United States
Environmental Protection
Agency
Environmental Monitoring
Systems Laboratory
Las Vegas, NV 89193-3478
Research and Development
EPA/600/SR-92/094 September 1992
EPA Project Summary
Tests of Indoor
Air Quality Sinks
James Quackenboss, Janet Remmers, James McHugh and Karin Bauer
Experiments were conducted in a
room-size test chamber to determine
the sink effects of selected materials
on indoor air concentrations of p-
dichlorobenzene (PDCB). These effects
might alter pollutant behavior from that
predicted using simple indoor air qual-
ity models, by reducing the peak con-
centrations during source usage and
by increasing the ventilation rates
needed to reduce pollutant concentra-
tions. Four experimental conditions
were tested: empty chamber (painted
gypsum wallboard), carpeting only, car-
peting and drapes, and carpeting plus
a full-size bed with a comforter. Cham-
ber temperature and relative humidity
were controlled; atmospheric pressure
was monitored. Air exchange rates were
monitored using the tracer-gas decay
method. Chamber air, sampled at six
minute intervals, was analyzed using a
gas chromatograph. Sink effects were
estimated by comparing decay curves
fit to the tracer gas (SF6) and the PDCB
concentrations. The influence of sink
effects on indoor air concentrations was
illustrated by estimating the ventilation
rates required to reduce PDCB concen-
trations from 14 ppm to 2 ppm. In the
absence of sink effects, the theoretical
rate was 1.95 air changes per hour
(ACH). With sinks, the rates predicted
for each experimental condition were:
2.5 (empty test chamber), 3.21 (carpet
only), 6.64 (carpet and drapes), and 3.75
ACH (carpet and bed).
This Project Summary was developed
by EPA's Environmental Monitoring
Systems Laboratory, Las Vegas, NV, to
announce key findings of the research
project that is fully documented in a
separate report of the same title (see
Project Report ordering information at
back).
Introduction
The Toxic Substances Control Act
(TSCA) mandates the assessment of risks
associated with the manufacture and use
of new and existing chemicals. In 1985,
the Interagency Committee on Indoor Air
Quality called for developing an under-
standing of human exposures to indoor
air pollutants, the contributions of various
energy conservation measures, and the
impact of introducing new consumer prod-
ucts and building materials. This effort re-
quires assessing exposure to chemical re-
leases from consumer products and build-
ing materials in indoor environments un-
der various conditions.
Due to the number and variety of prod-
ucts on the market, it would be extremely
costly to rely exclusively on the monitoring
of indoor air concentrations in order to
determine all possible exposure implica-
tions of these products. A preferable op-
tion is the use of predictive indoor air
quality models to estimate the air concen-
trations or. personal exposures that can
be expected under various conditions.
One of the most commonly used pre-
dictive mathematical models is the simple
dilution model. In a well-mixed chamber
with a constant air exchange rate (A), the
air concentration of a nonreactive pollut-
ant in air (C(), generated at a constant
Printed on Recycled Paper
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emission rate (G), can be predicted at
time (t) by the following mathematical
model1:
C, - G(1 - e-*1) / (AV)
(1)
where
C, - air concentration at time (t),
G « emission rate,
A - air exchange rate,
V - exchange volume, and
t - time.
After the source is removed, the air
concentration decays exponentially and is
defined by the following equation:
ff*
(2)
where C0 is the initial air concentration at
t-0, the time at which the source is re-
moved.
The dilution mode! assumes that no pol-
lutant is tost within the chamber from any
mechanisms other than dilution (e.g.,
through chemical decomposition, chemi-
cal reaction, or mass transfer between the
gas or vapor phase and solid surfaces
inside the chamber). These other mecha-
nisms are often referred to as "sinks."
The dilution model also assumes that the
inlet air stream delivers "clean" air to the
chamber at the same temperature and
pressure as those of the outlet stream,
and that there is complete mixing of the
chamber air.
The application of the dilution model to
predict exposure to a pollutant in an in-
door environment is complex if materials
(such as ftoor and wall coverings, window
drapes, and upholstered furniture) appre-
ciably adsorb or absorb the pollutant. For
purposes of this study, a "sink effect" is
defined as the mass transfer of an air
pollutant between its gas or vapor phase
and any surfaces inside the chamber. The
adsorption of the nonreactive pollutant to,
and subsequent desorption from,
nonreactive surfaces is assumed to be a
reversible process.
