THE EFFECT OF PENETRATION ON THE
INDOOR/OUTDOOR RATIO OF FINE PARTICLES
R. B, Mosley* , D, J. Greenwell, Z. Guo, U. S. Environmental Protection Agency, Office of
Research and Development, National Risk Management Research Laboratory, Research Triangle
Park, NC 27711.
R. Fortmann and C. Whitfield. ARCADIS Geraghty & Miller Inc., P.O. Box 13109, Research
Triangle Park, NC 27709.
ABSTRACT
Several recent studies, indicating significant health risks associated with exposure to fine particles
as measured outdoors, have led to increased interest in the relationship between indoor and
outdoor fine particles. This paper discusses some measured values of penetration into an
unoccupied research house in which most of the usual indoor sources of particles are absent, and in
which the rate of entry of outdoor particles can be controlled by applied pressure differentials.
Mathematical expressions are developed to compute the characteristic time of response of a
building to a change in its environment such as a change in particle concentration or a sudden
change in pressurization. Equations are also presented to compute the particle penetration from
measured responses of indoor concentration to either a pressurization system or air cleaners.
Preliminary measurements indicate that penetration of 0.5 urn aerodynamic diameter particles into
the closed research house with an air exchange rate of 0.45 h"1 is 0.96. The penetrations of 0.2 um
physical diameter particles with an air exchange rate ranging from 0.35 to 0.43 h"1 ranged from 0.5
to 0.88.
INTRODUCTION
Lately there is increased concern over exposure to fine particles (< 2.5 um aerodynamic diameter),
Dockery and Spengler (1981), Dockery et al. (1993), Pope, Schwartz, and Ransom (1992), and
Schwartz and Markus (1990). There is also heightened interest in understanding the exposure one
obtains indoors to fine particles that originated outdoors, Lioy et al. (1990), Ando et al. (1996),
Lin and Tai (1998), Parkhurst et al. (1999), and Jamriska et al. (1999). In order to identify indoor
exposure to outdoor particles, it is necessary to identify the source of the particles as indoors or
outdoors. Few unique markers are known that label a particle's source as either indoors or
outdoors. The entry mechanisms of particles into buildings are not well understood. The sizes and
distribution of openings in building shells are especially unclear. Recent studies, Wallace (1996),
Ozkaynak et al. (1996), and Thatcher and Layton (1995), have concluded that the penetration
factor for all particles smaller than 10 um aerodynamic diameter is unity. This seems to imply that
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particles enter buildings as easily as the air that carries them. The question of whether particles
penetrate through the openings in buildings with perfect efficiency plays an important role in
understand the relationship between outdoor particle concentrations and human exposure to those
particles, especially for individuals who are largely confined to the indoors. However, since people
typically spend the majority of their time indoors, this issue is very important to everyone's
exposure.
Most of the information available on penetration into buildings has come from the three studies
mentioned above [Wallace (1996), Ozkaynak et al. (1996), and Thatcher and Layton (1995)].
Since all the data in these papers came from studies in California, they are not necessarily
representative of U.S. housing stock. While Lewis (1995) used controlled experiments with well
defined apertures to demonstrate that particle penetration is a function of applied pressure and
particle size, little additional work has been done to study the mechanisms that control particle
entry into the indoor environment.
The objective of the present study is to better understand the mechanisms by which outdoor
particles enter the indoor environment. The study is conducted at both the laboratory and full-
scale levels. In the laboratory study, carefully controlled experiments are performed in airtight
chambers so that only particles intentionally injected will be observed in the measurements. In the
laboratory, the infiltration routes are simulated by well defined geometric shapes that are more
easily modeled than those of the real world. The laboratory studies are intended to result in
mathematical models that will be validated with well defined entry routes and have the ability to
extrapolate to the non-ideal entry routes of real construction. The real world is represented by an
instrumented research house that is unoccupied and devoid of furniture. When the research house
has been fully characterized in terms of flow/pressure relationships, measured penetration, and
deposition rate constants for different particle sizes, the model developed in the laboratory will be
applied to yield the best possible description of the full scale data. Further, an attempt will be
made to simulate the full-scale results in the laboratory using the characterizing data from the
research house. This paper will present some preliminary results from the studies in the research
house.
