SC IEN

CE

United States

Environmental Protection

Agency

EPA/600/R-09/009 | Feburary 2009 | www.epa.gov/ord

Resuspension of Fibers From Indoor 

Surfaces Due to Human Activity 

FINAL REPORT

Office of Research and Development

National Homeland Security Research Center
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EPA/600/R-09/009 | Feburary 2009 | www.epa.gov/ord

FINAL REPORT ON

Resuspension of Fibers From Indoor 

Surfaces Due to Human Activity

Prepared by

Jonathan Thornburg and Charles Rodes

Center for Aerosol Technology

RTI International

Post Office Box 12194

Research Triangle Park, NC 27709

Order Number: 3C-R321-NANX

Submitted by

Charles Rodes, PhD

Project Manager

Prepared for

Jacky Rosati, PhD

Project Officer

U.S. Environmental Protection Agency

National Risk Management Research Laboratory

Research Triangle Park, NC 27711
iv

RTI/08924 March, 2005

Notice

This report is submitted in fulfillment of Order Number 3C-R321-NANX by RTI International under 

the sponsorship of the United States Environmental Protection Agency. This report covers a period 

from October 2003 to March 2005, and work was completed as of March 4, 2005.
v

Disclaimer

The U.S. Environmental Protection Agency through its Office of Research and Development funded 

and managed the research described here under contract to RTI International. It has been subjected to 

the Agency’s review and has been approved for publication. Note that approval does not signify that 

the contents necessarily reflect the views of the Agency.
vi

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:

a. MicroVac method following a modified version of ASTM D5755-95,

b. Ultrasonication method developed by Millete et al. (1993),

c. 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.
vii

Table of Contents

Notice...................................................................................................................................................iv

Disclaimer ...........................................................................................................................................v

Abstract...............................................................................................................................................vi

List of Figures.....................................................................................................................................viii

List of Tables.......................................................................................................................................ix

1.0 Introduction..................................................................................................................................1

1.1 Scope ........................................................................................................................................1

1.2 Research Objectives.................................................................................................................1

1.3 General Approach.....................................................................................................................1

1.4 Underlying Variables................................................................................................................2

1.5 Data Presentation......................................................................................................................2

2.0 Experimental Methods ................................................................................................................3

2.1 Instrumentation & Procedures..................................................................................................3

2.2 Tests Conducted........................................................................................................................7

2.3 Emission Factor Calculations...................................................................................................7

3.0 Results & Discussion....................................................................................................................11

3.1 General Results.........................................................................................................................11

3.2 Wollastonite Available Method Comparison............................................................................12

3.3 Resuspended Wollastonite Measurement Method Comparison...............................................19

3.4 Wollastonite Size Distribution..................................................................................................19

3.5 Emission Factors......................................................................................................................21

3.6 K-Factors..................................................................................................................................24

4.0 Quality Assurance ........................................................................................................................31

4.1 URG Samples...........................................................................................................................31

4.2 MicroVac Samples....................................................................................................................31

4.3 Deposition Chamber Samples..................................................................................................32

4.4 Ultrasonication .........................................................................................................................32

4.5 SEM Image Analysis................................................................................................................34

4.6 Aerodynamic Particle Sizer......................................................................................................34

4.7 Temperature & Relative Humidity ...........................................................................................34

5.0 Conclusions...................................................................................................................................35

6.0 References.....................................................................................................................................37

Appendix A: SEM Image Analysis...................................................................................................A-1

Appendix B: Relationship between Aerodynamic and Fiber Diameters......................................B-1
viii

List of Figures

Figure 2-1. Aerosol generator and deposition chamber used to load Wollastonite on carpet..............5

Figure 2-2. Relationship between mass injected into deposition chamber with mass 

deposited on carpet surface ...............................................................................................5

Figure 2-3. Schematic of Large Dynamic Chamber used in resuspension tests..................................6

Figure 2-4. Particle transport efficiency in APS isokinetic sample line in Large 

Dynamic Chamber ............................................................................................................7

Figure 3-1. Conversion of total Wollastonite mass to total number of Wollastonite fibers 

per 1296 in2 of carpet.........................................................................................................16

Figure 3-2. Relationship between total Wollastonite mass resuspended during a 5 minute 

testand the average number concentration ........................................................................17

Figure 3-3. Relationship between mass resuspended estimates measured by URG and APS.............20

Figure 3-4. Relationship between resuspended Wollastonite counts measured by URG 

and APS.............................................................................................................................21

Figure 3-5. Average size dependent, mass based emission factors calculated from APS and 

SEM Fiber data for high and low relative humidity tests.................................................26

Figure 3-6. Average size dependent, count based emission factors calculated from APS and 

SEM Fiber data for vacuuming and walking resuspension method tests..........................28

Figure A-1. Comparison of RTI and METI aerodynamic diameters...................................................A-2

Figure B-1 Comparison of fiber aerodynamic (from APS) and fiber cross sectional diameters..........B-1
ix

List of Tables

Table 2-1. Nyglos M3 Wollastonite fiber size distribution ..................................................................3

Table 2-2. Carpet characteristics..........................................................................................................4

Table 2-3. Description of tests conducted in RTI exposure chamber ..................................................8

Table 3-1. Background particle mass found in tested carpets by multiple methods 

for determining mass available for resuspension................................................................11

Table 3-2. Experimental conditions recorded during each test............................................................13

