NEW f,T "~

NBSIR 88-3761

Predicting Formaldehyde

Concentrations in Manufactured

Housing Resulting From

Medium-Density Fiberboard

Samuel Silberstein

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Environment Division

Gaithersburg, MD 20899

Prepared for: U.S. Department of Housing and Urban Development

Washington, DC

April 1988

75 Years Stimulating America's Progress

1913-1988

NBSIR 88-3761

PREDICTING FORMALDEHYDE

CONCENTRATIONS IN MANUFACTURED

HOUSING RESULTING FROM

MEDIUM-DENSITY FIBERBOARD

Samuel Silberstein

U.S. DEPARTMENT OF COMMERCE

National Bureau of Standards

National Engineering Laboratory

Center for Building Technology

Building Environment Division

Gaithersburg, MD 20899

April 1988

Prepared for: U.S. Department of Housing and Urban Development

Washington, DC

U.S. DEPARTMENT OF COMMERCE, C. William Verity, Secretary

NATIONAL BUREAU OF STANDARDS, Ernest Ambler, Director

PREDICTING FORMALDEHYDE CONCENTRATIONS IN MANUFACTURED

HOUSING RESULTING FROM MEDIUM- DENSITY FIBERBOARD

1 . Introduction

In 1984, HUD issued Manufactured Home Construction and Safety Standards

limiting formaldehyde emissions of particleboard and plywood paneling

that was manufactured using urea- formaldehyde resins for use in

manufactured homes [1]. The standards specified that the formaldehyde

concentration of particleboard present in a measuring chamber at a loading (that is, the exposed pressed-wood product surface area per unit

volume of the space) of 0.13 ft^/ft^ (0.4265 m^/m^) be limited to 300

ppb , and that the concentration of plywood present in a measuring chamber

at a loading of 0.29 ft^/ft^ (0.9514 m^/m^) be limited to 200 ppb. The

chamber, which is specified in the standard, must be operated at 25 °C and

50% RH, with an air exchange rate of 0.5 h"^. Because concentration is

invariant with volume or pressed-wood product surface area as long as loading is maintained constant, the same loadings of these products would

result in the same formaldehyde concentrations in manufactured homes,

provided that temperature, relative humidity and air exchange rate are

kept constant.

The other major pressed-wood product manufactured from urea- formaldehyde

resins, but not covered by the standards, is medium- density fiberboard

(mdf) . The purpose of this study is to predict the formaldehyde

concentration of a chamber containing only medium- density fiberboard, so that if particleboard and plywood meeting the emissions limits of the HUD

standards were added to the chamber at the maximum loadings permitted by

the standards, the formaldehyde concentration would remain below 400 ppb.

Formaldehyde concentrations predicted for each type of pressed-wood

product may not simply be added together to predict the concentration

resulting from a combination of products. For example, if both

particleboard and plywood were present at the loadings and concentrations

permitted by the HUD standards, then the resulting concentration at 25 °C,

50% RH, and an air exchange rate of 0.5 would be 330 ppb, which is

far lower than the sum of 300 ppb and 200 ppb. In fact, the plywood

could be said to make a nearly negligible contribution to the

concentration. This is because the formaldehyde emission rate decreases

with increasing formaldehyde concentration. The emission rate of plywood

at 300 ppb formaldehyde is far lower than at 200 ppb, so it contributes

much less than an additional 200 ppb formaldehyde to an atmosphere

already containing 300 ppb.

In order to study the effects of combinations of pressed-wood products,

the concentrations of individual products must first be expressed in

terms of formaldehyde surface emission rates. According to Fick's

diffusion law, the surface emission rate is linear in concentration with

negative slope. Oak Ridge National Laboratory (ORNL) [2] developed

models that extrapolate the linear regression coefficients for the Fick's

Law expression at standard conditions of 23 °C, 50% RH, and 100 ppb

concentration, to any other combination of temperature, relative humidity

1
and concentration. A simple mass-balance indoor air quality model

developed at the National Bureau of Standards (NBS) [3] can then be used

to obtain concentrations resulting from combinations of products having

different emission rates.

Validation studies of the ORNL and mass-balance indoor air quality models

were performed at NBS [3]. These studies showed that the models

performed well for particleboard and mdf. They did not perform well for

plywood; however, the model's predictions for plywood emission rates were

higher than those actually obtained so that they still provide an upper

bound

.

