NISTIR 7068

Deposition Rates of Fine and Coarse

Particles in Residential Buildings:

Literature Review and Measurements

in an Occupied Townhouse

Cynthia Howard-Reed

Lance Wallace

Steven J. Emmerich
NISTIR 7068

Deposition Rates of Fine and Coarse

Particles in Residential Buildings:

Literature Review and Measurements

in an Occupied Townhouse

Cynthia Howard-Reed

Steven J. Emmerich

Building Environment Division

Building and Fire Research Laboratory

National Institute of Standards and Technology

Lance Wallace

National Exposure Research Laboratory

U.S. Environmental Protection Agency

October 2003

U.S. Department of Commerce

Donald L. Evans, Secretary

Technology Administration

Phillip J. Bond, Undersecretary of Commerce for Technology

National Institute of Standards and Technology

Arden L. Bement, Jr., Director
Acknowledgements and Disclaimer

We thank Andrew Persily of NIST and Tracy Thatcher of the Lawrence Berkeley National 

Laboratory for reading the entire paper and supplying helpful comments. This study was partially 

funded by an EPA Internal Grant to the second author. It has been reviewed and cleared for 

publication. Certain commercial equipment, instruments, or materials are identified in this paper 

in order to specify the experimental procedure adequately. Such identification is not intended to 

imply recommendation or endorsement by the National Institute of Standards and Technology, 

nor is it intended to imply that the materials or equipment identified are necessarily the best 

available for the purpose. 

i 
Abstract 

Several studies have shown the importance of particle losses in real homes due to deposition and 

filtration; however, none have quantitatively shown the impact of using a central forced air fan 

and in-duct filter on particle loss rates. In an attempt to provide such data, we measured the 

deposition of fine and coarse particles following specific source events in an occupied 

townhouse and also in an unoccupied test house. Experiments were run with three different 

sources (cooking with a gas stove, citronella candle, pouring kitty litter), with the central heating 

and air conditioning (HAC) fan on or off, and with two different types of in-duct filters 

(electrostatic precipitator and ordinary furnace filter). These tests resulted in a database of 

deposition rates for particles ranging from 0.3 μm to 10 μm under a wide range of occupancy 

conditions. Particle size, HAC fan operation, and the electrostatic precipitator had significant 

effects on particle loss rates. The standard furnace filter had no effect on loss rates. Surprisingly, 

the type of source (combustion vs. mechanical generation) and the type of furnishings (fully 

furnished including carpet vs. largely unfurnished including mostly bare floor) also had no 

measurable effect on the deposition rates of particles of comparable size. With the HAC fan off, 

average deposition rates varied from 0.3 h-1 for the smallest particle range (0.3 μm to 0.5 μm) to

5.2 h-1 for particles greater than 10 μm. Operation of the central HAC fan approximately doubled 

these rates for particles < 5 μm, and increased rates by 2 h-1 for the larger particles. An in-duct 

electrostatic precipitator increased the loss rates compared to the fan-off condition by factors of 5 

to 10 for particles < 2.5 μm, and by a factor of 3 for the larger particles. 

Keywords: fine particles, coarse particles, deposition, filtration, residential indoor air quality

ii 
Table of Contents 

Acknowledgements and Disclaimer ....................................................................... i

Abstract.................................................................................................................... ii

1 Introduction..........................................................................................................1

2 Experimental methodology .................................................................................2

2.1 Deposition Calculation.........................................................................................3 

Figure 1. Example calculation of decay rate for kitty litter event.. .........................................5 

Figure 2. Concentration of 0.5 μm to 1.0 μm particles following candle burned in basement with 

central fan off. ...............................................................................................................6 

3 Results and discussion .........................................................................................7

Figure 3. Comparison of deposition rates for different particle sizes and sources with HAC fan 

off ....................................................................................................................................................7 

Table 1. Deposition rates for cooking source events.............................................................8 

Table 2. Deposition rates for citronella candle source events................................................9 

Table 3. Deposition rates for kitty litter source events in sealed basement.............................12 

Figure 4. Comparison of particle deposition rates for different HAC configurations...............14 

Figure 5. Comparison of average deposition rates for particles from a citronella candle in two 

locations.....................................................................................................................15 

Table 4. Effect of HAC fan and in-duct filters on particle deposition rates (h-1)......................16 

3.2 Other Studies......................................................................................................18 

Table 5. Characteristics of previous studies of deposition rates for particles > 0.3 μm ............18 

Figure 6. Measured deposition rates in previous studies and this study. ...............................20 

Figure 7. Decay rates in this study by source type and fan-filter combination. ......................21 

Table 6. Effect of HAC fan and in-duct filters on the fraction of outdoor particles penetrating 

indoors. ......................................................................................................................22 

4 Conclusion...........................................................................................................23

5 References...........................................................................................................24

iii 
 

iv
1 Introduction 

There have been numerous studies documenting the contribution of indoor sources (e.g., 

smoking and cooking) to elevated concentrations of particles indoors (Özkaynak et al., 1996; 

Wallace, 1996). Indoor sources such as combustion tend to elevate ultrafine and fine particle 

concentrations whereas mechanically generated sources (sweeping, dusting, resuspension from

clothes and carpets) tend to elevate concentrations in the coarse fraction. Following the 

generation of particles indoors, concentration levels may be reduced through several mechanisms

including exfiltration through air change, filtration using portable or in-duct air cleaners, and 

deposition. Of these mechanisms, exfiltration losses are relatively easy to quantify for a given 

space, based on the air change rate, and these losses apply equally to all particle sizes. The other 

loss rates, however, are dependent on several factors including particle size, shape, composition, 

concentration, room air velocity, room surface characteristics, and volume flow of air through 

filters and duct work (Nazaroff and Cass, 1989). Coagulation is another important mechanism

that affects particle concentrations in a specific size range. However, Xu et al. (1994) estimated 

coagulation loss rates for environmental tobacco smoke in a room-size chamber and determined 

it only affected the concentration of particles less than 0.5 μm in diameter and for concentrations 

greater than 8000 particles cm-3, a very high concentration for typical residential environments. 

