 18

Inhalation Rate (m3/day) 13.3

a

Sources are PB underlayment, paneling, interior doors, countertop (particleboard), 

cabinets, and closet shelving (PB/MDF); sinks are carpet/padding and wallboard.

.

Table B-4. Model Results for Base Condition for the Manufactured Home

Initial Conc in 

Zone 1, 

mg/m3

Conc 24 Months 

Later in Zone 1, 

mg/m3

Average Daily 

Concentration, 

mg/m3

Average Daily 

Dose, 

mg/kg-day

% of Time 

Concentration 

≥ 0.02 ppm

0.1507 0.0937 0.0178 0.0033 35.0

Figure B-2. Relative Model Sensitivity for the Manufactured Home.

0.00%

10.00%

20.00%

30.00%

40.00%

50.00%

60.00%

70.00%

80.00%

90.00%

100.00%

Percent Change in Model Output Due to 20 % Change in Input Value

Initial Concentration

Conc @ 24 months

ADC

ADD

% Time ≥ 0.02 ppm
Appendix C

Basis for Emission Classes
C-1

As described in Sections 2.3.1 and 2.3.3 of the main document, default values for FIAM

emission-rate parameters have been developed for four emission classes: Baseline; CARB1; 

CARB2; and NAF. Further details on the basis for these default values are described below, 

under the headings of Baseline Emissions Class (Section C.1) and Reduced Emissions Classes 

(Section C.2; includes CARB1, CARB2, and NAF). This appendix has been extracted/adapted 

from the draft formaldehyde exposure assessment report cited in Appendix B (see Section B.2).

C.1 BASELINE EMISSIONS CLASS

Between June and October 2010, EPA administered a survey to domestic manufacturers of 

composite wood panels; the survey asked about both current and planned emission levels. 

Because many CWP manufacturers are in the midst of modifying their production technology 

and raw materials to achieve compliance with Phase 2 of the CARB Air Toxics Control Measure 

(ATCM), using current emission levels to estimate emission levels in 2013 would not accurately 

reflect the 2013 baseline. Instead, baseline emission levels were estimated using responses to a 

survey question about emission levels that mills planned to achieve within the next three years. 

These levels were weighted by production levels that mills reported in the survey to estimate 

average emission levels by certification standard. The survey data also were used to estimate the 

share of total production volume represented by products meeting each of the emission 

standards. The survey results are reported in Table C-1. For comparison purposes, the CARB 

emission limits are shown in Table C-2. 

Table C-1. Baseline Emission Levels (2013) For Domestically Produced CWPs, 

by Product Type and Emissions Certification Standard

Emissions Standard

Product Type

HWPW MDF PB

Weighted Average Emissions (ppm)

None 0.058 --- 0.058

CARB 1 a

--- --- 0.090

CARB 2 0.032 0.082 0.057

CARB ULEF b

0.024 0.042

CARB NAF c

0.013 0.035 0.013

Share of Production

None 0.4% --- 0.2%

CARB 1 --- --- 1.8%

CARB 2 60.2% 86.1% 88.5%

CARB ULEF 4.6% 2.8%

CARB NAF 39.4% 9.3% 6.7%

a Some mills producing products currently certified under CARB Phase 1 (CARB 1) reported that they would 

continue this product line after the CARB Phase 2 (CARB 2) effective date.

b Hardwood plywood mills meeting CARB 2 and CARB Ultra Low Emission Formaldehyde (ULEF) were 

grouped together in the CARB 2 category. Several HWPW mills appeared to be meeting CARB ULEF 

standards but did not report this in their response. For example, they listed the certification standard as “CARB 

2” but the resin type as “ULEF” and/or reported maximum emissions that satisfy ULEF requirements. Due to 

difficulties in distinguishing between CARB 2 and CARB ULEF product lines and the fact that emission levels 

appeared similar among mills reporting either CARB 2 or CARB ULEF certification standards, these mills 

were grouped together.

c NAF = No Added Formaldehyde.
C-2

Table C-2. Formaldehyde Emissions Standards in the CARB Air Toxic Control Measure 

for Formaldehyde Emissions from Compressed Wood Productsa(ppm)

Emissions Standard HWPW MDF Particleboard

CARB 1 0.08 0.21 0.18

CARB 2 0.05 0.11b

0.09

CARB ULEF – 

less frequent testing 0.05 90% of samples < 0.06

All samples < 0.09c

90% of samples < 0.05

All samples < 0.08

CARB ULEF –

exemption from third 

party certification

90% of samples < 0.04

All samples < 0.05

90% of samples < 0.04

All samples < 0.06

90% of samples < 0.04

All samples < 0.06

CARB NAF 90% of samples < 0.04

All samples < 0.05

90% of samples < 0.04

All samples < 0.06

90% of samples < 0.04

All samples < 0.06

a The Formaldehyde Standards for Composite Wood Products Act (FSCWPA) emission limits are the same as 

the CARB2 ATCM limits.

b The CARB 2 emission limit for thin MDF is 0.13 ppm.

c The CARB ULEF limits for thin MDF are 90% of samples < 0.08 ppm and all samples < 0.11 ppm.

Among imported products, baseline emission levels were estimated separately for products from 

Canada versus those from the rest of the world. The known composite wood manufacturers in 

Canada were compared to the CARB list of certified mills, and most Canadian mills were found 

to be CARB-certified. As a result, Canadian mills were assumed to achieve the same baseline 

emissions as U.S. mills (whose emissions are shown above in Table C-1).

The baseline emission levels for non-Canadian imported CWPs were estimated using data from a 

2009 presentation by Professional Service Industries66, a CARB Third Party Certifier (TPC). 

Although PSI is located in the U.S., the mills that it certifies are located mainly in Asia.67 The 

mills certified by PSI are located in countries that are major sources of pressed wood products 

imported into the United States; thus, the data from the PSI presentation were assumed to 

represent emission levels from all non-Canadian CWPs imported into the U.S. 

The PSI presentation included histograms of observed formaldehyde emission levels for 

hardwood plywood, MDF, and particleboard. The data included emission levels for 387, 198, 

and 193 samples of hardwood plywood, MDF, and particleboard, respectively, and for all three 

product types combined. After imputing the data underlying these graphs, the average emission 

values reported in the presentation were successfully replicated.68 This information was used to 

estimate the number of samples at different emission levels.

 

66 PSI. 2009. Professional Services Industries, Inc. (PSI). “The TPC Perspective,” available at 

http://university.ahfa.us/documents/fmhyde309_schutfort.pdf.

67 According to CARB’s list of certified mills, as of November 12, 2010, PSI was the TPC for 95 mills in 9 

countries. The countries, and the number of mills certified by PSI in each, are as follows: Argentina (1); Chile 

(1); China (55); Ecuador (2); Indonesia (2); Malaysia (26); the Philippines (1); Taiwan (1); and Thailand (6).

68 The emission levels in the graphs seemed to be labeled according to the interval midpoints (e.g., a reported 

value of 0.05 ppm represented an interval of 0.0375 to 0.075 ppm). The maximum values reported in the 

presentation for MDF and particleboard were used for the maximum intervals. The median value was used for 

the median interval for MDF. The data in the hardwood plywood graphic were consistent with an average value 

of 0.07 ppm rather than the 0.05 ppm value that could be calculated from the summary statistics shown in the 

presentation for all three product types combined. For hardwood plywood, the three highest emission levels (all 

of which exceeded 1.0 ppm) seemed to be driving this discrepancy; thus, they were dropped from the analysis. 