The ORNL and indoor air quality models will be used to give mdf

concentrations that satisfy the requirements specified above. These

concentrations will be translated into permissible surface emission

rates, which will depend on mdf loading. (A small loading will give the

same concentration with a higher surface emission rate than a larger

loading.) Next, a sensitivity study will be performed. The effects of

small variations in temperature, relative humidity, and air exchange rate

on formaldehyde concentrations and surface emission rates due to mdf,

will be analyzed.

2. Methods

Assume in all that follows that the external formaldehyde concentration =

0 ppb . The external concentration is that outside the chambers and

outside the home. Further, let subscripts 1, 2, and 3 refer to

particleboard underlayment , hardwood-plywood paneling, and mdf,

respectively

.

From Fick's diffusion law:

seri/seri } 23 , 50 , 100 = a i

" b i’ c C 1 ) where

C = formaldehyde concentration, ppb.

ser d = formaldehyde surface emission rate, mg/m^«h.

ser i , 23 , 50 , 100 = ser i at 23°C, 50% RH and C = 100 ppb.

ORNL empirically determined the coefficients a^ and b^ to be as follows:

b t = [1 + B«(T - 296 . 15) ] •

[ 1 + E«(RH - 50)]/(Cbstd - 100) (2)

ai = b i .exp[-c-(T- 1 -296.15- 1 )].(RH/50) A -Cbstd (3)

where

A, B, c, E, and Cbstd are given in table 1 for each type of

pressed wood product.

T = absolute temperature, K.

RH = relative humidity, %.

2
According to the NBS indoor air quality model:

Z 1-j/ser^ = C»g*AI (4)

where

C = formaldehyde concentration at equilibrium, ppb

.

li = loading = area-j[/V, m^/m^

.

area^ = exposed surface area of boards of class i, m^

.

V = volume of space, m^

.

g = formaldehyde concentration conversion factor from ppb to

mg/m^ = 1.228 x 10 “^ mg/m^»ppb.

AI = air exchange rate, h

- ^.

[Unspecified summations should be taken from i=l to i=3 .

]

Equation 4 gives the relationship between ser 3 and I 3

:

l 3 »ser 3 = C*g»AI - l^«ser^ - l 2 *ser 2 (5)

ser^ and ser 2 can be calculated from equations 2 and 3. For any value of

I 3 , ser3 23 50 100 may be calculated from ser 3 , as determined by equation

5, and from a3 and b 3 , as determined by equations 2 and 3:

ser3, 23,50, 100 = ser 3 /(a3- b 3 «C) . ( 6 ) In the present report, C is always taken to be 400 ppb in the home.

The mdf measuring chamber concentration can be calculated for any

combination of mdf loading and ser 3

:

l 3 »ser3 = C«g*AI (7)

According to equation 1

:

i 3* ser3,23,50,100* a 3

“ 1 3

# ser3 , 23 , 50 , 100 #b 3 = C*g-AI ( 8 )

C = (l3*ser3

j 23,50,100‘ a3)/( 1 3* ser3,23,50,100 ,b 3 + g‘ AI ) ( 9 )

3. Results

3.1. Mdf concentrations and emission rates

Suppose that the temperature, relative humidity, and air exchange rate of

a manufactured home are 25 °C, 50%, and 0.5 h" 1

, respectively. Suppose

that particleboard and plywood are present at the maximum loadings

permitted by the HUD standards. Their formaldehyde emission rates are

such that when the two types of pressed-wood products are tested

separately in chambers, their concentrations are as large as permitted,

i. e. 300 ppb and 200 ppb, respectively. Then the two pressed-wood

3
products in combination would give a concentration of 330 ppb . How much

mdf could be added to the building in order to just reach 400 ppb? The

answer is that any combination of mdf that results in a chamber- test

concentration of 300 ppb would result in a total home concentration of

400 ppb, provided that the loading, air exchange rate, temperature and

relative humidity are the same as for the chamber. This still does not

answer the question of how much mdf can be added to the building. This

depends on the loading and on the surface emission rate, which can be

conveniently expressed as ser23 50 100- The higher the loading one

wishes to permit, the smaller must be ser23 ( 50 , 100

•

The results discussed in the previous paragraph are shown in figure 1 and

table 2. The curves are iso-concentration curves; each point on each

curve yields an mdf chamber-test concentration as indicated. The

combinations of loadings and ser23 50 100

' s f° r the curve marked "300

ppb" would result in a total concentration of 400 ppb in the presence of

particleboard and plywood. The curve marked "400 ppb" shows the

permitted combinations of loadings and ser23 50 100 >s the absence of

formaldehyde emitters other than mdf. Both the house and chamber- test

concentrations would be 400 ppb in this case. The curve marked "200 ppb"

shows permitted combinations of loadings and ser23 50 100* s if one wishes

to incorporate a margin of error, as discussed below in the section on

sensitivity. The chamber- test concentration would be 200 ppb in this

case, and the house concentration would remain at 400 ppb even if the

temperature were slightly higher than 25 °C, the relative humidity

slightly higher than 50%, and the air exchange rate slightly higher than

0.5 h" 1

.