Thus, the more significant loss mechanisms for residential particle concentrations in need of 

further study are filtration and deposition. To date, there are only a few studies quantifying sizedependent particle loss rates in occupied homes due to deposition (Thatcher et al., 2002; Long et 

al., 2001; Vette et al., 2001; Abt et al., 2000; Fogh et al., 1997; Wallace, 1997; Thatcher and 

Layton, 1995), none of which also examine the loss due to operation of the ventilation system or 

use of air filtration devices. 

We conducted a multi-year monitoring study in an occupied three-story townhouse to increase 

the existing database of particle deposition rates for indoor sources representing a wide range of 

particle sizes. The study included real-time monitoring of tracer gas concentrations and indoor 

and outdoor concentrations of particles ranging from 0.3 μm to 10 μm. Following the peak 

concentration of an indoor particle source, the decay rate was calculated allowing for the 

determination of deposition and filtration particle losses. The heating and air conditioning (HAC) 

configuration was varied between off, on with no filter, and on with either a typical furnace filter 

or an electrostatic precipitator (ESP). The extended duration of this study allowed for 

determination of deposition and filtration rates for numerous indoor particle sources. 

The use of a single house normalized the impact of different surface characteristics on deposition 

losses. Thus, the relative importance of deposition as a loss mechanism may be determined 

between different types of sources. Additional decay experiments were completed in a second 

uninhabited house with the same source, thereby allowing another comparison of decay rates. 

1 
2 Experimental methodology 

The determination of decay rates for indoor particle sources is part of a larger monitoring study 

that has been described elsewhere (Wallace and Howard-Reed, 2002; Wallace et al., 2002). In 

summary, a three-story townhouse located in Reston, VA (approximately 35 km NW of 

Washington, D.C.) was equipped with several instruments to continuously monitor air change 

rates and indoor and outdoor particle concentrations. The end unit townhouse consists of three 

levels including a partial basement with recreation room, utility room, and bathroom on the 

bottom level; a kitchen/dining room, living room, and bathroom on the middle level; and four 

bedrooms and two bathrooms on the top level. The overall volume of the townhouse is 

approximately 400 m3

 with a floor area of approximately 50 m2 per level. The house’s primary 

heating system is a gas furnace and the cooling system includes central air conditioning and a 

temperature-actuated attic fan to vent the attic on hot days. The HAC system uses 100 % 

recirculated air and its ductwork does not enter the attic, resulting in minimal duct leakage to the 

outdoors. For the source decay rates described in this paper, the HAC system was either turned 

off, operated with no filter, or operated with either a typical panel furnace filter or an 

electrostatic precipitator (ESP). The ESP positively charges particles with ionizing wires at 6200 

V. The charged particles are then removed by ground collector plates. The ESP required 

frequent cleaning to maintain high removal efficiencies. 

The house’s infiltration rate was measured using the tracer decay method as described in ASTM 

Standard E741 (ASTM, 2001) with sulfur hexafluoride (SF6) as the tracer gas and a gas 

chromatograph with electron capture detector (GC-ECD) detection system. The system was 

automated to inject SF6 every 2 h to 4 h into the HAC system where it was distributed to the rest 

of the house. The GC-ECD measured tracer gas concentrations sequentially every minute from

10 locations in the house: two locations in the lower level, the central return of the HAC system, 

three locations on the middle level, three locations on the top level, and the attic. The GC-ECD 

was calibrated to measure SF6 concentrations between 30 μg/m3

 and 900 μg/m3 (5 ppb(v) and 

150 ppb(v)) with an accuracy of approximately ± 2 %. We observed that with the central HAC 

fan operating, it normally took about 30 min following injection to achieve relatively uniform

concentrations in all rooms. Therefore, hourly air change rates were calculated from 30 min past 

the hour to 30 min past the next hour, for all hours in which no injection took place. This 

allowed a calculated air change rate for about 4500 h out of 8760 possible hours in the year 

2000. Air change rates were estimated by linear regression of the natural logarithm of SF6

concentration versus time. The uncertainty of the calculated air change rates was approximately 

± 10 %. 

The duct velocity in the HAC system was measured every 5 min with a hot wire anemometer 

that had an accuracy of approximately ± 2.5 %. The duct velocity was converted to a volumetric 

airflow rate of approximately 5 h-1 when the central fan was operating. 

Particle concentrations were integrated over a 1 min time period every 5 min with a Climet 

(Climet Instruments Model 500-I, Redwood City, CA) optical scattering instrument that counts 

particles in the following six size ranges (modified to include one boundary at 2.5 μm) : 0.3 μm 

to 0.5 μm, 0.5 μm to 1.0 μm, 1.0 μm to 2.5 μm, 2.5 μm to 5.0 μm, 5.0 μm to 10 μm, and > 10 

2 
μm. Four Climets were used in this study, including one for counting outdoor particle 

concentrations. Limitations of this instrument have been discussed elsewhere (Wallace and 

Howard-Reed, 2002). All samples were collected directly by the Climet without tubing attached 

to the inlet. The precision of the Climet varies according to the size range considered, with the 

best precision at the smallest size ranges (which have the largest number of particles). Precision 

varies from a few percent at these small size ranges to much higher values at the largest size 

ranges, which may have less than 10 particles collected over a 1 min sampling period at typical 

indoor concentrations. 

Indoor and outdoor environmental conditions were also continuously monitored. Temperature 

was measured sequentially every minute in 10 indoor locations (same locations as SF6 samples) 

and outdoors with thermistors (accuracy of approximately ± 0.4 °C). Relative humidity (RH) was 

measured every 5 min in four indoor locations, the attic, and outdoors using bulk polymer 

resistance sensors with an accuracy of ± 3 % RH. Wind speed and direction were measured with 

a sonic anemometer (Climatronics, Inc.) mounted 2 m above the townhouse roof. The 

anemometer was capable of measuring wind speeds from 0 m s

-1 to 50 m s-1 (± 5 %) with a 

resolution of 0.1 m s

-1. For wind speeds above 4.5 m s-1, the wind direction had an accuracy of ±

5 % with no stated accuracy for lower wind speeds. Since the attic fan came on automatically 

when attic temperatures reached a certain point, the times it turned on or off were recorded 

electronically and transmitted to a computer. 

Finally, a detailed record of activities was kept by the home’s two non-smoking adult occupants. 