Table 3-3. General experimental results presented as particle mass....................................................14

Table 3-4. General experimental results presented as particle count...................................................15

Table 3-5. Correlation coefficients between different estimates of mass available 

for resuspension ..................................................................................................................18

Table 3-6. Correlation coefficients between different estimates of counts available 

for resuspension ..................................................................................................................18

Table 3-7. Size distribution of Wollastonite deposited on new and old carpet as measured

from ultrasonication filters and individual carpet fibers .....................................................22

Table 3-8. Size distribution of Wollastonite resuspended via walking and vacuuming 

as measured by the APS......................................................................................................22

Table 3-9. Emission factors calculated on total mass resuspended for each test condition.................23

Table 3-10. Table of p-values for each method of calculating mass emission factors (EmFa)............23

Table 3-11. Emission factors calculated on total count resuspended for each test condition ..............25

Table 3-12. Table of p-values for each method of calculating count emission factors (EmFa)...........25

Table 3-13. Table of size dependent APS/SEM Fiber mass emission factors for each test 

condition using new carpet................................................................................................25

Table 3-14. Table of p-values for APS/SEM Fiber mass emission factors including particle

diameter as an independent variable .................................................................................26

Table 3-15. Table of size dependent APS/SEM Fiber count emission factors for each test 

condition using new carpet................................................................................................27

Table 3-16. Table of p-values for APS/SEM Fiber count emission factors including particle 

diameter as an independent variable .................................................................................27

Table 3-17. Aggregate K-factors for each test .....................................................................................29

Table 3-18. Size specific K-factors for each test using new carpet. K-factors calculated 

from emission factors in Table 3-15..................................................................................29

Table 4-1. QA/QC criteria for measurements collected.......................................................................31

Table 4-2. Quality control measures to implement during testing.......................................................32

Table 4-3. URG sample completeness, filter blank, precision, and accuracy statistics.......................33

Table 4-4. MicroVac sample completeness, filter blank, precision, and accuracy statistics................33

Table 4-5. Deposition chamber filter sample completeness, filter blank, precision, 

and accuracy statistics.........................................................................................................33

Table 4-6. Gravimetric mass as measured by ultrasonication: sample completeness, 

blank, precision, and accuracy statistics .............................................................................33

Table 4-7. Size distribution from SEM images of ultrasonication filters: sample 

completeness, blank, precision, and accuracy statistics......................................................33

Table 4-8. Aerodynamic Particle Sizer sample completeness statistics...............................................34

Table 4-9. QA/QC results for the HOBO H8 temperature and relative humidity data........................34
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1

1.0

Introduction

1.1 Scope

The events of 9/11 and the subsequent related activities 

released an enormous particulateload into the environment 

proximal to the WTC complex. This aerosol burden included 

building material fragments that contained asbestos and 

other mineral fibers known or suspected to be health hazards. 

Although much of the aerosol subsequently deposited 

outdoors, a significant portion penetrated into nearby 

residences and businesses. The size distributions and quantity 

of the aerosol relative to the outside environment were 

strongly affected by the filtration ability of the ventilation 

system and building envelope. The model of Thornburg et al. 

(2001) provides a means of predicting the resultant aerosol 

sizes and quantities that would have been found in these 

buildings. A large fraction of the aerosols that penetrated 

these buildings deposited onto horizontal and vertical 

surfaces. Normal occupant activities can resuspend portions 

of the deposited aerosol on horizontal surfaces. Walking 

on medium pile carpeting is known to resuspend particles 

in the 2 to 10 mm range (Rodes and Wiener, 2001). This 

aerosol can result in unhealthful asbestos and other mineral 

fiber aerosol concentrations within the general vicinity of 

the activity and translocate the aerosol to other locations and 

surfaces within the building.

The research described here seeks to relate concentrations of 

asbestos-type fibers deposited upon carpeting by processes 

similar to those that occurred after 9/11 to the airborne 

concentrations that may have resulted from normal resident 

activities, such as walking and vacuuming. The general 

focus of this pilot effort was problem-identification rather 

than phenomenon characterization, since a) it is clearly 

retrospective, b) only simulants representing the real aerosol 

are available, and c) only a limited number of samples will be 

collected and analyzed within the resources available. This 

work built upon dust resuspension characterization methods 

for generic PM on carpeting studying the resuspension 

of generic particulate matter and metals (respectively) 

from flooring surfaces due to human activity (Rodes and 

Thornburg, 2004; Thornburg and Rodes, 2004). These pilot 

efforts provided the fundamental experimental methodologies 

and limited data to determine whether the resuspension of a 

tracked-in particles and metals from soil dust deposited on 

medium pile carpeting was significant and warrants further 

study. An important outcome of this research was collection 

of data that could be used in subsequent multi-pathway 

exposure modeling. This focus involved an experimental 

design that carefully considered how lifestyle factors such 

as adult and children’s activity levels, residence volumes, 

fraction of flooring with carpeting, etc. could be used in 

exposure models in conjunction with the more fundamental 

data from this study.

1.2 Research Objectives

1. Determine quantity (mass and number) of simulant 

fibers available for resuspension, as compared to 

deposited mass, using a variety of techniques that 

included:

a. MicroVac method following a modified version of 

ASTM D5755-95.

b. Ultrasonication method developed by Millete et al. 

(1993).

c. Individual carpet fiber analysis via scanning 

electron microscopy method (Thornburg et al., 

2006)

2. Calculate simulant fiber emission factors at two heights 

while walking on and vacuuming seeded carpet. 