DESCRIPTION OF THE RESEARCH HOUSE
The research house is a three-bedroom ranch with a den and fireplace. For a diagram of the floor
plan see Sparks et al. (1999). A living room is adjacent to a front entrance foyer. The attic has a
pull down set of stairs located in the hallway in front of the bedrooms. It has a main bathroom off
the hallway. There is a second bathroom, associated with the master bedroom. It has a central air
conditioner and a natural gas furnace. The furnace and air handler are located in a closet off the
hallway across from the main bath. The single return air vent is in the ceiling in the hallway. There
is an attached garage which serves as the instrumentation room for the research house. All
furniture, except for a couple of tables to support measurement instruments, has been removed.
Air exchange rates are measured on a nearly continuous basis using sulfur hexafluoride (SF6) as a
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tracer gas. The gas cylinder is in the garage, and injection occurs at the entrance to the air return
vent. When the air handler is running, the SF6 is quickly distributed uniformly throughout the
house. A measured dose of SF6 is injected every 6 hours, and concentrations are measured
continuously at four locations throughout the house. The SF6 is sampled by a Bruel and Kjaer
1320 acoustic analyzer. The air exchange rate is determined from the measured rate of decay of
the SF6. In addition to the SF6 sampling, the house is instrumented to measure temperature,
relative humidity, and indoor/outdoor pressure differences at several locations. Generally, for
pressure measurements, the outdoor probes on three sides of the house are manifolded to form an
average outdoor reference.
A stainless steel tube 10 cm in inside diameter is used to provide outdoor air samples. The tube
extends through the back wall by 1.3 m. The pipe runs 4 m across the master bedroom, makes a
right angle turn, and runs 4 m to exit through the end wall. This tube contains an in-line fan that
draws outdoor air through the tube and exhausts it through the end wall. Isokinetic sampling
probes are mounted on the center line of the tube. The speed of the fan in the tube can be varied
to match the flow velocity in the pipe to the flow velocity of the sampling probes. Timed
switching valves allow the particle counters to alternately sample outdoor particles in the tube and
indoor particles in the room. In this way, the same particle counter is used to measure both indoor
and outdoor particle concentration. Particle-laden air from the switching valves goes to a flow
splitter that feeds four particle analyzers. In most cases, simultaneous measurements are
performed with four instruments: 1) An aerodynamic particle sizer with 52 size channels spanning
the range 0.5 to 20 urn in aerodynamic diameter, 2) A scanning mobility particle sizer with 64
channels ranging from 0.05 to 1 urn in (approximate) physical diameter, 3) An optical particle sizer
with 17 size bins ranging from 0.1 to 7 urn in (approximate) physical diameter, and 4) An optical
particle sizer with 16 size bins ranging from 0.3 to 8.5 urn in (approximate) physical diameter. An
electrical low pressure impactor with 12 stages ranging in size from 0.03 to 10 urn in aerodynamic
diameter is sometimes available to sample at either the switching valves or other locations in the
house.
The house is also equipped with a whole house pressurization system consisting of two 10 in. (25
cm) diameter in-line fans that pull outdoor air through a high-efficiency particle air (HEPA) filter
and exhaust it into the indoor space. This system is capable of maintaining the house under
positive pressure except when the outdoor temperature is below freezing and during very windy
periods. A recently installed higher capacity pressurization system is capable of maintaining the
house under positive pressure during any anticipated weather conditions. Several portable air
cleaners are used to temporarily reduce the indoor concentrations in order to observe their
recovery rate. A meteorological station in the back yard continuously measures barometric
pressure, temperature, relative humidity, and wind speed and direction.