Specifically, all cooked meals, cleaning events, and combustion activities were recorded as well 

as ventilation status (e.g., windows open or closed, central fan off, patio door open, etc.). Sources 

causing substantial increases in indoor particle concentrations suitable for calculating decay rates 

included burning a citronella candle, pouring kitty litter, frying tortillas, and other cooking 

events. 

2.1 Deposition Calculations 

The following mass balance equation was used to estimate particle decay rates due to infiltration, 

deposition, and filtration following a source event (assuming no particle generation or 

coagulation during decay): 

p out,p p p p p,HAC p p,ac p

p P aC aC k C k C k C

dt

dC = − − − − (1)

where, 

Cp = indoor particle concentration for specific particle size p (particles m-3) 

Pp = penetration coefficient for specific particle size p (-) 

a = air change rate (h-1) 

Cout,p = outdoor particle concentration for specific particle size p (particles m-3) 

kp = deposition loss rate coefficient for specific particle size p describing losses to room

surfaces (h-1) 

kp,HAC = deposition loss rate coefficient for specific particle size p describing losses during 

HAC operation (h-1) 

3 
kp,ac = equivalent loss rate coefficient for air cleaner, where kp,ff corresponds to typical 

furnace filter and kp,ESP corresponds to electrostatic precipitator (h-1). 

Assuming Pp, Cout,p, a, kp, kp,HAC, and kp,ac are constant, the solution to Equation 1 is: 

( ) − [ ] − ( ) + + + +

+ + + = 1 exp a k k k ta k k k

P aC

C p p,HAC p,ac

p p,HAC p,ac

p out,p

p

C exp[ (a k k k )t] p,0 − + p + p,HAC + p,ac (2)

where, 

Cp,0 = indoor particle concentration for specific particle size p at time = 0 (particles m-3). 

At steady-state conditions, the solution to Equation 1 becomes: 

p p,HAC p,ac

p out,p

p,SS

a k k k

P aC

C

+ + + = (3)

where, 

Cp,SS = indoor particle concentration for specific particle size p at steady-state conditions 

(particles m-3). 

Equation 3 may be used to approximate particle concentrations during relatively stable ambient 

conditions with no indoor sources. This equation was used as an estimator of background particle 

concentrations in the townhouse. Using the background concentration and substituting Equation 

3 into Equation 2 results in: 

Cp = Cp, SS( ) 1− exp[− ( ) a + kp + kp,HAC + kp,ac t] + C exp[ ( ) a k k k t] p,0 − + p + p,HAC + p,ac

( ) C C (C C )exp[ ( ) a k k k t] p − p,SS = p,0 − p,SS − + p + p,HAC + p,ac

( ) [ ( ) ] ( p p,SS p p,HAC p,ac Cp,0 Cp,SS ln C − C = − a + k + k + k t + ln − ) (4)

The negative of the slope of the best-fit line of ln(Cp – Cp,SS) versus time is the value a + kp + 

kp,HAC + kp,ac. Subtracting the independently measured air change rate from the total decay results 

in an estimate of the loss rate due to deposition and filtration. An example plot of a decay rate for 

2.5 μm to 5.0 μm particles generated by pouring kitty litter with the fan off is shown in Figure 1. 

Several criteria were established to minimize the error associated with applying Equation 4 to 

calculate decay rates. First, the indoor source of interest must produce particle concentrations 

substantially higher than (e.g., 10 times) background levels. No other known indoor particle 

sources should be active and the outdoor concentration must be relatively constant during the 

decay calculation period. There also needed to be evidence of mixing of both the SF6 injection 

4 
-5

-4

-3

-2

-1

0

1

2

36.5 37 37.5 38 38.5 39 39.5 40

time (h)

ln (particles cm-3)

rec room

utility room

stairs

outside

Poured kitty litter

2.5 μm to 5.0 μm particles

Figure 1. Example calculation of decay rate for kitty litter event. Rec Room: a + kp = 1.26 h-1

(R2

 = 0.995). Utility room: a + kp = 1.41 h-1 (R2 = 0.99). Stairs: a + kp = 1.24 h-1 (R2 = 0.99). 

and particle emissions into all rooms or floors of the house. Figure 2 shows the concentration of 

0.5 μm to 1.0 μm particles resulting from a candle burned in the basement. For this case, the fan 

was not on, thereby showing the maximum time it takes to reach a relatively uniform

concentration in the house. All calculations of deposition rates were made after complete mixing 

had occurred. Figure 2 also illustrates the order of magnitude increase in particle concentration 

above background and the relatively low and constant outdoor concentration. If particle 

concentrations were not measured on every floor, then the decay rate was not calculated until the 

fan had been on for at least 10 min (except for kitty litter experiments conducted in the basement 

which was purposely closed off from rest of house). In addition, the R2

 for the decay of a given 

particle size needed to be greater than 0.9. Also the air change rates for all rooms were required 

to be similar (i.e., having a relative standard deviation (RSD) <15 %). Although there were 

hundreds of particle source events in the townhouse, only 15 cooking events, 18 candle burns, 

and 12 kitty litter pours met all of the criteria and are presented in this paper. 

5 
0.1

1

10

100

1000

35 35.5 36 36.5 37

time (h)

Particles cm-3

Basement

Kitchen

Office

Outside

0.5 μm to 1.0 μm particles

Citronella candle 

burned in basement

Figure 2. Concentration of 0.5 μm to 1.0 μm particles following candle burned in basement 

with central fan off. 

6 
3 Results and discussion 

Results from the 45 applicable tests are listed in Tables 1 – 3. Each table includes a single source 

type, within which results are organized by particle size, sampling location, and HAC fan/air 

cleaner status. Figure 3 shows average deposition rates for each source type as a function of 

particle size for all cases when the central fan was off. As expected, deposition for each source 

increased with increasing size. This effect was most dramatic once the particle size was greater 

than 2.5 μm. 