Aggregate and size dependent emission factors were 

calculated on a mass and count basis.

1.3 General Approach

The key elements of the research approach defined to address 

the objectives given in Section 1.1 were:

1. Select an appropriate asbestos fiber simulant

2. Develop or identify a methodology to deposit and 

embed (seed) known quantities of fiber simulant on 

carpet that emulates probable post-9/11 activities.

3. Select a vacuum cleaner representative of a 

homeowner quality unit rather than a HEPA unit that 

would be used in remediation efforts.

4. Develop a project Quality Assurance Project Plan 

before beginning experimental work (see: “Indoor PM 

Resuspension Testing of Fibers - Quality Assurance 

Project Plan For Basic Research Projects, EPA Order 

No. 3C-R321-NANX, EPA/NRMRL under Contract 

No. QT-OH-03-00572 RTI Project No. 08924)

5. Obtain new and old carpet for experimental use. 

Characterize background loading of particles on carpet. 

Condition carpet as necessary to obtain acceptable 

background levels.

6. Use existing RTI standardized test methodologies, 

incorporate other established procedures, and devise 

new methods as necessary that consider fiber simulants 

by particle size that define:

a. How to characterize the fiber simulant loadings on 

carpet fibers by methods specified in the project 

objectives that consider how much dust is available 

for resuspension, as a function of particle size.

b. How to characterize carpeting in terms of pile 

height (fiber length), age, soiling history, and 

surface loading level.
2

c. How to resuspend particles in a controlled 

environment with minimal interference from 

background particles. load carpeting with defined 

areal loadings of a selected dust,

d. How to quantify air concentrations as function of 

height, fully correcting for transmission line losses 

between the measurement points and the sensing 

zone, thus facilitating mass balance closure by 

accounting for the (large) fraction of particle mass 

often lost to internal surfaces

7. Conduct controlled walking and vacuuming 

experiments on new and/or old carpet loaded 

with up to three different quantities of fiber simulant.

1.4 Underlying Variables

Two variables that probably influence resuspension of 

particles from carpets were not studied during this project 

because of the limited scope of work. However, our 

hypotheses and suppositions regarding these variables based 

on our carpet research experience are presented below 

because these concepts aid in the interpretation of the data 

collected during these experiments.

1.4.1 Flooring Surfaces

Previous RTI resuspension data showed that as the pile 

height decreased, the level of resuspension decreased 

substantially from normal walking. No measurable dusts 

were observed from dusts on bare flooring. It could be 

conjectured that resuspending dust from bare floors requires 

substantial turbulence from either stomping or very fast 

walking to provide the energy to both release particles from 

the surface and elevate them into the air sufficiently to add 

to the air concentration. Low pile, indoor-outdoor carpeting 

also provided essentially immeasurable air concentration 

levels. Thus, the current work focused only on medium pile 

carpeting (~70% of all new carpeting sold) which has been 

shown to contribute significantly to resuspension.

1.4.2 Particle Adhesion

An important factor felt to be extremely important in 

understanding resuspension of particles from carpet fibers is 

adhesion. Adhesion of particles to carpet fibers is influenced 

by relative humidity (Rodes and Thornburg, 2004), thought 

to be controlled primarily by electrical charging of both the 

fibers and the particles. Very low humidities are routinely 

found to increase the charging of certain formulations of 

carpet fibers. An additional adhesive force considered here 

is surface tension: particle-to-particle and particle-to-fiber. 

Surface tension forces bonding particles together or to the 

fibers is felt to be very important at relative humidities above 

~45%. At 45% Rh, sufficient water is present to increase 

the surface adhesion force by increasing the contact angle 

between the particle and a second surface (Ranade, 1987). 

Both of these types of adhesive forces work together to bond 

particles to surfaces. Undoubtedly, resuspension occurs when 

sufficient energy is imparted to exceed the cumulative force 

levels.

Our extensive scanning electron imaging of carpet fibers has 

shown that as carpeting ages or becomes significantly soiled 

(including coating of the fibers over time by grease aerosol 

from cooking), the carpet fibers ability to generate static 

charging appears to decrease substantially. This substantial 

change in potential adhesion characteristics for particles to 

fibers between new and old carpeting was addressed in the 

current research by considering either new, unsoiled carpeting, 

or soiled carpeting that was at least 1 year (or substantially 

more) old as a binomial variable.

No efforts were made to measure either type of adhesion in 

these experiments, but temperature and relative humidity 

were recorded during all tests to determine if the influences 

from these types of adhesions could be estimated categorically. 

Successful modeling of particle resuspension will at some 

point require more detailed investigation of the relationships 

among these factors.

1.5 Data Presentation

The wide array of methods used to measure the asbestos 

fiber simulant size distribution and concentration on the carpet 

fibers and resuspended required selection of a reference metric. 

Asbestos simulant size was measured via microscopy to yield 

a projected area diameter, and via aerosol instrumentation 

to yield an aerodynamic diameter. In addition, the bulk 

asbestos fiber simulant size distribution was measured in 

terms of optical diameters. For the purposes of this research, 

aerodynamic diameter was selected to characterize the 

asbestos simulant size distribution. Aerodynamic diameter is 

the standard reference for all aerosol research. As such, factors 

are available for converting other diameter measurements to 

aerodynamic diameter. For example, projected area diameters 

can be converted to aerodynamic diameters using established 

conversion factors as described in Section 2.1.9. However, 

a conversion between optical and aerodynamic diameters 

does not exist because of the complexity surrounding 

particle refractive indices. Therefore, a basis does not exist 

for comparing the aerodynamic diameters reported from the 

microscopy measurements and aerosol instrumentation with 

the optical diameters used to characterize the bulk asbestos 

simulant size distribution.