After dropping these outliers the imputed average emissions matched the 0.05 ppm value that could be 

calculated from the other summary statistics shown in the presentation.
C-3

The emission levels reported in the PSI presentation ranged from near zero to levels exceeding 

the CARB 2 emission standards by a substantial margin. The emission levels observed in these 

data were from a period after CARB 1 took effect but before CARB 2 did. Because many of the 

mills that met the CARB 1 standards at the time when samples were taken may subsequently 

achieve CARB 2 levels, baseline emissions were predicted by assuming that products with 

emission levels between the CARB 1 and CARB 2 levels in the PSI data would achieve CARB 2 

levels by 2013. Emission levels of 0.06 and 0.05 ppm were used for CARB 2 baseline emissions 

for MDF and particleboard, respectively, because these levels represent the midpoints of the 

highest emission intervals in the PSI data that meet the CARB 2 limits. None of the intervals in 

the PSI data for HWPW reflected the CARB 1 emission limits (the CARB 1 limit of 0.08 ppm is 

spanned by the interval of 0.075 to 0.125 ppm in the PSI data and the CARB 2 limit of 0.05 ppm 

is spanned by the interval of 0.0375 to 0.075 ppm). Therefore, no adjustments were made to 

hardwood plywood for the shift from CARB 1 to CARB 2. After making the MDF and PB 

adjustments, the average emissions by product type were calculated (see Table C-3).

Although a number of samples in the PSI data had emission levels close to zero, it was not 

possible to determine whether the products qualify for certification as ultra low-emitting 

formaldehyde (ULEF) or no added formaldehyde (NAF)69; the PSI presentation did not identify

the resin type used for each sample. Also, as indicated in a footnote to C-2 above, NAF and 

ULEF have emissions requirements that must be met by 90% of the samples from a mill as well 

a ceiling value that must be met by all samples. This distribution of emissions could not be 

determined from the PSI presentation; thus, baseline emission levels were not estimated 

separately for non-Canadian imports of ULEF and NAF products. Such products are included in 

the baseline emission estimates for the CARB-2 category.

Table C-3 also includes the share of imports for each product type and emissions standard. This 

share was estimated by assuming that the fraction of samples for each category in the PSI

presentation is equivalent to the share of imports for that category across all non-Canadian 

composite wood products imported into the U.S.

Table C-3. Baseline Emission Levels for Non-Canadian Imported Composite Wood Products, by 

Product Type and Emissions Standard

Emissions Standard Hardwood Plywood MDF Particleboard

Average Emissions (ppm)

None 0.163 0.502 0.384

CARB 2 0.015 0.047 0.036

Share of Imports

None 26% 11% 38%

CARB 2 74% 89% 62%

 

69 Resin formulations that meet the NAF definition are those that do not contain any added formaldehyde in their 

formulation.
C-4

The estimates presented above in Table C-3 rely on several critical assumptions that are a source 

of uncertainty in this analysis, the most important being as follows:

 The sample is representative of imported non-Canadian composite wood products.

− It is not known whether the sample is representative of products destined for the 

United States. The destination of these products could be other countries or their 

country of origin. On the one hand, U.S. end users may be more likely to insist on 

lower-emitting products. On the other hand, U.S. end users not shipping to 

California face less stringent formaldehyde emissions standards than many other 

potential destinations. (Although many end users prefer to handle a single type of 

product, rather than one product that meets the California requirements and 

another type for the other 49 states). The fact that the samples were being tested 

for formaldehyde emissions by a CARB TPC suggests that many of them were 

produced by mills whose products ultimately end up in the U.S. 

 The distribution of emission levels will not change for mills that were producing products 

with emission levels that currently exceed the CARB 1 standards.

− If some of these mills decide to obtain CARB 2 certification, it is not known 

whether these will be the higher or lower emitting mills on average. Some 

additional mills (aside from those below the CARB 1 limits but above the CARB 2 

limits) may subsequently decide to lower their emission levels. 

− Conversely, the PSI data may under-represent imported products with very high 

emission levels. Mills making products likely to exceed the CARB levels may not 

have had their products tested because they intended to sell them in the other 49 

states.

 The percentage of samples with current emissions below CARB 1 levels is a reasonable 

estimate of the percentage of mills that will be achieving CARB 2 levels in the baseline 

scenario:

− The PSI presentation was given after CARB 1 was in effect but before CARB 2 

was. It is not known whether more overseas producers ultimately will meet the 

Phase 2 emission limits than initially met the Phase 1 limits.

 

For the purposes of TSCA Title VI, laminated products are a subset of hardwood plywood. The 

statute defines a laminated product as one made by affixing a wood veneer to a particleboard, 

medium-density fiberboard, or veneer-core platform. The statutory definition goes on to state 

that laminated products are component parts used in the construction or assembly of a finished 

good, and that a laminated product is produced by the manufacturer or fabricator of the finished 

good in which the product is incorporated. EPA is given the authority to modify the statutory 

definition of laminated product through rulemaking. EPA also is directed to use all available and 

relevant information to determine whether the definition of hardwood plywood should exempt 

engineered veneer or any laminated product. 

Note that baseline emissions data are not available for hardwood plywood defined as laminated 

products. The CARB 1 standard for hardwood plywood is used to represent the average baseline 

emission level for laminated products made in the U.S. and Canada. The CARB 1 level 

represents the average emission level that CARB found prior to promulgating the ATCM. It

reflects a mix of products using different resin types, including both UF and NAF resins. This 

level is considered to be a reasonable proxy for domestically produced laminated products. These 

products are thought to generally be made with UF or PVA resin, so their emissions may be 

similar to those from stock hardwood panel products before the ATCM went into effect. 
C-5

For laminated products made outside the U.S. and Canada, average baseline emissions were 

assumed to be the same as the levels for imported hardwood plywood that does not meet an 

emission standard (estimated from the PSI presentation). Emissions data prior to the CARB 

ATCM are not available for non-Canadian imported hardwood plywood. Because the PSI 

presentation was given after CARB 1 was in effect, it presumably reflects a reduction in 

emissions compared to pre-ATCM levels. Therefore, if baseline laminated product emissions are 

similar to pre-ATCM emissions, using the average of all non-Canadian imported hardwood 

plywood would underestimate emission levels for laminated products. Using the estimated 

average emissions from hardwood plywood that does not meet an emissions standard reflects the 

possibility that many of these imported laminated products are made with UF resins.

Table C-4 lists the emission levels used for the baseline 2013 scenario for CWPs, including 

laminated products; the values listed reflect the assumptions and choices described above. The 

estimated share of U.S. consumption also is listed in the table for each product type and 

associated country of origin.

Table C-4. Baseline Emission Levels for Composite Wood Products by Product Type, Country of 

Origin, and Emission Standard

Emissions 

Standard

Hardwood Plywood MDF Particleboard Laminated Products

U.S./Can. NonU.S./Can. U.S./Can. NonU.S./Can. U.S./Can. NonU.S./Can. U.S./Can. NonU.S./Can.

Average Emissions (ppm)

None 0.058 0.163 --- 0.502 0.058 0.384 0.058 0.163

CARB 1 --- --- --- --- 0.090 --- --- ---

CARB 2 0.032 0.015 0.082 0.047 0.057 0.036 --- ---

CARB ULEF 0.024 0.042

CARB NAF 0.013 0.035 0.013

Share of U.S. Consumption

None 0.1% 16.6% --- 0.8% 0.2% 1.1% 36.0% 64.0%

CARB 1 --- --- --- --- 1.7% --- --- ---

CARB 2 19.7% 47.4% 79.2% 7.2% 85.0% 1.9% --- ---

CARB ULEF 4.2% 3.0%

CARB NAF 16.2% 8.6% 7.1%
C-6

C.2 REDUCED EMISSIONS CLASSES

Seven analytical options were evaluated in the EPA exposure assessment, as listed and described 

briefly in Table C-5. The first four options – CARB1, CARB2/Statutory Option, 

CARB2/Statutory Option including laminated products at NAF, and NAF – assume that 

progressively higher percentages of CWPs meet stricter emission standards. The last three 

scenarios are similar to CARB1, CARB2/Statutory Option, and NAF, with the single exception 

that laminated products are not regulated as HWPW under TSCA. 