In addition, Figure 3 also illustrates the lack of influence of source type on deposition rates for a 

given particle size. The types of particles generated by the sources included cooking oil droplets, 

combustion particles, and coarse particles. Although the particle characteristics between the 

sources are dissimilar, this difference did not appear to significantly impact deposition rates. For 

example, the average deposition rates for 0.5 μm to 1.0 μm particles were 0.41 h-1(SD ± 0.10 h-1) 

for cooking, 0.41 h-1(SD ± 0.24 h-1) for kitty litter, and 0.47 h-1(SD ± 0.12 h-1) for the citronella 

candle (HAC fan off). In fact, for each size range, decay rates for the three sources (cooking, 

citronella candle, kitty litter) were not significantly different whether the fan was off, on, or on 

with a filter present. This finding allowed us to combine results from all sources in considering 

the effect of the central fan and the various air filtration devices. 

0

1

2

3

4

5

6

7

0.3 - 0.5 0.5 -1.0 1.0 - 2.5 2.5 - 5.0 5.0 - 10 > 10

Particle Diameter (μm)

Average Deposition Rates (h-1)

Citronella Candle

Cooking

Kitty Litter

Figure 3. Comparison of deposition rates for different particle sizes and sources with HAC fan 

off. Error bars are ± one standard deviation. 

7 
Table 1. Deposition rates for cooking source events. 

Date Source 

Measurement 

Location 

Air Change 

Rate 0.3 μm to 0.5 μm 

Particle Size

0.5 μm to 1.0 μm 1.0 μ

Fan Off a (h-1) kp (h-1) kp (h-1) 

3/10/00 Tortillas Basement

Kitchen 

0.29 0.39 

0.47 0.39 

0.61 0.49 

3/24/00 Tortillas Kitchen

Upstairs 

0.37 0.44 

0.27 0.13 

0.42 0.30 

5/30/00 Tortillas Basement

Kitchen 

0.29 0.27 

0.31 0.30 

0.32 0.33 

4/4/00 Fritata Basement

Kitchen 

Upstairs 

0.23 0.52 0.54 

0.32 0.21 0.18 

0.48 0.38 0.34 

AVERAGE: 0.29 0.41 

STANDARD DEVIATION: 0.11 0.10 

Fan On a (h-1) kp+kp,HAC (h-1) kp+kp,HAC (h-1) kp+

4/8/00 Tortillas Basement

Living Room

Upstairs 

0.79 0.81 0.83 

1.2 1.5 

0.97 

0.88 

1.1 

0.61 

4/14/00 Tortillas Basement

Living Room

Upstairs 

0.40 0.37 0.31 

0.90 0.86 0.93 

0.85 0.85 0.89 

5/12/00 Tortillas Cent. Return

Kitchen 

Upstairs 

0.17 0.17 0.14 

1.4 1.6 1.0 

1.0 1.2 

0.69 

3/24/00 Fried

eggs 

Basement 

Kitchen 

Upstairs 

0.52 0.51 0.57 

0.56 0.49 0.60 

1.0 

0.88 

1.1 

2/17/00 Sautéed

Clam

Sauce 

Upstream

Downstream

Kitchen 

0.60 0.60 0.58 

0.90 

1.0 1.0 

1.2 1.2 1.4 

8 
5/8/00 Stir-fry Basement

Living Room

Upstairs 

0.32 0.32 0.34 

0.27 0.35 0.28 

0.62 0.59 0.50 

AVERAGE: 0.88 0.92 

STANDARD DEVIATION: 0.40 0.25 

Fan On and ESP On a (h-1) kp+kp,HAC+kp,ESP (h-1) kp+kp,HAC+kp,ESP (h-1) kp+kp,H

3/31/00 Tortillas Basement 0.64 4.6 5.0 

4/8/00 Tortillas Basement

Living Room

Upstairs 

0.79 0.81 0.83 

3.8 4.3 4.1 

4.1 4.9 4.1 

4/14/00 Tortillas Basement

Living Room

Upstairs 

0.44 0.43 0.35 

2.2 2.3 2.3 

2.5 2.6 2.7 

5/12/00 Tortillas Cent. Return

Kitchen 

Upstairs 

0.17 0.17 0.14 

4.9 5.8 4.4 

8.6 8.3 5.1 

5/30/00 Tortillas Basement

Kitchen 

0.29 0.27 

2.6 3.0 

2.9 6.3 

AVERAGE: 3.7 4.8 

STANDARD DEVIATION: 1.2 2.1 

Table 2. Deposition rates for citronella candle source events. 

Date 

Source 

Location 

Measurement 

Location 

Air Change 

Rate 

0.3 μm to 

0.5 μm 

0.5 μm to 

1.0 μm 

1.0 μm to 

2.5 μm 

Fan Off a (h-1) kp (h-1) kp (h-1) kp (h-1) 2/24/99 Kitchen Basement

Kitchen 

Office 

0.42 0.42 0.42 

0.37 0.43 0.26 

0.45 0.66 0.41 

0.36 0.78 0.61 

4/3/00 Kitchen Basement

Living Room

Upstairs 

0.18 0.18 0.18 

0.29 0.32 0.31 

0.47 0.39 0.38 

0.75 0.63 0.61 

5/22/00 Basement Basement 0.29 - 0.66 0.92 

9 
Kitchen 

Upstairs 

0.29 0.29 

- - 

0.35 0.35 

0.77 0.67 

AVERAGE: 0.33 0.47 0.68 

STANDARD DEVIATION: 0.06 0.12 0.16 

Fan On w/o Furnace Filter a (h-1) kp+kp,HAC (h-1) kp+kp,HAC (h-1) kp+kp,HAC (h-1) 

3/2/99 Kitchen Basement

Kitchen 

Upstairs 

0.41 0.41 0.41 

0.83 0.85 0.83 

1.4 1.3 1.3 

2.0 2.0 2.6 

3/3/99 Kitchen Basement

Kitchen 

Upstairs 

0.29 0.29 0.29 

1.0 

0.97 0.74 

1.6 1.5 1.3 

2.4 2.5 2.4 

2/25/00 Basement Basement

Bathroom

Kitchen 

Upstairs 

0.35 0.35 0.36 0.32 

0.34 0.39 0.34 0.44 

0.75 0.69 0.69 0.74 

1.7 1.7 1.7 1.6 

3/15/00 Upstairs Basement

Kitchen 

Upstairs 

0.24 0.25 0.26 

0.68 0.72 0.69 

1.1 1.1 1.1 

1.9 2.0 1.8 

4/14/00 Kitchen Basement

Living Room

Upstairs 

0.38 0.43 0.28 

0.43 0.36 0.41 

0.78 0.65 0.73 

1.6 1.2 1.3 

6/6/00 Kitchen Kitchen

Upstairs 

0.40 0.29 

0.93 

1.0 

1.0 1.1 

1.3 1.4 

AVERAGE: 0.66 1.0 1.8 

STANDARD DEVIATION: 0.25 0.31 0.43 

Fan On w/Furnace Filter a 

(h-1) 

kp+kp,HAC+kp,ff

(h-1) 

kp+kp,HAC+kp,ff

(h-1) 

kp+kp,HAC+kp,ff

(h-1) 