Similarly, microscopy measurements provided asbestos 

simulant fiber count data, realtime aerosol instrumentation 

provided either count or mass concentration data, and 

gravimetric analysis yielded mass concentration data. 

Gravimetric analysis determined the quantity of asbestos 

simulant deposited on the test surfaces and one method 

for measuring the quantity of simulant resuspended during 

the experiments. Without a priori knowledge of the bulk 

simulant aerodynamic size distribution (due to the lack 

of an optical-aerodynamic conversion discussed above) 

an established method for converting mass data to count 

data was not available. Asbestos simulant count data from 

microscopy and real-time aerosol instrumentation easily were 

converted to a mass basis from the volume and density of the 

measured particles. To provide an estimate of the deposited 

and resuspended asbestos simulant count concentrations, 

regression curves relating the mass to the count concentrations 

are presented in Section 3.1.
3

2.0

Experimental Methods

2.1 Instrumentation & Procedures

The instrumentation and procedures selected determined the 

particle aerodynamic diameter and concentration, either mass 

or number, of the particles resuspended from the carpet or 

embedded on the carpet fibers.

2.1.1 Asbestos Fiber Simulant

Calcium Silicate (CaSiO3), commonly known as 

Wollastonite, was the asbestos fiber simulant selected. 

Wollastonite fibers were the simulant of choice for asbestos 

occupational exposure studies because of their acicular 

nature. Nyglos M3 (NYCO Minerals, Willsboro NY) was 

used (Table 2.1). Wollastonite fibers had a mean aspect ratio 

of 3.33 ± 1 and density of 2.9 g/cm3.

2.1.2 Carpet Characteristics

New and old carpets were obtained for the resuspension 

experiments (Table 2-2). New carpet was beige, medium 

pile carpet manufactured by Shaw Carpets or Beaulieu. The 

carpet was purchased retail and was removed directly off the 

main roll. New carpet was vacuumed multiple times with a 

standard household vacuum to remove excess loose fibers 

and dirt. Old carpet was obtained from the same retailer after 

removal from unknown private residences. The age, use, 

manufacturer, and history of the old carpet was unknown. 

Old carpet exhibiting normal wear and soiling were selected; 

carpets with excessive wear and obvious soiling were 

avoided. Old carpet was cleaned thoroughly with a rental 

hot water carpet cleaner and vacuumed multiple times with a 

standard vacuum before experimental use.

2.1.3 Deposition Chamber

New and old carpet was seeded with Nyglos M33 

Wollastonite fibers using a flow through deposition chamber 

(Figure 2-1). Simulant dust was aspirated and dispersed into 

a laminar flow transport pipe that carried the fibers to the 

injection head at the top of the chamber. A mixing baffle at 

the injection head evenly dispersed the fibers across the cross 

section of the chamber. The velocity at the injection head 

outlet was balanced with the downward air velocity generated 

by a fan that pulled the fibers into the carpet.

A known mass of Wollastonite was injected into the chamber. 

47 mm Teflo® filters placed in a 9-point grid pattern across 

the surface of the carpet collected samples of deposited 

Wollastonite to determine gravimetrically the total mass 

deposited. Mass deposited on the carpet was calculated by 

taking the average mass loading (mg/in2) measured per filter 

and multiplying by the area of exposed carpet (Ac),

(2-1)

where Fm is the filter mass and d2 is the filter diameter.

Carpet loadings in the chamber were found to be linear with 

injected dust mass, and predictable, having an R2 value of 

0.96. Figure 2-2 illustrates the experimental utility of the 

chamber, showing the linearity and strong correlation of the 

regression of injected mass versus deposited mass. Without 

demonstrating this linearity, it would have been inappropriate 

to normalize the emission factors by loading. Note a leak in 

the injection nozzle was identified after all experiments were 

completed. This leak limited the percentage of Wollastonite 

deposited to 2%. However, the percentage deposited was 

linear with quantity injected. As a result, the quality of the 

Wollastonite resuspension data was not compromised.

Table 2-1. Nyglos M3 Wollastonite fiber size distribution. 

Median diameter = 2.5 μm by Cilas Granulometer

Diameter (μm) Mass Fraction Cumulative Fraction

<0.5 0.09 0.09

1 0.11 0.20

2 0.22 0.42

3 0.16 0.58

4 0.14 0.72

5 0.09 0.81

7 0.08 0.89

10 0.07 0.96

>10 0.04 1
4

Table 2-2. Carpet characteristics

Carpet Mfg Material # Fibers/in2 Pile Height Test Use

New #1 Shaw Nylon 6,6 4000 10 mm Walking

New #2 Beaulieu Nylon 6,6 4000 8 mm Vacuuming

Old #1 unknown unknown 3000a 10 mm Walking

Old #2 unknown Nylon 6,6 2500a 12 mm Walking &

Vacuuming

a

Estimated by counting number of fibers per loop and number of loops per in2.