4. Sensitivity

One might want to ensure that the formaldehyde concentration of a home

remains below 400 ppb for temperatures greater than 25 °C, relative

humidities greater than 50%, and air exchange rates smaller than 0.5 h”^.

In that case, one would choose combinations of mdf loadings and

ser 23 50 100 >s that yield lower formaldehyde concentrations in chamber

tests than required to meet the 400 ppb limit. For example, figure 2 shows that to ensure that the concentration remains below 400 ppb at a loading of 0.1 m^/m^ at 26 °C, one would require that ser 23 50 100 he kept

below 1 mg/m^-h. For 25°C, ser23 50 100 would only have to be kept below

2 mg/m^*h. At 27 °C, 50% RH, and an air exchange rate of 0.5 h" , it is

impossible to keep the concentration below 400 ppb because the permitted

loadings of other pressed- wood products already result in a greater

concentration before addition of any mdf.

Table 3 and figure 3 show the sensitivity of chamber-test concentrations

to temperature. The curve shows that the mdf chamber- test concentration

changes by about 35 ppb for a change of 1°C to either direction from

25 °C. This means that if excursions of 1°C in the chamber temperature

commonly occur or cannot be distinguished by the measurement system, one

should limit the chamber concentration to about 260 ppb rather than 300

ppb.

4
Table 4 and figure 4 show the sensitivity of chamber- test concentrations

to relative humidity. The curve shows that the mdf chamber- test

concentration changes by about 50 ppb for a change of 5% to either

direction from 50% RH. This means that if excursions of 5% in chamber

relative humidity commonly occur or cannot be distinguished by the

measurement system, one should limit the chamber concentration to about

250 ppb rather than 300 ppb.

Table 5 and figure 5 show the sensitivity of chamber- test concentrations

to air exchange rate. The curve shows that the mdf chamber- test

concentration changes by about 30 ppb for a change of 0.1 h"^ to either

direction from 0.5 h

- ^. This means that if excursions of 0.1 h_ ^ in the

chamber air exchange rate commonly occur or cannot be distinguished by

the measurement system, one should limit the chamber concentration to

about 270 ppb rather than 300 ppb.

These results show that excursions of about 50 ppb can be expected from

divergences of the temperature, relative humidity and air exchange rate

from their specified values. Table 6 shows the effect of two

combinations of such divergences. For example, suppose that instead of

being at the specified conditions, the chamber is at the first

combination of conditions in table 6. If the chamber concentration were

limited to 150 ppb at those conditions, then that would be equivalent to

a concentration of 300 ppb if at specified conditions (last line of table

6). If one obtained a concentration of 300 ppb instead of 150 ppb, then

the equivalent concentration at the specified conditions would be well

over 100 ppb higher than 300 ppb at the specified conditions. The

divergence at the second combination of conditions would not be as great.

Neither of these combinations of conditions would be difficult to obtain

during a chamber test. Thus, in order to ensure that mdf emissions truly

result in a concentration of 300 ppb at specified conditions, one might

want to allow for a margin of error and permit a concentration on the

order of 200 ppb. Figure 1 shows the relationship between mdf loading

and ser 23 50 100 i- n order to obtain a chamber-test concentration of 200

ppb

.

5. References

[1] Department of Housing and Urban Development, "Manufactured home

construction and safety standards," Fed. Reg. 49, 31996-32008, 1984.

[2] T. G. Matthews, T. J. Reed, B. J. Tromberg, K. W. Fung, C. V.

Thompson, J. 0. Simpson, and A. R. Hawthorne, "Modeling and Testing

of Formaldehyde Emission Characteristics of Pressed-Wood Products,"

Report no. 18 to CPSC, Oak Ridge National Laboratory Report no. TM

9867, 1985.