12/31/98 Kitchen Basement

Kitchen 

Office 

0.82 0.82 0.82 

0.34 0.40 0.34 

1.1 1.3 1.1 

1.9 1.7 1.8 

1/1/99 Basement Basement 

Kitchen 

Upstairs 

0.71 0.71 0.71 

1.0 1.3 1.2 

1.4 1.5 1.4 

2.2 2.4 1.8 

10 
2/26/99 Kitchen Basement

Kitchen 

Upstairs 

0.53 0.53 0.53 

0.73 

1.0 

0.94 

1.3 1.7 1.5 

1.9 2.4 2.2 

AVERAGE: 0.81 1.4 2.0 

STANDARD DEVIATION: 0.37 0.19 0.27 

Fan On and ESP On a 

(h-1) 

kp+kp,HAC 

+kp,ESP (h-1) 

kp+kp,HAC+ 

kp,ESP (h-1) 

kp+kp,HAC+ 

kp,ESP (h-1) 

3/28/00 Basement Basement 0.33 2.0 4.1 4.7 4/14/00 Kitchen Basement

Living Room

Upstairs 

0.43 0.33 0.34 

2.4 2.6 2.4 

2.2 2.6 2.3 

3.2 3.2 2.8 

4/19/00 Basement Basement

Living Room

Upstairs 

0.56 0.59 0.55 

2.2 1.8 2.0 

2.1 2.0 2.3 

4.2 3.1 3.9 

5/18/00 Basement Basement

Kitchen 

Upstairs 

0.30 0.51 0.45 

1.9 1.9 2.2 

3.1 3.0 3.3 

4.0 3.3 4.3 

5/31/00 Basement Basement

Kitchen 

Upstairs 

0.25 0.23 0.20 

2.3 2.3 2.2 

3.1 3.1 2.9 

3.6 3.7 3.2 

6/5/00 Kitchen Kitchen

Upstairs 

0.30 0.33 

3.5 3.4 

4.2 4.8 

5.3 5.3 

AVERAGE: 2.3 3.0 3.9 

STANDARD DEVIATION: 0.50 0.83 0.78 

11 
Table 3. Deposition rates for kitty litter source events in sealed basement. 

D

ate

S

o

u

r

c

e

Location 

Measurement 

Location 

Air Change 

Rate 

0.5 

μm to 

1.0 

μm 

1.0 

μm to 

2.5 

μm 

2.5 

μm to 

5.0 

μm 

5

Fan Off 

a (h-1) kp (h-1) kp (h-1) kp (h-1) 

2/21/0

0

B

ase

m

e

nt Rec Room

Utility Room

Stairs 

0.53 0.49 0.51 

0.32 0.29 0.17 

1.3 1.5 1.1 

1.7 1.7 1.6 

2/2

8/0

0

B

ase

m

e

nt Rec Room

Utility Room

Stairs 

0.70 0.70 0.70 

- - - 

0.63 0.53 0.70 

1.2 1.2 1.2 

3/19/0

0

B

ase

m

e

nt Rec Room

Stairs 

Rec Room

Utility Room

0.60 0.56 0.60 0.51 

0.89 0.68 0.51 0.57 

1.3 1.6 1.2 1.3 

1.8 2.2 1.5 1.9 

3/2

7/0

0

B

ase

m

e

nt Rec Room

Stairs 

Utility RoomUtility Room

0.58 0.50 0.42 0.42 

- - - - 

1.1 1.4 1.4 

0.91 

1.7 2.1 1.9 1.2 

4/3/00 Basem

ent Rec Room

Rec Room

Utility Room

Stairs 

0.69 0.69 0.69 0.69 

0.25 0.29 0.13 0.23 

0.64 0.71 0.57 0.72 

1.3 1.4 1.3 1.5 

4/15/0

0

B

ase

m

e

nt Rec Room

Utility Room

Rec Room

Stairs 

0.86 0.70 0.86 0.78 

- - - - 

0.34 0.47 0.37 0.48 

1.1 1.1 

0.98 

1.1 

5/2

0/0

0

B

ase

m

e

nt Rec 

Flo

or

Stairs 

Rec Table 

Utility Room

0.65 0.64 0.65 0.62 

- - - - 

0.76 0.85 0.96 0.82 

1.6 1.6 1.5 1.7 

12 
5/29/00 Basement Rec Room

Utility Room

Stairs 

0.75 0.74 0.75 

0.62 0.64 0.53 

0.76 0.75 0.69 

1.5 1.5 1.4 

6/19/00 Basement Rec Room

Utility Room

0.31 0.31 

- - 

- - 

1.3 1.1 

7/7/00 Basement Rec Room

Utility Room

0.65 0.89 

0.36 

0 

0.96 0.55 

1.5 1.1 

AVERAGE: 0.41 0.88 1.5 

STANDARD DEVIATION: 0.24 0.35 0.31 

Fan On w/o Filter kp+kp,HAC

(h-1) 

kp+kp,HAC

(h-1) 

kp+kp,HAC

(h-1) 

k

12/20/99 Basement Rec Room

Rec RoomRec Room

0.56 0.56 0.56 

0.60 

1.1 

0.74 

1.4 2.0 1.5 

2.4 2.9 2.6 

2/19/00 Basement Rec Room

Utility Room

Kitchen 

0.53 0.59 0.56 

- - - 

1.4 1.3 1.0 

3.6 3.6 3.0 

AVERAGE: 0.80 1.4 3.0 

13 
With the HAC fan off, deposition of particles is confined to room surfaces. As expected, this test 

condition consistently resulted in the lowest deposition rate. Once the HAC fan was turned on, 

even without a filter present, particle deposition rates increased (see Figure 4). For example, for 

1.0 μm to 2.5 μm particles, the average deposition rate doubled when the fan was turned on. 

