NIST Technical Note 1841

Modeling Particle Resuspension for Estimating 

Potential Exposure to Bacillus Spores

W. Stuart Dols

Andrew K. Persily

This publication is available free of charge from:

http://dx.doi.org/10.6028/NIST.TN.1841

NIST Technical Note 1841 

Modeling Particle Resuspension for Estimating 

Potential Exposure to Bacillus Spores 

W. Stuart Dols 

Andrew K. Persily 

Engineering Laboratory 

Prepared for: 

U.S. Environmental Protection Agency 

Office of Research and Development 

Funded under Interagency Agreement DW-13-92340101-0 

This publication is available free of charge from: 

http://dx.doi.org/10.6028/NIST.TN.1841 

June 2015 

U.S. Department of Commerce 

Penny Pritzker, Secretary 

National Institute of Standards and Technology 

Willie May, Under Secretary of Commerce for Standards and Technology and Director
Certain commercial entities, equipment, or materials may be identified in this document in 

order to describe an experimental procedure or concept 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 entities, materials, or equipment are necessarily 

the best available for the purpose.

National Institute of Standards and Technology Technical Note 1841

Natl. Inst. Stand. Technol. Tech. Note 1841, 77 pages (June 2015)

CODEN: NSPUE2
iii

Abstract

A key challenge in assessing the risks of low level contamination following biological threat

agent releases and subsequent decontamination is developing an understanding of the 

potential for airborne resuspension of agents that have deposited on surfaces within a building. 

Resuspension is a particularly difficult and important problem for the persistent and deadly 

biological threat agent that causes anthrax, Bacillus anthracis. The National Homeland Security 

Research Center of the U.S. Environmental Protection Agency, in collaboration with several 

other federal agencies, conducted the Biological Operation Testing and Evaluation (BOTE) study 

[1] to evaluate B. anthracis spore decontamination technologies and to better understand the 

potential inhalation exposures to spores before and after decontamination. The data collected 

in this study provide a unique resource to support the development of microbial exposure 

assessment methodologies, including the application of particulate modeling. Particulate 

transport modeling approaches that consider particulate characteristics and building features 

provide a means to quantify the distribution, transport and fate of agents in a building. Models 

exist to predict resuspension rates as a function of particle size, surface characteristics, human 

disturbance and other factors; however, these models have not been evaluated for their 

applicability and accuracy for determining potential inhalation exposure doses of Bacillus 

spores. Using the field sampling data collected in the BOTE study and resuspension rates 

determined using existing resuspension models, particulate transport models were used within 

building airflow and contaminant transport simulation tools to estimate potential inhalation 

exposure. Approaches to the use of such exposure modeling approaches for future risk 

assessment are also summarized.

Key Words: airflow, bioaerosols, CONTAM, modeling, particles, multizone modeling, risk 

assessment, resuspension, sensitivity analysis, simulation
iv
Table of Contents

Abstract...........................................................................................................................................iii

List of Figures.................................................................................................................................vii

List of Tables ..................................................................................................................................vii

Acronyms and Abbreviations........................................................................................................viii

Executive Summary......................................................................................................................... 1

1 Introduction .............................................................................................................................. 3

2 Literature Review...................................................................................................................... 4

2.1 Background.....................................................................................................................................4

2.2 Literature Review Categories.........................................................................................................6

2.2.1 Relevant resuspension models...........................................................................................6

2.2.2 Chamber measurements of resuspension rates.................................................................7

2.2.3 Other chamber measurements...........................................................................................7

2.2.4 Field measurements of resuspension rates or particle concentrations.............................7

2.2.5 Other model development and application .......................................................................7

2.2.6 Reviews...............................................................................................................................8

2.3 Summary of Existing Resuspension Rate Data...............................................................................8

2.4 Conclusions from the Literature Review......................................................................................13

3 Description of BOTE Measurement Data Used for Modeling Study ...................................... 13

3.1 Building Description .....................................................................................................................13

3.2 BOTE Experimental Measurements.............................................................................................14

3.2.1 Particle Measurements.....................................................................................................15

3.2.2 Video of Sampling Activities .............................................................................................24

4 Whole-Building Resuspension Simulations Results................................................................ 25

4.1 CONTAM Representation of PBF-632 ..........................................................................................25

4.1.1 Weather Data....................................................................................................................26

4.1.2 Resuspension ....................................................................................................................26

4.1.3 Initial Loading....................................................................................................................27

4.1.4 Activity Schedule...............................................................................................................28

4.2 Simulation Results and Comparison with Measurements...........................................................28

5 Sensitivity Analysis of Resuspension Exposure Model ........................................................... 33

5.1 Single-zone Simulation Model......................................................................................................33

5.2 Model Inputs................................................................................................................................34

5.3 Results of Sensitivity Analysis.......................................................................................................34

6 Proposed Resuspension Exposure Tool .................................................................................. 38

7 Summary and Conclusions...................................................................................................... 41

8 References .............................................................................................................................. 43

Acknowledgments......................................................................................................................... 46

Appendix A. Summaries of Documents from Literature Review .............................................. 47

vii

List of Figures

Figure 1. Summary plot of resuspension rates.........................................................................................9

Figure 2. INL building PBF-632 and decontamination containment tent...............................................13

Figure 3. 2

nd floor plan of PBF-632 .........................................................................................................14

Figure 4. 1

st floor plan of PBF-632..........................................................................................................14

Figure 5. 1st floor plan of PBF-632 building shows locations of rooms 101A and 102, the UVAPS and 

IBAC devices, and the spore release .......................................................................................16

Figure 6. Particle measurements for Round 1, Room 101A...................................................................18

Figure 7. Particle measurements for Round 1, Room 102 .....................................................................19

Figure 8. Particle measurements for Round 2, Room 101A...................................................................20

Figure 9. Particle measurements for Round 2, Room 102 .....................................................................21

Figure 10. Particle measurements for Round 3, Room 101A...................................................................22

Figure 11. Particle measurements for Round 3, Room 102 .....................................................................23

Figure 12. CONTAM representation of 1st Floor and Plenum .................................................................25

Figure 13. CONTAM representation of 2nd Floor and Plenum..................................................................25

Figure 14. Simulation vs. measurement, Round 1, with detailed resuspension schedule ......................30

Figure 15. Simulation vs. measurement, Round 1, resuspension 10 x rbase, loading 10 x Lbase................30

Figure 16. Simulation vs. measurement, Round 2, resuspension 10 x rbase, loading 10 x Lbase................31

Figure 17. Simulation vs. Measurement, Round 3, resuspension 10 x rbase, loading 10 x Lbase ...............31

Figure 18. Ordered Data Plot ...................................................................................................................35

Figure 19. Main Effects Plot .....................................................................................................................35

Figure 20. Absolute Effects Plot ...............................................................................................................36

Figure 21. Interactive Effects Matrix (color-coded to match the Absolute Effects Plot)..........................37

Figure 22. Exposure tool sample screen ..................................................................................................40

List of Tables

Table 1. Resuspension rate data from literature review – Chamber measurements (part 1)..............10

Table 2. Resuspension rate data from literature review – Chamber measurements (part 2)..............11

Table 3. Resuspension rate data from literature review – Field measurements..................................12

Table 4. Release and resuspension information for three tests...........................................................15

Table 5. Building level properties..........................................................................................................26

Table 6. Release and initial loading assumptions used in simulations .................................................28

Table 7. Summary of average measured and simulated resuspended airborne concentrations.........32

Table 8. Range of input values for sensitivity analysis..........................................................................34

Table 9. Key inputs for resuspension exposure calculations................................................................39
viii

Acronyms and Abbreviations

BG Bacillus globigii

BOTE Biological Operation Testing and Evaluation

CBR chemical, biological and radiological

CFU colony-forming unit

EPA U.S. Environmental Protection Agency

FPSL fluorescent polystyrene latex

IBAC (trademark name, not an acronym)

INL Idaho National Laboratory

NHSRC National Homeland Security Research Center

NIST National Institute of Standards and Technology

NOAA National Oceanic and Atmospheric Administration

PBF Power Burst Facility

PSU Pennsylvania State University

UVAPS Ultraviolet Aerodynamic Particle Sizer
1

Executive Summary

The U.S. Environmental Protection Agency (EPA) is charged with planning for and responding to 

intentional and unintentional releases of airborne chemical, biological and radiological (CBR) 

agents. The EPA conducts research to support site-specific contamination characterization and 

remediation decisions during such incidents. As part of these research activities, the EPA’s 

National Homeland Security Research Center, in collaboration with several other federal 

agencies, conducted the Biological Operation Testing and Evaluation (BOTE) study at the Idaho 

National Laboratory to evaluate several Bacillus anthracis decontamination technologies and to 

better understand the potential exposure to spores before and after decontamination[1, 2]. 

The data collected in this study provide a unique resource to support the development of 

microbial exposure assessment methodologies, including the application of particulate 

transport modeling.

Given the great variability among built environments, modes of agent release, and subsequent 

occupant activities that may impact exposure, modeling offers an important tool to support 

exposure assessments. Particle transport modeling capabilities that consider specific particle 

characteristics, along with the impacts of building construction and building systems, have been 

incorporated into multizone building airflow and contaminant transport simulation software 

tools. Such simulation tools provide a means to quantify the distribution and transport of 

contaminants within buildings as well as the potential exposure associated with the resultant 

spatial and temporal variations in indoor contaminant concentrations. Also, simulation 

parameters can be adjusted to study the impact of variations in building, agent and release

parameters on potential exposure more easily than can be done in experimental studies, which 

require significant time and resources.

To demonstrate and assess the applicability of said simulation tools to calculate potential 

exposure to resuspended particles during decontamination-related sampling activities, a 

modeling study based upon the BOTE experiments was performed. This modeling study 

involved the following tasks:

 Perform a literature review of resuspension rate measurement studies to identify 

existing resuspension models and data

 Develop a whole-building representation of the BOTE resuspension experiments within 

a multizone modelling tool

 Compare resuspension of particles determined from the BOTE experimental 

measurements with those based on simulation results using a multizone model

 Perform a sensitivity analysis of inputs for a simplified, single-zone exposure model

 Propose a software tool to estimate exposure due to resuspension for generalized 

application to exposure events

The review of existing resuspension studies identified resuspension models that could be used 

within the existing multizone modeling software, CONTAM, for the purpose of estimating 

exposure due to resuspension. A wide range of resuspension rates associated with various 

types and levels of activity was identified spanning almost ten orders of magnitude. However, 

the methods of measuring and reporting resuspension rates were inconsistent and the rates 
2

varied widely, revealing the need for more rigorous and consistent means of determining and 

presenting resuspension rates for use in multizone modeling software. 

The whole-building particle resuspension simulations required many assumptions related to 

particle transport and building conditions. Among the most significant assumptions were initial 

particle loadings, resuspension rates and resuspension activity levels. Considering the fairly 

uncertain nature of these assumptions, the order of magnitude agreement between measured 

and simulated results that was obtained is encouraging.

To address the wide range of uncertainty in the model inputs, a sensitivity analysis was 

performed. This analysis, aimed at identifying the most significant effects of several inputs to a 

single-zone resuspension exposure model, revealed that initial surface loading, resuspension 

rate and their interactions were most significant; deposition rate, outdoor air change rate and 

their associated interactions were somewhat less significant but still significant; and deposition 

surface area and its interactions were relatively insignificant. These results were utilized in 

considering input parameters to a proposed software tool to estimate exposure due to 

resuspension.

An outline was presented of such a proposed tool that could be used to provide a quick and 

simple means to estimate the potential for exposure due to resuspension by those who 

respond to and then decontaminate facilities that experience a CBR event. Such a tool could be 

developed for laptop platforms and/or adapted for use on hand-held devices.
3

1 Introduction

The U.S. Environmental Protection Agency (EPA) is charged with planning for and responding to 

intentional and unintentional releases of airborne chemical, biological and radiological (CBR) 

agents [3-6]. The EPA conducts research to support site-specific contamination characterization 

and remediation decisions during these potential incidents. To facilitate greater risk-based 

remediation decision making, it is important to better understand agent transport into and 

within buildings, potential occupant exposure, and the effectiveness of various 

decontamination strategies [7, 8]. A key challenge associated with assessing the exposure due 

to potential releases of CBR agents is evaluating the risks of low level contamination following 

biothreat agent releases and subsequent decontamination efforts. This issue is of particular 

interest for Bacillus anthracis, the causative agent of anthrax [9, 10]. Given the great variability 

among built environments, potential modes of agent release, and subsequent occupant 

activities that may lead to exposure, modeling offers an important tool to support exposure 

assessments. Particulate transport modeling capabilities that consider specific particulate 

characteristics, building construction and building system (e.g., ventilation and filtration) 

features have been incorporated into multizone building airflow and contaminant transport 

simulation software tools [11, 12]. Such simulation tools provide a means to quantify the 

distribution and transport of contaminants within buildings as well as the potential exposure 

associated with resultant spatial and temporal variations of indoor contaminant concentrations. 

Also, simulation parameters can more easily be adjusted to study the impact of variations in 

building, agent and release parameters on potential exposure than experimental studies, which 

require significant time and resources.

The EPA’s National Homeland Security Research Center (NHSRC), in collaboration with several 

other federal agencies, conducted the Biological Operation Testing and Evaluation (BOTE) study

[1] at the Idaho National Laboratory (INL) to evaluate several Bacillus anthracis 

decontamination technologies and to better understand the potential exposure to spores 

before and after decontamination. The data collected in this study provide a unique resource to 

support the development of microbial exposure assessment methodologies, including the 

application of particulate transport modeling. 

Models have been developed to predict resuspension rates as a function of particle size, 

surface characteristics, human disturbances and other factors, but their applicability and 

accuracy for use in exposure assessment have not been fully determined. Whole-building 

airflow and contaminant transport simulation software was used to determine the applicability 

of resuspension models to estimate potential human exposures to biological contaminants. The 

applicability of resuspension models was tested using the field sampling data collected from the 

BOTE study and resuspension rates determined from existing resuspension models from the 

literature.
4

2 Literature Review

A literature review was conducted to identify models that predict resuspension rates for use in 

the analysis performed as part of this study. The literature review was focused on rate-based 

resuspension models.

2.1 Background

In describing the work performed under this project, it is important to distinguish between 

resuspension models, resuspension rates determined from these models and building 

simulation tools in which resuspension rates are applied. The following describes the 

terminology and mathematical context within which the resuspension rates identified in the 

literature review were considered. A description of the literature review that was conducted as 

part of this effort is then provided, followed by a summary of resuspension rate data found 

during the review.

Zhang [13] defines particle resuspension as “a process in which particles detach from the 

surface and become airborne again.” For the purposes of this study, particle resuspension 

models were characterized into two categories: physical and rate-based. Resuspension 

simulation tools are the software within which such a resuspension model is implemented. 

Physical resuspension models characterize particle resuspension based on the interaction 

between specific forces that attach a particle to and detach a particle from a surface, e.g., 

gravitational, mechanical and electrostatic. Rate-based models provide resuspension rates 

based on general parameters including particle size and type, surface characteristics such as 

carpet or tile, and disturbance type or activity such as walking or vacuuming. Rate-based 

models are more relevant to this effort than the fundamental physical models as the latter are 

more difficult to apply, particularly on a whole-building scale, without very detailed inputs.

The whole-building multizone airflow and contaminant transport simulation software CONTAM 

[11] is a building simulation tool in which resuspension modeling has been implemented.

1

CONTAM is arguably the most widely used software for simulating contaminant transport on a 

whole-building scale and is used in the current study. To provide context for the use of 

resuspension rates within CONTAM, the mass balance model employed by CONTAM is 

presented. This explanation is also helpful in identifying those resuspension studies presented 

in the literature review that are most relevant to this project. Note that the results of these

studies are presented in several different forms that are not always amenable to application 

within the mass balance model employed by CONTAM and similar simulation tools.

CONTAM employs the following mass balance equations to represent a two-compartment 

model of resuspension in a well-mixed zone or room. The two compartments refer to the air 

and surfaces for a given zone. These equations account for particle generation within the zone 

and entry of particles from outside the zone and assume that particles are removed only by 

deposition and airflow out of the zone.