DATA COLLECTION
One of the major challenges in understanding the relationship between indoor and outdoor particle
concentrations is to identify the indoor particles that originated outdoors. It is not possible to
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directly measure the penetration factor for outdoor particles entering the building if we cannot
identify which particles entered from the outdoors. The house pressurization system and the air
cleaners are intended to help with this challenge. We rationalize that, when the house is under
positive pressure, no outdoor particles will enter. Recall we are filtering the particles from the air
used for pressurization (removal efficiency of 99.999% at 0.3 urn particle diameter). Entry of
particle-free air dilutes the particles in the indoor air and greatly reduces the indoor concentration.
When the pressurized house is in a steady state with very reduced particle concentration, we turn
off the pressure and observe the indoor particle concentration reestablish itself. By measuring the
rate at which the particle concentration increases, while monitoring the outdoor concentration and
air exchange rate, we can estimate the contribution of outdoor particles to the rate of increase of
indoor concentration if we assume a value for the penetration factor. In a similar manner, air
cleaners can be used to evaluate the strength of indoor particle sources.
While the house is in a steady state under positive pressure, air cleaners can be used to further
reduce the indoor concentration and then turned off to observe the generation rate associated with
indoor sources. If we assume that the source generating rate is independent of pressure, then the
difference in the total rate of increase in particle concentration and the rate of increase from indoor
sources alone can be inferred to represent the entry rate of particles from outdoors.
Field studies often try to interpret routine measurements of indoor and outdoor particle
concentrations in terms of a steady state condition. The current study questions the validity of
those assumptions. It is recognized that there are only certain relatively short periods of time when
the outdoor concentration and other weather related parameters are sufficiently constant to allow a
quasi-steady-state condition. The approach to studying these effects will be to perturb the existing
quasi-steady-state and observe the relaxation back to equilibrium. One must look at the rate of
change of outdoor concentration, air exchange rate, and perhaps other weather related parameters
to determine whether steady state conditions apply.
DEVELOPMENT OF EQUATIONS FOR THE RESEARCH HOUSE
For simplification, we will imagine a house in which the air is well mixed so that its consideration
as a single zone is justified. Under many of the measurement circumstances, this is valid. For
instance, many of the measurements are performed with the air handler running, high volume air
cleaners running, and with numerous mixing fans running. Future experiments will attempt to
quantify the effectiveness of the mixing fans.
A schematic of a single-zone house illustrating components described in this paper is shown in
Figure 1. Flow rates, Q (nvV1), between different components are illustrated by arrows. The
indoor concentration is a function of the outdoor concentration, the infiltration rate, the
penetration, indoor particle loss mechanisms, and indoor sources. A four-step experimental
process was developed to determine the values of all the parameters. The process uses two air
infiltration situations: when the pressurization system is on, only particle-free air enters the house;
however, when the pressurization system is off (the normal situation), particle-laden air enters the
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Figure 1. Schematic of a single-zone house illustrating a pressurization system and all the
processes modeled in this paper.
Infiltration, Qinf
Exfiltration, Qexf
Airhandler Flow, Qah
Indoor Generation
~. ...
Deposition
house through the normal infiltration process. The experiment consists of four steps. In step 1,
the pressurization system is turned on to keep outdoor particles from entering the house. Once a
steady condition is established under a constant pressure, the air cleaners are turned on (step 2) to
further reduce the indoor concentration. After a new steady condition has been established, the air
cleaners are turned off (step 3). During step 3, the particle concentration may increase to recover
its previous steady condition. Since no particles are entering from the outdoors, the increase
during step 3 must be due to emissions from indoor sources. Finally, after a steady condition is
established in step 3, the pressurization system is turned off (step 4). While the house is under
pressurization, the indoor concentration will experience a true steady state. This steady state
occurs because the pressurization system dominates over the influence of the outdoor conditions.