2.1.4 Exposure Chamber

All tests were conducted in the RTI Large Dynamic Chamber 

(Figure 2-3). This positive pressure chamber equipped with 

six pre-filters and six ultra low penetration air (ULPA) filters 

can provide essentially particle-free air at flows ranging from 

0.25 to 80 m3/min. For these tests, the flow was 13.2 m3/

min; equivalent to a linear face velocity in the test section 

of 10 cm/s. A flow distribution baffle preceding the filters 

creates a uniform velocity profile through the chamber. The 

test space within the chamber is 4 ft wide x 7 ft long x 6 ft 

high. Carpet (3 x 3 ft) samples were placed in this space. 

A contraction of 12° over 2.4 meters with a mixing baffle 

immediately upstream of the 20 cm diameter outlet pipe 

uniformly concentrates the aerosol generated in the test 

space. Mass samplers were installed in this section to collect 

PM10 filter samples isokinetically for gravimetric analysis. 

Sampling ports in the contraction allow direct sampling from 

within the test space at the desired heights and with multiple 

instruments. A Model 3321 Aerodynamic Particle Sizer (TSI 

Inc., Minneapolis MN) was connected to sample ports in 

the outlet pipe for isokinetic collection of resuspended PM 

samples for size distribution measurements. Temperature and 

relative humidity were monitored with a HOBO H8 Data 

Logger (Onset Computer Corp., Bourne MA) installed in 

the test space. Preliminary tests confirmed air entering the 

chamber and activities conducted by test personnel wearing 

cleanroom coveralls inside the chamber did not generate 

a significant number of particles above background. As 

an extra precaution the chamber test section surfaces were 

vacuumed and wiped with clean, damp cloths prior to 

each test.

2.1.5 Resuspension Methods

Walking and vacuuming were the resuspension methods 

selected. A single volunteer walked randomly across the 

carpet area for 5 minutes during an experiment. The volunteer 

was a 73 inch, 170 lb male with size 12 shoes. A particle free 

cleanroom garment was worn for all tests. The same shoes 

were worn during all walking experiments. A pedometer 

worn by the volunteer determined the number of steps 

taken by the volunteer during the experiment. The volunteer 

walked across each test carpet for 3 minutes prior to seeding 

with Wollastonite to determine the background resuspended 

particle concentration. The subject attempted to apply a 

constant foot pressure (energy) to the carpet during since 

previous work showed emission factors varied with energy 

level. A standard 13 amp Mach 2.1 Hoover (Model # U5330-

900) upright vacuum with beater bar was used. The entire 

vacuum was cleaned thoroughly prior to the experiments. A 

new Hoover Type Y Allergen® vacuum bag (99.98% filtration 

efficiency) was installed for each experiment. The vacuum 

was ON for approximately 5 minutes before an experiment 

began to allow particles generated from the starting of the 

motor to leave the chamber. A different volunteer, wearing 

a particle free cleanroom garment, randomly moved the 

vacuum across the carpet in all directions for 5 minutes 

during a test. The volunteer did not walk on the carpet during 

the tests.

2.1.6 Carpet Fibers

Individual carpet fibers were collected from two locations 

within each piece of test carpet after seeding with 

Wollastonite. Fibers were analyzed via SEM to quantify 

the size distribution of the Wollastonite on the carpet fibers 

available for resuspension. A dial micrometer was used to 

estimate the carpet pile height at multiple locations for each 

type of carpet.
5

Figure 2-1. Aerosol generator and deposition chamber used to load Wollastonite on carpet

Figure 2-2. Relationship between mass injected into deposition 

chamber with mass deposited on carpet surface.
6

Figure 2-3. Schematic of Large Dynamic Chamber used in resuspension tests.

2.1.7 Carpet Vacuum Samples

The quantity of particulate matter potentially available 

for resuspension was estimated by collecting two vacuum 

samples onto 47 mm Teflo® filters from an area adjacent to 

where the carpet fibers were extracted per Section 2.1.6. 

The vacuum used was custom designed by RTI. A modified 

ASTM method D5755-95: Standard Test Method for 

Microvacuum Sampling and Indirect Analysis of Dust by 

Transmission Electron Microscopy for Asbestos Structure 

Number Concentrations was followed for the sample 

collection. The RTI modifications included: using a brush 

on the vacuum nozzle to trap carpet fibers, reducing the 

sampling velocity to 45 cm/s to minimize the aggressiveness 

of the procedure, manual removal of obvious (visible 

to naked eye) carpet fibers collected on the filter, and 

gravimetric analysis of the filter instead of liquid extraction 

for microscopic analysis.

2.1.8 Ultrasonication Method

Samples of carpet for ultrasonic extraction of particles 

were collected following a modified version of Millette 

et al. (1993). Approximate 20 cm2 pieces of carpet were 

carefully removed from the test carpet following seeding 

with Wollastonite fibers for the ultrasonic extraction. Carpet 

pieces were immersed fibers down in a 1 L beaker containing 

100 ml of a 0.002% solution of methyl cellulose surfactant 

in particle free water. The entire beaker was placed in an 

ultrasonic bath for 30 minutes. The carpet piece then was 

removed and rinsed with 100 ml of particle free water, 

raising the total volume to 200 ml. The entire solution was 

shaken vigorously to disperse particles then allowed to sit 

for 2 minutes. Only 50 ml aliquots of solution were extracted 

from 1⁄4 to 1⁄2 inch below the surface. Smaller aliquots did not 

yield sufficient number of particles for accurate SEM image 

analysis. The 50 ml aliquot was passed through a coarse 

metal screen to remove large carpet material pieces placed 

over a buchner funnel. The coarsely screened aliquot was 

filtered through a 47 mm, 0.2 μm pore polycarbonate filter, 

backed by a 0.45 μm pore cellulose ester filter placed inside 

the funnel. The filters were equilibrated inside a temperature 

(23°C) and humidity (35%) controlled chamber for at least 24 

hours before transfer to U.S. EPA for SEM imaging. A subset 

of the ultrasonication filters underwent gravimetric analysis 

to provide comparative data for the Wollastonite fiber mass 

measured via SEM.