 

1 While other multizone airflow and contaminant transport simulation tools do exist, COMIS is the only tool known 

to the authors that is similar to CONTAM in its capabilities. However, COMIS does not directly support 

resuspension and is no longer under development within the U.S.
5

For the air:

V dCz⁄dt = PQCo

(t) + rArLs(t) + G − QCz(t) − kdVCz(t) (1)

For the surface:

As dLs⁄dt = kdVCz

(t) − rArLs(t) (2)

where:

V = zone volume [m3]

Ar = resuspension surface area [m2]

As = deposition surface area [m2]

Cz = zone particle concentration in air [kg/m3]

Co = particle concentration of air flowing into the zone [kg/m3]

Ls = particle surface loading [kg/m2]

Q = volumetric airflow rate [m3/s]

r = particle resuspension rate [1/s]

kd = particle deposition rate [1/s] (kd = d As /V )

d = particle deposition velocity [m/s]

G = particle generation rate [kg/s]

P = particle penetration factor [-]

t = time [s]

As outlined below in discussing the literature review, measurements have been performed to 

estimate resuspension rates using various methods, with results being reported in various 

forms including resuspension rate, resuspension factor, resuspension fraction and emission 

factor. The following presents some background information on how these various forms of 

resuspension rates relate to one another. 

To relate these forms of resuspension measurements, steady state conditions must apply. 

Assuming steady state conditions in equation (1), the following equation provides the steady 

state concentration of particles in the air.

Css =

G + PQCo + rArLs

Q + kdV

(3)

Resuspension measurement conditions and results are sometimes reported in ways that are 

not conducive for them to be applied readily in the above equations. In some cases, the 

deposition surface area is associated with the resuspension rate, as opposed to the smaller area 

involved in resuspension. In other cases, a resuspension factor, K, is reported as the ratio of the 

air concentration to the surface concentration in units of inverse length, e.g., m-1. However, it is 

not always clear whether or not the reported values were measured under steady conditions. 

The following equation provides the relationship between K and r at steady state.

K =

Css

Ls

=

G + PQCo + rArLs

Ls

(Q + kdV)

(4)
6

In other cases, results are reported as a dimensionless emission factor, E, which is the ratio of 

particles suspended in the air to particles available for resuspension on the surface. The 

emission factor can be related to the resuspension factor according to the following equation.

E = K

V

As

(5)

These resuspension and emission factors are sometimes presented in terms of count 

concentrations or mass concentrations for various particle size ranges. To be useful in applying 

equations (1) and (2), sufficient information on measurement conditions and parameters must 

be provided to allow resuspension factors to be converted to a resuspension rate as presented 

in equation (6).

r =

KLs

(Q + kdV) − G − PQCo

ArLs

(6)

For the purposes of this modeling study, resuspension rates, as addressed later in Section 2.3, 

were employed because they can readily be applied to predict airborne particle concentrations 

for conditions other than those under which the resuspension rate measurements were 

performed. Specifically, these values can be used directly within models using equations (1) and 

(2) to estimate potential exposure due to resuspension. 

The resuspension fraction, F, is the particle resuspension intensity based on the actual area 

disturbed by the activity instead of the entire deposition surface area loaded with particles. For 

walking, F is the resuspension rate times the ratio of the floor area (As) to the foot contact area 

(Ar) divided by the contact frequency,  [1/time] as presented in the following equation.

F =

rAs

ωAr

(7)

2.2 Literature Review Categories

The documents reviewed are grouped into the following categories:

 Relevant resuspension models

 Chamber measurements of resuspension rates

 Other chamber measurements

 Field measurements of resuspension rates or particle concentrations

 Other model development and application

 Reviews.

This Section describes the types of studies in each category. Appendix A contains the reference 

and abstract of each publication examined.

2.2.1 Relevant resuspension models

Among all the literature identified in the review, only two publications described models to 

predict resuspension rates based on user inputs. These models were developed by the 

Pennsylvania State University (PSU) for the U.S. Army Center for Health Promotion and 

Preventive Medicine. The first, described by Bahnfleth et al. [14], is a Microsoft Excel 
7

spreadsheet that yields resuspension rates based on user input of a number of variables: 

particle size, particle type (from a limited set of choices), and flooring (carpet or linoleum). The 

second model, Freihaut et al. [15], presents an approach using “look-up” tables, which yield 

resuspension rates adjusted for factors such as particle type, flooring, and relative humidity. 

Note that the resuspension rates obtained from these two approaches do not agree with each 

other. This discrepancy, which is more than an order of magnitude in most cases, was

investigated and the spreadsheet results are considered to be more reliable based on their 

being better referenced and explained in the source documents.

2.2.2 Chamber measurements of resuspension rates 

The papers in this category include chamber studies in which resuspension rates were

measured in laboratory test chambers, as well as a small number of chamber studies that do 

not provide resuspension rates but which are nonetheless of interest. The chamber 

measurements of resuspension rates involved either actual or simulated human walking to 

induce resuspension for different types and sizes of particles, flooring types and environmental 

conditions, e.g., relative humidity. The results of these measurements are summarized 

graphically and in tabular form in Section 2.3, below.

2.2.3 Other chamber measurements 

Several other papers describe chamber studies that did not measure resuspension rates but 

measured other quantities of interest. These studies included detailed measurements of 

adhesion forces holding particles to a surface. Other studies described approaches to 

measuring resuspension rates but did not present any measurement results. One study was 

focused on outdoor resuspension as opposed to the indoor environments of interest to this 

effort. There were a number of particle resuspension studies conducted in nuclear facilities, 

starting in the 1960s time frame, based on concerns about radioactive dust [15]. These studies 

are not covered in this report based on the unique aerosols and space types involved. Similarly, 

there has been and continues to be research on aerosol resuspension outdoors induced by 

wind and considering various environmental and soil properties [16, 17]. This work is also not 

considered relevant to the present study. 

2.2.4 Field measurements of resuspension rates or particle concentrations

There have been a number of studies in which resuspension rates were measured in actual 

buildings, in many cases residences. Most of these measurements involved walking, but other 

occupant activities were studied as well. In other field studies, only airborne particle 

concentrations were measured as opposed to the resuspension rates.

2.2.5 Other model development and application 

The papers in this category include model development and application efforts of interest but 

were not directly applicable to the current effort in that they do not provide methods for 

estimating resuspension rates. For example, simulation studies of resuspension impacts on 

particle concentrations in buildings have been conducted using building airflow and 

contaminant transport models. Other studies have modeled the detailed processes impacting 

particles on surfaces.
8

2.2.6 Reviews

This last category includes articles that have reviewed the literature on resuspension. Two of 

these were focused on outdoor resuspension rather than indoor. The reviews of indoor 

resuspension studies were useful in identifying relevant references and for verifying the 

information obtained in this literature review. 

2.3 Summary of Existing Resuspension Rate Data 

As noted above, a number of studies report measured resuspension data from chamber and 

field experiments. Table 1 and Table 2 provide a summary of chamber measurements and Table 

3 provides a summary of field measurements. All three tables summarize those reported values 

including information on the test space, particle type, environmental conditions, floor type, and 

particle size. Figure 1 provides a plot of resuspension rates based on Table 1 through Table 3.

Where possible, resuspension data were converted from reported units to resuspension rates

in units of h-1. However, this was not always possible based on reported measurement 

conditions and results. Each point in Figure 1 represents the individual results of each study, so 

the x-axis is not quantitative but serves to report the individual test results. The Reference 

column of Table 1 through Table 3 provides the citation from which the data were obtained. 

These citations are all contained in Appendix A. The particle sizes in Table 1 through Table 3 are 

those reported in each of the cited studies, typically the aerodynamic diameter.

As seen in Figure 1 and Table 1 through Table 3, resuspension values in the literature vary 

widely, covering a range of almost ten orders of magnitude, with strong dependencies on 

particle size, particle type, floor type and activity. While these values do not necessarily support 

predictions for a given set of field conditions, they do provide an indication of the range of 

resuspension rates that might exist in the field.
9

Figure 1. Summary plot of resuspension rates
10

Table 1. Resuspension rate data from literature review – Chamber measurements (part 1)

Reference Test Space and

Particle Type Floor Type Particle size

(m) Activity

Reported Results 

(entries that don't use default units are noted)

Factor (m-1) Rate (h-1) (mg min-1 person-1) Fraction

CHAMBER MEASUREMENTS

Gomes et al. 16 L chamber Olefin carpet 2.1 to 16 Simulated walking with sample 

vibration and air puff 1.00E-04 6.00E-02

(2005 and 2007)

[18, 19]

Quartz and Roach dust Linoleum 1.00E-08 6.00E-06

Gomes et al. All values are resuspension rates (h-1)

(2005) [18] Quartz Spore Dust mite

as reported in Linoleum 0.3 to 0.39 Walking @ 114 steps/min 2.74E-04 1.64E-03 2.12E-04

Bahnfleth et al. 0.4 to 0.49 1.90E-04 1.21E-04 3.59E-05

(2007) [14] 0.5 to 0.64 1.94E-04 2.23E-04 3.10E-05

0.65 to 0.79 1.77E-04 2.13E-04 4.07E-05

0.8 to 0.99 1.83E-04 2.75E-04 2.15E-05

1.0 to 1.59 7.62E-05 3.66E-04 5.83E-05

1.6 to 2.0 1.26E-04 3.08E-04 2.11E-05

>2.0 2.83E-05 2.47E-04 8.40E-05

Carpet 0.3 to 0.39 5.57E-04 5.00E-02 8.28E-05

0.4 to 0.49 4.44E-04 3.34E-03 8.88E-05

0.5 to 0.64 6.06E-04 5.75E-03 2.74E-04

0.65 to 0.79 2.99E-04 7.08E-03 1.90E-04

0.8 to 0.99 3.68E-04 7.80E-03 1.45E-04

1.0 to 1.59 5.02E-04 1.06E-02 2.44E-04

1.6 to 2.0 2.80E-04 8.40E-03 2.36E-04

>2.0 7.80E-05 7.62E-03 1.67E-04

Hu 16 L chamber Vinyl tile Alumina: Air puffs to simulate human step All values are resuspension rates [h-1]

(2008b) [20] Alumina Loop carpet 0.35 to 10 Vinyl tile Alumina Silica Spore Dust mite

Silica sand Spore: w/ high electrostatic field 2.98E-05 4.28E-01 1.47E+00 7.88E-01

Spore 0.58 to 16.57 w/ medium electrostatic field 1.82E-05 3.00E-01 1.32E+00 4.60E-01

Dust mite control (no field applied) 1.62E-05 2.98E-01 1.00E+00 5.21E-01

Carpet w/ high electrostatic field 5.15E-05 2.66E-01 6.24E-01 2.59E-01

 w/ medium electrostatic field 1.19E-04 1.96E-01 1.25E+00 3.67E-01

 control (no field applied) 7.71E-05 3.43E-01 8.10E-01 5.50E-01

Qian and Ferro 54.4 m3 chamber Vinyl tile 0.8 to 1.0 30 min cycle of walking and tapping 

feet 1.40E-03

(2008a) [21] Test dust New loop carpet 1.0 to 2.0 1.60E-03

Old loop carpet 2.0 to 5.0 5.20E-03

5.0 to 10 1.30E-02

Thornburg et al. 4.75 m3 chamber Carpet 1.0 Walking (min) 3.47E-04 3.93E-02

(2009) [22] Asbestos fiber (max) 3.15E-03 3.56E-01

simulant Vacuuming (min) 5.22E-03 4.95E+00

 (max) 7.19E-02 6.81E+01
11

Table 2. Resuspension rate data from literature review – Chamber measurements (part 2)

Reference Test Space and

Particle Type Floor Type Particle size

(m) Activity

Reported Results 

(entries that don't use default units are noted)

Factor (m-1) Rate (h-1) (mg min-1 person-1) Fraction

CHAMBER MEASUREMENTS (continued)

Rosati et al. Test chamber New loop carpet 1.0 Walking for 5 minutes 2.03E-07 3.91E-06

(2008) [23] Arizona test dust Old carpet (range of all values measured) 2.62E-05 2.50E-03

Shaughnessy and Vu Test chamber 25 m3 Carpet (FT) 0.8 to 1.5 Walking for 10 minutes All values are resuspension rates [h-1]

(2012) [24] Textile (VCTT) 105 steps/min FT VCTT VCT

Vinyl tile (VCT) loading 18 g/m2(2.65 ± 0.78)E-03 (7.47 ± 0.30)E-04 (9.54 ± 1.95)E-03

loading 100 g/m2(3.32 ± 1.10)E-03 (6.36 ± 1.95)E-04

loading 150 g/m2(5.25 ± 0.43)E-03 (1.49 ± 5.26)E-03

1.5 to 3.0 loading 100 g/m2(2.08 ± 0.75)E-02 (4.43 ± 0.82)E-03

loading 150 g/m2(4.18 ± 0.22)E-02 (1.52 ± 0.71)E-02

Manthena and Ferro Test chamber Vinyl tile (T) Walking for 10 minutes All values are resuspension rates [h-1]

(2009) [25] Arizona test dust (ATD) New carpet (C) ATD-T RD-T ATD-C RD-C

Real house dust (RD) 0.4 to 0.8 0.0001 to 0.0002 0.0002 to 0.0006 0.0003 to 0.0006 0.0002 to 0.0004

0.8 to 1.0 0.0002 to 0.0004 0.0004 to 0.002 0.0003 to 0.002 0.0006 to 0.002

1.0 to 2.0 0.0003 to 0.0005 0.0008 to 0.004 0.0006 to 0.002 0.001 to 0.003

2.0 to 5.0 0.001 to 0.003 0.004 to 0.03 0.003 to 0.01 0.007 to 0.03

5.0 to 10 0.009 to 0.01 0.01 to 0.2 0.008 to 0.03 0.01 to 0.08

Tian et al. Test chamber 0.12 m3 Hard (2 types) 0.4 to 10 Simulated walking for 55 sec Reported range of values 

(2014) [26] Real house dust Carpet (3 types) 9.24 E-05 1.0E-07

9.24 E-02 1.0E-04
12

Table 3. Resuspension rate data from literature review – Field measurements

Reference Test Space and

Particle Type Floor Type Particle size

(m) Activity

Reported Results 

(entries that don't use default units are noted)

Factor (m-1) Rate (h-1) (mg min-1 person-1) Fraction

FIELD MEASUREMENTS

Ferro et al. Single family Wood PM2.5 2 persons walking, 1st floor 0.22

(2004) [27] residence Rug 1 person walking, 1st floor 0.13

Dust 1 person walking, basement 0.06

PM5 2 persons walking, 1st floor 0.72

1 person walking, 1st floor 0.50

1 person walking, basement 0.28

Hambreaus operating suite Vinyl Not reported Moist floor mopping 2.00E-04 1.22E-03

(1978) [28] bacteria 4 people walking for 30 min 3.50E-03 2.13E-02

Karlsson et al. Experimental room Not reported 6.3 1 person, table work activity 4.68E+00

(1996) [29] Grass pollen

Karlsson et al. Experimental room PVC floor 12

Walking and paperwork; not clear to 

which the resuspension rate 

corresponds

6.25E-04 5.00E-04

(1999) [29] Freeze dried spores 

(Bacillus subtilis)

(clusters of 

1.8 x 0.9 

spores)

Qian et al. Dust Loop carpet PM10 Walking for 30 min 1.40E-06

(2008b) [30]

Thatcher and Residential building Carpet 0-0.5 4 residents, normal activity 9.90E-07

Layton Inert dust Wood 0.5-1.0 4.40E-07

(1995) [31] Vinyl 1-5 1.80E-05

5-10 8.30E-05

10-25 3.80E-04

>25 3.40E-05

Buttner et al. 35.2 m3 chamber Vinyl 1.8 - 3.5 Walking for 1 minute 4.30E-05

(2002) [32] Spore Loop carpet 4.30E-05

Pile carpet 3.90E-03

Shaughnessy and Vu School floors w/ Carpet (FT) Walking for 10 minutes All values are resuspension rates [h-1]

(2012) [24] mobile chamber Textile (VCTT) 105 steps/min FT VCTT

8 m3 Vinyl tile (VCT) 0.8 to 1.5 Loading ≈ 60 g/m2(1.79 ± 0.82)E-02 (4.15 ± 2.40)E-03

Loading ≈ 160 g/m2(7.25 ± 1.98)E-03 (2.76 ± 0.69)E-03

1.5 to 3.0 Loading ≈ 60 g/m2(5.52 ± 1.98)E-02 (1.73 ± 1.59)E-02

Loading ≈ 160 g/m2(2.48 ± 1.00)E-02 (8.65 ± 2.26)E-03
13

2.4 Conclusions from the Literature Review

Based on the literature review, the spreadsheet model presented in Bahnfleth et al. [14], 

referred to herein as the PSU model, is the only viable option identified for calculating 

resuspension rates from walking based on independent parameters of particle size, floor 

loading, floor type, number of occupants and walking speed. The PSU model was therefore 

used to estimate resuspension rates for the subsequent modeling using the BOTE INL data. In 

addition, given the uncertainty in resuspension rate prediction, the measured rates from the 

literature and other relevant parameters were also considered in conducting the sensitivity 

analysis presented below. 