Table C-5. Alternative Analytical Options Evaluated in the Exposure Assessment

Analytical Option Description

CARB1, Including 

Laminated Productsa

All CWPs, including laminated productsb, are at emission levels that meet 

CARB Phase 1 standards1. Products that meet CARB Phase 1 standards 

at baseline are assumed to remain at that emission level.

CARB2 / Statutory Option,

Including Laminated 

Productsa,c

All CWPs, including laminated products, are at emission levels that meet 

CARB Phase 2 standards. Products that meet CARB Phase 2 standards at 

baseline are assumed to remain at that emission level.

CARB2 / Statutory Option, 

with Laminated Products

at NAF

As above, except that laminated products are assumed to meet NAFd

standards.

NAF, Including Laminated 

Productsa

All CWPs, including laminated products, are at emission levels that meet 

NAF standards. Products that meet NAF standards at baseline are 

assumed to remain at that emission level.

CARB1, Excluding 

Laminated Products

Like CARB1 above, except that laminated products are not regulated as 

HWPW and, thus, are assumed remain at baseline emission levels.

CARB2/Statutory Option,

Excluding Laminated 

Products

Like CARB2 above, except that laminated products are not regulated as 

HWPW and, thus, are assumed remain at baseline emission levels.

NAF, Excluding 

Laminated Products

Like NAF above, except that laminated products are not regulated and, 

thus, are assumed remain at baseline emission levels.

a

Included among the emission classes in FIAM-pwp

b

30% of all HWPW products are assumed to be laminated products.

b The CARB emission standards for this scenario are equivalent to those specified in the FSCWPA.

d NAF = no added formaldehyde.

In the economic analysis presented to the Small Business Advocacy Review (SBAR) Panel for 

the formaldehyde rule, EPA estimated that HWPW production (excluding engineered wood 

flooring) was 1,749 thousand square feet (on a 3/8-inch basis) in 2006, and that laminated 

product production (excluding engineered wood flooring) was about 408 million square feet on a 

5/8-inch basis, equivalent to about 680 million square feet on a 3/8-inch basis. Thus, based on 

these estimates, laminated products account for about 28% of combined HWPW production. The 

Hardwood Plywood and Veneer Association has indicated that laminated products represent 

about 30% of engineered wood flooring production. Therefore, this exposure analysis assumes 

that laminated products represent 30% of the volume of HWPW.
C-7

Table C-6 lists emission levels and market shares for the same set of CWPs under the 

assumption that CARB1 emission limits would be in effect for all CWPs, including laminated 

products. In compiling this table, it was first assumed that emission levels of all products 

meeting CARB1 or stricter (CARB2/ULEF/NAF) standards at baseline would not change. Next, 

it was assumed that any products listed as not meeting any emission standard at baseline, but 

whose emission levels would comply with CARB1, also would remain at their baseline emission 

levels. For example, some U.S.-produced HWPW and PB products are not certified as meeting 

an emission standard at baseline but nonetheless have emission levels that comply with CARB

Phase 1 emission limits; thus, their emission levels are assumed to remain at the baseline level 

under this scenario/analytical option. As noted in a table footnote, for the CARB1 analytical 

option laminated products were assumed to be at the same emission levels as HWPW if 

regulated and at baseline emission levels (see Table C-4) if not regulated.

Table C-6. Emission Levels for CARB1 Analytical Option by CWP Type, Origin, and Emissions Standard

Emissions 

Standard

Hardwood Plywooda MDF Particleboard

U.S. Non-U.S. U.S. Non-U.S. U.S. Non-U.S.

Average “Emissions” (ppm)

None --- --- --- --- --- ---

CARB1 

(previously NOT

CARB1)

0.058b0.058 --- 0.14c0.058b0.090

Already at

CARB1 --- --- --- --- 0.090 ---

CARB2 0.032 0.015 0.082 0.047 0.057 0.036

ULEF --- --- 0.024 --- 0.042 ---

NAF 0.013 --- 0.035 - 0.013 -

Share of U.S. Consumption

None --- --- --- --- --- ---

CARB1 

(previously NOT

CARB1)

0.1% 16.6% --- 0.8% 0.2% 1.1%

Already at

CARB1 --- --- --- --- 1.7% ---

CARB2 19.7% 47.4% 79.2% 7.2% 85.0% 1.9%

ULEF --- --- 4.2% --- 3.0% ---

NAF 16.2% --- 8.6% --- 7.1% ---

a

For the CARB1 scenario that includes laminated products, those products are assumed to have the same emission 

levels as hardwood plywood; for the CARB1 scenario that excludes laminated products, those products are 

assumed to have baseline emission levels as listed in Table C-4.

b

Products in these cells have baseline emission levels that comply with CARB1, even though producers reported 

that they were not intentionally meeting any particular standard. Because the baseline levels already comply with 

CARB1, the emission levels for these products are assumed not to change and, thus, are the same as listed in 

Table C-4.

c

Products in this cell are not CARB1-compliant at baseline; thus, their emission levels are assumed to change for 

the CARB1 scenario. The assumed emission level is 67 % of the level that would exactly comply with CARB1; as 

noted in the text, industry generally sets target emission levels below the standard to ensure that it is being met, 

given variability across/within production lots as well as measurement uncertainty.
C-8

For the CARB1 scenario it was further assumed that products listed as not meeting any emission 

standard at baseline, and whose emission levels exceed the CARB Phase 1 limit, would attain 

emission levels equal to those of corresponding U.S. products listed as not meeting any emission 

standard but nonetheless complying with CARB1 at baseline. For example, non-U.S. HWPW not 

complying with CARB1 at baseline, with an average emission level of 0.163 ppm, was assumed 

to match the level (0.058 ppm) of U.S.-produced HWPW complying with CARB1 at baseline, 

and its market share (16.6%) was shifted from “None” to “CARB1” in the table. 

Lastly, it was assumed that products not meeting any emission standard at baseline, and for 

which there are no comparable products complying with CARB1 at baseline, would obtain an 

emission level equal to 67% of the CARB1 limit – the CWP industry generally sets targets for 

emission levels below the standard to ensure that it is being met, considering the variability 

across/within production lots as well as measurement uncertainty. For example, non-U.S. MDF, 

with an emission level at baseline (0.502 ppm) that is not CARB1-compliant, was assumed to

change to a level equal to 67 percent of the CARB1 standard (0.67 x 0.21 ppm = 0.14 ppm).

Table C-7 lists emission levels and market shares for the analytical option where CARB2/TSCA 

Title VI statutory emission limits would be in effect. As with the CARB1 scenario, it was 

assumed that emission levels of all products already meeting CARB2 or stricter standards 

(ULEF/NAF) would not change. Next, it was assumed that products not meeting CARB2 at 

baseline (including those meeting the less strict CARB1 standard) would obtain emission levels 

equal to U.S. products70 of the same CWP type that comply with CARB2 at baseline. For 

example, both U.S.-produced HWPW (baseline level of 0.058 ppm) and non-U.S. HWPW 

(baseline level of 0.163 ppm) were assumed to shift to a CARB2-compliant emission level of 

0.032 ppm. For this analytical option, laminated products were assumed to be at the same 

emission levels as HWPW if regulated and at baseline emission levels (see Table C-4) if not 

regulated.

 

70 Emission levels for U.S. CARB2-compliant products (as opposed to non-U.S. products) were chosen because 

the data set from which average emission levels were determined was larger and, thus, considered more reliable.
C-9

Table C-7. Emission Levels for CARB2 Analytical Option by CWP Type, Origin, and Emissions Standard

Emissions 

Standard

Hardwood Plywooda MDF Particleboard

U.S. Non-U.S. U.S. Non-U.S. U.S. Non-U.S.