The objectives of this project were two-
fold. First, experimental tests in a environ-
mental chamber were performed to as-
sess the sink effects of selected surfaces
and building materials on the concentra-
tions of an indoor air pollutant. Second,
the importance of these sink effects to
existing predictive mathematical models
of indoor air pollutants was to be deter-
mined. The project report presents the
data for 12 experimental runs using one
chemical, p-dichlorobenzene (PDCB). Re-
sulls of 11 of these runs are presented in
the body of the report. Due to problems in
1 Ootm.J.E. 1987. Models and Statistical Methods for
Gaseous Emission Testing of Finite Sources in Well-
mixed Chambers. Atmos. Environ. 21:425-430.
run 11, all data and results pertaining to
that run are considered suspect and are
presented separately (in Appendix H of
the project report).
PDCB was selected for study because
it is widely used in the indoor environment
as an insecticide (moth repellant), a disin-
fectant, or a deodorant (room freshener).
It was also selected to minimize the sink
effects from chemical reaction or degra-
dation mechanisms, allowing the study to
focus on adsorption and desorption mecha-
nisms.
Procedure
Experiments were conducted in a room-
size (1,261-ft3) test chamber at the Mid-
west Research Institute's (MRI's) Air Con-
sumer Exposure (ACE) Laboratory under
stable environmental conditions. The test
chamber simulates a residential room
where household consumer products
would be used. Interior surfaces were cov-
ered with gypsum wallboard. The wall-
board was sealed with ready-mix joint com-
pound and painted with one coat of inte-
rior latex primer and one coat of semi-
gloss interior latex paint. Twelve experi-
mental runs were conducted, three tests
for each of the following configurations:
empty test chamber, test chamber with
carpeting, test chamber with carpeting and
drapes, and test chamber with carpeting
and a full-size bed covered with a com-
forter. The carpet pile was made of 100%
polyester fibers. The drapes were made
of 72% rayon and 28% polyester. The
mattress consisted of 45% cotton felt, 40%
polyurethane foam, 15% polyester fibers
on a wire spring unit. The box spring con-
sisted of a wire spring unit covered with
100% polyester fabric. The mattress was
covered with a comforter consisting of 50%
cotton, 50% polyester fabric and filled with
,100,% polyester ,fjberfi|L._T,he,,carpeting,
drapes, and bed were replaced after each
experimental run.
Real-time temperature, humidity, and
barometric pressure measurements were
collected during all experimental runs.
Temperature and relative humidity were
controlled; barometric pressure was moni-
tored but not controlled. Air mixing was
achieved using two commercially avail-
able box fans. Air exchange rates within
the test chamber were controlled for ap-
proximately 1 exchange per hour, and
monitored using the tracer gas decay
method.
Each experimental run began by simul-
taneously releasing PDCB vapor and a
tracer gas, sulfur hexafluoride (SF6) into
the test chamber at constant emission
rates. The flow of SF6 into the test cham-
ber from a compressed gas cylinder was
controlled by a fine-metering valve, and
adjusted to maintain the air concentration
inside the test chamber to below 500 ppb.
PDCB vapor was generated in the test
chamber by sublimation of pure PDCB
crystals placed in a flat metal pan that
was positioned at the air inlet.
A continuous sample of test chamber
air was collected at the outlet. After both
compounds were maintained at a steady-
state equilibrium for a 6-hr period, injec-
tion was stopped. Air monitoring for PDCB
and SF6, using a gas chromatograph (GC),
began prior to the introduction of both
compounds into the test chamber and was
continued throughout the air concentra-
tion buildup period, a 6-hr steady-state
equilibrium period, and the period of de-
cay (after removal of the sources)rThe air
monitoring ended when concentrations of
both compounds fell below their respec-
tive detection limits.
The air concentrations of SFe and PDCB
measured at the time the sources were
removed were the initial concentrations
(C0) used in the decay model (Eq. 2) to
approximate a decay curve which is rep-
resentative of decay caused solely by
dilution. The concentrations of SF6 and
PDCB measured from the time the sources
were removed were plotted against time,
and curves were fitted to the data. The
empirical decay curves for each chemical
were compared to the theoretical decay
curves predicted by Eq. 2. Given that Eq.
2 provided an adequate fit to the SF6
data, sink effects were indicated by an
observed rate of decay for PDCB which
deviated from that in Eq. 2. This is ex-
plained by the fact that other sources than
the PDCB concentration at time t = 0 (C0)
are contributing to PDCB concentrations
(Ct) in the test chamber air during the
decay period. This would result in the
.. measured air concentrations exceeding
those predicted by Eq. 2. The most likely
source is PDCB absorbed into surfaces
during the experiment, which subsequently
desorbs and reenters the air. No other
source was identified.