3 Description of BOTE Measurement Data Used for Modeling Study

Three rounds of biological release and decontamination experiments took place in April and 

May of 2011 in a building located at INL. Complete descriptions and BOTE study results can be 

found in the BOTE report [1]. This Section contains a description of that building and the 

experimental measurements that were performed. Measurement results are presented against 

which simulation results will be compared herein for the purposes of evaluating the 

applicability and accuracy for determining potential inhalation exposure due to particle 

resuspension. 

3.1 Building Description

The building used in the BOTE study and for the modeling analysis described in this report is the

Power Burst Facility 632 (PBF-632) located at INL and shown in Figure 2. This building is the 

same building that was used in previous exercises aimed at the evaluation of sample planning 

methods and multizone modeling validation of particle release experiments [1, 33-35]. Floor 

plans for the two floors of PBF-632 are shown in Figure 3 and Figure 4. Each floor is 

approximately 24.4 m x 15.2 m for a total of 372 m2 per floor. Most of the floor is covered with 

laminate floor tile with several rooms on each floor covered with carpet including rooms 101A 

and 102 shown in Figure 5. Each floor contains a constant volume air handler located within a 

mechanical room on the floor it serves with no provision for bringing in outdoor air. Supply air 

ducts are located above suspended ceilings on the floor that they serve. The building was 

modified for decontamination studies by installing dedicated return ducts and decontamination 

distribution ducts below the suspended ceiling, i.e., within the occupiable space on each floor. 

A large tent (shown in Figure 2) was erected around the entire building for the decontamination 

studies.

Figure 2. INL building PBF-632 and decontamination containment tent
14

Figure 3. 2

nd floor plan of PBF-632

Figure 4. 1

st floor plan of PBF-632

3.2 BOTE Experimental Measurements

The three particle resuspension experiments consisted of the dissemination of Bacillus

atrophaeus subsp. globigii (BG), which is used as a surrogate for Bacillus anthracis. During each 

of these experiments, 200 mg were released on the 1st floor and 0.5 mg were released on the 

second floor through the use of automated nebulizers placed on the downstream side of the 

filter banks of the recirculating air handling system. Releases took place in the early afternoon. 

The fans of the air handlers were left on for approximately two hours after each release and 

then turned off for the remainder of each experiment. Particles were allowed to settle 

overnight, and sampling took place during the following days as detailed in Table 4.

1

2

3

4

5

6

7

8

9

10

11

12

13 13 R @ 9 15/64" = 10'- 0" 12 T @ 12" = 12'-0"

Janitor

Men

Office Office Office Office Women

Office Office Office Office Office

Office

Office

Office

Office

Storage

Office Office

Mechanical Room

1

2

3

4

5

6

7

8

9

10

11

12

13 13 R @ 9 15/64" = 10'-0 " 12 T @ 12" = 12'-0"

Lobby

Office Office Office

Office Office Office

Office

Office

Office

Office

Office Mechanical Room

Women

Men
15

Table 4. Release and resuspension information for three tests

Test

BG Release (2 mg)

Resuspension Period

Building Entry Room 101A Room 102

Date Fan on Fan off Date Time Enter Exit Enter Exit

Round 1 2011-04-16 13:18 15:18 2011-04-17 09:18 12:06 n/a n/a 16:47

Round 2 2011-04-25 14:16 16:16 2011-04-26 08:09 09:28 11:31 11:59 13:38

Round 3 2011-05-10 15:22 17:22 2011-05-11 08:42 11:51 14:34 16:02 17:54

In addition to the three BG releases, a set of so-called building characterization tests was 

performed prior to the BG decontamination experiments. These characterization tests were 

performed by releasing 1 m mono-dispersed fluorescent polystyrene latex (FPSL) particles in an 

attempt to establish target release levels for the BG experiments. While this simulation study 

does not address these FPSL particle releases, these particles did contribute to initial loadings of 

non-viable particles during the resuspension tests.

3.2.1 Particle Measurements

The BOTE study utilized two types of real-time aerosol monitors referred to as Ultraviolet 

Aerodynamic Particle Sizer (UVAPS) [36] and IBAC [37]. The UVAPS provides counts in 52 

individual bins of particle diameter ranging from 0.5 m to 20 m at approximately one-minute 

intervals. The manufacturer of the UVAPS provides software to read the raw data files and 

export them to text files in units of number of particles per cubic centimeter, which were then 

imported into a spreadsheet for further analysis. The IBAC counter provides total counts of 

particles ranging from 0.7 m to 10 m in diameter at one-second intervals, and data are 

written to files in units of number of particles per liter. Note that the IBAC did not provide 

counts in individual particle size bins as did the UVAPS. Detailed descriptions of this equipment 

can be found in the BOTE report [1].

Both the UVAPS and the IBAC are capable of distinguishing viable and non-viable particles 

based on fluorescence [38]. However, for the data collected during these experiments, the 

particles were not distinguished by fluorescence. Therefore, the BG data from these

experiments do not distinguish the particles as viable or non-viable. UVAPS aerosol monitors 

measured real-time particle aerosol levels in two rooms, 101A and 102, both located on the 

first floor of the building as shown in Figure 5. Of these two rooms, only Room 101A also 

contained a single IBAC sampler. The corresponding figures reveal that the total UVAPS count 

matches that of the IBAC count very well, indicating that particles sizes were well within the 

range of the IBAC counter. There was no real-time particle monitoring of the outdoor air.
16

Figure 5. 1st floor plan of PBF-632 building shows locations of rooms 101A and 102, the 

UVAPS and IBAC devices, and the spore release

Plots of particle measurements reveal the time history over the range of particle sizes 

measured by the UVAPS and IBAC instruments during the simulant release phase through the 

resuspension phase. Figure 6 through Figure 11 show these plots for the three rounds of BG 

release in rooms 101A and 102. Each plot shows the release event occurring in the early 

afternoon, followed by the resuspension activity beginning the next morning between 6 a.m. 

and 12 noon. The time the building was first entered and the sampling period are provided in 

the title of each plot. To reduce clutter, only bins in the range from 0.5 m to 5 m are plotted 

for the UVAPS measurements (Also to reduce clutter, not all of the bins in this range are shown, 

for example particles larger than 5 m, which were not elevated above background levels). The 

UVAPS curves include all one-minute data as obtained from the aforementioned data files. 

However, IBAC curves were obtained using one-minute data gleaned from the one-second raw 

data files. Both UVAPS and IBAC one-minute data were smoothed using ten minute time 

averages to improve the clarity of the plots.

The plots reveal the significant increase in particle concentrations associated with the initial 

releases soon after the indicated times of release. Particles in the size ranges plotted (from 

0.5 m to 5 m diameter) were all elevated by at least an order of magnitude above their 

respective background levels. This release period is followed by decay of concentrations until 

the times at which the buildings were entered for the performance of the sampling 

experiments. (The perfectly linear portions in some UVAPS plots indicate data missing from raw 

data files.) The plots reveal increased levels in both rooms 101A and 102 very close to the 

reported building entry times even though building entry occurs typically an hour or more prior 

1

2

3

4

5

6

7

8

9

10

11

12

13 13 R @ 9 15/64" = 10'-0 " 12 T @ 12" = 12'-0"

Lobby

Office Office Office

Office Office Office

Office

Office

Office

Office

Office Mechanical Room

Women

Men

101A

101

109 107 105 103

110 108 106

104

102

Lobby

Women

Men

Mechanical

Room

UVAPS IBAC

UVAPS

IBAC IBAC IBAC

IBAC

IBAC IBAC IBAC IBAC

IBAC

IBAC

Spore release location
17

to entry into the two rooms. These elevations in concentrations were assumed to be the result 

of resuspension due to human activity elsewhere in the building and not in the specific rooms.

These elevations are an indication that inter-zone particle transport was occurring within the 

building as a result of airflow and diffusion transport processes. More discussion on these 

measurement results is provided later when comparisons are made with simulation results. 
18

Figure 6. Particle measurements for Round 1, Room 101A
19

Figure 7. Particle measurements for Round 1, Room 102
20

Figure 8. Particle measurements for Round 2, Room 101A
21

Figure 9. Particle measurements for Round 2, Room 102
22

Figure 10. Particle measurements for Round 3, Room 101A
23

Figure 11. Particle measurements for Round 3, Room 102
24

3.2.2 Video of Sampling Activities

Video of the BOTE testing was analyzed in an attempt to obtain detailed information on 

occupant activity to support the development of resuspension schedules for use in the 

CONTAM simulations. Seventeen video cameras were located throughout both floors of the 

building and at the building entrance through which the sampling personnel entered. Each 

camera continuously captured video during the three rounds of sampling. The three sampling 

periods covered between 5 hours and 9 hours each, for a total of approximately 23 hours of 

experimental time yielding approximately 380 hours of video.

The video was reviewed for the first round of sampling to determine the types of resuspensionrelated activities that were occurring during the post-contamination sampling periods. Video 

revealed multiple teams of sample collectors moving throughout the building simultaneously;

utilizing carts to transport sampling equipment; performing multiple sampling-related activities

including: vacuuming, climbing ladders and removing ceiling tiles, and shuffling papers. During 

the sampling period there were anywhere from one to five 3-person teams within the building. 

Discerning the exact time and location of all members was ultimately determined not to be 

considered a critical issue due to the uncertainty in attributing resuspension rates to the range 

of occupant activities occurring within the building and the wide range of particle sizes that 

were resuspended into the air as a result of these activities.
25

4 Whole-Building Resuspension Simulations Results

Simulations were performed to compare the airborne particle concentrations provided by field 

sampling data collected during the BOTE study to simulated airborne concentrations calculated 

based on resuspension rates evaluated from the literature. The objective of these simulations 

was to demonstrate that multizone building airflow and contaminant modeling can be used to 

predict resuspension of deposited/settled particles given relevant input data (building and 

system characteristics, particle size, deposition rates, initial loading, information on activities 

inducing resuspension, and resuspension rates) in support of estimations of potential human 

exposures to residual biocontamination.

4.1 CONTAM Representation of PBF-632

The CONTAM representation of PBF-632 was based largely on a previous version developed for 

other studies carried out in the same building [33-35]. Each level of the building was 

represented in CONTAM by a schematic of the floor plan via the CONTAM sketchpad. The 

representation used in this study consisted of four levels – the 1st and 2nd floors and their 

respective plenums that contain the air distribution ductwork. CONTAM sketchpads are shown 

for each of the four levels in Figure 12 and Figure 13. The nominal floor area of the building is 

372 m2 with a nominal volume of approximately 2610 m3. Individual level properties are 

provided in Table 5.

 

Figure 12. CONTAM representation of 1st Floor and Plenum

 

Figure 13. CONTAM representation of 2nd Floor and Plenum
26

Table 5. Building level properties

Level Nominal Height

[m]

Nominal Volume

[m3]

1

st floor 2.44 906

1

st floor plenum 0.61 227

2

nd floor 2.44 906

2

nd floor plenum/attic 1.22 570

The building representation was modified for the purposes of this project by utilizing the 

weather data for the BOTE test period, placing resuspension source/sinks in every zone 

depending on the type of flooring located in the zone, setting initial zone concentrations and 

surface loadings, and scheduling system airflows and resuspension activity. These modifications 

are described below.

4.1.1 Weather Data

NIST obtained weather data from the “PBF” weather station of the National Oceanic and 

Atmospheric Administration (NOAA) INL Weather Center website 

(http://www.noaa.inel.gov/metgraph). Weather data included barometric pressure, outdoor 

temperatures, wind speed and wind direction at five-minute intervals. These data were 

reformatted into the format required by CONTAM. There were no indoor temperature 

measurements available and the building was unconditioned. Therefore, indoor and outdoor 

temperatures were set to be the same leading to mostly wind-driven air infiltration and twoway airflow through interior doorways as described in [35]. The tent around the building was

accounted for by utilizing a wind pressure modifier based on urban terrain to reduce the effect 

of wind pressure on the building [35].

4.1.2 Resuspension

Resuspension source/sinks were placed in each zone over the entire floor area of the two 

floors. Only the floor surfaces were modeled, either as carpet or laminate according to the type 

of flooring installed for the purposes of the BOTE study. It was assumed for the purposes of 

simulation that all deposition occurred onto the floors, i.e., deposition onto vertical surfaces 

was considered negligible.

Resuspension rates were selected based on the literature review presented previously. 

Resuspension was attributed solely to people walking on the two types of flooring: carpet and 

laminate. Baseline resuspension rates (rbase) for each floor type were selected from the ratebased PSU resuspension model [14] for 1 m diameter particles, a walking rate of 114 

steps/min and a resuspension surface area of 0.028 m2(the size of a single foot print). These 

baseline rates are:

Carpet = 1.06 x 10-2 h

-1

Laminate = 3.67 x 10-4 h

-1

These rates are referred to as baseline rates, because they are for a single person walking. As 

described below, they were adjusted in the simulations to reflect increased levels of activity.
27

4.1.3 Initial Loading

CONTAM is capable of simulating particle releases in units of mass or number per unit time 

over a schedule based on the contaminant source of interest. However, for the purposes of 

these simulations, the BG releases via nebulizers were not included as part of the analysis for 

two reasons. First, the rate of emission associated with the nebulizers as a function of particle 

size was not well characterized. Also, the surfaces accumulated particles throughout the entire 

set of experiments. Decontamination targeted the deactivation of viable particles but not 

removal of the BG (and other) particles. As previously mentioned, non-viable particles include 

FPSL and other unknown background particles. Therefore, simulations were performed only

during the sampling phase with initial loadings estimated for all of the resuspension 

source/sinks prior to the start of the simulation. Surface sampling was performed using various 

methods to determine the number of viable particles, or colony forming units (CFU), but the 

total loadings (viable and non-viable) of particles were not measured. The latter quantity was 

required for the simulations. Therefore the initial loadings were estimated based on the known, 

intentional releases of BG during the BOTE project. These initial loadings did not include 

background particle loadings that may have been present prior to starting the release tests.

According to the BOTE Final Test Plan [39], FPSL releases were intended to determine the 

amount of BG that would be required to establish target floor loadings between 

1.08 x 105 CFU/m2 (1 x 104 CFU/ft2) and 1.08 x 107 CFU/m2(1 x 106 CFU/ft2) on the 1st floor and 

between 1 076 CFU/m2(100 CFU/ft2) and 2 153 CFU/m2(200 CFU/ft2) on the 2nd floor. Each of 

the three rounds of BG release consisted of 200 mg on the 1st floor and 0.5 mg on the 2nd floor

(the assumed viability rate was not available). For the purposes of estimating initial floor 

loadings for simulations performed herein, all contaminant released was assumed to be

dispersed evenly throughout the entire volume of the respective building level in which it was 

released, contaminant released was all assumed to be deposited onto the floor before 

resuspension activity occurred, and contaminant released was all assumed to be viable. As a 

result, the initial particle loadings were calculated according to equation (8).

Li =

Mrel

Vpρp

1

As

(8)

where:

Li = particle surface loading [particles/m2]

Mrel = mass released [kg]

Vp = volume of a single particle [m3]

p = density of a single particle [kg/m3]

As = deposition surface area [m2]

Initial loadings were characterized by particles/area as opposed to CFU/area. The release and 

loading assumptions used in the simulations are provided in Table 6.
28

Table 6. Release and initial loading assumptions used in simulations

Floor Area 372 m2(4 000 ft2)

Particle diameter 1.0 m, corresponds to a particle volume, Vp = 5.236 x 10-19 m3

Particle density 1 000 kg/m3

BG mass release 1

st floor = 200 mg, 2nd floor = 0.5 mg

1

st floor loading

(baseline value, Lbase)

1.03 x 109 particles/m2(9.55 x 107 particles/ft2)

5.38 x 10-7kg/m2

2

nd floor loading

(baseline value, Lbase)

2.57 x 106 particles/m2(2.39 x 105 particles/ft2)

1.35 x 10-9kg/m2

Note that the loadings in the last two rows of Table 6 are approximately two orders of 

magnitude greater than the target loadings that were presented above in terms of CFU. This 

difference may be a function of assumed viability of the BG spores by experimenters when 

establishing release amounts or anticipated losses of particles through deposition to other 

surfaces, e.g., ducts and plenums, and/or removal by ventilation system filters.