Average “Emissions” (ppm)

None --- --- --- --- --- ---

CARB1 --- --- --- --- --- ---

CARB2 

(previously NOT

CARB2)

0.032 0.032 - 0.082 0.057 0.057

Already at

CARB2 0.032 0.015 0.082 0.047 0.057 0.036

CARB2 --- --- 0.024 --- 0.042 ---

ULEF 0.013 --- 0.035 --- 0.013 ---

NAF --- --- --- --- --- ---

Share of U.S. Consumption

None --- --- --- --- --- ---

CARB1 --- --- --- --- --- ---

CARB2 

(previously NOT

CARB2)

0.1% 16.6% --- 0.8% 1.9% 1.1%

Already at

CARB2 19.7% 47.4% 79.2% 7.2% 85.0% 1.9%

CARB2 --- --- 4.2% --- 3.0% ---

ULEF 16.2% --- 8.6% --- 7.1% ---

NAF --- --- --- --- --- ---

a

For the CARB2 scenario that includes laminated products, those products are assumed to have the same emission 

levels as hardwood plywood; for the CARB2 scenario that excludes laminated products, those products are 

assumed to have baseline emission levels as listed in C-4.

Table C-8 lists emission levels and estimated percentages under the analytical option whereby

NAF emission limits would be in effect. First, it was assumed that emission levels of all products 

meeting the NAF standard at baseline would not change. Next, it was assumed that products not 

meeting NAF at baseline (including those meeting less strict standards) would attain the 

emission levels for U.S. products of the same CWP type that are NAF-certified at baseline. For 

example, U.S.-produced HWPW accounts for 36 percent of all HWPW production. A subset of 

the U.S. HWPW production, accounting for 19.8 percent, does not comply with NAF at baseline 

but attains a NAF-compliant emission level of 0.013 ppm under the NAF analytical option; the 

remaining 16.2 percent, already NAF-compliant, likewise has an emission level of 0.013 ppm. 

Lastly, it was assumed that MDF products meeting ULEF at baseline, but with average emission 

levels lower than those for NAF-compliant MDF products at baseline, would not change. As 

with the analytical options described above (CARB1 and CARB2), for the NAF analytical option 

laminated products were assumed to be at the same emission levels as HWPW meeting the NAF 

standard if regulated and at baseline emission levels (see Table C-4) if not regulated.
C-10

Table C-8. Emission Levels for NAF Scenario by CWP Type, Origin, and Emissions Standard

Emissions 

Standard

Hardwood Plywooda MDF Particleboard

U.S. Non-U.S. U.S. Non-U.S. U.S. Non-U.S.

Average “Emissions” (ppm)

None --- --- --- --- --- ---

CARB P1 --- --- --- --- --- ---

CARB P2 --- --- --- --- --- -

NAF (previously 

not NAF with 

non-NAF 

emissions)

0.013 0.013 0.035 0.035 0.013 0.013

NAF (previously 

not NAF with 

NAF emissions)

--- --- 0.024 --- --- ---

Already at NAF 0.013 --- 0.035 --- 0.013 ---

Share of U.S. Consumption

None --- --- --- --- --- ---

CARB P1 --- --- --- --- --- ---

CARB P2 --- --- --- --- --- ---

NAF (previously 

not NAF with 

non-NAF 

emissions)

19.8% 64.0% 79.2% 8.0% 89.9% 3%

NAF (previously 

not NAF with 

NAF emissions)

--- --- 4.2% --- --- ---

Already at NAF 16.2% --- 8.6% --- 7.1% ---

a

For the NAF scenario that includes laminated products, those products are assumed to have the same emission 

levels as hardwood plywood; for the NAF scenario that excludes laminated products, those products are assumed to 

have baseline emission levels as listed in Table C-4.

Table C-9 lists emission levels and market shares under the analytical option whereby CARB2

emission limits would be in effect, but with the added assumption that all laminated products

(assumed to account for 30 percent of all HWPW production) are NAF-compliant (e.g., using 

NAF cores laminated with NAF resins) at an emission level of 0.013 ppm. Note that the 

estimated percentages for the U.S. and non-U.S. HWPW columns have been adjusted in the table 

such that all HWPW production, including laminates, sums to 100 percent.
C-11

Table C-9. Emission Levels for CARB2 Scenario with Laminates at NAF by CWP Type, Origin, 

and Emissions Standard

Emissions 

Standard

Hardwood Plywood MDF Particleboard Laminated 

Productsa

U.S. Non-U.S. U.S. Non-U.S. U.S. Non-U.S. U.S. Non-U.S.

Average “Emissions” (ppm)

None --- --- --- --- --- --- --- ---

CARB1 --- --- --- --- --- --- --- ---

CARB2 

(previously 

NOT CARB2)

0.032 0.032 --- 0.082 0.057 0.057 --- ---

Already at

CARB2 0.032 0.015 0.082 0.047 0.057 0.036 --- ---

CARB2 --- --- 0.024 --- 0.042 --- --- ---

ULEF 0.013 --- 0.035 --- 0.013 --- --- ---

NAF --- --- --- --- --- --- 0.013 0.013

Share of U.S. Consumption

None --- --- --- --- --- --- --- ---

CARB1 --- --- --- --- --- --- --- ---

CARB2 

(previously 

NOT CARB2)

0.1% 11.6% --- 0.8% 1.9% 1.1% --- ---

Already at

CARB2 13.8% 33.2% 79.2% 7.2% 85.0% 1.9% --- ---

CARB2 --- --- 4.2% --- 3.0% --- --- ---

ULEF 11.3% --- 8.6% --- 7.1% --- --- ---

NAF --- --- --- --- --- --- 10.8% 19.2%

a Assumed to account for 30% of all HWPW production.
C-12

C.3 SLOPES AND INTERCEPTS FOR EMISSION RATES

Appendix D lists emission rates for various types of PWPs that were derived from sets of 

chamber emission tests conducted during the 1980s and up through the mid-1990s. A 

distinguishing feature of these chamber tests, although somewhat dated, is that they were 

conducted at multiple air exchange rates and/or product loading ratios, enabling estimation of 

model parameters – slope and intercept – that reflect the dependence of the emission rate on the 

indoor formaldehyde concentration. The slope is a mass transfer coefficient that reflects the 

“backpressure” effect of the indoor formaldehyde concentration on the emission rate, whereas 

the intercept reflects the hypothetical emission rate when the indoor formaldehyde concentration 

is zero.

The following equation, identical to Eqn. 3-4 in Section 3.2 of the main document, was used as a 

basis for determining the intercept (b) associated with products meeting certain emission 

standards:

[CH2O]SS = [CH2O]out + (CH2OER * Area)/Q (Eqn. C-1)

where:

[CH2O]SS = steady-state formaldehyde concentration inside the 

compartment (mg/m3)

[CH2O]out = steady-state formaldehyde concentration outside the 

compartment (mg/m3)

CH2OER’ = the emission rate of a solid formaldehyde source inside the 

compartment (mg/h)

Area = the surface area of the formaldehyde source (m2)

Q = airflow rate into and out of the compartment (m3/hr)

The emission rate of a solid source is:

CH2OER = -m * [CH2O]V + b (Eqn. C-2)

where:

m = the mass transfer coefficient (m/hr)

[CH2O]V = the CH2O concentration in the vapor phase (mg/m3)

b = a constant; the emission rate at zero CH2O concentration 

in the air (mg/m2-hr).