The data used in modeling the decay of
SPe and PDCB concentrations over time
were restricted to the data starting with
the first data point obtained just prior to
the withdrawal of the gas sources from
the test chamber (see data in Appendix
G). Further, the data were truncated at
the end of each run. Data following the
first occurrence when the SF6 concentra-
tion fell below 5 ppb were discarded from
the statistical analysis. A 1 -ppm cutoff for
runs 1 through 6 and a 0.3-ppm cutoff for
runs 7 through 12 were used for PDCB
concentrations. These figures correspond
to the limits of quantitation for the two
compounds.
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Data from each run were analyzed sepa-
rately. A series of decay models were
fitted to the SF6 and PDCB concentration
data over time. These models included
first- and second-order exponential decay
models for both compounds. A segmented
model, consisting of a second-order expo-
nential decay curve followed by a first-
order linear equation, was also fitted to
the PDCB concentration. Each model was
forced through C0, the concentration cal-
culated at time t=0 when the source was
removed. The equations considered take
the general forms:
First-order exponential decay:
Cone = C0 exp(-At) (3)
Second-order exponential decay:
' """ Cone = C0 exp(-Bf + Ct2) ' " ' (4)
Segmented second-order exponential
decay and first-order linear:
Cone = C0 exp (~Dt + Et2)
If t < T, the time of model transition,
Cone = F + Gt
otherwise , (5)
where A, B, C, D, E, F, G, and T are
model parameters (coefficients) to be esti-
mated from the data. The coefficient A in
Eq. 3, obtained for SF6, provides an esti-
mate of the air exchange rate within the
test chamber. To model Eqs. 3 or 4 above,
the concentrations were first log-trans-
formed and the regression parameters es-
timated on the log-scale. To use a stan-
dard linear regression analysis procedure,
the response variable modeled versus time
was [log(conc) - log(C0)], selecting the
no-intercept model option. The regression
coefficients B and C were directly ob-
tained from that regression analysis.
For each individual run, the sink effect
was estimated as a function of time, the
initial PDGB concentration at time t = 0
(C0), the air exchange rate (A) of SF6
during the same run, and the parameter
estimates (B and C or D, E, F, G, and T)
of the PDCB decay function. If Eq.4 above
best described the PDCB concentration
decay curve, then the sink effect, at time
t, expressed in ppm, was estimated by
the following difference:
Estimated sink effect =
C0 exp(-Bt + Ct2) - C0 exp(-At).
If Eq. 5 best described the PDCB con-
centration decay curve, then the sink ef-
fect, at time t, was estimated as follows:
Estimated sink effect =
C0exp(-Dt+Et2)-C0exp(-At)
if t < T,
Estimated sink effect =
F + Gt - C0 exp(-At)
otherwise.
Results and Discussion
Sink effects were observed in all ex-
perimental runs; that is, PDCB air concen-
trations measured after the source was
removed exceeded that predicted by the
simple dilution model. The most likely
source of excess PDCB was from the
chemical's adsorption to the walls, drapes,
carpeting, bed, and comforter during the
experiments and subsequent desorption
and reentry into the air. No other possible
source was identified.
The maximum differences between
PDCB air concentrations predicted from
the models fitted to the calculated con-
centrations and those of the air concen-
trations predicted from Eq. 2 are as fol-
lows:
Empty test chamber 0.9 ppm
Test chamber with carpet 1.6 ppm
Test chamber with carpet
and drapes 7.9 ppm
Test chamber with carpet
and bed 2.6 ppm
The PDCB decay models from each
run were also used to predict the number
of air exchanges needed to reduce PDCB
air concentrations from a high to a low
level. As an example, the models fitted to
the PDCB data in the presence of sinks
and the model described by Eq. 2 were
used to calculate the number of air ex-
changes per hour (ACH) required to re-
duce the air concentration of PDCB from
14ppm to 2ppm (Tablel). Under the as-
sumption of no sink effect (Eq. 2), it would
require 1.95 ACH to reduce the PDCB
concentration from 14 to 2ppm, regard-
less of chamber configuration. (The num-
ber 1.95 is obtained by solving the equa-
tion, 2 = 14e'At, for At.) The average
number of air exchanges necessary to
reduce the PDCB air concentration within
the empty test chamber from 14 to 2 ppm,
as calculated from the decay models, is
2.50 ACH. It would require an average of
3.21 ACH for the test chamber with car-
peting, 6.64 ACH for the test chamber
with carpeting and drapes, and 3.75 ACH
for the test chamber with carpeting and a
bed.
Summary and Conclusions
The project report presents the data
from experimental runs using a chemical,
p-dichlorobenzene, in a room-size test
chamber. Triplicate experimental runs were
conducted in the test chamber using four
configurations: empty test chamber; test
chamber outfitted with wall-to-wall carpet;
test chamber with carpet and drapes; and
test chamber with carpet and a full-size
bed with a comforter.