4.1.4 Activity Schedule

For the purposes of comparing simulation results to measurements, a constant activity 

schedule and resuspension rate of ten times that of rbase (to roughly account for pre-existing 

particle loadings) were assumed throughout the two floors of the building and during the entire 

sampling period. The schedule activates the resuspension component of the source/sink

models to simulate resuspension at the rate established by the source/sink properties. 

CONTAM does allow for very complex scheduling of resuspension, but as discussed previously, 

complex scheduling was not implemented in the study due to the uncertainty in associating 

resuspension rates with the various types and locations of activity, surface types and sizes of 

particles being resuspended.

4.2 Simulation Results and Comparison with Measurements

Simulations of the sampling period during which resuspension would be occurring due to 

occupant activity were run for the three rounds of BG release. Inputs were as described above. 

The measurements used for comparison are based on a limited range of particle sizes as 

measured with the UVAPS, specifically the four bins of particle diameters between 0.835 m 

and 1.037 m. The comparisons between measured and simulated airborne particle 

concentrations focused on the results within rooms 101A and 102, primarily because the 

UVAPS measurements enable the consideration of a specific particle size range that showed the 

most significant increase above background levels and is close in size to the nominal 1 m 

diameter of the BG particles. This approach greatly simplified the simulation inputs to those for 

a single particle size as opposed to the wide range of particles whose properties can vary 

greatly, e.g., deposition rate and resuspension rate.

Figure 14 through Figure 17 provide plots of the comparisons between measured and predicted 

airborne particle concentrations for the three rounds of BG sampling tests. The dashed lines 

provide the measured UVAPS airborne concentrations and the solid lines provide the simulation 
29

results. Chart titles provide information on resuspension rates and initial loadings used, i.e., ten 

times the base resuspension rate, rbase, and ten times the base loading, Lbase. 

In Figure 14, the measured concentrations of Round 1 revealed that particle concentrations 

were clearly elevated in rooms 101A and 102 during the resuspension (sampling) periods. The 

review of the detailed occupancy video for Round 1 revealed that a sampling team was working 

in Room 101A from 12:00 to 14:30 and Room 102 from 14:30 to 16:45. The plot of measured 

results for Round 1 reveals that the timing of the elevated airborne concentrations do not 

correspond directly to the time frames of the observed activity within rooms 101A and 102 and 

that the airborne concentration at the measurement locations was not indicative of a constant 

resuspension rate and a constant air change rate, i.e., a build-up in concentration to a steady 

level. This discrepancy between observed activity schedules and measurements of elevated 

particle concentrations attributed to resuspension could be the result of variations in 

resuspension rates due to various types of activity, non-uniform room concentrations due to 

airflow patterns or resuspension location within the rooms, or some combination thereof. 

These observations of variability and the lack of key input variables (e.g., activity-based 

resuspension rates) led to the decision not to pursue detailed modeling of the resuspension 

activities themselves. However, for the purposes of illustration, Figure 14 provides simulation 

results obtained by assuming rooms were occupied during the periods obtained from the video 

analysis for Round 1 showing that CONTAM is capable of simulating such details if warranted.
30

Figure 14. Simulation vs. measurement, Round 1, with detailed resuspension schedule

Figure 15. Simulation vs. measurement, Round 1, resuspension 10 x rbase, loading 10 x Lbase
31

Figure 16. Simulation vs. measurement, Round 2, resuspension 10 x rbase, loading 10 x Lbase

Figure 17. Simulation vs. Measurement, Round 3, resuspension 10 x rbase, loading 10 x Lbase
32

Table 7 provides a summary of comparisons between measurements and simulations for 

average airborne concentrations in rooms 101A and 102 based on data shown in Figure 15

through Figure 17. Average and standard deviation of air change rates determined from the 

CONTAM simulation are also presented. Decay rates in Room 101A were determined from 

measured IBAC data for several periods of each round during fan-off conditions. While these 

values are not directly comparable to the air change rate (and are not presented in the table),

they ranged between 0.12 h

-1

to 0.49 h

-1

.

Table 7. Summary of average measured and simulated resuspended airborne concentrations

Test

Air Change

Rate [h-1]

Integration Period

Average Concentration [mg/m3]

Measured Simulation Percent Diff.

AVG STD BEGIN END [h] 101A 102 101A 102 101A 102

Round 1 0.29 0.04 4/17/2011 9:15 4/17/2011 17:00 7.75 3.65E-04 4.63E-04 4.09E-04 3.91E-04 12 -16

Round 2 0.19 0.08 4/26/2011 8:05 4/26/2011 13:40 5.58 1.68E-04 1.93E-04 4.50E-04 4.10E-04 168 112

Round 3 0.08 0.03 5/11/2011 8:40 5/11/2011 18:00 9.33 3.46E-04 3.59E-04 5.64E-04 5.38E-04 63 50

The measured average concentrations ranged from approximately 1.7 x 10-4 mg/m3to 

4.6 x 10-4 mg/m3(or assuming a 1 m particle diameter, from 3 x 105 particles/m3to 

9 x 105 particles/m3), and simulated values were within approximately one order of magnitude 

of the measured values. One order of magnitude agreement is thought to be reasonable 

agreement considering the uncertainty in simulation inputs, e.g., activity-related resuspension 

rates, initial loadings and building airflows. Further, the variation in agreement between 

Rounds 1 through 3 (percent differences ranging from -16 % to 168 %) is indicative of the wide 

variation in concentration time histories as depicted in Figure 15 through Figure 17 and is likely 

due to variation in activity-related resuspension rates and occupant movement patterns.
33

5 Sensitivity Analysis of Resuspension Exposure Model

The whole-building simulation results revealed that the potential exposure, i.e., average 

airborne concentration, was very sensitive to the simulation inputs. Future prospects of 

establishing a means to predict potential exposure to spore resuspension should be based on a 

sound basis for modeling resuspension activity and establishing the inputs to such a model. 

Therefore, a single-zone simulation model is applied herein to examine the sensitivity of 

predicted exposures to key inputs. This resuspension exposure model is then presented as a 

candidate for incorporation into a future software tool for resuspension exposure assessment.

In the discussion of resuspension simulations presented in Section 3, several inputs were 

identified as having a wide range of possible values that presumably led to some of the 

observed disagreement between measured and predicted values. Some inputs to the wholebuilding simulation were building-specific, e.g., building layout and occupancy schedules, which

should not generally be associated with a high degree of uncertainty. However, other inputs are 

more difficult to estimate and are therefore more likely to be associated with a range of 

potential values, e.g., resuspension rates and particle loadings. 

To examine the impact of the variations in these factors on the simulation results, a sensitivity 

analysis was performed by conducting a series of single zone simulations. This sensitivity 

analysis consisted of a two-level, full factorial analysis [40, 41] based on the Design of 

Experiments methodology as presented in [41]. This analysis uses graphical methods to screen 

for main effects and interactive effects among factors or inputs affecting the outcome of an 

experiment, or in this case, a simulation. This analysis provides a relatively simple method to 

qualify those input variables that have the most significant influence on the outcome of the 

calculation in question, which in this case will be the potential exposure due to resuspension for 

the model presented in the following section. This discussion uses several terms specific to the 

Design of Experiments methodology, and those terms are presented in italics below to make 

them easy to identify.

5.1 Single-zone Simulation Model

To focus on factors that are not building-specific, this sensitivity analysis employed CONTAM to 

model a single well-mixed zone with deposition and resuspension. This model is described by 

equations (1) and (2) with the added assumption that the outdoor concentration and the 

indoor source terms are both zero, leading to the following set of two equations.

For the air:

dCz⁄dt = −(

Q

V

+ vd

As

V

)Cz

(t) + r

Ar

V

Ls

(t) (9)

For the surface:

dLs⁄dt = υdCz

(t) − r

Ar

As

Ls

(t) (10)

This initial value problem assumes that the initial zone concentration was zero, and the floor 

contained an initial loading of particles. Simulations were run for an eight-hour period. The 

average zone concentration, Cavg, was selected as the outcome or response factor to be 
34

representative of the potential exposure to which one performing sampling would be 

subjected.

5.2 Model Inputs

The inputs to the sensitivity analysis were selected to represent the ranges of values that might 

be found in realistic circumstances; however, most of the inputs can vary widely and may not 

be known definitively. A set of five inputs was selected for consideration in the analysis,

including: 

Li = initial surface loading [kg/m2]

Q = volumetric outdoor airflow rate [m3/s]

r = particle resuspension rate [1/s]

d = deposition velocity [m/s]

As = deposition surface area [m2]

Zone volume was set to 1 000 m3 and resuspension area Ar was set to 0.028 m2(the size of a 

single foot print) for all simulations. Table 8 provides the ranges of the five input values that 

were varied. The minimum value for initial loading is based on that which would occur for a 

single release of BG particles in a single BOTE experiment, and the maximum was somewhat 

arbitrarily set to 100 times the minimum. Minimum deposition surface area was based on the 

zone volume divided by a ceiling height of 2.44 m and allowed to vary by two times to vary the 

surface-to-volume ratio from 0.41 m-1to 0.82 m-1. The range of resuspension rates was selected 

based on the 25th and 75th quartiles of the data presented in Figure 1 of Section 2.3. Volumetric 

airflow rates were selected to include air change rates between 0.25 h

-1 and 3.0 h-1

, which 

cover a reasonable range of building air change rates. Deposition velocities were determined 

based on measured deposition rates for 0.5 m to 2.5 m diameter particles as presented in

[42] and [43].

Table 8. Range of input values for sensitivity analysis

Li

[kg/m2]

As

[m2]

r

[1/s]

Q

[m3/s]

d

[m/s]

Minimum 5.38E-07 410.10 2.328E-08 250 2.5E-05

Maximum 5.38E-05 820.21 1.164E-05 3000 5.0E-03

Ratio 100:1 2:1 500:1 12:1 200:1

These minimum/maximum pairs of values are referred to in the sensitivity analysis as the two 

levels for each input. The pairs are depicted in the following discussion and graphs as “-” and

“+” (and as “-1” and “+1”), respectively.

5.3 Results of Sensitivity Analysis

Five inputs with two levels each yields a set of 32 (25) simulations for which the resultant 

average airborne concentrations are presented in Figure 18. This Ordered Data Plot presents 

resultant values in order from smallest to largest and provides the combination of input levels 
35

that pertain to each result at the top of the chart. From this chart, one can readily discern which 

combination of inputs yields the most significant results, i.e., higher average concentration.

Figure 18. Ordered Data Plot

One of the main purposes of this sensitivity analysis is to identify those factors that have the 

main effects on the outcome. Figure 19 is referred to as the Main Effects Plot and provides an 

indication of the effect a single variable has on the outcome by plotting the mean of the 

responses for each variable at the indicated levels, -/+. Those factors having the steepest slope 

are considered most significant to the outcome relative to others; those factors with flatter

lines are less significant. This plot indicates that initial surface loading and resuspension rate 

have the most significant effect, while deposition surface area has the least significant effect.

Figure 19. Main Effects Plot
36

While the Main Effects Plot provides information on effects related to individual variables, the 

Absolute Effects Plot (Figure 20) provides information on the interaction between pairs of 

inputs in conjunction with the main effects. Absolute effects, |Exy|, are determined by the 

absolute value of the difference between the means of the responses for those variables at the 

levels determined by the so-called multiplicative cross products as explained in Section 5.5.9.4. 

of [41]. For example, |ELiQ | as depicted in the Absolute Effects Plot is the absolute value of the 

difference between the “+1” and “-1” values of the corresponding Interactive Effects Matrix

shown in Figure 21. These values of |Exy| provide a quantitative relationship among the main 

and interactive effects as depicted in Figure 20, which shows that the initial loading (Li), 

resuspension rate (r) and their interactive component (Li r) have nearly equally significant 

effects. Further, all factors not including the deposition surface area would appear to be 

relatively significant factors as demonstrated by the sharp drop-off in |Exy| at the “As” factor.

Figure 20. Absolute Effects Plot
37

Figure 21. Interactive Effects Matrix (color-coded to match the Absolute Effects Plot)

This sensitivity analysis was based on the Design of Experiments methodology as presented in 

[41]. While this methodology is geared towards identifying those factors that have a significant 

effect on an outcome of an experiment, the methodology can also be used to develop a model 

of the experimental outcome based on the input levels utilized in the analysis, i.e., the “+” and 

“-” values for each factor (see Section 5.5.9.9.5 of [41]). However, this model would be for the 

specific ranges of inputs and assumptions under which this analysis was performed, e.g., fixed 

building volume and 8-hour exposure time. It would be more useful to develop a more flexible 

modeling tool that would allow for a broader range of inputs and outputs as discussed in the 

following section.
38

6 Proposed Resuspension Exposure Tool

A key objective of the EPA BOTE project was to provide first responders and others with 

techniques to estimate residual risks associated with biological agents remaining after releases 

and/or decontamination efforts. One technique would be a software tool that would allow 

users to estimate indoor agent concentrations and the resultant exposures due to 

resuspension. This Section describes a resuspension tool that would allow the user to select key 

inputs and provide estimates of potential exposure as a result of resuspension.

The whole-building simulations of the BOTE resuspension experiments and the sensitivity 

analysis of the inputs to the proposed single-zone resuspension model presented above provide 

insight into the development of such a tool. While a whole-building airflow and contaminant 

transport representation of a specific building would be ideal for addressing particle 

release/resuspension events, such a representation, i.e., a multizone resuspension model, 

would be difficult to develop quickly and accurately for situations as they arise. The wholebuilding simulations presented above (including work presented in the establishment of the 

building representation [35]) and the review of resuspension rate studies revealed several 

potential issues related to application of this type of simulation to a specific building including:

 establishing initial floor loadings

 establishing schedules of resuspension activity, i.e., number of occupants and walking 

rate

 associating resuspension rates with other activities, e.g., vacuuming and shuffling 

papers

 associating resuspension rates with flooring material (and other surfaces)

 establishing a building representation that captures building airflows (ventilation 

system, infiltration, and interzone) for multiple building operating conditions

 establishing boundary conditions that drive airflow, i.e., internal and external 

environmental parameters such as temperatures, wind speed and direction

Based on these issues, it would be more reasonable to develop a simpler model that could be 

used to perform quick estimations based on a well-established set of inputs. The sensitivity 

analysis provided such a set of inputs based on building-related information (e.g., volume and 

air change rate), the literature review (e.g., walking-induced resuspension rates), and 

information related to release events (e.g., floor loadings based on release amounts and 

particle size).

The key inputs to calculating exposure due to resuspension are listed in Table 9, along with an 

assessment of the ability to determine reliable values for use in exposure calculations. The 

current view of the tool is that it would perform single-zone calculations of indoor agent levels 

assuming an initial loading on the interior surfaces and resuspension of that agent based on 

indoor activities. The first four parameters in the table, volume, air change rate, particle size 

and deposition rate, are fairly straightforward to specify. The tool could have default air change 

rates that vary by building type and operating conditions, which the user could select from a 

menu or other input scheme. Particle size could also be menu driven, and the deposition rate as 
39

a function of particle size could be provided by the tool with the ability to be overridden by the 

user if they desire.

The next three variables in Table 9 are more challenging to estimate. The initial loading of the 

agent depends on the agent release itself, including the quantity released and the amount that 

enters the building in the case of an outdoor release or the amount that remains in the building 

if the agent release takes place indoors. If the user has this information, the loading value will 

be more reliable than it would be otherwise. If the user does not know the nature of the 

release, they will have to estimate the initial loading. The viable fraction of the agent that is 

available for resuspension is a key factor to determining the fraction of resuspended particles 

that contributes to the risk. If the user knows this fraction, the exposure estimate will be more 

accurate. Otherwise a conservative assumption of 100 % viability may be reasonable. 

The resuspension rate is probably the most challenging input parameter to determine for this 

calculation. The discussion in the Literature Review Section of this report presents the range of 

values that has been reported in the literature and describes the dependence on particle size, 

surface type and activity. The tool could incorporate the PSU model, as it is the most complete 

approach that existed when the literature review was performed, but the user would still need 

to describe the activity, e.g., number of people walking and duration of their activity. It may 

also be possible to fit a model to other experimental measurements of resuspension rate, in 

particular the measurements performed by Tian et al. [26].