Assuming that [CH2O]out is zero and that [CH2O]V = [CH2O]SS , and substituting Eqn. C-2 into 

Eqn. C-1, we can solve for b as follows:

b = [CH2O]SS * (1 + m * Area / Q) * (Q / Area) (Eqn. C-3)

ASTM E 1333, Standard Test Method for Determining Formaldehyde Concentrations in Air and 

Emission Rates from Wood Products Using a Large Chamber, is a commonly used method for 

demonstrating compliance with formaldehyde emission standards. The method prescribes an air 

exchange rate for testing of 0.5 hr-1(i.e., 0.5 ACH) and product loading ratios of 0.43 m

2

/m3for 

PB, 0.95 m

2

/m3for HWPW, and 0.26 m

2

/m3for MDF. Using these rates along with an assumed 

slope (m) for a given PWP type, the corresponding intercept can be determined for a hypothetical 

source that just meets a given emissions standard. 
C-13

For example, the CARB2 emission standard for MDF is 0.11 ppm or 0.135 mg/m3. An arbitrary 

chamber volume of 100 m

3

, together with a value of 50 m

3

/hr for Q (corresponding to an air 

exchange rate of 0.5/hr), was used in the calculation. The volume assumed does not matter, as 

the exposed surface area scales to the volume via the loading ratio – with a volume of 100 m3the 

corresponding surface area is 26 m2. A slope of 1.06 m/hr was assumed; this value is the average 

slope from about 30 tests of different MDF specimens, as listed in Appendix D. 

Substituting the assumed values given above in Eqn. 4-2, the intercept (b) that corresponds to the 

CARB2 emission standard for MDF is calculated as follows:

b = 0.135 mg/m3* (1 + 1.06 m/hr * 26m2/ 50 m3) * (50 m3/ 26 m2)

= 0.40 mg/m2-hr

Table C-10 lists the calculated intercepts associated with various emission levels for HWPW, 

MDF and PB, as listed previously in Tables C-6 through C-9, using the calculation method 

described above and an assumed slope for each CWP type as indicated in the table.

Table C-10. Calculated Intercepts for Various Emission Levels for Each CWP Type 

Emission Standard – Origin Emission Levela Assumed 

Slope

Calculated 

Intercept

ppm mg/m3 m/hr mg/m2/hr

Hardwood Plywood

None – U.S. 0.058 0.072 0.27 0.057

None – non-U.S. 0.163 0.201 0.27 0.160

CARB1 – U.S. & non-U.S. 0.058 0.072 0.27 0.057

CARB2 – U.S. 0.032 0.040 0.27 0.031

CARB2 – non-U.S. 0.015 0.019 0.27 0.019

NAF – U.S. 0.013 0.016 0.27 0.013

Medium Density Fiberboard

None – U.S. --- --- 1.06 ---

None – non-U.S. 0.502 0.620 1.06 1.849

CARB1 – U.S. 0.140 0.173 1.06 0.516

CARB2 – U.S. 0.082 0.101 1.06 0.302

CARB2 – non-U.S. 0.047 0.058 1.06 0.173

ULEF – U.S. 0.024 0.030 1.06 0.088

NAF – U.S. 0.035 0.043 1.06 0.129

Particleboard

None – U.S. 0.058 0.072 0.7 0.133

None – non-U.S. 0.384 0.474 0.7 0.883

CARB1 – U.S. 0.058 0.072 0.7 0.133

CARB1 – non-U.S. 0.090 0.111 0.7 0.207

CARB2 – U.S. 0.057 0.070 0.7 0.131

CARB2 – non-U.S. 0.036 0.044 0.7 0.083

NAF – U.S. & non-U.S. 0.013 0.016 0.7 0.030

a mg/m3

formaldehyde = ppm formaldehyde * 1.23
C-14

Providing standard-specific slopes and intercepts for each CWP type would prove cumbersome, 

exceeding the number of input rows available for sources in the FIAM model. It was determined 

experimentally that using a composite intercept together with the corresponding total exposed 

surface area for a given CWP type yielded identical modeling results to that from the more 

tedious approach of using values for the individual components meeting different emission 

standards. The composite intercept was calculated as a weighted average of the individual 

intercepts, using the market share as the weight; the resultant composite intercepts are shown for 

the baseline scenario in Table C-11 for each CWP type, along with standard-specific intercepts. 

Table C-11. Composite Intercepts by CWP Type – Baseline Scenario

Emission Standard – Origin Slope Intercept Market 

Share

Hardwood Plywood

None – U.S. 0.27 0.057 0.1%

None – non-U.S. 0.27 0.160 16.6%

CARB2 – U.S. 0.27 0.031 19.7%

CARB2 – non-U.S. 0.27 0.015 47.4%

NAF – U.S. 0.27 0.013 16.2%

Composite 0.27 0.042 100 %

Medium Density Fiberboard

None – non-U.S. 1.06 1.849 0.8%

CARB2 – U.S. 1.06 0.302 79.2%

CARB2 non-US 1.06 0.173 7.2%

ULEF – U.S. 1.06 0.088 4.2%

NAF – U.S. 1.06 0.129 8.6%

Composite 1.06 0.281 100 %

Particleboard

None – U.S. 0.70 0.133 0.2%

None – non-U.S. 0.70 0.883 1.1%

CARB1 – U.S. 0.70 0.207 1.7%

CARB2 – U.S. 0.70 0.131 85.0%

CARB2 – non-U.S. 0.70 0.083 1.9%

ULEF – U.S. 0.70 0.097 3.0%

NAF – U.S. 0.70 0.030 7.1%

Composite 0.70 0.131 100 %

Composite intercepts for baseline and other emission scenarios – CARB1, CARB2 and NAF –

are shown by CWP type in Table C-12, without the underlying details. The most notable 

difference across scenarios is that the NAF intercepts are substantially lower than for the other 

three scenarios, for each CWP type.
C-15

Table C-12. Composite Intercepts by CWP Type and Emissions Scenario

CWP Type

Composite Intercept

Baseline CARB1 CARB2 NAF

Hardwood Plywood 0.042 0.025 0.021 0.013

Medium Density Fiberboard 0.281 0.271 0.269 0.127

Particleboard 0.131 0.124 0.122 0.030

The modeling also accounted for some PWP types that would not be subject to emission 

standards because they are specifically exempt from TSCA Title VI. Underlayment in homes 

typically is either tongue-and-groove softwood plywood (SWPW) or oriented strand board 

(OSB). The emission rates from earlier chamber tests on SWPW are thought to be applicable to 

either type; for example, a study by Hodgson et al.71 reported virtually the same intercept (0.029) 

for OSB as did the earlier tests for SWPW (0.030). 

Interior doors were represented using estimated emission rates from chamber testing of 6-panel 

white doors that were used in the EPA pilot study; this is the only known study in which such 

doors have been tested at multiple air exchange/loading rates, enabling calculation of both a 

slope and an intercept. The emission rates for these doors also were used to represent those for 

coated (but not laminated) CWP materials, assuming a similar barrier effect.

Because the interiors of such coated materials presumably would be in compliance with any rules 

governing CWP emissions, the intercept for interior doors and coated CWPs was varied across 

emission scenarios. It was observed that, at baseline, the intercept for these products was close to 

halfway between the intercepts for HWPW and PB. Thus, the average of those two intercepts for 

other emission scenarios – CARB1, CARB2 and NAF – was used for doors and coated CWPs; 

the average intercepts were 0.074 for CARB1, 0.071 for CARB2, and 0.0215 for NAF. 

Emission factors for underlayment, interior doors, and coated CWPs are listed in Table C-13.

Table C-13. Emission Rates for Underlayment and Coated CWPs – Baseline Scenario

PWP Type/Class Proxy Used for Emission Rate Slope Intercept

Underlayment (OSB) Softwood Plywood

(PF Resin) 0.61 0.030

Interior Doors & Coated CWPsa 6-panel Doors Used

in EPA Pilot Study 0.52 0.082

a CWPs could be coated, for example, with primer/paint, vinyl film, or thermally fused paper.