5
[3] R. A. Grot, S. Silberstein, and K. Ishiguro, "Validation of Models

for Predicting Formaldehyde Concentrations in Residences due to

Pressed Wood Products -- Phase I," National Bureau of Standards

Interagency Report no. 85-3255, Gaithersburg, MD , 1985.

Table 1. Coefficients for the ORNL model

pressed-wood product c A ^bstd B E

K ppb K~ 1 %~ 1

particleboard underlayment 9,400 0.37 360 0.025 0.016

hardwood-plywood paneling 6,500 0.66 410 0.053 0.029

medium- density fiberboard 5,000 1.90 900 0.090 0.000

6
Table 2. Mdf loading vs. ser23 50 100 i- n t^ie

presence of particleboard and plywood

particleboard ser 23 50 100

plywood ser 2 3 } 50,100

particleboard loading

plywood loading

home HCHO concentration

chamber test temperature

relative humidity

air exchange rate

0.736 mg/m^»h

0.132 mg/m^ »h

0.427 m^/m^

0.951 m^/m^

400 ppb

25 °C

50%

0.5 h' 1

mdf loading mdf ser 2 3 j 50 > 100

m^/m^ at chamber test concentration of

20C) ppb 30C) ppb 400 ppb

0 ..05 2. 061 3. 448 5. 479

0 ..10 1 . 031 1 . 724 2. 739

0..15 0 . 687 1 . 149 1 . 826

0 ,20 0 . 515 0 . 862 1 . 370

0 ,.25 0 . 412 0 . 690 1 . 096

0 ..30 0 . 344 0 . 575 0 . 913

0 ..35 0 . 294 0 . 493 0 . 783

0 ,40 0 . 258 0 . 431 0 . 685

0..45 0 . 229 0 . 383 0 . 609

0 ..50 0 . 206 0 . 345 0 . 548

0 ..55 0 . 187 0 . 313 0 . 498

0 .,60 0 . 172 0 . 287 0 . 457

0 .,65 0 . 159 0 . 265 0 . 421

0 .,70 0 . 147 0 . 246 0 . 391

0 .,75 0 . 137 0 . 230 0 . 365

0 .,80 0 . 129 0 . 216 0 . 342

0 .,85 0 . 121 0 . 203 0 . 322

0 .,90 0 . 115 0 . 192 0 . 304

0 .,95 0 . 108 0 . 181 0 . 288

1 .,00 0 . 103 0 . 172 0 . 274

7
Table 3. Temperature vs. mdf chamber test concentration

Table

relative humidity

air exchange rate

mdf ser2 3 j 50,100

mdf loading

50%

0.5 h" 1 1.724 mg/m2 »h

0 . 1 m

2 /m2

temperature mdf chamber concentration

°c ppb

22.5 220

23.0 230

23.5 250

24.0 260

24.5 280

25.0 300

25.5 310

26.0 330

26.5 350

Relative humidity vs. mdf chamber test concentration

chamber test temperature

air exchange rate

mdf ser2 3 > 50,100

mdf loading

relative humidity

%

35.0

40.0

45.0

50.0

55.0

60.0

65.0

25°C

0.5 h

-1

1.724 mg/m2 *h

0 . 1 m

2 /m2

mdf chamber concentration

ppb

150

190

240

300

350

420

490

8
Table 5. Air exchange rate vs. mdf chamber test concentration

chamber test temperature

relative humidity

mdf ser2 3 } 50,100

mdf loading

25 °C

50%

1.724 mg/m^ *h

0 . 1 m^/m^

air exchange rate mdf chamber concentration

h' 1 ppb

0.30 410

0.35 370

0.40 340

0.45 320

0.50 300

0.55 280

0.60 260

0.65 240

0.70 230

Table 6. Combined effect on mdf chamber concentration

of temperature, re lative humidity and air exchange rate

mdf ser23 , 50 , 100 1,,724

o mg/m^ *h

mdf loading 0.1 ur/m^

temp RH air exchange rate mdf chamber

°C % h' 1 ppb

24.0 40 0.60 150

24.5 45 0.55 210

25.0 50 0.50 300

cone

.

9
HCHO

ser23.50.100.

mgAn2.h

6

5 -

4 -

3 ~

2 -

1 -

mdl loading, m2An3

Figure 1. Isoconcentration curves (200, 300, and 400 ppb) showing the

combinations of mdf loading and ser 23 50 100 required to obtain the

indicated chamber-test concentrations in the absence of any other

formaldehyde emitters. The same combination of mdf loading and

ser 23 50 100 woulh presumably result in the indicated concentration in a manufactured home in the absence of other emitters. The temperature is

25 °C, the relative humidity is 50%, and the air exchange rate is 0.5 h~ ^

.