During step 4, outdoor particles once again enter the house through the usual infiltration
processes. The mass balance equation that applies under pressurization (step 1) is:
dC
dt
(1)
-------
where C is the indoor concentration of particles, t is time, G is the generation rate of indoor
sources, V is the building volume, Xf is the rate at which the pressurization system replaces the
indoor air, k is the equivalent rate at which the air in the house is cleaned by particle deposition on
surfaces, T] is the size-specific removal efficiency of the air handler system (filter and surfaces), T is
the duty cycle (fractional time of operation) of the air handler fan, and X^ is the rate at which the
air handler circulates the house air. Except that the air exchange rate due to natural infiltration has
been replaced by Xf, this equation is equivalent to one presented by Thornburg et al. (2000) with
air flows expressed in the form of equivalent air exchange rates. Thornburg et al. show that eq 1
has a solution:
C(f) = (C. - C/)exp(- */T) + Cf
where Q is the initial indoor concentration at t = 0, Cf is the final or steady state concentration
with
-
and
Cf = - T (4)
The parameter, T, is clearly a characteristic time that describes the rate at which the house
approaches a new equilibrium condition while the pressure is applied. Typical values of Xf are 5 h"
l, of k are 0.01 - 2 h'1 (depending on particle size), of X,,, are 4 - 6 h'1, and of the product iiT are 0 -
0.2. Thus typical values of T are 0.12 - 0.2 h. A convenient way to obtain the value of T from the
measurements is to plot the data in the form:
C(t) - C t
-c~^ - -, ^
which should yield a straight line with slope of I/T.
-------
In step 2, where the air cleaners are used to further reduce the concentration, the mass balance
equation is:
f
where r\K is the size-specific particle removal efficiency of the air cleaners, and Xac is the rate at
which the air cleaners circulate the indoor air. The solution to eq 6 is identical in form to that of
eq2:
C(t) = (C.t - C)exp(-[t - J-.J/T) + C
f
where tj is the initial time when the air cleaners are turned on,
and
(8)
Typical values of TiAc are 3 - 5 h'1, k are 0,1 - 2 h'1, and nTX* are 0-1.2 h'1. Thus typical values
of tare 0.12 -0.3 h.
The solution in step 3 is identical to that in step 2 with A^ = 0. Notice that in steps 1, 2, and 3,
particles do not penetrate from the outdoors. In step 4 both the pressure fan and the air cleaners
are off and normal infiltration and penetration processes are operative. The mass balance equation
is:
where P is the penetration factor for outdoor particles entering the house, C0 is the outdoor
particle concentration (assumed to remain constant), and N (the air exchange rate) is the rate at
which the house air is being exchanged by infiltration (also assumed to be constant). It follows
that the solution to eq 9 has the same form as for eq 7:
-------
where t; is the initial time when the pressure fan is turned off. It also follows that:
and
(12)
Typical values of N are 0.2 - 0.6 h'1, k are 0.1 - 2 h"1, and r\\h are 0. - 1.2 h'1. Thus typical values
oft are 0.26 - 3.3 h. Since a multiple of five characteristic times are required to reach 99% of the
final steady state value, this experiment needs to run a minimum of 1.3 - 16.5 h (depending on the
combination of parameter values). In the case of the longer run times, it is likely that the outdoor
concentrations will change appreciably. By substituting eq 4 into 12, the penetration factor
becomes:
C C i
P~-(-^-^c^ (»)
where Cf4 is the steady value of concentration during step 4, T4 is the characteristic time associated
with step 4, Cn is the steady value of concentration during step 1, xt is the characteristic time
during step 1, and C0 is the outdoor particle concentration during step 4 of the measurements.
Actually, any one of the first three steps can be combined with the fourth step to determine the
penetration factor. If there were no indoor sources, 0 = 0 and eq 12 can be rewritten as:
p ~
o
-------
In this case, only step 4 is required to determine the penetration. By using eqs 11 and 12 in eq 5
when G=0, we obtain:
Ln
C(0-
PNC.