 

71 A.T Hodgson, A.F. Rudd, D. Beal, and S. Chandra. 2000. Volatile Organic Compound Concentrations and 

Emission Rates in New Manufactured and Site-Built Houses. Indoor Air 10: 178-192.
Appendix D

Historical Testing of PWPs to Derive Slope and Intercept Values
D-1

D.1 BACKGROUND

During the 1980s and up through the mid-1990s, a number of organizations conducted sets of 

chamber emission tests at multiple air exchange rates and/or product loading ratios, enabling the 

estimation of model parameters – slope and intercept – that reflect the dependence of the 

emission rate on the indoor formaldehyde concentration. The following source types were tested:

 Particleboard underlayment

 Mobile home decking

 Industrial particleboard

 Medium density fiberboard

 Hardwood plywood paneling (with a print face)

 Hardwood plywood paneling (with a paper face)

 Hardwood plywood paneling (with a wood veneer face)

 Hardwood plywood paneling (unspecified face material)

 Softwood plywood (phenol-formaldehyde resin)

 Hardboard (phenol-formaldehyde resin)

 Kitchen cabinets

 Interior doors. 

D.2 DETAILED CHAMBER TESTING RESULTS

Table D-1

72 lists, by product type, products for which multiple chamber tests were conducted in 

the 1980s-1990s to enable estimation of emission rate model parameters (slope and intercept). 

The table also provides references for the data and additional information, as available, on 

chamber testing conditions (air exchange rate and loading ratio). The R

2

values listed in the 

right-hand column of the table, which indicate the degree of correspondence between calculated 

and predicted emission rates, are based on regression analysis of calculated emission rates 

against measured chamber concentrations for the various products tested, from which the slope 

and intercept were estimated (see Section D.4 for an example of this type of analysis). Not all 

investigators reported these values; values are listed in cases where they were reported.

The abbreviated names assigned to individual available sources in the table reflect the 

organization (e.g., EPA, ORNL) or individual that performed the testing from which modeling 

parameters were estimated. Several Available Sources – EPA-PBU for Particleboard 

Underlayment, EPA-HWP for Hardwood Plywood Paneling (Unspecified), EPA-CABINET for 

Kitchen Cabinets, and EPA-DOOR for Interior Doors – are based on chamber testing performed 

as part of the EPA pilot study on formaldehyde (see Appendix A).

 

72 The degree to which the historical data in Table D-1 reflects current/recent domestic or imported production is 

not known.
D-2

Table D-1. Calculated Slopes and Intercepts for Matthews Model from Chamber Tests Conducted at 23 C, 50 % RH or at 25 C, 50 % RHa

(Listed Cases are for Conditions of 23 C and 50 % RH)

Product Type Product Codeb

No. of Data 

Pointsc N/L Ranged

Measured 

Concentration 

Range (ppm)

Matthew’s Emission Modele

ER = -m (conc) + b

Slope (m/hr) Intcpt. (mg/m2/hr) R

2 Valuef

Particleboard 

Underlayment

ORNL-PCB #1

ORNL-PCB #2

ORNL-PCB #3

ORNL-PBU 1 #4

ORNL-PBU 3 #3

ORNL-PBU 3 #6

GP-2

GP-4

GP-5

NBS-USM2-2B

NBS-USM2-3B

NBS-USM5-1A

NBS-U-5,8,9,12,18

EPA-PBU

3

4

4

4

4

3

0.13 to 0.63

0.32 to 5.70

0.10 to 0.95

0.16 to 9.18

0.44 to 6.67

0.60 to 2.37

0.077 to 0.162

0.057 to 0.354

0.084 to 0.158

0.084 to 0.215

0.058 to 0.442

0.062 to 0.118

0.32

0.70

0.88

1.72

0.70

0.93

0.84

1.57

0.76

0.36

0.38

0.62

0.60

1.25

0.09

0.44

0.19

0.46

0.51

0.47

0.40

0.37

0.58

0.18

0.27

0.33

0.18

0.28

0.92

0.98

0.92

0.77

0.96

0.52

0.81

0.98

0.42

0.76

Mobile Home 

Decking

LEH-B

GP-MHD

1.35

0.77

0.66

0.39

Industrial 

Particleboard

ORNL-PBI 3#2

LEH-C

LEH-D

LEH-H

LEH-I

LEH-J

LEH-K

LEH-L

LEH-M

LEH-N

LEH-O

LEH-P

LEH-Q

4

8

9

6

6

6

6

6

6

6

6

6

6

0.32 to 3.31

0.76 to 6.10

0.31 to 6.10

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.59 to 3.82

0.096 to 0.457 0.47

1.00

0.54

1.02

0.87

1.30

0.96

1.07

0.40

1.32

0.58

1.58

0.98

0.42

1.24

0.36

0.45

0.60

0.27

0.32

0.53

0.14

0.24

0.22

0.37

0.39

0.88
D-3

Product Type Product Codeb

No. of Data 

Pointsc N/L Ranged

Measured 

Concentration 

Range (ppm)

Matthew’s Emission Modele

ER = -m (conc) + b

Slope (m/hr) Intcpt. (mg/m2/hr) R

2 Valuef

Industrial 

Particleboard 

(continued)

GP-1A

GP-3A

GP-6A

GP-7A

GP-8A

GP-9A

GP-11A

NBS-USM7-1B

NBS-USM2-1A

NBS-USM6-1B

NBS-USM6-2A

NBS-USM6-3B

NBS-USM4-1B

GP-1N

GP-2N

GP-3N

GP-4N

GP-5N

GP-6N

GP-7N

0.41

0.63

0.46

0.38

0.63

0.62

0.55

0.45

0.39

0.80

0.33

0.64

0.28

0.70

0.72

0.81

0.55

0.50

0.46

0.58

0.12

0.33

0.34

0.24

0.33

0.35

0.29

0.12

0.30

0.18

0.18

0.29

0.20

0.53

0.53

0.35

0.27

0.37

0.34

0.48

0.98

0.67

0.98

0.32

0.94

0.98

Medium Density 

Fiberboard

ORNL-MDF 1 #5

ORNL-MDF 3 #5

ORNL-MDF 1 #4

ORNL-MDF 2 #4

ORNL-MDF-U

LEH-E

LEH-F

LEH-G

GP-1

GP-2

GP-3

GP-4

GP-5

GP-6

4

4

9

9

15

1.47 to 12.5

1.33 to 12.5

0.31 to 6.10

0.31 to 6.10

0.15 to 7.09

0.170 to 0.952

0.160 to 0.936

0.89

1.00

1.25

1.58

0.91

0.48

0.80

0.80

4.98

1.17

2.00

2.22

3.87

0.78

2.78

2.49

1.62

1.09

4.24

0.32

0.79

0.83

1.27

0.77

0.84

1.33

0.80

0.50

0.98

0.81
D-4

Product Type Product Codeb

No. of Data 

Pointsc N/L Ranged

Measured 

Concentration 

Range (ppm)

Matthew’s Emission Modele

ER = -m (conc) + b

Slope (m/hr) Intcpt. (mg/m2/hr) R

2 Valuef

Medium Density 

Fiberboard 

(continued)

GP-7

GP-8

GP-9

GP-10

GP-11

GP-12

GP-13

GP-14

GP-15

NBS-LMDF-1C

NBS-LMDF-2B

NBS-MDF 1

NBS-MDF 2

NBS-MDF 3

NBS-MDF 4

NBS-MDF 5

0.73

0.43

0.69

1.83

0.81

1.45

0.90

1.30

0.77

0.88

0.71

1.50

1.16

1.09

0.76

0.65

0.58

0.46

0.32

0.92

0.58

1.31

0.63

0.75

0.39

0.64

0.73

1.73

1.14

1.31

1.29

1.43

Hardwood 

Plywood Paneling 

(Print)