Possible reasons for this increase include the additional surface area of the HAC ducts and other 

system components (e.g., heat exchanger, fan blades, etc.), as well as the increase in room air 

velocity and turbulent kinetic energy with the fan system on. As noted earlier, approximately 5 

house volumes of air pass through the ducts each hour. Although an HAC system was not used, 

an increase in particle deposition due to use of room mixing fans has been reported by other 

researchers (Xu et al., 1994; Mosley et al., 2001; Thatcher et al., 2002). We also investigated the 

possibility that increased air change rate alone might lead to increased particle deposition rates 

because of increased air velocity and turbulent energy. However, no increase in deposition rate 

with increasing air change rates was noted for any of the six particle sizes studied under the fanoff condition. Interestingly, the addition of a standard furnace filter had no observable effect on 

deposition rates (beyond the HAC system effect) for particles less than 5 μm, whereas the use of 

an electronic air cleaner had a significant impact on particle removal rates. It should be noted 

that the furnace filter and ESP were only used during events that generate smaller particles (i.e., 

cooking and candle burns). Thus, measurable decay rates were not available for particles > 5 μm.

The effectiveness of these different types of air cleaners will be explored in more detail in a 

future report. 

0

1

2

3

4

5

6

7

8

9

10

0.3 - 0.5 0.5 -1.0 1.0 - 2.5 2.5 - 5.0 5.0 - 10 > 10

Particle Diameter (μm)

Average Deposition Rates (h-1)

Fan On w/o Filter

Fan Off

Fan On w/Furnace Filter

Fan On w/ESP

Figure 4. Comparison of particle deposition rates for different HAC configurations. Error bars 

are ± one standard deviation. 

14 
To investigate the potential impact of room surface area and types of furnishings, similar 

deposition tests were completed in a second house. For these tests, the same citronella candle 

was burned in a single room unfurnished test house in Gaithersburg, MD. As shown in Figure 5, 

in cases with the HAC fan on and off, there was no significant difference in particle deposition 

rate between the two locations. The ratio of floor surface area to volume for the two houses was 

very similar with a value of approximately 0.38 m-1 per floor for the townhouse and 0.44 m-1 for 

the test house. However, if wall surfaces and room furnishings were to also be included, the 

townhouse surface area to volume ratio would be far greater than that of the unfurnished test 

house, indicating a lack of importance of increased surface material for a given space on most 

particle deposition rates. Thatcher et al. (2002) investigated the effect of room furnishings on 

particle deposition and found a more significant impact on particles ≤ 1.0 μm, which is also 

indicated in Figure 5. As the authors point out, this result follows deposition theory where loss of 

larger particles is dominated by gravitational settling, thereby being less affected by room

furnishings. The ratio of volumetric flow through the HAC system to room volume was slightly 

higher for the test house with a value of 7.2 h-1, whereas the townhouse was 5 h-1. This 

difference in flow rate through the duct system was apparently not enough to cause a noticeable 

difference in deposition rates. 

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

0.3 - 0.5 0.5 -1.0 1.0 - 2.5 2.5 - 5.0

Particle Diameter (μm)

Average Deposition Rates (h-1)

Test House - Fan Off

Townhouse - Fan Off

Townhouse - Fan On (no filter)

Test House - Fan On (no filter)

Figure 5. Comparison of average deposition rates for particles from a citronella candle in two 

locations. Error bars are ± one standard deviation. 

The effect of having the central fan on and using an in-duct filter is summarized in Table 4. 

Using the central fan increased the deposition rate by between 0.5 h-1 and 2 h-1. Adding the induct electrostatic precipitator increased the loss rate by an additional 1.4 h-1 to 2.8 h-1. 

15 
Table 4. Effect of HAC fan and in-duct filters on particle deposition rates (h-1). 

Particle Size Range (μm) 

0.3 to 0.5 0.5 to 1 1 to 2.5 2.5 to 5 5 to 10 

Ventilation/ Filtration Setting Mean SD N Mean SD N Mean SD N Mean SD N Mean SD N HAC Fan Off (kp) 

(h-1) 

0.30 0.09 15 0.42 0.18 34 0.78 0.37 48 1.4 0.34 40 2.7 0.52 3

HAC Fan On (h-1) 

0.77 0.34 36 0.97 0.28 39 1.5 0.46 40 3.2 0.80 22 4.8 0.54 6 

Air Cleaner (ESP) on (h-1) 

2.9 1.1 27 3.8 1.7 27 4.0 1.0 21 4.6 0.82 14 - - 

kp,HAC (h-1) 0.47 0.36 * 0.55 0.33 * 0.72 0.56 * 1.8 0.87 * 2.1 0.75 * kp,ac (h-1) 2.1 1.2 * 2.8 1.7 * 2.5 1.1 * 1.4 1.1 * - - *: Deposition rate based on difference of mean values above. Standard deviation is the square root of the sum of the squares of the asso

16 
3.2 Other studies 

Over 20 previous studies have determined deposition rates for particles > 0.3 μm in diameter. Lai 

(2002) provides an extensive summary of 12 studies completed in experimental chambers. Other 

tests have been conducted either in controlled test houses (Offermann et al., 1985; Xu et al.

1994; Thatcher et al., 2002; Emmerich and Nabinger, 2001) or in occupied or unoccupied houses 

(Thatcher and Layton 1995; Wallace et al., 1997; Fogh et al., 1997; Abt et al., 2000; Long et al., 

2001; Vette et al., 2001). A summary of these 10 studies is provided in Table 5. 

Test houses 

Offermann et al. (1985) measured the performance of several portable air cleaners using 

cigarette smoke in one room of a three-room test house. In order to estimate the effectiveness of 

each air cleaner, particle deposition was measured during a period without the air cleaners 

operating. The resulting deposition rates were at a minimum of 0.05 h-1 for 0.3 μm particles and 

reached 0.38 h-1 for 1.5 μm particles. 

Xu et al. (1994) also employed machine-smoked cigarettes in one room of an experimental 

house. An LAS-X monitor was used to count particles in 16 bins ranging from 0.09 μm to > 3 

μm. However, only eight calculated deposition rates were reported between 0.3 μm and about 

1.5 μm. The range of deposition rates was from 0.1 h-1 to 1.2 h-1 for the fan-off case and from

0.4 h-1 to 2.3 h-1 for the fan-on case. 