Table 9. Key inputs for resuspension exposure calculations

Input Parameter Assessment

Building volume Straightforward to determine; 

High accuracy is not critical

Building outdoor air change rate Depends on building, ventilation system type and operating conditions; 

Existing data to support estimates

Particle size of agent Key input that user must select

Particle deposition rate Size dependent; 

Values exist in literature

Initial agent loading on surfaces Challenging to determine but can be estimated from agent release 

information

Viable fraction of agent Potentially important for risk assessment; 

Availability unclear

Resuspension rate Depends on surface, particle size, activity type and activity level; 

Challenging to estimate

Exposure period User input

As noted in the last row of Table 9, the user would also need to specify the exposure period, 

which constitutes the start and stop time for the exposure calculation. This period may or may 

not include the period during which the resuspension activity occurs. Therefore, the user would 

need to separately specify the start and stop time of the resuspension activity and of the 
40

exposure period. The tool would then calculate and output the average airborne agent 

concentration over the exposure period.

Figure 22 shows a schematic of how the exposure tool screen could appear, with the inputs 

identified for user entry in the upper box. The outputs, including a plot of agent concentration 

over time, are shown in the bottom half of the screen. This presentation is only conceptual at 

this time, and if the tool is developed it will likely change as programming decisions are made 

and input is received from beta users.

Figure 22. Exposure tool sample screen
41

7 Summary and Conclusions

This modeling study was predicated on the availability of resuspension rate models that would 

be applicable for use in the CONTAM multizone airflow and contaminant transport simulation 

tool or other similar simulation tools. The intent was to use the rates determined from such 

resuspension models to simulate the conditions of the BOTE resuspension experiments, and 

then to compare the simulation results to those obtained from the BOTE experiments. 

The literature review revealed one resuspension rate model that could be used to identify 

resuspension rates based on a well-defined set of inputs. This model is referred to as the PSU 

model and provides resuspension rates that could be directly applied within CONTAM based on 

the following inputs: particle size, particle type, flooring type, walking rate and resuspension 

surface area (area of a single foot print). Several other critical observations were made based 

on the literature review related to the use of resuspension rates determined from experimental 

results within CONTAM or other similar simulation models. The methods of reporting 

resuspension results varied widely and included multiple forms, e.g., resuspension factor, 

resuspension fraction and resuspension rate. However, some results proved inapplicable, 

because they could not be converted to a resuspension rate parameter that can be applied in a 

building contaminant transport model. Conversion proved difficult due to inconsistency in 

reporting important experimental information, e.g., the steady-state assumption being met, 

resuspension surface area and air change rate of the test volume. These inconsistencies lead to 

the conclusion that particle resuspension experimentation requires a more rigorous approach 

and means of reporting results as also noted by Qian et al.[44].

Simulation results compared favorably with experimental results in that order of magnitude 

agreement was obtained between average airborne concentrations of resuspended particles 

over the period of resuspension activity for all three rounds of BOTE experiments. This result

was very encouraging due to the uncertain nature of resuspension rates that vary widely 

depending on particle size, resuspension activity type (walking, vacuuming, shuffling papers, 

etc.), resuspension activity schedules (number of occupants, location of occupants, etc.), 

particle surface loadings, and other environmental factors (airflow rate, relative humidity, etc.).

It should also be noted that the properties upon which the PSU model was based (e.g., flooring,

particle types and resuspension mechanisms) did not necessarily represent those encountered 

in the BOTE study.

Comparisons between the experimental results and simulations revealed the inherent difficulty 

in capturing the detailed nature of the range of activities and associated resuspension rates.

Nevertheless, given a well-formed building representation and assuming one could associate 

resuspension rates with particle size, flooring surfaces, activity types and schedules, a building 

simulation could predict potential exposure to resuspension activities. However, development 

of such well-formed and detailed building models and associated resuspension-related inputs 

would be quite resource-intensive and perhaps reserved for those buildings that warrant such 

attention. More work is needed to evaluate application of whole-building simulation for these 

more detailed analysis methods. Previous work has been performed to simulate building 

protection schemes with respect to CBR events [45, 46], but there does not appear to have 
42

been as much done on simulating resuspension due to decontamination-related sampling after 

such events.

The uncertain nature of the threat that motivated this study makes it difficult to anticipate 

when and where the need for a detailed resuspension exposure tool would arise. Therefore, it 

would be more reasonable to develop a simpler tool that could be used to perform quick 

estimations based on a well-established set of inputs. A sensitivity analysis defined such a set of 

inputs based on building-related information (e.g., volume and air change rate), the literature 

review (e.g., walking-induced resuspension rates), and information related to release events

(e.g., floor loadings based on release amounts and particle size). A relatively simple tool was 

proposed that would capture the main inputs and provide an estimate of potential exposure 

during a resuspension event. The preliminary design of the proposed tool could be modified or 

enhanced to accommodate additional requirements.
43

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Coarse Particles in Residential Buildings: Literature Review and Measurements in an 

Occupied Townhouse, NISTIR 7068. National Institute of Standards and Technology.

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indoor environments. Atmospheric Environment, 89(0): p. 464-481.

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46

Acknowledgments

This work was sponsored by the U.S. Environmental Protection Agency (EPA) through its Office 

of Research and Development under Interagency Agreement DW-13-92340101-0 under the 

direction of Sarah Taft (EPA). The authors would like to thank Brian Polidoro and Brady 

Lindemon for their assistance in reduction of measured data. We also appreciate Alan Heckert 

and James Filliben of the NIST Statistical Engineering Division for their insights into the topic of 

sensitivity analysis. 
47

Appendix A. Summaries of Documents from Literature Review

This appendix lists the articles and reports considered by NIST as part of this literature review

organized into the following categories: Relevant resuspension models, Chamber measurements 

of resuspension rates, Other chamber measurements, Field measurements of resuspension rates 

or particle concentrations, Other model development and application, and Reviews. Each 

reference is followed by the published abstract. For publications that do not contain an 

abstract, the listed abstract is an excerpt from the publication’s introduction.

Relevant resuspension models 

Bahnfleth, W.P., Freihaut, J.D., and Aumpansub, P. 2007. Implementation of resuspension factors in building air flow simulations for cross contamination risk assessment in 

multi-zone buildings and determination of source strengths associated with given levels of risk 

reduction. U.S. Army Center for Health Promotion and Preventive Medicine, Phase III Task 7 

Report.

ABSTRACT There is a lot of interest in the assessment of risk resulting from bioterrorism attacks 

on buildings. Exposure of occupants can result from both primary aerosolization (initial airborne 

release of an agent) and secondary aerosolization (re-aerosolization of agent deposited on 

various surfaces). Reaerosolization can also cause exposures to occupants who are far away 

from the concentrated residual contamination sites. Consequently, it is necessary to be able to 

assess occupant risk in both the immediate zone of residual contamination (reservoir) and the 

resulting airborne contamination of other building zones due to the indigenous air flow paths of 

the given building. 

The objective of this task is to develop a tool to help users to create key coefficients required to 

simulate particle deposition and re-suspension in the CONTAMW multi-zone modeling program 

(NIST 2006). The tool utilizes experimental re-suspension rate data for several types of 

particles, information extracted from CONTAMW simulations and other information to 

compute deposition and resuspension schedules.

A Microsoft Excel spreadsheet application was created that can be easily modified to 

incorporate existing and future resuspension rate data. The use of the tool is demonstrated in a 

parametric study for representative buildings, release types, HVAC systems and other key 

parameters including deposition and re-suspension of aerosols. Scenarios included exposure 

due to re-suspension of residual contaminant left in the floor reservoir after clean-up and 

reoccupation of a building.

Freihaut JD, Hu, B., Kremer P, Bahnfleth WP, and Gomes C. 2008b. Development of a 

Lookup Table of Resuspension Rates and Factors to be Utilized in Indoor Air Flow Simulation 

Models. Pennsylvania State University. Phase II Task 5 Report to U.S. Army Center for Health 

Promotion and Preventive Medicine, Contract DABJ05-03-P-1210.

ABSTRACT In this Phase II Task 5 report, entitled “Development of a Lookup Table of 

Resuspension Rates and Factors to be Utilized in Indoor Air Flow Simulations Models” the 

results of extensive literature reviews undertaken as well as the experimental results obtained 

in Phase II Task 4 and Phase III Task 10, are “synthesized into a set of recommended particle 

resuspension rates and factors. It is suggested that these values be utilized in Indoor Modeling 
48

of Air Pathogens to estimate occupant exposure risks for conditions in which contaminated 

airborne particles are dominated by the reservoir resuspension processes

Chamber measurements of resuspension rates

Freihaut JD, Kremer P, Gomes C, Bahnfleth WP, and Hu B 2008a. Quantification of Floor 

Vibration and Aerodynamic Swirl Effects on Particle Resuspension Properties. Pennsylvania 

State University. Phase 2 Task 4 Report to U.S. Army Center for Health Promotion and 

Preventive Medicine, Contract DABJ05-03-P-1210.

ABSTRACT Phase 2 Task 4 (and Phase 3, Tasks 10 and 11) focuses on the development of 

experimental resuspension rates and resuspension factors in well-controlled laboratory 

conditions which mimic typical occupant activity levels in a building in the presence of building 

surface particle reservoirs. Phase 2 Task 4 focuses on the resuspension phenomenology 

determined by mechanical vibration (floor) and aerodynamic drag and lift forces created by 

occupant walking.

In Phase 2 Task 4 a look-up table of resuspension rates and factors is developed from the 

experimental work of the other tasks and the available, applicable literature values. These 

resuspension values can then be utilized to model particle resuspension and estimate occupant 

inhalation exposure risks for a contaminated site in which field determined particle reservoir 

particle size concentrations and contaminant size distributions have been determined (Phase 2 

Task 6). 

Given the above background, the Phase II Task 4 report:

1. establishes occupant activity related, fundamental aero-mechanical disturbance drive 

resuspension rates of stand-alone spores and allergen carrier particles relative to 

“reference quartz” particles;

2. places the measured resuspension rates in the context of reported literature values.

Gomes, C., Freihaut, F. and Bahnfleth, W. 2005. Resuspension of allergen-containing

particles under mechanical and aerodynamic disturbances from human walking – introduction 

to an experimental controlled methodology. Proceedings of Indoor Air 2005.

ABSTRACT Epidemiological evidence indicates that common environmental allergens found in 

building reservoirs are strongly associated with the development of bronchial hyper-reactivity 

(BHR) or asthma, affecting up to half the population in North America and Europe. Although 

they are rarely life threatening, these diseases cause much distress and lost time from school 

and work. These diseases are believed inhalation sensitized and developed, suggesting an 

aerobiological pathway of allergen-containing carrier particles from reservoir to occupant 

respiration. This study presents and develops a controlled and characterized method to explore 

the influence of human walking on the aerosolization of allergen-containing particles. Time 

resolved particle size distribution and allergen content are measured for particles resuspended 

from representative samples of flooring materials and for different sets of floor disturbances in 

an environmentally controlled experimental chamber. Initial results, when placed in the context 

of previous investigations; indicate the method can be utilized to develop a database for 

particle resuspension rates.
49

Gomes, C., Freihaut, F. and Bahnfleth, W. 2007. Resuspension of allergen-containing

particles under mechanical and aerodynamic disturbances from human walking. Atmospheric 

Environment. 41(25): 5257-5270.

ABSTRACT This study presents and develops a controlled and characterized method to explore 

the influence of specific occupant activity on the aerosolization of allergen-containing particles. 

Indoor allergen-related diseases are primarily inhalation sensitized and developed, suggesting 

an aerobiological pathway of allergen-containing carrier particles from dust reservoir to 

occupant respiration. But the pathways are not well understood or quantified. The influence of 

occupant walking on particle aerosolization is simulated by a system in which complex floor 

disturbances are deconvoluted into aerodynamic and mechanical components. Time resolved 

particle size distributions are measured for particles resuspended from representative samples 

of flooring materials and different types of floor disturbances in an environmentally controlled

experimental chamber. Results indicate aerodynamic disturbances, relative to mechanical, 

dominate the particle resuspension behavior. Dust type, dust load and floor type showed 

marginal influences on a normalized surface loading basis. Humidity effects were not clear since 

during experiments the floor samples may not have reached moisture partitioning equilibrium 

with the controlled air humidity. Average resuspension rates ranged from 10-7to 10-3 min-1,

having phenomenological consistency with previous, large room or chamber investigations, 

suggesting the method can be utilized to develop a database for particle resuspension rates.

Hu, B. 2008. An investigation of walking induced electrostatic field effects on indoor particle

resuspension. PhD thesis, Architectural Engineering, Pennsylvania State University.

ABSTRACT Airborne concentration of particulate matter (PM) is an important index of indoor 

air quality. Researches have demonstrated the strong correlation between airborne particulate 

concentration and human health. Micron-sized particulate sources are commonly found in the 

enclosed indoor environment. Human activity is considered the main reason causing indoor 

particle resuspension, which leads to human secondary exposure to PM. However, no general 

rules have been established to empirically relate airborne particle concentration from 

resuspension with specific types of human activity. 

This work studies the effects of electrostatic field, generated by human walking, a common 

human activity, on indoor particle resuspension. Particle resuspension forces, generated by 

human walking on indoor surface particles, can mainly be decomposed into three types: 

mechanical vibration, aerodynamic drag and electrostatic forces. A parametric study is carried 

out to compare the magnitudes of different particle resuspension force components, as well as 

the particle-surface adhesion force. The comparison shows that electrostatic force is a 

significant particle resuspension force, while the theoretically calculated particle adhesion force 

is much larger than each of the resuspension force components. This work suggests that real 

particle adhesion force is much smaller than the theoretically calculated values due to 

imperfect contact and thus the resuspension forces, introduced by human activity, can 

overcome the adhesion force to resuspend particles into the air. To testify this idea, 

experiments are designed to measure the real adhesion force between different particles and 

flooring materials with the electrostatic detachment method.
50

This work uses particle resuspension chamber as the major facility to systematically study the 

walking-induced electrostatic effects on particle resuspension. Experiments are designed to 

measure the floor surface electrostatic field strength that can be generated by human walking 

in indoor environment. The measured field strength profile is used for the electrostatic field 

simulation in the resuspension chamber experiments. The validity of the chamber experiments 

is testified by comparing the resuspension coefficients concluded from the resuspension 

chamber experiments with the results reported from field experiments. The results of chamber 

experiments show that the electrostatic fields introduced by human walking can have a 

significant influence on indoor particle resuspension. The electrostatic effects are strongly 

related with such factors as particle type, flooring type, relative humidity and field polarity.

Besides, this work also studies the applicability of multizone airflow and contaminant dispersion 

model in simulating particle resuspension and dispersion in indoor environment with a heating, 

ventilation and air-conditioning (HVAC) system.

Qian, J. and A. Ferro. 2008a. Resuspension of dust particles in a chamber and associated

environmental factors. Aerosol Science and Technology, 42: 566-578.

ABSTRACT Experiments investigating particle resuspension from human activities were 

conducted in a full-scale experimental chamber. The experiments tested three types of flooring 

(vinyl tiles, new and old level-loop carpets) and two ventilation configurations (ceiling and side 

wall supply systems). The floorings were seeded with 0.1–10 μm test particles. The airborne 

particle concentration was measured by an array of optical particle counters (OPCs) in the 

chamber. Resuspension rates were estimated in size ranges of 0.8–1, 1.0–2.0, 2.0–5.0, and 5.0–

10 μm ranging from 10−5–10−2 hr−1, with higher resuspension rates associated with larger 

particles. Resuspension via walking activity varied from experiment to experiment. “Heavy and 

fast” walking was associated with higher resuspension rates than less active walking, most likely 

due to a combination of increased pace, increased air swirl velocity, and electrostatic field 

effects established by the walking. The type of floorings also influenced the particle 

resuspension. Given the same size and mass distribution of test particles per unit floor area, 

resuspension rates for the seeded new level-loop carpet were significantly higher than those 

for the vinyl tile flooring for larger particles (1.0–10 μm) under the ceiling air supply system. 

Sohn, C.W. 2006. Resuspension physics of fine particles. U.S. Army Engineer Research and 

Development Center, ERDC/CERL SR-06-18.

ABSTRACT Different release models can yield significantly different dynamic concentration 

profiles in a room depending on the release rates chosen. Results from two different cases 

demonstrated significantly different concentration profiles in the room of interest. This work 

was undertaken to: (1) Critically review the current model, (2) formulate a new, 1st order 

Algebraic model, and (3) use experimental data to validate the modeled theory. This report 

documents preliminary work that was suspended after Fiscal year 2005 (FY05). No follow-on 

research is currently funded.