Points on the curve marked "300 ppb" would result in a total

concentration of 400 ppb, either in chambers or manufactured homes, in

the presence of the maximum loadings of particleboard and plywood

permitted by the HUD standards. Particleboard and plywood are assumed to

emit formaldehyde at the maximum rate permitted by the HUD standards.

10
HCHO

ser23.50.100.

mgAn2.h

28

mdt loading, m2/m3

Figure 2. Isoconcentration curves showing combinations of mdf loading

and ser 23 50 100 required to obtain a concentration of 400 ppb at the

indicated temperatures (in °C) . The relative humidity is 50%, and the

air exchange rate is 0.5 h”^. Particleboard and plywood are assumed to

be present at the maximum loadings, and to emit formaldehyde at the

maximum rates, permitted by the HUD standards.

11
HCHO

concentration,

ppb

temperature, oC

Figure 3. Sensitivity of chamber- test formaldehyde concentrations

resulting from mdf to temperature. The relative humidity is 50%, the air

exchange rate is 0,5 h"^, and the mdf loading is 0.1 m^/m^

.

12
HCHO

concentration,

ppb

500

relative humidity, %

Figure 4. Sensitivity of chamber- test formaldehyde concentrations

resulting from mdf to relative humidity. The temperature is 25 °C, the

air exchange rate is 0.5 h

- ^, and the mdf loading is 0.1 m^/m^

.

13
HCHO

concentration,

ppb

air exchange rate, h-1

Figure 5. Sensitivity of chamber-test formaldehyde concentrations

resulting from mdf concentrations to air exchange rate. The temperature

is 25 °C, and the relative humidity is 50%, and the mdf loading is 0.1

m^/m^

.

14
NBS»114A (REV. 2-80

U.S. DEPT. OF COMM.

BIBLIOGRAPHIC DATA

SHEET (See instructions)

1. PUBLICATION OR

REPORT NO.

NBSIR 88-3761

2. Performing Organ. Report No 3. Publ i cation D ate

4. MAY 1988

TITLE AND SUBTITLE

Predicting Formaldehyde Concentrations in Manufactured Housing Resulting From

Medium-Density Fiberboard 5.

AUTHOR(S)

Samuel Silberstein

6.

PERFORMING ORGANIZATION (If joint or other then NBS. see in struction s) 7. Contract/Grant No. NATIONAL BUREAU OF STANDARDS -

U.S. DEPARTMENT OF COMMERCE

GAITHERSBURG, MD 20899

9.

SPONSORING ORGANIZATION NAME AND COMPLETE ADDRESS (Street. City. State. ZIP)

. Type of Report & Period Covered

National Bureau of Standards

Gaithersburg, MD 20899

10.

SUPPLEMENTARY NOTES

|

1 Document describes a computer program; SF-185, FIPS Software Summary, is attached.

11.

ABSTRACT (A 200-word or less factual summary of most si gn i fi cant in formation . If document includes a si gnificant

bi bliography or literature survey, mention it here)

HUD previously issued Manufactured Home Construction and Safety Standards limiting

formaldehyde emissions of particleboard and plywood paneling that were ianuf ac tured

using urea-formaldehyde resins for use in manufactured homes. This report uses indoor

air quality models to predict how much medium-density fiberboard may be added to

manufactured homes already containing maximum loadings of particleboard and plywood

paneling, without raising the formaldehyde concentration beyond 400 ppb. It was

found that any combination of mdf that results in a chamber-test concentration of

300 ppb may be added to such a home.

A sensitivity analysis was done to predict how this formaldehyde concentration limit

is affected by variations in temperature, relative humidity, and air exchange rate.

It was concluded that limiting chamber concentrations to 200 ppb would allow for small

errors in temperature, relative humidity, and air exchange rate that might be expected

to arise in practice.

12.

KEY WORDS (Six to twelve entries ; alphabetical order; capitalize only proper names; and separate key words by semi colon s) formaldehyde, medium-density fiberboard,

modeling

.

13.

availability

^

Xi Uni imited For Official Distribution. Do Not Release to NTIS

manufactured homes, indoor air quality,

14. NO. OF

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