Thatcher et al. (2002) measured polydisperse olive oil particles between 0.5 μm and 20 μm in a

small experimental room. Three different levels of furnishing and four fan speeds were 

employed. Increased furnishing and higher fan speeds generally led to higher deposition rates, 

although for large particles the bare room had higher deposition rates than when it was fully 

furnished. Deposition rates varied from 0.10 h-1 for 0.55 μm particles to greater than 10 h-1 for 

8.66 μm particles. 

As part of a study to determine the impact of using different particle air cleaners in a single room

unfurnished test house, Emmerich and Nabinger (2001) measured the decay rates of 0.3 μm to 

5 μm particles. Average deposition rates ranged from 0.4 h-1 to 1.0 h-1 for cases with the HAC 

fan off, and 0.74 h-1 to 2.0 h-1 for cases with the HAC fan on. The deposition and filtration 

removal rates for an electronic air cleaner similar to the one used in this study ranged from 

6.2 h-1 to 7.7 h-1. They also investigated the effect of a mechanical air cleaner and found its 

removal rate to range from 0.54 h-1 to 2.1 h-1. 

Homes 

Thatcher and Layton (1995) measured particles in a residence using Climets (Climet 

Instruments, Redlands CA), optical scattering devices. The Climet 208 collects particles in 100 

size ranges between 0.5 μm and 6 μm and the Climet 500-CI in six size ranges from 0.3 μm to

0.5 μm, 0.5 μm to 1 μm, 1 μm to 5 μm, 5 μm to 10 μm, 10 μm to 25 μm, and > 25 μm. Data 

from only three days were presented, with deposition rates varying from 0.27 h-1 to 1.3 h-1 for 

size ranges between 1.5 μm and 7 μm. 

17 
Table 5. Characteristics of previous studies of deposition rates for particles > 0.3 μm. 

Reference Site Type Unoccupied

Occupied/

# sites Aerosola Source Fan

Exhaust

Fanb

Duc

Filter

Offermann et al.(1985) Test house Unoccupied 1 Poly Cigarette Off N/A N/A Xu et al. (1994) Test house Unoccupied 1 Poly Cigarette On/off N/A N/A Thatcher et al. (2002) Test house Unoccupied 1 Poly Generator

Off/On 

(3 speeds) N/A N/A 

Emmerich and Nabinger (2001) Test house Unoccupied 1 

Mono and 

Poly 

Generator, 

Various On/off N/A On/O

Thatcher and Layton (1995) House Occupied 1 Poly Various Off No No Wallace et al. (1997) House Occupied 1 Poly candle, kitty litter 

Cooking oil, 

On/off On/off No 

Fogh et al. (1997) House Occupied 4

Mono 

(5 sizes) Generator Off

No 

No 

Abt et al. (2000) House Occupied 3 Poly Various Mixed

No 

No 

Long et al. (2001) House Occupied 9 Poly Various Mixed

No 

No 

Vette et al. (2001) House Occupied 1 Poly Various Off No No This study (2003) House Occupied 1 Poly candle, kitty litter 

Cooking oil, 

On/off On/off On/o

a: Mono = monodisperse; Poly = polydisperse b: N/A = not applicablec: APS = aerodynamic particle sizer; EC/CNC = electrostatic classifier with condensation nucleus counter; OPC = optical particle counspectrometer.

18 
Wallace et al. (1997) measured particles in an occupied home using the Climet 500-CI with 

modified size ranges: 0.3 μm to 0.5 μm, 0.5 μm to 1 μm, 1 μm to 2.5 μm, 2.5 μm to 5 μm, 5 μm 

to 10 μm, and > 10 μm. Data from 27 days were presented, with average deposition rates 

varying from 0.20 h-1 to 2.8 h-1 for size ranges between 0.4 μm and >10 μm.

Fogh et al. (1997) investigated four houses, some occupied, using a particle generator to create 

monodisperse particles at 0.5 μm, 2.5 μm, 3.0 μm, 4.5 μm, and 5.5 μm diameter. Particle 

numbers were counted using an aerodynamic particle sizer (APS). Houses were measured with 

and without furniture. Average measured deposition rates in the unfurnished case ranged from

0.33 h-1 to 1.77 h-1, with slightly higher values of 0.47 h-1 to 1.88 h-1 for the furnished case. 

Abt et al. (2000) investigated four Boston homes using an APS switched every hour between 

indoor (45 min) and outdoor (15 min) measurements. Overnight measurements, when there were 

few or no indoor sources, were used to calculate values of the penetration coefficient P and the 

deposition rate k. It was not possible to determine these coefficients separately for each home, 

but an average deposition rate was determined for the sizes from 0.35 μm to 10 μm. The

deposition rate varied from 0.7 h-1 to 3.0 h-1. 

Long et al. (2001) used the same sampling methods and equipment as Abt et al. (2000) to sample 

nine homes in the Boston area. Once again only an average deposition rate could be determined 

for the nine homes. The rates varied from 0.15 h-1 to 0.72 h-1, considerably lower than the rates 

found in Abt et al. (2000). A puzzling result was the observed decline of the deposition rate for 

particles 4.5 μm and larger. Armendariz and Leith (2000) found the APS to mix particles in this 

size range with smaller particles, which would lead to a reduced apparent deposition rate. This 

effect was also observed by Wallace et al. (2002). 

Vette et al. (2001) studied an unoccupied detached residence using a laser aerosol spectrometer 

(LAS-X) for particle sizes from 0.1 μm to 2.5 μm. The author states that only a few 

measurements of deposition rates for particles >1 μm were made and that the particle numbers 

were also very low, which reduced the reliability of the counting statistics of the LAS-X. The 

HAC system was turned off during the measurements. Observed deposition rates were 0.45 to 

0.6 h-1 for particles between 0.3 μm and 0.6 μm, rising to1.3 h-1 between 0.75 μm and 0.85 μm. 

For particles >1 μm, very large uncertainties were reported; the average deposition rates ranged 

from 2.4 (+ 1.0) h-1 to 2.95 (+ 2.2) h-1 for 1 μm to 2 μm particles, rising to 3.9 (+ 1.4) h-1 for 

2.5 μm particles. 