2.1.9 SEM Image Analysis

Mantech Environmental (METI) provided scanning electron 

microscopy (SEM) images of individual carpet fibers and the 

ultrasonication filters. The aerodynamic diameter of each

particle contained in the images was determined using 

image processing software and established algorithms. The 

algorithms disregarded particles with aspect ratios less than 2 

(non-Wollastonite fibers) and converted the measured particle 

projected area to an equivalent diameter then an aerodynamic 

diameter using standard dynamic shape and volume shape 

factors for fibers (Hinds, 1982). SEM images at 880x 

magnification were provided, equivalent to 5.059 pixels per 

micrometer, providing a particle minimum detection limit of 

0.4 mm. Additional details are provided in Appendix A.

2.1.10 Aerodynamic Particle Sizer

The Aerodynamic Particle Sizer was used to obtain resuspended PM mass and count size distribution data from 

0.5 to 20 μm. The instrument was operated according to the 

instructions provided by the manufacturer. Isokinetic samples 

were collected through the sample probe installed in the 

outlet pipe. Particle losses within the sampling tube, shown 

in Figure 2-4, were estimated using standard aspiration 

and transport equations for laminar flow accounting for 

gravitational and inertial deposition losses (Brockman, 2001). 

Counting efficiency errors were corrected using the data from 

Peters and Leith (2003).

2.1.11 URG Mass Sampler

URG mass samplers (University Research Glassware, 

Chapel Hill, NC) collected PM10 samples isokinetically at 

16.7 Lpm within the test chamber. Two samplers were used 

per test: one at 18” and the other at 36” above the floor. 

However, mixing within the chamber probably eliminated 

any gradient in resuspended PM concentration. Samples were 

collected on 47 mm Teflo® filters. Filter mass was determined 

gravimetrically.

2.1.12 Gravimetric Analysis

Aerosol mass collected on filters were weighed in RTI’s 

temperature and humidity controlled (23°C, 35% Rh) weigh 

chamber on a Mettler Toledo MT2 balance with 0.1 μg 

resolution.
7

2.1.13 Temperature & Relative Humidity

Temperature and relative humidity within the homes was 

measured with a HOBO H8 Data Logger. Temperature and 

relative humidity were recorded at 1-minute intervals.

2.1.14 Statistical Analysis

All statistical analyses were performed using SAS version 

9.1 (SAS Inc., Cary NC). The General Linear Models module 

was used to determine the significance of experimental 

parameters on emission factors. Forward addition and 

backward subtraction methods were used to identify the 

statistical significance of the experimental parameters.

2.2 Tests Conducted

The experimental design selected provided Wollastonite fiber 

emission factor data for various resuspension mechanisms, 

carpet characteristics, and environmental conditions. Walking 

and vacuuming were the resuspension mechanisms. Carpet 

characteristics encompassed new/unmatted carpet with strong 

static charge and old/soiled/matted carpet with little static 

charge. Humidity ranged from typical, indoor air conditioned 

values to extremely high simulating buildings without air 

conditioning during precipitation events. Potential variations 

in emission factors as a function of Wollastonite loading were 

investigated as well. Total Wollastonite mass deposited on 

the carpet surface varied from ~25 mg to ~60 mg to ~100 

mg, representing low, medium, and high loading levels, 

respectively.

Emission factors were calculated multiple ways. 

Resuspended Wollastonite concentrations were measured 

with the APS and URG instruments. The Wollastonite 

available for resuspension was calculated from vacuum 

samples, SEM images, and ultrasonication of carpet sections.

2.3 Emission Factor Calculations

Emission factors were calculated multiple ways. 

Resuspended mass concentrations were measured with the 

APS and URG instruments. Corresponding resuspended 

Wollastonite count data were provided by the APS or 

calculated from the URG gravimetric data. The mass 

available for resuspension was calculated from vacuum 

samples, SEM images, and ultrasonication of carpet sections. 

SEM images provided available Wollastonite count data 

directly. Wollastonite count data from gravimetric samples 

were estimated. Emission factors were normalized by the 

number of steps made during an experiment.

Figure 2-4. Particle transport efficiency in APS isokinetic sample 

line in Large Dynamic Chamber.
8

Table 2-3. Description of tests conducted 

in RTI exposure chamber.

Test # Carpet Method Loading Humidity

1 New #1 Walk Low Low

2 New #1 Walk Medium Low

3 New #1 Walk High Low

4 Old #1 Walk Low Low

5 Old #1 Walk Medium Low

6 Old #1 Walk High Low

7 New #1 Walk Medium High

8 Old #2 Walk Medium Low

9 New #2 Vacuum Low Low

10 New #2 Vacuum High Low

11 Old #2 Vacuum Low Low

12 Old #2 Vacuum High Low

13 New #2 Vacuum High High

14 Old #2 Vacuum High High

15 Old #1 Walk Medium High

2.3.1 Wollastonite Resuspended by APS

Mass resuspended measured by APS was calculated using Eq. 