Thornburg, J and C, Rodes. 2009. Resuspension of fibers from indoor surfaces due to human

activity. U.S. Environmental Protection Agency. EPA/600/R-09/009.
51

ABSTRACT This research was directed toward 1) determining the quantity of asbestos simulant 

fibers resuspended (emitted) as normalized by the amount deposited, and 2) calculating 

asbestos simulant fiber emission factors at two heights while walking on and vacuuming seeded 

carpet. The asbestos fiber simulant selected for this research was calcium silicate, commonly 

known as Wollastonite. Three methods for measuring the quantity available for resuspension 

were studied:

1. MicroVac method following a modified version of ASTM D5755-95,

2. Ultrasonication method developed by Millete et al. (1993),

3. Individual carpet fiber analysis via scanning electron microscopy.

Wollastonite resuspension during walking and vacuuming was studied. Total quantity and size 

dependent fractions of resuspended Wollastonite were measured gravimetrically and with realtime aerodynamic particle instrumentation, respectively. Established experimental procedures 

were followed to seed new and old carpet with Wollastonite, characterize the quantity and size 

distribution of simulant fibers deposited on the carpet, resuspend the simulant fibers within an 

exposure chamber, and collect representative samples of resuspended fibers. The research 

defined a fractional carpet resuspension emission factor as ratio of Wollastonite resuspended 

and Wollastonite available for resuspension on the surface. The best method for estimating the 

amount available for resuspension was the modified MicroVac technique. This simple method 

only collected Wollastonite from the upper carpet fiber surfaces that were potentially available 

for resuspension; Wollastonite fibers embedded deep in the carpet with low probability of 

being resuspended were not collected. SEM analysis of individual carpet fibers worked only for 

new carpet fibers. Old carpet samples typically had too many “background” particles that 

confounded the analysis. The Millete et al. ultrasonication method poorly estimated the 

quantity available. The removal of a carpet plug and sonic bath released a very high number of 

carpet material particles that completely overwhelmed the ability to detect Wollastonite. 

Simulant fiber emission factors ranged from < 0.01 to 0.45, with the majority falling between 

0.01 and 0.10. As expected, experimental conditions (primarily resuspension method, carpet 

age, and relative humidity) affected the emission factors. The majority of Wollastonite fibers 

resuspended from carpets were between 2 and 10 mm, with particles between 2 and 6 mm 

yielding the highest mass emission factors. The vacuum beater bar did resuspend a significant 

number of sub-micrometer particles that did not contribute much to the mass resuspended. 

Emission factor testing did not elucidate the influences of electrostatic and surface tension 

adhesion forces between the Wollastonite and carpet fibers in determining the amount 

available for resuspension. Further investigation of these mechanisms and their influence on 

emission factors will provide the requisite data needed for robust modeling exposure to 

resuspended particles.

Manthena, S., Ferro, A.R. 2009. Resuspension rate estimation of PM from human activities in 

an indoor particle transport chamber, in Healthy Buildings 2009: Syracuse, NY.

ABSTRACT Resuspension is the phenomenon of reentrainment of a previously deposited 

material into the atmosphere. It has been identified as a major source of indoor airborne 

particulate matter (PM). Human activities can increase PM concentration up to several orders 

of magnitude above the background levels in indoor environments. To determine the affect of 
52

resuspension and the impacts of various environmental factors on resuspension, more accurate 

characterization is required. We conducted experiments in the Indoor Particle Transport 

Chamber (IPTC) at Clarkson University to characterize the resuspension from human activities 

from real floorings using both Arizona test dust and house dust collected from residences. For 

each experiment, a participant walked on different types of floorings such as hard floor and 

carpet inside the chamber. The floorings were seeded with Arizona test dust and house dust 

particles with a known size distribution of 0.1-20 μm in diameter. A number of factors, including 

resuspension mechanism, walking pace, shoe wear, temperature, and air exchange rate, were 

kept constant while relative humidity, particle type, and flooring type were varied. 

Resuspension rates were estimated from an air-surface compartment model using real-time 

particle concentration data. Resuspension rates were significantly higher for carpeting as 

compared with hard flooring and for house dust as compared with Arizona test dust.

Tian, Y., K. Sul, J. Qian, S. Mondal, and A.R. Ferro. A Comparative Study of Walking-induced 

Dust Resuspension using a Consistent Test Mechanism. Indoor Air, 2014: p. n/a-n/a. (accepted 

for publication 2014-03-01)

ABSTRACT Human walking influences indoor air quality mainly by resuspending dust particles 

settled on the floor. This study characterized walking-induced particle resuspension as a 

function of flooring type, relative humidity (RH), surface dust loading, and particle size using a 

consistent resuspension mechanism. Five types of flooring, including hardwood, vinyl, high 

density cut pile carpet, low density cut pile carpet and high density loop carpet, were tested 

with two levels of RH (40% and 70%) and surface dust loading (2 g/m2 and 8 g/m2), respectively. 

Resuspension fraction ra (fraction of surface dust resuspended per step) for house dust was 

found to vary from 10-7to 10-4(particle size: 0.4-10 μm). Results showed that for particles at 

0.4-3.0 μm, the difference in resuspension fraction between carpets and hard floorings was not 

significant. For particles at 3.0-10.0 μm, carpets exhibited higher resuspension fractions 

compared to hard floorings. Increased RH level enhanced resuspension on high density cut pile 

carpet, whereas the opposite effect was observed on hard floorings. Higher surface dust 

loading was associated with lower resuspension fractions on carpets, while on hard floorings 

the effect of surface dust loading varied with different RH levels. 

Other chamber measurements

Gomes, C.A.S. 2004. Resuspension of allergen-containing particles subject to mechanical and

aerodynamic disturbance - introduction to an experimental controlled methodology. MS thesis, 

Architectural Engineering, Pennsylvania State University.

ABSTRACT Epidemiological evidence indicates that common environmental allergens found in

building reservoirs are strongly associated with the development of bronchial hyper-reactivity

(BHR) or asthma, affecting up to half the population in North America and Europe. Although

they are rarely life threatening, these diseases cause much distress and lost time from school 

and work. There is evidence that allergen induced respiratory distress diseases are inhalation

sensitized and developed, suggesting that there must be an aerobiological pathway of 

allergenic containing carrier particles from building reservoir to occupant respiration. This study 

presents and develops an accurate method to explore the influence of human walking on the 

aerosolization of allergen-containing particles using an environmental controlled chamber.
53

Hu, B, J. Freihaut, W. Bahnfleth, and B. Thran. 2008a. Measurements and factorial 

analysis of micron-sized particle adhesion force to indoor flooring materials by electrostatic 

detachment method. Aerosol Science and Technology, 42: 513-520.

ABSTRACT Airborne concentration of micron-sized particulate matter (PM) is an important 

index of indoor air quality. While human activity is considered the main reason causing indoor 

particle resuspension, theoretical particle adhesion force models give predictions of adhesion 

force much larger than the disturbance forces introduced by human activity. This work suggests 

that the imperfect contact between particles and surfaces can greatly reduce the adhesion 

bond. Electrostatic detachment method is used to measure the actual adhesion force 

distribution of micron-sized particles to such common indoor flooring materials as vinyl and 

rubber. Comparisons are made between the theoretical predictions and experimental 

measurements. Factorial experiments are also designed to study the influence of particle type, 

flooring type and contact time on particle adhesion force.

Kildeso, J., Vinzents, P. and Schneider, T. 1998. Measuring the potential resuspension of 

dust from carpets. Journal of Aerosol Science, 29(Suppl. 1): S287-S288.

ABSTRACT Particles resuspended from carpets are likely to affect the indoor air quality. A 

simple instrument has been developed which measures the potential resuspension of dust from 

carpets through the use of a falling weight.

Loosmore, G.A. 2003. Evaluation and development of models for resuspension of aerosols at 

short times after deposition. Atmospheric Environment. 37: 639-647

ABSTRACT Resuspension is known to transport hazardous particles in the natural environment, 

moving a fraction of deposited material back into the atmosphere. This process is notoriously 

difficult to model, given the complexity of the turbulent boundary layer and chemistry of the 

three-phase interface (air, liquid, solid) typically found at the land surface. Wind tunnel studies 

have demonstrated the importance of resuspension within a short time after deposition, but 

there exists no robust model for short-term resuspension. Numerical simulations of accidental 

or terrorist releases of hazardous materials need such a model to accurately predict fate and 

transport of the materials within hours to days after release. Many accepted conventional 

models were derived from resuspension data for aged sources, such as former weapons test

sites; these data sets, and the associated models, may not be appropriate for short-time 

resuspension. The study described here reexamined historical wind tunnel data on short-term 

resuspension, with the goal of developing a model appropriate for numerical simulations.

Empirical models are derived from these data using a suite of parameters (friction velocity, 

particle diameter, surface roughness, particle density, and time). These empirical models, and 

the wind tunnel data, are compared quantitatively with existing conventional models from the 

literature. The conventional models underpredict short-time resuspension, resulting in orderof-magnitude errors in predictions of resuspended mass. Only three models perform 

reasonably well: the empirical models derived from the data and an adaptation of the NCRP 

129 model. More data are needed to validate the empirical models and build the physical 

understanding of the processes involved.
54

Mukai, C., Siegel, J.A., Novoselac, A. 2009. Impact of Airflow Characteristics on Particle 

Resuspension from Indoor Surfaces. Aerosol Science and Technology. 43: 1022-1032

ABSTRACT Resuspension is an important source of indoor particles. We measured the 

resuspension of 1 to 20 μm particles on common indoor materials and explored the importance 

of turbulence to the resuspension process. Experimental variables included materials (linoleum, 

carpet, and galvanized sheet metal) and bulk air velocity (5, 10, 15, 20, and 25 m/s). At each of 

these conditions the turbulence intensity in the boundary layer was varied between a low, 

medium, and high state and ranged from 9 to 34% at the surface. For comparison of 

resuspension from the considered surfaces and at different flow conditions, we use the relative 

resuspension, which quantifies resuspension without requiring knowledge of the number of 

particles initially seeded on the surface. The relative resuspension compares the fraction of 

particles resuspended at the experimental conditions to the maximum achieved with a

controlled impinging jet. In general, the results show that for the ranges considered, increasing 

velocity caused the largest increase in resuspension, followed by increasing turbulence intensity 

and then increasing particle diameter. All three material types showed consistent patterns with 

carpet having the largest resuspension for a given set of conditions, followed by linoleum and 

then by galvanized sheet metal. High turbulence and high velocity conditions minimized the 

differences between materials. An understanding of the relative magnitudes of these effects 

allows for better analysis and mitigation of indoor resuspension.

Shaughnessy R., Vu, H. 2012. Particle loadings and resuspension related to floor coverings in 

chamber and in occupied school environments. Atmospheric Environment. 55: 515-524.

ABSTRACT A series of experiments were conducted to evaluate characteristics of varied flooring 

types which may influence dirt buildup and airborne particle levels. The experiments included 

investigation of typical dirt loadings found on flooring in school environments, the time 

necessary to effectively vacuum dirt from textile flooring (both in chamber and school 

conditions), and the resuspension of particles generated from walking activity on various 

flooring types in chamber and classroom environments. The flooring types studied included 

textile floored surfaces (flow through (FT) carpet and variable cushion tufted textile (VCTT) 

floorings) and hard surface (vinyl composition tile (VCT)).

The study demonstrates that dirt buildup on textile floors will vary based on the location within 

the classroom. One of the primary findings of the study is that textile floor covering in schools is 

not a homogeneous medium. Physical characteristics vary based on the type of backing, carpet 

tufted type, face weight, gauge (including stitches/inch), and adhesive requirements. This

variance in physical parameters contributes to significant differences in the resuspension rates 

(RRs) of particles into the air, generated by walking activity. FT carpet displayed significantly 

greater (2.5 - 5 times) RRs than VCTT in both chamber conditions and classroom environments 

at similar floor loadings and particle size ranges. Controlled chamber test runs revealed VCT

flooring exhibiting 3.6 time higher RRs than FT flooring and 12.8 time greater RRs than VCTT

flooring. Moreover, particle RRs and airborne concentrations are also a function of time of 

walking activity, floor dust loadings and particle size ranges.
55

Field measurements of resuspension rates or particle concentrations

Ferro, A.R., Kopperud, R.J, Hildemann, L.M. 2004. Source Strengths for Indoor Human 

Activities that Resuspend Particulate Matter. Environmental Science & Technology. 38: 1759-

1764.

ABSTRACT A mathematical model was applied to continuous indoor and outdoor particulate 

matter (PM) measurements to estimate source strengths for a variety of prescribed human

activities that resuspend house dust in the home. Activities included folding blankets, folding 

clothes, dry dusting, making a bed, dancing on a rug, dancing on a wood floor, vacuuming, and 

walking around and sitting on upholstered furniture. Although most of the resuspended particle

mass from these activities was larger than 5 um in diameter, the resuspension of PM2.5 and 

PM5 was substantial, with source strengths ranging from 0.03 to 0.5 mg min-1 for PM2.5 and 

from 0.1 to 1.4 mg min-1 for PM5. Source strengths for PM > 5 um could not be quantified due 

to instrument limitations. The source strengths were found to be a function of the number of 

persons performing the activity, the vigor of the activity, the type of activity, and the type of 

flooring.

Hambreaus, A., Bengtsson, S., Laurell, G. 1978. Bacterial contamination in a modern

operating suite. 3. Importance of floor contamination as a source of airborne bacteria. Journal 

of Hygiene, 80: 169-174. 

ABSTRACT The redispersal factor for bacteria-carrying particles from a contaminated floor was 

determined after mopping, blowing and walking activity. Walking gave the highest redispersal 

factor, 3-5 x 10-3 m-1, which was three times higher than for blowing and 17 times higher than

for mopping. The mean die-away rate for the bacteria-carrying particles used was 1.9/h without 

ventilation and 14.3/h with ventilation. It was calculated that in the operating rooms less than 

15 % of the bacteria found in the air were redispersed floor bacteria.

Karlsson, E., Fängmark, I., Berglund, T. 1996. Resuspension of an indoor aerosol. Journal 

of Aerosol Science. 27, Suppl. 1: S441-S442

ABSTRACT Results on one resuspension rate measurement in an office using pollen particles, 

about 5 um. Measured air concentration with person working at table for 1 h, yielding 

resuspension rate of 1.3 x 103s

-1

.

Karlsson, E., Berglund, T., Fängmark, I. 1999. The effect of resuspension caused by human 

activities on the indoor concentration of biological aerosols. Journal of Aerosol Science. 30, 

Suppl. 1: S737-S738.

ABSTRACT Two page paper on model and measurements but not much detail and model based 

on non-dimensional resuspension fraction (defined as fraction of particles emitted from contact 

area during one cycle of activity, e.g. one foot step) and frequency of human activity.

Experiments using spores of about 1 um; 2 activities, walking and paperwork at desk. Results 

showed similar values of resuspension fraction independent of contact frequencies.

Qian, J, A, Ferro, and K, Fowler. 2008b. Estimating the resuspension rate and residence 

time of indoor particles. Journal of the Air and Waste Management Association. 58(4): 502-

516.
56

ABSTRACT Resuspension experiments were performed in a single-family residence. 

Resuspension by human activity was found to elevate the mass concentration of indoor 

particulate matter with an aerodynamic diameter less than 10 μm (PM10) an average of 2.5 

times as high as the background level. As summarized from 14 experiments, the average

estimated PM10 resuspension rate by a person walking on a carpeted floor was (1.4 ± 0.6) × 10-4

hr-1. The estimated residence time for PM in the indoor air following resuspension was less than 

2 hr for PM10 and less than 3 hr for 2-μm tracer particles. However, experimental results show 

that the 2-μm tracer particles stayed in the combined indoor air and surface compartments 

much longer (>>19 days). Using a two-compartment model to simulate a regular deposition 

and resuspension cycle by normal human activity (e.g., walking and sitting on furniture), we 

estimated residence time for 2-μm conservative particulate pollutants to be more than 7 

decades without vacuum cleaning, and months if vacuum cleaning was done once per week. 