Findings from all of these studies are compared with our findings in Figure 6. All studies agree 

in finding increased deposition rates for larger particles. There is also agreement that increased 

surface area (furnishings) and increased air speeds are associated with higher deposition rates, 

although the effect of both these parameters is not large compared with the effect of particle size. 

However, comparing the controlled chamber/test house studies to the occupied house studies, it 

is possible to see that the range of deposition rates is larger in the controlled studies. In 

particular, at the smallest particle sizes of 0.3 μm to 0.7 μm, the deposition rates measured in the 

controlled studies may be an order of magnitude smaller than those in the occupied house 

studies. At these particle sizes, the controlled studies agree better with predicted values than the 

19 
occupied house studies. For larger particles, both types of studies agree reasonably well with 

theoretical predictions (Lai and Nazaroff, 2000). The reason for this divergence for smaller 

particles is not clear. 

0.01

0.1

1

10

100

0.1 1 10

Particle diameter (μm)

Deposition rate (h-1)

This Study (Duct Fan Off) Thatcher et al. (2002) (Fan Off, Bare Floor)

Emmerich and Nabinger (2001) (Duct Fan Off) Long et al. (2001) (Winter)

Long et al. (2001) (Summer) Vette et al. (2001)

Abt et al. (2000) Fogh et al. (1997) (Unfurnished)

Wallace et al. (1997) Xu et al. (1994) (no fan)

Thatcher and Layton (1995) Offermann et al. (1985)

This Study (Duct Fan On) Thatcher et al. (2002) (Fan On, Furnished)

Emmerich and Nabinger (2001) (Duct Fan On) Fogh et al. (1997) (Furnished)

Xu et al. (1994) (fan) This Study (Duct Fan & ESP On)

Emmerich and Nabinger (2001) (ESP) Emmerich and Nabinger (2001) (mech air cleaner)

Figure 6. Measured deposition rates in previous studies and this study. 

This study is the first to provide quantitative estimates of the decay rates associated with 

unfiltered and filtered ductwork in an occupied house. Figure 7 shows that the unfiltered duct 

increased the deposition rate over the fan-off condition for all particle sizes, and that the filtered 

duct increased the decay rate still further for all particle sizes. These results indicate that for 

homes with central air, increasing the time that the duct fan is on will reduce the residence time 

of particles in the home, and installing a high-efficiency in-duct filter will reduce the particle 

levels still further. For example, use of the HAC fan can reduce particle levels by 23 % to 50 %, 

and use of an in-duct ESP can reduce the levels by 57 % to 85 % (Table 6). This is comparable to 

the finding by Fugler and Bowser (2002) that an in-duct ESP reduced particle levels in five 

homes by 30 % to 70 %, depending on resident activity level. However, a standard furnace filter 

had no effect on decay rates for particles less than 5 μm beyond the HAC system effect. 

Additional implications of these results include the comparison of control strategies to reduce 

particle concentrations indoors. For example, use of the duct fan and air cleaner will reduce 

particle concentrations of indoor and outdoor origin both, whereas tightening the house’s 

envelope only reduces the influx of outdoor particles and actually increases the residence time of 

indoor-generated particles. 

20 
0.1

1

10

0.1 1 10

Particle diameter (μm)

Deposition rate (h-1)

Cooking (fan off) Cooking (fan on) Cooking (fan on, ESP on)

Candle (fan off) Candle (fan on) Candle (fan on, ESP on)

Kitty litter (fan off) Kitty litter (fan on)

Figure 7. Decay rates in this study by source type and fan-filter combination. The type of source 

(cooking, candle, pouring kitty litter) has little effect on deposition rate. 

21 
Table 6. Effect of HAC fan and in-duct filters on the fraction of outdoor particles penetrating indoors. 

Infiltration Factors: Tight House (

a = 0.2 h-1, P = 1) 

Ventilation/Filtratio

n Midpoint of Particle Siz

e Range (

μm) 

Setting 

0.3-0.5

0.5-1 1-2.5

2.5-5

5-10

HAC Fan Off 

0.4

0

0.3

2

0.2

0

0.13

0.0

7

HAC Fan On 

0.21 0.17

0.12

0.0

6

0.0

4

Air Cleaner On 

0.0

6

0.0

5

0.0

5

0.0

4 - 

% Reduction (Fan) 

4

8

4

7

4

2

5

3

4

2

% Reduction (Air Cleaner) 

8

4

8

5

7

7

6

7 - 

Infiltration Factors: Drafty House (

a = 1 h-1, P = 1) 

Ventilation/Filtratio

n Midpoint of Particle Siz

e Range (

μm)

Setting 

0.3-0.5

0.5-1 1-2.5

2.5-5

5-10

HAC Fan Off 

0.7

7

0.7

0

0.5

6

0.4

2

0.2

7

HAC Fan On 

0.5

6

0.51 0.4

0

0.2

4

0.17

Air Cleaner On 

0.2

6

0.21 0.2

0

0.18 - 

% Reduction (Fan) 

2

7

2

8

2

9

4

3

3

6

% Reduction (Air Cleaner) 

6

7

7

0

6

4

5

7 - 

22 
4 Conclusion 

This study has presented some of the first measurements of fine and coarse particle decay rates 

associated with a central forced-air fan and an in-duct electronic air cleaner under realistic 

conditions in an occupied home. The effect of each of these actions is generally larger than the 

deposition rate under fan-off conditions. Since all homes with central forced air heating and air 

conditioning will employ the central fan on an intermittent basis, and some use the fan constantly 

to increase air circulation, the effect in these homes will be to reduce concentrations of indoor air 

particles up to twice as fast as in homes not using a central fan. The standard furnace filter was 

ineffective at removing particles, while the ESP was able to greatly increase the decay rate of 

particles. 

We also found that the source of the particles appeared to have little if any influence on 

deposition rates, even though the particles varied from cooking oil droplets to combustion 

particles to coarse particles. Only the size of the particles and not their composition affected 

deposition rates within the uncertainty of these tests; but size alone determined about an order of 

magnitude difference in deposition rates (from < 0.5 h-1 to > 10 h-1). 

23 
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25