'alt
-Ln
PNC,
'ah
(15)
Eq 15 is transcendental and can not be solved algebraically for any of the parameters. Specifically,
P, k, and T\ are the parameters that were not independently measured in this experiment and,
therefore, are the quantities we would like to determine. Clearly, independent values for all three
parameters cannot be uniquely determined by one equation even if we could solve it in closed
form. Actually, X^ was not measured in this experiment, but was recently measured under similar
conditions. However, eq 15 can be solved either graphically or numerically for P if the other
parameters are known. In the current experiments, the parameters can all be determined from the
measurements except for k, P, and r|. While values of k have been measured in the laboratory for
the appropriate particle sizes, a full range of values have not yet been measured in the research
house. Consequently, only the probable ranges of k are known. In the absence of directly
measured values of k and T|, the best that can be done is to simultaneously choose values of P, k,
and t| that yield the best fit to eq 15.
RESULTS
Concurrent measurements of outdoor and indoor concentrations of 0.5 um aerodynamic diameter
particles over a 48 hour period (covering 12/3- 4/99) are shown in Figure 2.
Figure 2. Indoor and outdoor concentrations of 0.5 um aerodynamic diameter particles at the
research house on 12/3-4/99.
-------
During the 4 hour period starting at 1130, the house was under a positive pressure that varied
between 1 and 2 Pa. It can be noted from the figure that the indoor concentration dropped rapidly
to a low value. The pressurization system was turned off at 1530 and the indoor concentration
soon began to increase. During the time required for the indoor concentration to approach a
constant value, the outdoor concentration varied by 180%, but by only 24% during the initial rise.
Note that the indoor concentration exceeds the outdoor concentration beginning at about hour 32
which corresponds to the beginning of the work day when the house was entered to set up another
experiment (consequently, terminating the current experiment). The apparent indoor source is
activity within the house. Because we assume no indoor sources, analysis of only step 4 described
earlier will yield a calculation of the penetration factor. For purposes of this analysis, the data
illustrated in Figures 2, 3, and 4 will be referred to as experiment 1. Figure 3 shows a plot of the
data that are fitted to eq 15 using the values of parameters shown in Table 1.
Figure 3. Semi-log plot of the transient recovery of perturbed indoor particle concentration in
the research house for experiment 1.
y = -0.5021x- 0.0099
R2 = 0.9995
Figure 4 shows the data from experiment 1 fitted to eq 10. This is the traditional representation of
a characteristic grow-in response. Note that the short segment of the initial data appears to vary
exponentially with time as indicated by the nearly linear nature of the semi-log plot of Figure 3. The
line through the data represents a regression curve from which the fit equation was computed.
From eq 15, we see that the slope of the curve contains the sum of the air exchange rate, the
particle deposition rate constant, and the loss rate in the air handler system. In experiment 1, the air
handler was turned off. Table 1 shows measured values of parameters (C0i N, X^, and T) in the first
10
-------
four columns and calculated values (k, n, and P) in the last three columns. The air exchange rate
was measured using the decay method for SF6. The average values of C0 and N during the time
period over which the data are being fitted are used. The variation of C0 during the initial rise time
is much less than for the entire grow-in period. An average air exchange rate of 0.45 h"1 is used.
The characteristic time of response can be obtained as the reciprocal of the slope of Figure 3 (T =
1/0.502 = 2 h). This analysis yields a value of penetration for 0.5 urn aerodynamic diameter
particles of 0.83 and a deposition rate constant of 0.05 h'1. A value of penetration for 0.5 urn
aerodynamic diameter particles close to 1 is to be expected. This size particle tends to lie very near
the maximum in the penetration curve for all deposition processes. This size particle settles very
slowly, but is too large to diffuse effectively.
Figure 4. Comparison of measurements and model for 0.5 |im aerodynamic diameter particles
at the research house on 12/3-4/99.