This finding supports the observed long residence time of persistent organic pollutants in 

indoor environments. This study introduces a method to evaluate the particle resuspension rate 

from semicontinuous concentration data of particulate matter (PM). It reveals that 

resuspension and subsequent exfiltration does not strongly affect the overall residence time of 

PM pollutants when compared with surface cleaning. However, resuspension substantially 

increases PM concentration, and thus increases short-term inhalation exposure to indoor PM 

pollutants. 

Rosati, J.A., Thornburg, J, Rodes, C. 2008. Resuspension of particulate matter from carpet 

due to human activity. Aerosol Science and Technology. 42: 472-482.

ABSTRACT This work investigated the resuspension and subsequent translocation of particulate 

matter (PM) from carpeted flooring surfaces due to walking. In addition, the effect of HVAC 

systems and ceiling fans on mixing and/or translocation of resuspended PM was studied. 

Testing took place both in a residence with a well worn, soiled carpet and in an environmental 

test chamber. Prescribed walking occurred with PM measurements taken at multiple sampling 

heights. Scanning electron microscopy (SEM) of carpet fibers was used to determine the 

fraction of dust available for resuspension. These data, in conjunction with resuspended mass 

concentrations from this study, were used to generate emission factors by particle size for 

walking on both new and worn carpet.

Carpet loading does not affect the emission factor, indicating that the amount of resuspended 

PM is directly proportional to the available PM in the carpet. While relative humidity (RH) plays 

an important role in resuspension from new carpets, with high RH enhancing resuspension, it 

has the opposite affect with old carpets, with increased RH decreasing resuspension. With the 

HVAC system on, translocated particles 1.2 m horizontally from the source had number 

concentrations of approximately 20–40% of those at the source. With a ceiling fan on, extensive 

mixing was noted with little difference seen in particle resuspension by height. With the ceiling 

fan off, there was very little mixing present and particle size varied substantially by height.

Thatcher, T.L., Layton, D.W. 1995. Deposition, Resuspension, and Penetration of Particles 

within a Residence. Atmospheric Environment. 29(13): 1487-1497.

ABSTRACT Aerosol concentrations and particle size distributions were measured indoors and 

outdoors at a two-story residence in California during the summer months. A single central 
57

sampling point in the downstairs living area was used for all indoor samples. The deposition 

rate for supermicron particles was measured by raising the particle concentration indoors and 

simultaneously measuring air infiltration rates and part concentration decay rates. For particles 

between 1 and 5 um diameter, the deposition velocity closely matched the calculated settling 

velocity. For particles larger than 5 um, the deposition velocity was less than the calculated 

settling velocity, probably due to the nonspherical nature of these particles. The penetration 

factor for supermicron particles, a measure of the amount of filtration achieved by the building 

shell, was calculated using experimentally determined deposition velocity and indoor/outdoor 

particle ratios when no resuspension or generation activities were present. A penetration factor

of one was found, indicating that the building shell was not effective at removing infiltrating 

particles. Resuspension was measured under several different conditions and was found to 

have a significant impact on indoor concentrations. Just walking into a room can increase

particle concentration by 100% for some supermicron particle sizes. For light activity with four

people in the residence, a resuspension rate between 1.8E-5 and 3.8E-4 h-1 was found for 

supermicron particles assuming a particle density of 1 g m-3. These calculated rates may be 

lower than the actual rates due to assumptions made about particle size distribution of floor 

dust.

Abt, E., Suh, H.H., Catalano, P. and Koutrakis, P. 2000. Relative contribution of outdoor 

and indoor particle sources to indoor concentrations. Environmental Science and Technology, 

34(17): 3579-3587.

ABSTRACT The effect of indoor particle sources on indoor particle size distributions and 

concentrations was previously investigated using real-time indoor and outdoor particle size 

distribution data collected in four homes in Boston in 1996. These data demonstrated the 

importance of indoor sources (i.e., cooking, cleaning, and movement of people) and air 

exchange rates on observed indoor concentrations. As part of the continued analyses of these 

data, a simple physical model was used to determine the source emission and infiltration rates 

for specific particle sizes. Decay rates were also estimated. Cooking, cleaning, and indoor work 

(characterized by movement of people) significantly increased PM(0.7-10) concentrations by 0.27, 

0.27, and 0.25 μm3cm-3 min-1, respectively. Cooking was the only variable significantly 

associated with generation of particles less than 0.5 μm in diameter. Outdoor particles (0.02-

0.5 and 0.7-10 μm) were found to contribute significantly to indoor particle levels. Effective 

penetration efficiencies ranged from 0.38 to 0.94 for 0.02-0.5 μm particles and from 0.12 to 

0.53 for 0.7-10 μm particles. Estimates for 0.7-10 μm particles decreased with increasing 

particle size, reflecting the influence of deposition losses from gravitational settling. The realtime particle size distribution data in conjunction with time-activity information provides 

valuable information on the origin and fate of indoor particles.

Buttner, M.P., Cruz-Perez, P., Stetzenbach, L.D., Garrett, P.J., Luedtke, A.E. 2002.

Measurement of airborne fungal spore dispersal from three types of flooring materials. 

Aerobiologia. 18: 1-11.

ABSTRACT Research was conducted in an experimental room to measure the effect of human 

activity on airborne dispersal of settled fungal spores from carpet and vinyl tile flooring. A series 

of experiments were conducted in which commercial loop pile carpet, residential cut pile 
58

carpet, or vinyl tile installed in the experimental room were contaminated with Penicillium 

chrysogenum spores. The flooring materials were contaminated to two different levels (106 and 

107colony forming units per square meter [c.f.u./m2] of flooring surface). Airborne culturable 

and total P. chrysogenum concentrations were measured using Andersen single-stage impactor 

samplers and Burkard personal slide impactor samplers, respectively. Bioaerosol concentrations 

were measured at floor level, 1 meter, and the adult breathing zone (1.5 meter) heights before 

and after human activity consisting of walking in a prescribed pattern for 1 minute in the room. 

Airborne P. chrysogenum concentrations were greater with the higher surface loading for all 

three flooring materials. For all flooring materials there was no significant difference between 

sampler locations, although the data from the 1-meter location were the highest, followed by 

the floor level and the breathing zone locations, respectively. The data from these experiments 

indicate that while a very small fraction of culturable P. chrysogenum spores present on flooring 

materials were aerosolized by walking, relatively high airborne concentrations of spores maybe 

re-entrained from contaminated materials. The airborne P. chrysogenum concentrations were 

significantly higher after walking on cut pile carpet than with the other two flooring materials at 

both contamination levels, with the differences in concentration often ≥ 2 orders of magnitude. 

No differences were measured in airborne culturable P. chrysogenum between vinyl flooring 

and loop pile carpet at both contamination levels. Total spore data from the experiments with 

the 107c.f.u./m2contamination level indicated that walking on loop pile carpet produced 

higher airborne spore concentrations than similarly contaminated vinyl tile although no 

significant difference was observed at the 106c.f.u./m2level.

Cheng, K-C, M, Marian, and L, Hildemann. 2010. Association of size-resolved airborne 

particles with foot traffic inside a carpeted hallway. Atmospheric Environment. 44: 2062-2066.

ABSTRACT The effect of foot traffic on indoor particle resuspension was evaluated by 

associating non-prescribed foot traffic with simultaneous size-resolved airborne particulate 

matter (PM) concentrations in a northern California hospital. Foot traffic and PM were 

measured every 15 min in a carpeted hallway over two 27-h periods. The PM concentration in 

the hallway was modeled based on the foot traffic intensity, including the previous PM 

concentration via an autocorrelation regression method based on the well-mixed box model. All 

5 size ranges of PM, ranging from 0.75–1 μm to 5–7.5 μm, were highly correlated with foot 

traffic measurements for both monitoring periods (p < 0.001, R2 = 0.87–0.90). However, 

correlations during daytime hours were less significant than nighttime. Coefficients found via 

this autoregressive analysis can be interpreted to reveal (i) time-independent contributions of 

walking activities on PM levels for a specific location; and (ii) size-specific characteristics of the 

resuspended PM.

Corsi, R.L., Siegel, J.A., Chiang, C. 2008. Particle Resuspension During the Use of Vacuum 

Cleaners on Residential Carpet. Journal of Occupational and Environmental Hygiene. 5: 232-238.

ABSTRACT Vacuuming is generally considered to be an important activity with respect to the 

cleanliness of indoor environments but may lead to short-term resuspension of particulate 

matter and elevated particle mass in indoor air. Because resuspended particles often contain 

toxicants, such as lead and pesticides, or consist of biological agents that can trigger allergic 

reactions, it is important to understand the role of vacuuming on short-term variations in 
59

indoor particulate matter concentrations. The inhalation of particles during vacuuming events 

may affect adversely those whose occupation requires them to clean a wide range of indoor 

environments, from homes to schools and offices, as well as those who occupy those 

environments. In response, a series of 46 experiments was completed to determine timevariant concentrations of both PM10 and PM2.5 during various vacuuming activities in 12 

separate apartments. Experiments involved the use of two different non-HEPA vacuum cleaners 

and were completed with a vacuum cleaner activated (switched on) as well as deactivated 

(switched off). The latter was intended to provide insight on the potential for resuspension of 

particles by the mechanical agitation of vacuum cleaner movement across carpet. Separate 

experiments were completed also using “mock” vacuuming simulations, that is, walking on the 

carpet in a manner consistent with using a vacuum cleaner. Results are presented as 

incremental particulate matter concentration increases, relative to background (prevacuum) 

concentrations, and peak-to-background particle concentration ratios. Results indicate 

significant resuspension of PM10 mass during vacuum cleaning, with a mean time-averaged 

PM10 increase of greater than 17 μg/m3 above background. Resuspension of PM2.5 mass was 

determined to be small, that is, PM10 mass was dominated by particles greater than 2.5 μm. 

The frequency of vacuuming (between a 10-day standard frequency and several experiments at 

>24 days between vacuuming) had little influence on resuspended particle mass. Resuspension 

by mechanical agitation (rolling of vacuum cleaner across carpet) with the vacuum cleaner 

switched off was determined to be substantial, with a mean time-averaged (during vacuuming) 

PM10 increase of 35 μg/m3 relative to background. Peak-to-background PM10 concentrations 

exceeded 6 for some experiments and averaged between approximately 3 and 4 for 

experiments when the vacuum cleaner was switched on.

Long, C.M., Suh, H.H, and Koutrakis, P. 2000. Characterization of indoor particle sources

using continuous mass and size monitors. Journal of the Air & Waste Management Association, 

50(7): 1236-1250.

ABSTRACT A comprehensive indoor particle characterization study was conducted in nine 

Boston-area homes in 1998 in order to characterize sources of PM in indoor environments. 

State-of-the-art sampling methodologies were used to obtain continuous PM2.5 concentration 

and size distribution particulate data for both indoor and outdoor air. Study homes, five of 

which were sampled during two seasons, were monitored over week-long periods. Among 

other data collected during the extensive monitoring efforts were 24-hr elemental/organic 

carbon (EC/OC) particulate data as well as semi-continuous air exchange rates and time activity 

information.

This rich data set shows that indoor particle events tend to be brief, intermittent, and highly 

variable, thus requiring the use of continuous instrumentation for their characterization. In 

addition to dramatically increasing indoor PM2.5 concentrations, these data demonstrate that 

indoor particle events can significantly alter the size distribution and composition of indoor 

particles. Source event data demonstrate that the impacts of indoor activities are especially 

pronounced in the ultrafine (da ≤ 0.1 μm) and coarse (2.5 ≤ da ≤ 10 μm) modes. Among the 

sources of ultrafine particles characterized in this study are indoor ozone/terpene reactions. 

Furthermore, EC/OC data suggest that organic carbon is a major constituent of particles 

emitted during indoor source events. Whether exposures to indoor-generated particles, 
60

particularly from large short-term peak events, may be associated with adverse health effects 

will become clearer when biological mechanisms are better known.

Raunemaa, T., M., Kulmala, H., Saari, M. Olin, and M. Kulmala. 1989. Indoor aerosol 

model: transport indoors and deposition of fine and coarse particles. Aerosol Science and 

Technology, 11: 11-25.

ABSTRACT Mass and elemental concentrations of particles were determined in indoor and

outdoor air in urban and suburban Helsinki, Finland. The effects of outdoor-to indoor transport 

and indoor sources on indoor particle concentrations were studied in detail. Resuspension and 

deposition to surfaces were also investigated. The experimental results are used to develop and 

specify an aerosol physical model for particle behavior in indoor air, which is presented at the 

end.

Weis, C, A, Intrepido, A., Miller, P., Cowin, M., Durno, J., Gebhardt, J. and R. Bull . 2002.

Secondary aerosolization of viable Bacillus anthracis spores in a contaminated US senate office.

Journal of the American Medical Association, 288(22): 2853-2858.

ABSTRACT Context Bioterrorist attacks involving letters and mail-handling systems in 

Washington, DC, resulted in Bacillus anthracis (anthrax) spore contamination in the Hart Senate 

Office Building and other facilities in the US Capitol’s vicinity.

Objective To provide information about the nature and extent of indoor secondary 

aerosolization of B. anthracis spores.

Design Stationary and personal air samples, surface dust, and swab samples were collected 

under semiquiescent (minimal activities) and then simulated active office conditions to 

estimate secondary aerosolization of B anthracis spores. Nominal size characteristics, airborne 

concentrations, and surface contamination of B anthracis particles (colony-forming units) were 

evaluated.

Results Viable B anthracis spores reaerosolized under semiquiescent conditions, with a marked 

increase in reaerosolization during simulated active office conditions. Increases were observed 

for B anthracis collected on open sheep blood agar plates (P_.001) and personal air monitors 

(P=.01) during active office conditions. More than 80% of the B anthracis particles collected on 

stationary monitors were within an alveolar Respirable size range of 0.95 to 3.5 μm.

Conclusions Bacillus anthracis spores used in a recent terrorist incident reaerosolized under 

common office activities. These findings have important implications for appropriate 

respiratory protection, remediation, and reoccupancy of contaminated office environments.

Other model development and application

Firrantello, J.T., Aumpansub, P., Bahnfleth, W.P., Hu, B., Freihaut, J.D., Thran, B., and 

Hutchens, S. 2007. Effects of HVAC system and building characteristics on exposure of 

occupants to short-duration point source aerosol releases. Journal of Architectural Engineering, 

13(2): 84-94.

ABSTRACT This paper presents results from the simulation of localized, short-duration 

bioaerosol releases in a hypothetical building similar to a dormitory or barracks using public 

domain multizone air flow and contaminant dispersion modeling software. The primary
61

purpose of the modeling was to generate example exposure data to be used in the 

development of a comprehensive microbial risk assessment methodology. However, these 

results are also of intrinsic interest for what they reveal about the contribution of various

building characteristics to risk from airborne contaminants. A variety of parameters were 

varied, including building construction, heating, ventilating, and air-conditioning (HVAC) system 

design, and release characteristics, among others. Results of these simulations demonstrate the 

variability of exposure possible under different scenarios and, more particularly, the impact 

that HVAC design decisions can have on risk. Although a single building and restricted set of 

scenarios was investigated, several general conclusions could be drawn regarding factors, such 

as HVAC zoning and filter maintenance, that intrinsically contribute to vulnerability reduction.

Freihaut J.D., Bahnfleth W.P., and Hu, B. 2005. Modeling of Transient Aerosol Deposition 

and Resuspension (Secondary Aerosolization) with Multizone Air Flow and Contaminant 

Transport Software. Pennsylvania State University. Phase II Task 2 Report to U.S. Army Center 

for Health Promotion and Preventive Medicine, Contract DABJ05-03-P-1210.

ABSTRACT The objective of the project Indoor Air Modeling for Human Pathogens is to develop 

indoor contaminant transport modeling procedures for use in a microbial risk assessment 

procedure being developed by the U.S. Army Center for Health Promotion and Preventive 

Medicine (CHPPM). Phase II, Task 1 of this project (Bahnfleth, et al. 2005) identified the 

absence of the ability to model contaminant deposition and resuspension. Phase II, Task 2 of 

this project, which is summarized in this report, examines the incorporation of the missing 

“critical variables” for deposition and resuspension into the modeling process. Specifically, this 

report describes how literature-based deposition and resuspension models and data can be 

used as the basis for simulating resuspension in the public domain CONTAM version 2.1 

multizone air flow and contaminant transport modeling software (Dols and Walton 2002), 

which is developed and distributed by the National Institute of Standards and Technology 

(NIST). 

Hong, T., Gurian, P.L., and Dudley Ward, N.F. 2010. Setting risk-informed environmental

standards for bacillus anthracis spores. Risk Analysis, 30(10): 1602-1622.