ORNL-PNPR 2 #3

ORNL-PNPR 3 #3

ORNL-PNPR 2 #5

ORNL-PNPR 3 #1

ORNL-PAN #2

4

4

4

0.31 to 6.57

0.44 to 3.34

0.32 to 1.90

0.085 to 0.719

0.029 to 0.127

0.021 to 0.055

0.46

0.37

0.40

0.85

0.48

0.67

0.13

0.25

0.26

0.05

0.86

0.94

0.77

Hardwood 

Plywood Paneling 

(Paper)

ORNL-PNP 2 #4

ORNL-PNP 3 #1,2g

ORNL-PNP 3 #1,2h

4

4

3

0.31 to 6.57

0.20 to 1.11

0.17 to 1.11

0.037 to 0.426

0.063 to 0.225

0.129 to 0.402

0.27

0.10

0.27

0.27

0.09

0.22

0.61

0.69

0.98

Hardwood 

Plywood Paneling 

(Veneer)

ORNL-PND 1 #1,2g

ORNL-PND 1 #1,2h

ORNL-PND 3 #5

ORNL-PAN #1

4

4

3

4

0.15 to 0.35

0.12 to 1.06

0.32 to 1.11

0.14 to 0.95

0.105 to 0.194

0.055 to 0.248

0.062 to 0.137

0.072 to 0.162

0.19

0.11

0.34

0.45

0.08

0.08

0.11

0.13

0.36

0.81

0.90

0.85

Hardwood NBS-P6-1 8 0.04 to 1.91 0.022 to 0.253 0.12 0.05 0.83
D-5

Product Type Product Codeb

No. of Data 

Pointsc N/L Ranged

Measured 

Concentration 

Range (ppm)

Matthew’s Emission Modele

ER = -m (conc) + b

Slope (m/hr) Intcpt. (mg/m2/hr) R

2 Valuef

Plywood Paneling 

(Unknown)

NBS-P8-1

NBS-P10-1

NBS-P14-1

NBS-P17-1

NBS-P21-1

EPA-HWP

5

11

8

5

0.03 to 1.14

0.04 to 4.86

0.18 to 5.20

0.08 to 7.36

0.26 to 1.04

0.027 to 0.239

0.045 to 0.370

0.014 to 0.098

0.020 to 0.224

0.066 to 0.106

0.12

0.45

0.67

0.52

0.22

0.96

0.04

0.30

0.08

0.15

0.03

0.16

0.96

0.71

0.66

0.72

0.18

0.98

Softwood 

Plywood 

(PF Resin)

ORNL-PFPLY #1 5 0.23 to 1.05 0.017 to 0.029 0.61 0.03 --

Hardboard

(e.g., "Masonite")

ORNL-HBD 1 7 -- 0.02 to 0.095 1.39 0.17 0.98

Kitchen Cabinets EPA-CABIN 3 0.39 to 2.29 0.023 to 0.075 0.49 0.08 0.94

Interior Doors EPA-DOOR 3 0.58 to 2.44 0.022 to 0.061 0.52 0.08 0.86

a The data listed in this table are based on the results of emission rate tests performed at Oak Ridge National Laboratory (ORNL) for the Consumer Product 

Safety Commission (CPSC), at the National Bureau of Standards (NBS) for EPA and CPSC, at Georgia Pacific Corp. (GP), and at Weyerhauser Co. by Dr. 

W. F. Lehmann. The data listed in the table for individual sources were obtained from or are based on the results reported in Progress Reports No. I, II, XIV, 

XV, XVI, and XVIII submitted to CPSC by ORNL; NBS Report #NBSIR 85-3255 to CPSC; a 1987 NBS report to EPA (Grot et al. 1987); comments 

submitted by Georgia Pacific Corp. to EPA (Howlett 1988); Lehmann (1987) (also personal communication between B. Lehmann of Weyerhauser Co. and 

G. Schweer of EPA on June 30, 1987); and Koontz et al. 1996.

The ORNL reports listed the Matthew’s model parameters for the individual sources and presented summary test data for the tested N/L conditions. The 

Matthews model parameters listed in the table are as listed in the ORNL reports. Lehmann (1987) and Howlett (1988) reported HBF model parameters for 

the tested boards; Matthews model parameters for these products were calculated from the HBF model parameters. The NBS report listed Matthews model 

parameters.

b Product codes are reported as listed in the ORNL Progress Reports (except for interior plywood, hardboard, and MDF-U) for the ORNL products. The 

products reported in Lehmann (1987) are denoted by "LEH" followed by the letter code used in his report. The products reported in the NBS reports are 

denoted by "NBS" followed by the code used in the reports. The products tested by Georgia Pacific Corp. are denoted by "GP" followed by a digit 

representing the order presented in the report’s tables. The products tested for the EPA Pilot Study (Koontz et al. 1996) are denoted by "EPA."

c Refers to the number of distinct N/L experiments conducted.
D-6

d N/L is the ratio of the experimental air exchange rate (N), in air changes per hour, to the product loading in the chamber (L), in m

2

of product surface area 

per m3of chamber volume.

e ER is the emission rate of the product in units of mg of formaldehyde per m

2

of product surface area per hour; the intercept has the same units. The slope 

units are m/hr.

f Indicator of the degree of correspondence between calculated emission rates and rates predicted by the regression equation.

g Only decorative side exposed.

h Both sides exposed.

REFERENCES CITED IN TABLE D-1:

ANSI 1999. American National Standard: Particleboard. ANSI 208.1-1999, approved February 8, 1999.

ANSI 2002. American National Standard: Particleboard. ANSI 208.2-2002, approved May 13, 2002.

R.A Grot, S. Silberstein, and K. Ishiguro. 1985. Validation of Models for Predicting Formaldehyde Concentrations in Residences Due to Pressed Wood 

Products. U.S. Dept. of Commerce, National Bureau of Standards. Report #NBSIR 85-3255.

R.A. Grot, S. Nabinger, and S. Silberstein. 1987. Formaldehyde from Low-emitting Pressed Wood Products and the Effectiveness of Various Remedial Measures 

for Reducing Formaldehyde Emissions. U.S. Dept. of Commerce, National Bureau of Standards.

C.T. Howlett. 1988. Data provided to EPA on January 14, 1988, and on June 6, 1988 by C.T. Howlett (Georgia Pacific Corp.).

M.D. Koontz, H.E. Rector, D.R. Cade, C.R. Wilkes, and L.L. Niang. 1996. Residential Indoor Air Formaldehyde Testing Program: Pilot Study. Report No. IE2814, prepared by GEOMET Technologies, Inc. for the U.S. Environmental Protection Agency, Washington, DC, under EPA Contract No. 68-D3-0013.

W.F. Lehmann. 1987. Effect of Ventilation and Loading Rates in Large Chamber Testing of Formaldehyde Emissions from Composite Panels. Forest Products 

Journal 37(4):31-37.
D-7

D.3 AVERAGE VALUES FOR SLOPES AND INTERCEPTS

Average slope and intercept values were determined from the data displayed in Table D-1. All 

data shown in the table were used for each of the product types summarized in Table D-2, except 

three cases for MDF that were deemed outliers because they had either a substantially higher 

slope (Product Codes GP-1 and GP-5) or a substantially higher intercept (Product Code ORNLMDF-U) than other cases within that product group. Although, as noted previously, the data on 

which these averages are based are somewhat dated, the slopes in the table are useful for 

estimating hypothetical intercept values for products assumed to meet certain emission 

standards. Illustrative calculations in this regard are provided in Appendix C (see Section C.1) 

and in Section 2.3.3 of the main document.