Sulfur hexafluoride was used to deter-
mine the air exchange rates of each run
within the chamber and to demonstrate
the actual decay of the chemical in the
absence of sink effects under the environ-
mental conditions set for each run. The
empirical decay curve for SFe, obtained
from fitting SF6 concentrations over time
after the source of the gas was withdrawn
from the chamber, coincided with the theo-
retical decay curve predicted by the simple
dilution model (Eq. 2).
Sink effects were observed when the
estimated rate of decay of PDCB deviated
from the rate predicted'by Eq. 2, using the
air exchange rate determined by SF6. This
effect can be explained by the presence
of sources other than C0, the concentra-
tion of PDCB in the air at the beginning of
the decay period, which are contributing
to PDCB concentration (C() in the cham-
ber during the decay period. This results
in measured PDCB air concentrations that
exceed the air concentrations predicted
by Eq. 2 (without sink effects). The sink
effect was found in all experimental runs.
The most likely source was PDCB
adsorbed to chamber surfaces during the
experiments, with PDCB subsequently de-
sorbing and reentering the air. The mag-
nitude of the sink effect was influenced by
furnishings added to the chamber.
In the presence of sinks, a higher num-
ber of air exchanges was required to re-
duce PDCB air concentrations within the
test chamber to background levels after
the source of the contaminant was with-
drawn. As an example, the models fitted
to the PDCB data in the presence of sinks
and the model described by Eq. 2 were
used to calculate the number of air ex-
changes required to reduce the air con-
centration of PDCB from 14 ppm to 2
ppm. The first-order decay model (Eq. 2)
would significantly underestimate the num-
ber of air exchanges necessary to reduce
air concentrations to background levels.
As shown in this study, the sorption of
PDCB onto chamber surfaces and its sub-
sequent desorption was significant rela-
tive to the PDCB levels in the room air.
Therefore, sink effect terms should be in-
corporated into predictive exposure mod-
els. Additional modeling of the data base
generated during this study should be un-
dertaken. Fitting other models to these
data, based on some type of equilibrium
adsorption phenomenon or other physi-
cally-based concept, should be attempted.
•U.S. Government Printing Office: 1992— 646-080/60140
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Table 1. Number of Air Exchanges Needed to Reduce PDCB Concentrations from 14 to 2 ppm
Run
no.
Time to
14 ppm
(V
Time to
2 ppm
(h)
Time
difference
(h)
Number of air
exchanges
w/o sink effect
Number of air
exchanges
w/sink effect
Empty tost chamber
1 0.65 3.13
2 0.28 2.13
3 0.36 2.41
Test chamber with carpet
4 0.47 3.25
5 0.31 2.86
6 0.62 3.73
Test chamber with carpet and drapes
7 0.63 5.99
8 0.05 4.91
9 1.00 6.40
Test chamber with carpet and bed
10 0.62 4.07
12 0.26 2.67
2.48
1.85
2.06
2.78
2.55
3.11
5.36
4.87
5.40
3.45
2.41
1.95
1.95
1.95
Avg. =
Std =
CV(%)
1.95
1.95
1.95
Avg. =
Std =
CV(%)
1.95
1.95
1.95
Avg. =
Std =
CV(%)
1.95
1.95
Avg. =
Std =
CV(%)
2.51
2.53
2.45
2.50
0.04
1.53%
3.27
3.22
3.14
3.21
0.07
2,14%
6.84
6.03
7.05
6.64
0.54
8.13%
3.99
3.51
3.75
0.34
9.05%
The information in this document has
been funded wholly or in part by the United
States Environmental Protection Agency
under Contract 68-DO-0137 to Midwest
Research Institute. It has been subjected
to the Agency's peer and administrative
review, and it has been approved for pub-
lication as an EPA document.
Mention of trade names or commercial
products does not constitute endorsement
or recommendation for use.
The EPA authors are James Quakenboss, Environmental Monitoring and Systems
Laboratory, Las Vegas, NV 89193-3478, and Janet Remmers, Office of Pollution
Prevention and Toxics, Washington, DC 20460. James McHugh andKarin Bauer are
With the Midwest Research Institute, Kansas City, MO 64110.
Joseph V. Behar is the EPA Project Officer (see below).
The complete report, entitled 'Tests of Indoor Air Quality Sinks," (Order No. PB92-
218346/AS; Cost: $43.00; subject to change) will be available only from:
National Technical Information Service
5285 Port Royal Road
Springfield, VA 22161
Telephone: 703-487-4650 , , ............ ...,.,.„....
The EPA Project Officer can be contacted at:
Environmental Monitoring and Systems Laboratory
U.S. Environmental Protection Agency
Las Vegas, NV 89193-3478
United States
Environmental Protection Agency
Center for Environmental Research Information
Cincinnati, OH 45268
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