ABSTRACT In many cases, human health risk from biological agents is associated with aerosol 

exposures. Because air concentrations decline rapidly after a release, it may be necessary to 

use concentrations found in other environmental media to infer future or past aerosol 

exposures. This article presents an approach for linking environmental concentrations of 

Bacillus anthracis (B. anthracis) spores on walls, floors, ventilation system filters, and in human 

nasal passages with human health risk from exposure to B. anthracis spores. This approach is 

then used to calculate example values of risk-informed concentration standards for both 

retrospective risk mitigation (e.g., prophylactic antibiotics) and prospective risk mitigation (e.g., 

environmental clean up and reoccupancy). A large number of assumptions are required to 

calculate these values, and the resulting values have large uncertainties associated with them. 

The values calculated here suggest that documenting compliance with risks in the range of 10−4 

to 10−6 would be challenging for small diameter (respirable) spore particles. For less stringent 

risk targets and for releases of larger diameter particles (which are less respirable and hence 

less hazardous), environmental sampling would be more promising.
62

Hu, B., J. Freihaut, W. Bahnfleth, P. Aumpansub, and B. Thran. 2007. Modeling particle

dispersion under human activity disturbance in a multizone indoor environment. Journal of 

Architectural Engineering. 13(4): 187-193.

ABSTRACT Human activity is an important factor influencing particle resuspension in the indoor 

environment. This work studies the applicability of a multizone airflow and contaminant 

transport model (CONTAM 2.1) in the simulation of indoor dispersion of particles under human 

activity disturbance. An iterative method is suggested to complement CONTAM 2.1, by tracking 

the transient particle concentration on floor surface due to dynamic deposition and 

resuspension process. A three-zone building with a heating, ventilation, and air conditioning 

system is used as the simulation case to test the convergence and accuracy of this algorithm 

under different particle-release scenarios. The algorithm shows a very fast convergence speed 

in the simulation. Comparisons of calculation results between the multizone model and the 

analytical model show good agreement and verify the accuracy of the multizone model 

simulation. The airborne particle concentration profiles and human breathing dose are also 

analyzed for the three-zone building model.

Hu, B. J. Freihaut, W. Bahnfleth, P. Aumpansub, and B. Thran. 2007a. Modeling particle

dispersion under human activity disturbance in a multizone indoor environment. Journal of 

Architectural Engineering. 13(4): 187-193.

ABSTRACT Human activity is an important factor influencing particle resuspension in the indoor 

environment. This work studies the applicability of a multizone airflow and contaminant 

transport model (CONTAM 2.1) in the simulation of indoor dispersion of particles under human 

activity disturbance. An iterative method is suggested to complement CONTAM 2.1, by tracking 

the transient particle concentration on floor surface due to dynamic deposition and 

resuspension process. A three-zone building with a heating, ventilation, and air conditioning 

system is used as the simulation case to test the convergence and accuracy of this algorithm 

under different particle-release scenarios. The algorithm shows a very fast convergence speed 

in the simulation. Comparisons of calculation results between the multizone model and the 

analytical model show good agreement and verify the accuracy of the multizone model 

simulation. The airborne particle concentration profiles and human breathing dose are also 

analyzed for the three-zone building model.

Hu, B., Freihaut, J., Bahnfleth, W., and Thran, B. 2007b. Simulating transient particle

deposition/re-suspension in indoor environments. Proceedings of Third International Building 

Physics Conference. Montreal. 

ABSTRACT The deposition and resuspension of particles are important factors in the 

determination of airborne concentrations resulting from accidental and intentional 

contamination of a building. Most multizone airflow and contaminant transport modeling 

software treats particulate contaminants as gases, and may not possess the capability to track 

the cumulative amount of aerosol deposited, which is needed for the simulation of transient 

resuspension processes. A methodology is described for implementing particulate dispersion 

modeling in a transient, multizone simulation that is consistent with these limitations. The key 

elements of the method are the calculation of particulate source/sink values in the form 

required by the simulation software and the use of an iterative procedure to establish the 
63

transient surface reservoir concentration. The method is demonstrated and tested using a 

widely used, public domain, multizone modeling program, CONTAM 2.1. It is shown that, 

although somewhat cumbersome given current capabilities, this method produces reasonable 

results. Modifications to existing software capabilities required for automated implementation 

of resuspension are also recommended.

Kim, Y., A. Gidwani, M. Sippola, and C. Sohn. 2008. Source term model for fine particle 

resuspension from indoor surfaces. Engineering Research and Development Center, Report

ERDC/CERL TR-08-4. 

ABSTRACT This Phase I effort developed a source term model for particle resuspension from 

indoor surfaces to be used as a source term boundary condition for CFD simulation of particle 

transport and dispersion in a building. Specifically, this work: (1) investigated responsible 

mechanisms for fine particle resuspension from indoor surfaces, (2) identified parameters 

relevant to resuspension, (3) performed a dimensional analysis and derivation of a 

resuspension model, and (4) evaluated the model against published experimental data on 

resuspension. Preliminary validation of the derived model was conducted based on a set of 

experimental data from the Lawrence Berkeley National Laboratory.

Kim, Y., A. Gidwani, B. Wyslouzil, and C. Sohn. 2010. Source term models for fine particle 

resuspension from indoor surfaces. Building and Environment. 45(8): 1854-1865.

ABSTRACT Understanding the dispersion of contaminants inside buildings is important for 

improving indoor air quality (IAQ). Detailed information on the dispersion profile within a room 

is required to design active protection systems and to develop countermeasure strategies 

against potential threats from particulate based agents. A number of computational fluid 

dynamics (CFD) codes in the public and commercial domain can simulate contaminant 

dispersion inside a building. One of the critical boundary conditions required by these CFD 

codes is a resuspension source term model. This paper develops general source term models 

for particle resuspension from indoor surfaces based on dimensional analysis. First, the physical 

mechanisms responsible for fine particle resuspension from indoor surfaces are investigated 

and relevant parameters are identified. Then, three different models are developed using 

dimensional analysis and published resuspension data in the literature. Finally, the models are 

evaluated against independent experimental data that were not used to determine the model 

coefficients.

Lazaridis, M. and Y. Drossinos. 1998. Multilayer resuspension of small identical particles by 

turbulent flow. Aerosol Science and Technology, 28(6): 548-560.

ABSTRACT A model for the resuspension of a multilayer deposit by turbulent flow is developed. 

The resuspension rate is obtained by solving a set of coupled, first order kinetic equations. The 

multilayer resuspension rate depends explicitly on single-particle resuspension rates that are 

determined from modified energy transfer model. The surface-particle and particle-particle 

interaction potentials are calculated by a microscopic approach based on the integration of the 

Lennard-Jones intermolecular interaction potential. The effect of the surface roughness, which 

leads to a distribution of the adhesive forces, is considered, as well as the energy transfer from 

the fluctuating part of the turbulent flow to the particle. It is shown that for a geometrical 
64

arrangement of deposited particles with a co-ordination number of two (particles stacked on 

top of each other) particles from the top layers resuspend at lower friction velocities than 

particles adjacent to the surface. The predicted long term resuspension rate decays 

algebraically with exposure time. Calculations are presented for a two-layer deposit of either 

SnO, and AZ,O, particles on a stainless steel surface.

Luoma, M., and Batterman, S.A. 2001. Characterization of particulate emissions from

occupant activities in offices. Indoor Air, 11: 35-48. 

ABSTRACT This paper characterizes the relationship between occupant activities and indoor air 

particulate levels in a non-smoking office building. Occupant activities were recorded on video. 

Particulate concentrations were monitored by three optical particle counters (OPCs) in five size 

ranges at three heights. Particulate mass concentrations were measured gravimetrically and 

bioaerosol concentrations were determined by impaction methods. Occupant activities and 

number concentrations were determined with 1-min resolution over a 1-week period. 

Occupant activities such as walking past or visiting the monitoring site explained 24–55% of the 

variation of 1- to 25-mm diameter particle number concentrations. Statistical models 

associating particulate concentrations with occupant activities depended on the size fraction 

and included an autocorrelative term. Occupant activities are estimated to contribute up to 

10 μg m-3in particulate concentrations per person. Number concentrations of particles smaller 

than 1 mm had little correlation with indoor activities other than cigarette smoking and were 

highly correlated with outdoor levels. The method can be used to characterize emissions from 

activities if rapid measurements can be made and if activities can be coded from the video 

record.

Schneider, T., Kildesø, J., and Breum, N.O. 1999. A two compartment model for 

determining the contribution of sources, surface deposition and resuspension to air and surface 

dust concentration levels in occupied rooms. Building and Environment, 34: 583-595. 

ABSTRACT A semi-empirical two-compartment, constant parameter model is used to predict 

airborne and surface dust concentrations. The model parameters are air in- and exfiltration, 

internal particle sources, surface deposition caused by settling, Brownian and turbulent

diffusion and thermophoresis, track-in of dust particles and resuspension. Model predictions 

are calculated for some typical scenarios, and the soiling rate of a vertical surface is calculated 

for a range of friction velocities and electric field strengths. Model sensitivity is determined 

based on input parameter value distributions for a population of rooms estimated from 

published data. The predictions are sensitive to track-in and resuspension rates on which field 

data thus are needed.

Sextro, R. D. Lorenzetti, M. Sohn, and T. Thatcher. 2002. Modeling the spread of anthrax 

in buildings. Proceedings of Indoor Air 2002.

ABSTRACT The recent contamination of several U.S. buildings by letters containing anthrax 

demonstrates the need to understand better the transport and fate of anthrax spores within 

buildings. We modeled the spread of anthrax for a hypothetical office suite and estimated the 

distribution of mass and resulting occupant exposures. Based on our modeling assumptions, 

more than 90% of the anthrax released remains in the building during the first 48 hours, with 
65

the largest fraction of the mass accumulating on floor surfaces where it is subject to tracking 

and resuspension. Although tracking and resuspension account for only a small amount of mass 

transfer, the model results suggests they can have an important effect on subsequent 

exposures. Additional research is necessary to understand and quantify these processes.

Stuempfle, A.K., Fischer, B.W. 2009. Biological aerosol hazard assessments of emergency 

responders in specific scenarios. Edgewood Chemical Biological Center, ECBC-CR-101 (OMI762). 

ABSTRACT The Biological-agent Indoor-Evaporation model was used to estimate the respirable 

particle concentration-time profile and the resuspension of particles by human activity in 

ventilated enclosures. Various venues, ventilation conditions, dissemination devices, Biological 

Warfare Agents (BWAs), and responder actions either during or following agent release were 

examined. The magnitude of the BWA hazards depended on the type of pathogen released, its 

quantity, dissemination method, and the incident site and conditions. "Scenario average 

particle concentration values" for the selected dissemination devices and indoor scenarios were 

estimated as a function of time. The expected indoor respirable particle concentrations 

following explosive and spray releases in typical meeting rooms depended on the device's fill 

weight and the agent's bulk concentration (colony forming units/gram). For the scenarios 

examined, maximum challenge levels/gram of agent disseminated ranged from 106—1010

particles/m3. The resuspension of deposited biological agents from flooring material by 

mechanical stress was a small fraction of the contamination density, but can result in a 

significant respiratory hazard. The reaerosolized and resuspended particulates could reach 

levels of ~103-105 particles/m3, depending on the flooring material and initial Hot-Zone 

conditions. Numerous data deficiencies and knowledge gaps regarding bioaerosols and their 

dissemination devices were identified. Additional experimental effort is needed.

Zhang, X., Ahmadi, G., Qian, J., and Ferro, A. 2008. Particle detachment, resuspension and

transport due to human walking in indoor environments. Journal of Adhesion Science and 

Technology, 22: 591-621.

ABSTRACT In this work, particles detachment, resuspension and transport due to indoor human 

walking were studied numerically and experimentally. The stepping motions of the foot, down 

and up, were modeled using a combination of two effective circular disks. The flow generated 

by the squeezing film at the shoe–floor interface was assumed to be laminar and the 

corresponding velocity field was evaluated. The flow outside of the foot was modeled based on 

a wall jet theory. The effects of adhesion force and surface roughness were included in the 

analysis. Models for particle detachment and resuspension were developed. The effects of 

particle-wall adhesion force and the hydrodynamic drag and lift forces were included in the 

particle detachment model. Spreading and dispersion of resuspended particle clouds was also 

evaluated. Particle deposition, turbulent diffusion and Brownian diffusion were also included in 

the particle transport model. Comparisons of the model predictions for particle concentration 

in the room and for particle resuspension rate with the obtained experimental data showed 

good agreements. The simulation results showed that shoe bottom roughness, foot size, 

walking velocity, background velocity as well as the foot stepping velocities, down and up, all 
66

affected particle resuspension rate from the floor as well as the corresponding particle

concentrations in the indoor environment.

Reviews

Hu, B., Freihaut, J.D., Bahnfleth, W., Gomes, C.A.S, Freihaut, F. and Thran, B. 2005.

Literature review and parametric study: indoor particle resuspension by human activity.

Proceedings of Indoor Air 2005.

ABSTRACT Once an aerosol contaminant is introduced into an indoor environment, it can 

remain in the air, deposit on interior surfaces or attach to dust particles already present. 

Human activity, such as walking and cleaning, resuspends contaminated particles, regenerating 

airborne contaminants. This report is a literature review on the effects of human activity on 

particle re-suspension in indoor environment. A parametric investigation is also made on 

particle resuspension effects of potential mechanical, aerodynamic and electrostatic force 

components, due to human activity over the particle-containing reservoir. It is shown that 

combinations of mechanical-aero-electro forces from human activity, can lead to significant 

particle resuspension in indoor environment. Additional experimental and modeling work is 

recommended to quantify the influence of human activity on indoor particle resuspension.

Nicholson, K.W. 1998. Review of Particle Resuspension. Atmospheric Environment. 22 (12): 

2639-2651.

ABSTRACT Some of the various types of studies on particle resuspension or re-entrainment are 

summarized along with shortcomings. General experimental aspects have been considered, 

rather than focusing on the numerical values of results, and research on erosion and 

resuspension by mechanisms other than wind has been included. It is evident that experiments 

have been performed in a wide range of environmental conditions but that additional research 

is required, in many areas, if a quantitative assessment of resuspension is to be achieved.

Qian, J., Peccia, J., Ferro, A.R. 2014. Walking-induced particle resuspension in indoor 

environments. Atmospheric Environment. 89: 464-481.

ABSTRACT Resuspension of particles indoors increases the risk of consequent exposure through 

inhalation and nondietary ingestion. Studies have been conducted to characterize indoor 

particle resuspension but results do not always agree, and there are still many open questions 

in this field. This paper reviews the recent research of indoor resuspension and summarizes 

findings to answer six critical questions: 1) How does the resuspension sources compared to 

other indoor sources; 2) How is resuspension determined and how does the resuspension 

measure change as a function of particle size; 3) What are the primary resuspension 

mechanisms; 4) What are the factors affecting resuspension; 5) What are the knowledge gaps 

and future research directions in this area; and 6) How can what we know about resuspension 

guide better exposure mitigation strategies? From synthesized results, we conclude that 

resuspension is an important source for indoor particulate matter, compared with other indoor 

sources. Among all existing quantification terms of resuspension, resuspension fraction has the 

least variation in its estimates by explicitly defining surface loading and walking frequency, and 

thus is recommended to be adopted in future research over other terms. Resuspension 

increases with particle size in the range of 0.7 - 10 m, although differences exist in 
67

resuspension estimates by orders of magnitude. The primary mechanism of particle 

resuspension involves rolling detachment, and the adhesive forces can be greatly reduced by 

microscopic surface roughness. Particle resuspension is by nature complicated, affected by 

various factors and their interactions. There are still many open questions to be answered to 

achieve an understanding of resuspension fundamentals. Given the complex and 

multidisciplinary nature of resuspension, understanding indoor particle resuspension behavior 

requires cross-disciplinary participation from experts in aerosol science, textile science, surface 

chemistry, electrostatics, and fluid mechanics.

Sehmel, G.A. 1980. Particle resuspension: A review. Environment International, 4: 107-127.

ABSTRACT Published numerical values of resuspension variables, rates, factors, and weathering 

half-lives are summarized. Results of the review show the great uncertainty in accurately 

predicting resuspension. Resuspension rates range over six orders of magnitude from 10-12 to 

10-4fraction/sec, resuspension factors over nine orders of magnitude from 10-10 to over 

10-2 m-1, and weathering half-lives from 35 days to years.