1.4
1.2 -
P 1 -
u
I 0.8 H
2
1 0.6 H
00.4
0.2 H
0
10
15
20
Time(h)
Measurements
Mod step 1
Mod step 4
25
30
Figure 5 shows another example of the recovery of the house from an applied pressure. It shows
three separate episodes in which grow-in occurs. Both the measured concentrations and the
modeled responses are shown. Only the recovery segments (step 4) of the curves are shown. In this
case the data are plotted in the form of eq 10. The data set represented in Figures 5 and 6 are
referred to as experiment 2. The individual responses are distinguished by their approximate
starting times. The parameters that yield the best fit to the model in eq 15 are also shown in Table
1. These data represent particles with physical diameters of 0.2 um . In experiment 2, the air
handler was running continuously.
11
-------
Figure 5. Comparison of measurements and model for three sets of transient recoveries for 0.2
um physical diameter particles.
•4-1
0)
u
c
o
o
45
40 -
35 -
30 -
25
20
15 -
10 -
5
0
36
48
60
Time(h)
Measurements
Model 1
Model 2
Model 3
72
84
Table 1. Parameters describing the results in experiments 1 and 2.
Experiment
1
2
hour 44
hour 56
hour 68
C0
(cin3)
1.2 ±0.38
72.1 ±20
40 ±1
105 ±18
N
(h-1)
0.45 ±0.05
0.37 ±0.02
0.43 ± 0.02
0.35 ±0.05
^ah
(h-1)
0
5
5
5
T
0
1
1
1
k
(h-1)
0.05
0.08
0.08
0.08
T|
NA*
0.006
0.006
0.006
P
0.83
0.72
0.88
0.5
* NA - Not applicable because the air handler was not running.
12
-------
Figure 6. Semi-log plot of the transient recovery of perturbed indoor particle
concentration in the research house for experiment 2.
= -0.4882x + 0.0015
R2 = 0.9993
-2
*v- *•
c
_i
-2.5
-3
FT = 0.9999
y =-0.5715x-0.0482
R2 = 0.989
2 3
Time(h)
DISCUSSION
The regression equations of best fit using the measured values of parameters when available are
shown in Figures 3 and 6. The criterion for determining the best fit to the data is to maximize the
coefficient of determination (R2). For the data shown, all the coefficients of determination exceed
99%. Note that, in experiment 1, two parameters were estimated to obtain the best fit.
Consequently, the pair of values for these parameters are not unique. Other sets of values may give
equally good fits. However, the value of k obtained is consistent with limited measurements both in
the laboratory and in one room of the research house. However, the weakest point of this analysis
is the lack of direct simultaneous measurements of the particle deposition rate constant. There are
plans to perform direct measurements of k in the research house in the future. During experiment 2,
the air handler fan was operating continuously. Therefore, three parameters (P, k, and TI) are
estimated to obtain the best fit. Since the particle deposition constant and the air handler loss rate
13
-------
are additive in eq 15, the fit is obtained by varying only two parameters (P and k+^TX^. When the
best fit is obtained, the second parameter is then separated into k and nTX^. This separation is
performed by assuming a value of k consistent with previous measurements in the research house as
well as experiment 1.
If all parameters except P are measured, this approach can yield reasonable estimates of particle
penetration into buildings. However, because the assumption of constant outdoor concentration
and air exchange rate during the periods of measurement is well satisfied only occasionally, one
sometimes has to collect several sets of data until one appropriate for analysis is obtained.
Unfortunately, periods of slowly varying outdoor concentration are not easily predictable. During
certain seasons, it is cumbersome to obtain sufficiently large data sets for which the model
assumptions remain valid. Sometimes it is possible to use only an initial segment of the data for
which the model assumptions are valid. While it does appear to be possible to modify the method
to incorporate the time dependence of the outdoor particle concentration and air exchange rate, the
development is too involved to include in this paper. Consequently, that development will be
presented later.