2-2. The mass concentration in each size bin (CR) corrected 

for the background resuspended particle concentration (CBkg) 

and transport efficiency into the APS (h) were summed to 

yield a total concentration. This concentration was multiplied 

by the flow through the chamber (Qc = 0.013 Lpm), the total 

time particles were resuspended (t), and a correction factor 

for APS counting efficiency ( =2). The total time particles 

were resuspended was not constant across all tests because of 

depletion of the particles available for resuspension.

(2-2)

The APS also provided resuspended Wollastonite number 

concentration data. The total counts resuspended (CAPS) 

were calculated using an equation similar to Eq. 2-2.

(2-3)

2.3.2 Wollastonite Resuspended by Gravimetric Data

Mass resuspended as measured by the URG samplers was 

calculated from the gravimetric mass collected on the filters, 

Mfilter divided by the URG sample flow (QURG = 20 Lpm) 

and multiplied by the flow through the chamber (Qc = 0.013 

Lpm). Mass resuspended was not corrected for background 

because concentrations either were not statistically different 

from zero or contributed less than 5% of the total mass 

collected (as measured by APS).

(2-4)

Resuspended Wollastonite number concentrations (CURG) 

were estimated from the gravimetric mass and APS mass size 

distribution data. The cumulative gravimetric mass (MURG)

was multiplied by the mass fraction in each APS size bin (f

i

) 

then converted to particle counts assuming each fiber was an 

aerodynamic sphere.

(2-5)

2.3.3 Wollastonite Available by MicroVac

Mass available by MicroVac was calculated from the 

gravimetric mass collected on the filters, Mfilter divided by the 

total area vacuumed (Avac = 9 in2) and multiplied by the area 

of the test carpet piece (Ac = 1296 in2).

(2-6)

The number of Wollastonite fibers available for resuspension 

was calculated from the MicroVac mass using an equation 

similar to Eq. 2-5. The Wollastonite size distribution data 

measured by the SEM images of the carpet fibers determined 

the mass fraction in each size range.

(2-7)
9

2.3.4 Wollastonite Available by Carpet Fiber SEM 

Images

Fiber SEM images provided cumulative and size dependent 

estimates of the Wollastonite mass available for resuspension. 

From the size distribution calculated in each image, the total 

mass, MSEM , of particles on the 36” x 36” carpet section was 

calculated based on the incremental mass in each size interval 

(i) via the following equation,

(2-8)

where MFi is the incremental particle mass divided by the 

total number of images processed, N is the fiber length 

visible per photo (101 μm), Nf is the number of millimeters 

of fiber available for resuspension, Lf is the number of fibers 

per unit area of carpet, and Ac is the area of carpet tested 

(36” x 36”). A Nf value for each carpet type was estimated 

by scanning the entire length of the fiber and determining the 

distance from the fiber tip that Wollastonite fibers were no 

longer present. Maximum Nf value was 4 mm, or 50% of the 

fiber length.

The number of Wollastonite fibers available for resuspension 

(CSEM ) was determined directly from the incremental count 

data in each size interval (CFi) provided by the SEM images.

(2-9)

2.3.5 Wollastonite Available by Ultrasonication

SEM images of ultrasonication filters provided a second 

cumulative and size dependent estimate of the Wollastonite 

mass available for resuspension. One photo per buchner 

funnel hole was collected. Usually 5 to 10 holes were 

photographed. The number of holes photographed depended 

on the number of particles surrounding the hole. A sufficient 

number of particles were desired to insure an accurate size 

distribution. From particle size distribution calculated from 

each image, the total mass, MSON , on the 36” x 36” carpet 

section was calculated via the following equation,

(2-10)

where MFi is the incremental particle mass per size bin, Aphoto

is the image area, Nphoto is the number of photos collected, 

Ahole is the area of a buchner funnel hole, Nhole is the total 

number of buchner funnel holes (91), Ac is the carpet section 

area tested, and Asam is the carpet area sonicated. Additionally, 

gravimetric analysis of the ultrasonication filters provided 

another estimate of the Wollastonite mass available for 

resuspension.

A similar equation estimated the total number of Wollastonite 

particles available for resuspension using the particle count 

data from the SEM images.

(2-11)
10

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11

3.0

Results & Discussion

3.1 General Results

The background mass of particles available for resuspension 

from the carpet and resuspended by walking and vacuuming 

were measured for all four carpets tested. The background 

mass does not contain Wollastonite particles, but may 

contain other fibrous particles that affected the SEM analysis. 

Background particle mass data affecting the mass available in 

the carpet is presented in Table 3.1. All resuspension data for 

a respective metric were corrected for the background mass.

As expected, the old carpets yielded higher background 

particle mass, even after extensive cleaning. The MicroVac 

yielded a significant quantity of particles in addition to 

carpet fiber fragments. The destructive ultrasonication 

method yielded a very high mass of particles as measured 

gravimetrically and via SEM. The high mass measured 

via SEM for ultrasonication filters resulted from a large 

number of fibrous type particles being collected on the filter. 

Fiber SEM analysis provided a much smaller background 

particle mass because it is a non-destructive technique. The 

differences between the two types of old carpet was caused 

by the presence of high concentrations of sawdust in Old #2 

that presented as fibrous particles with an aspect ratio similar 

to Wollastonite.