Table D-2. Calculated Average Slopes and Intercepts for Eight Product Types

Product Type Average Slope Average Intercept

Particleboard Underlayment 0.83 0.34

Mobile Home Decking 1.06 0.53

Industrial Particleboard 0.70 0.35

Medium Density Fiberboarda1.06 1.02

Hardwood Plywood Paneling (Print 0.51 0.27

Hardwood Plywood Paneling (Paper) 0.21 0.19

Hardwood Plywood Paneling (Veneer) 0.27 0.10

Hardwood Plywood Paneling (Unknown) 0.44 0.12

a Two outliers excluded from the calculation; see text.

D.4 ILLUSTRATIVE REGRESSION ANALYSIS ON CHAMBER DATA SET

The data set for this example originated from the EPA pilot study described in Appendix A. 

Among the product types subjected to chamber testing was a kitchen cabinet ensemble. Test 

conditions and results for kitchen cabinets prior to the first house loading are shown in Table D-3.

The chamber tests were conducted at three air exchange rates – 0.17 ACH, 0.51 CH, and 1.0 

ACH – to provide a basis for estimating the relationship between formaldehyde concentration 

and emission rate. The chamber tests were conducted at a target temperature of 77 °F (25 °C) 

and a target relative humidity (RH) of 50 percent. The measured “steady-state” chamber 

concentrations shown in the table are based on adjustment to standard conditions of 77 °F and 50 %

RH, per ASTM Standard E-1333. The chamber volume was 1080 ft3(30.6 m

3

) and the product 

loading ratio was 0.13 ft2/ft3(0.43 m2/m3).

Table D-3. Chamber Conditions and Test Results for Kitchen Cabinets – EPA Pilot Study

Air Exchange 

Rate, ACH

Temperature, 

°F

RH, 

%

Chamber 

Concentration, mg/m3

Calculated Emission 

Rate, mg/m2-hr

1.00 75.0 50 0.0353 0.0821

0.51 77.5 48 0.0645 0.0765

0.17 77.5 51 0.1143 0.0452
D-8

As shown in the table, the increase in the chamber concentration was less than proportional to 

the decrease in air exchange rate. For example, if there were no dependence of the emission rate 

on the air concentration (i.e., if there were no “backpressure” effect), then one would expect a 

six-fold decrease in the air exchange rate (from 1.0 to 0.17 ACH) to result in a six-fold increase 

in the concentration (from 0.035 to 0.212 mg/m3); however, the measured chamber concentration 

at 0.17 ACH (0.114 mg/m3) was about half the expected value, indicative of a backpressure effect.

The emission rates shown in the table were calculated under a steady-state assumption as 

follows:

ER = (Css * AER * V) / Area (D-1)

Where:

ER = Emission rate, mg/m2-hr

Css = Steady-state concentration, mg/m3

AER = Air exchange rate, 1/hr

V = Chamber volume, m3

Area = Exposed surface area, m2= loading ratio (m2/m3) * chamber volume (m3)

The volume term appears in both the numerator and denominator; thus, Eqn. D-1 can be simplified 

to: 

ER = (Css * AER) / L (D-2)

Where:

L = Loading ratio, m2/m3

The calculated emission rate is then regressed against the chamber concentration, as follows: 

ER = -m * Css + b (D-3)

Where:

m = Regression slope (reflecting the backpressure effect)

b = Regression intercept

The resultant regression estimates from this procedure are 0.49 for the slope and 0.10 for the 

intercept. The regression intercept obtained here is slightly different from that shown in Table 

D-1 (0.08), but there is uncertainty as to the specific data points that were used by the originating 

researchers. A comparison of the predicted and calculated emission rates (Table D-4) indicates a 

relatively high degree of correspondence, as reflected in an R2value of 0.94 for the regression.

Table D-4. Predicted vs. Calculated Emission Rates for Kitchen Cabinets – EPA Pilot Study

Air Exchange 

Rate, ACH

Temperature, 

°F

RH, 

%

Predicted Emission 

Rate, mg/m2-hr

Calculated Emission 

Rate, mg/m2-hr

1.00 75.0 50 0.0861 0.0821

0.51 77.5 48 0.0719 0.0765

0.17 77.5 51 0.0475 0.0452
Appendix E

Derivation of Two-zone Steady-state Solution
E-1

Mass balance equation for zone 1 (well-mixed assumption):

01 0 21 2 10 1 12 1 1

1

1 Q C Q C Q C Q C S

dt

dC V      (1)

where:

1 1 1 1 S   z C  y (Matthew’s implementation) (2)

1

1

1

z1 m Area

in Zone

num sources

k

 k



 m

3

/hr (3)

k

in Zone

num sources

k

y bk Area





1

1

1

 mg/hr (4)

m = mass transfer coefficient, m/hr

b = emission rate when the air concentration is zero, mg/(m2hr)

Q = flow rates, m3/hr

C = concentrations, mg/m3

Area = emission area, m2

t = time, hr

V = volume, m3

Mass balance equation for zone 2 (well-mixed assumption):

02 0 12 1 20 2 21 2 2

2

2 Q C Q C Q C Q C S

dt

dC V      (5)

where:

Q12

Q21

Q01 Q10 Q02

Q20

Zone 1

V1

S1

Zone 2

V2

S2
E-2

2 2 2 2 S   z C  y (6)

2

2

1

z2 m Z Area

in Zone

num sources

k

  k 

 (7)

2

2

1

y2 m C

in Zone

num sources

k

 k





(8)

Assume flows are constant and set dC/dt = 0 (steady state condition):

0 01 0 21 2 10 1 12 1 1 1 1  Q C  Q C  Q C  Q C  z C  y (9)

And:

0 02 0 12 1 20 2 21 2 2 2 2  Q C  Q C  Q C  Q C  z C  y (10)

Solving for C1:

10 12 1

01 0 21 2 1

1 Q Q z

Q C Q C y

C

 

 

 (11)

Solving for C2:

20 21 2

02 0 12 1 2

2 Q Q z

Q C Q C y

C

 

 

 (12)

Substitute equation 12 into equation 11:

10 12 1

1

20 21 2

02 0 12 1 2

01 0 21

1 Q Q z

y

Q Q z

Q C Q C y

Q C Q

C

 

















 

 





(13)
E-3

Rearranging and combining terms:

 

20 21 2

21 02 0 21 12 1 21 2

10 12 1 1 01 0 1 Q Q z

Q Q C Q Q C Q y

Q Q z C Q C y

 

 

    

     10 12 1 1 01 0 1 20 21 2 21 02 0 21 12 1 21 2 Q  Q  z C  Q C  y Q  Q  z  Q Q C  Q Q C  Q y

21 02 0 21 12 1 21 2

01 20 0 01 21 0 01 2 0 20 1 21 1 2 1

10 20 1 10 21 1 10 2 1 12 20 1 12 21 1 12 2 1 1 20 1 1 21 1 1 2 1

Q Q C Q Q C Q y

Q Q C Q Q C Q z C Q y Q y z y

Q Q C Q Q C Q z C Q Q C Q Q C Q z C z Q C z Q C z z C

  

     

       

 

    01 20 01 21 01 2 21 02 0 20 21 2 1 21 2

10 20 10 21 10 2 12 20 12 2 1 20 1 21 1 2 1

Q Q Q Q Q z Q Q C Q Q z y Q y

Q Q Q Q Q z Q Q Q z z Q z Q z z C

      

       

   

  10 20 10 21 10 2 12 20 12 2 1 20 1 21 1 2

01 20 01 21 01 2 21 02 0 20 21 2 1 21 2

1 Q Q Q Q Q z Q Q Q z z Q z Q z z

Q Q Q Q Q z Q Q C Q Q z y Q y

C

      

      



(14)

Similarly:

   

  20 10 20 12 20 1 21 10 21 1 2 10 2 12 2 1

02 10 02 12 02 1 12 01 0 10 12 1 2 12 1

2 Q Q Q Q Q z Q Q Q z z Q z Q z z

Q Q Q Q Q z Q Q C Q Q z y Q y

C

      

      

 (15)