CONCLUSIONS
This study presents some new techniques for measuring particle penetration factors by measuring
characteristic response times of buildings to perturbations that temporarily disturb equilibrium
values of indoor particle concentration. The resulting transient solutions to the transport equation
lead to reliable values of particle penetration so long as the outdoor concentration and other
weather-related parameters remain sufficiently constant that the assumptions of the transient
solutions are reasonably valid. From these preliminary measurements, the penetration of 0.5 urn
aerodynamic diameter particles into the closed research house with an air exchange rate of 0.45 h"1
is 0.83. For 0.2 urn physical diameter particles with air exchange rates ranging from 0.35 to 0.43 h'
', the measured values of penetration ranged from 0.5 to 0.88.
REFERENCES
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14
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15
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NRMRL-RTP-P-535
TECHNICAL REPORT DATA
(Please read Instate tions on the reverse before completing]
1. REPORT NO.
IPA/600/A-00/062
2.
3. REC
4. TITLE AND SUBTITLE
The Effect of Penetration on the Indoor/Outdoor
Ratio of Fine Particles
5. REPORT DATE
6. PERFORMING ORGANIZATION CODE
7.
f D> j, Greenwell, and Z. Guo
(EPA); -and R. Fortaiann and C. Whitfield (ARCADIS)
8. PERFORMING ORGANIZATION REPORT NO.
9. PERFORMING ORGANIZATION NAME AND ADDRESS
ARCADIS Geraghty and Miller, Inc.
P.O. Box 13109
Research Triangle Park, North Carolina 27709
tO. PROGRAM ELEMENT NO.
11. CONTRACT/GRANT NO.
68-C9-9201-02-4
12. SPONSORING AGENCY NAME AND ADDRESS
EPA, Office of Research and Development
Air Pollution Prevention and Control Division
Research Triangle Park, NC 27711
13. TYPE OF REPORT AND PERIOD COVERED
Published paper; FY99-00
14. SPONSORING AGENCY CODE
EPA/600/13
15.SUPPLEMENTARY NOTES APPCD project officer is Ronald B. Mosley, Mail Drop 54, 919/
541-7865. For presentation at Engineering Solutions to IAQ Problems, Raleigh, NC,
7/17-19/00.
16. ABSTRACT>r;ne paper discusses some measured values of penetration into an unoccu-
pied research house in which most of the usual indoor sources of particles are ab-
sent, and in which the rate of entry of outdoor particles can be controlled by applied
pressure differentials. (NOTE: Several recent studies, indicating significant health
risks associated with exposure to fine particles are measured outdoors, have led to
increased interest in the relationship between indoor and outdoor fine particles.)
Mathematical expressions are developed to compute the characteristic time of re-
sponse of a building to a change in its environment, such as particle concentration or
pressurization. Equations are also presented to compute the particle penetration
from measured responses of indoor concentration to either a pressurization system
or air cleaners. Preliminary measurements indicate that penetration of 0. 5-micro-
meter aerodynamic diameter particles into the closed research house with an air
exchange rate of 0.45/hr is 0.96. The penetrations of 0.2-micrometer physical dia-
meter particles with an air exchange rate ranging from 0.35 to 0.43/hr ranged from
0.5 to 0.88.
17.
KEY WORDS AND DOCUMENT ANALYSIS
DESCRIPTORS
b.lDENTIFIERS/OPEN ENDED TERMS
COS AT I Field/Group
Pollution
Particles
Penetration
Residential Buildings
Pressurizing
Air Cleaners
Pollution Control
Stationary Sources
Particulate
Indoor Air
Particle Concentration
Research House
13B
14G
13 M
13 A, 131
18. DISTRIBUTION STATEMENT
Release to Public
19. SECURITY CLASS (ThisReport}
Unclassified
21. NO. OF PAGES
2O. SECURITY CLASS (This page)
Unclassified
22. PRICE
EPA Form 222O-1 (9-73)
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