On the other hand, new carpets yielded much lower mass by 

all methods. The small quantity collected by the MicroVac 

probably was carpet fiber fragments. The gravimetric analysis 

of the ultrasonication filters from new carpets showed a 

significant mass of carpet backing is generated during the 

procedure. However, SEM analysis of these filters showed 

these particles were not fibrous (aspect ratios < 2) such that a 

mass and size distribution could not be determined. Similarly, 

the lack of any particles on the new carpet fibers prevented 

quantification of the mass and size distribution by the Fiber

SEM method.

Resuspension tests to characterize the background 

concentration of particles generated during walking and 

vacuuming were conducted. The background resuspended 

particle mass was less than 0.05 mg, equivalent to a 

concentration of 3 μg/m3

. Background concentrations 

were 10 times, minimum, lower than the concentration 

resuspended during a test. Resuspended fiber mass data 

measured by APS and URG systems were corrected for this 

background mass.

General experimental conditions for all tests are presented in 

Table 3-2. Note that depletion occurred during resuspension 

tests with low Wollastonite mass loadings. Depletion of 

the Wollastonite during these tests was accounted for in 

the data analysis via Eqs. 2-2 and 2-3. Also, some of the 

mass deposited estimates were invalid due to movement of 

deposition coupons within the chamber during the carpet 

loading. 

The mass available and mass resuspended data from each test 

are presented in Table 3-3. Overall, data quality objectives 

for each metric were achieved except for the ultrasonication 

SEM mass measurements. The total mass and number of 

particles collected on the ultransonication polycarbonate 

filters was sufficiently high that valid photographs over 

buchner funnel holes could not be obtained. The particle 

loading around the holes was sufficiently large for all 

filters that it was impossible to accurately assess the size 

distribution because of particle overlap. Only by moving 

the image away from the buchner funnel holes could image 

be collected of individually isolated particles so the size 

distribution could be assessed.

Table 3-1. Background particle mass found in tested carpets by multiple 

methods for determining mass available for resuspension. Mass data 

presented in mg. Mass median diameter (MMD) and geometric standard 

deviation (GSD) data are in mm.

Carpet MicroVac

Ultrasonication Fiber SEM

Grav

Mass

SEM

Mass MMD GSD Mass MMD GSD

New #1 9.0 482

Unable to Quantify Unable to Quantify New #2 4.6 238

Old #1 194 - 1161 5.05 2.01 7 7.29 1.88

Old #2 916 2049 3066 5.09 1.87 84 3.52 1.76

Ultrasonication gravimetric mass for Old #1 was invalid.
12

Otherwise, only a few data points noted by asterisks, 

collected during the experiments were invalid. Typical 

reasons included movement of filters during Wollastonite 

deposition on carpet, lost samples (Test 7a), accidental 

destruction of polycarbonate filters during the ultrasonication 

extraction procedure, or insufficient particles were measured 

via SEM on carpet fibers for a statistically valid assessment 

of mass and size distribution. Other samples, noted by 

dashes, were collected but not analyzed because analytical 

protocols were changed during sample analysis. The first 

couple ultrasonication filters were not gravimetrically 

analyzed until an adequate procedure for removal of 

electrostatic charge was developed. Other ultrasonication 

filters were not analyzed via SEM to expedite the data 

analysis and limited resources.

Table 3-4 is similar to Table 3-3, except all data are 

presented as Wollastonite counts. Because of the error in the 

ultrasonication measurements, these data were not included 

in the table.

As discussed in Section 1.5, the relationship between 

mass and count is required to link the data collected by the 

gravimetric and particle counter metrics. The Wollastonite 

mass loaded on the carpet and the calculated number of 

Wollastonite particles loaded onto the carpet is presented 

in Figure 3-3. Similarly, the relationship between the 

Wollastonite mass and number of Wollastonite particles 

resuspended is shown in Figure 3-4. The regression 

equations presented provide a conversion for comparison 

of the data collected presented here with current occupational 

exposure standards. The conversion from mass to counts 

per 1296 in2

 of carpet as measured by SEM images of carpet 

fibers is shown.

3.2 Wollastonite Available Method Comparison

3.2.1 Mass Comparison

Table 3-5 presents correlation coefficients for the various 

estimates of Wollastonite mass available for resuspension. 

The primary correlation of interest is the comparison 

between the mass deposited and the mass available. Direct 

correlations between the various methods determining mass 

available are shown for completeness. Since the background 

particle and fiber masses associated with new and old carpets 

were drastically different, correlations also are presented 

separated by age of carpet.

The MicroVac estimate of mass available correlated best 

with the mass deposited on the carpet, regardless of carpet 

age. Fiber SEM method correlated significantly only for 

new carpet. The insignificant correlation for old carpet 

indicates a large number of particles with aspect ratios 

similar to Wollastonite were present. It was noted during 

carpet preparation that Old #2 had a significant quantity 

of wood chips, either sawdust or pet bedding, present. 

These chips possibly degraded into micrometer size fibers 

indistinguishable from Wollastonite by SEM. Gravimetric 

mass measured on the ultrasonication filters were not 

correlated with the mass deposited. The destructive nature 

of the technique generated a very large quantity of carpet 

backing particles with a total mass that easily exceeded the 

quantity of Wollastonite deposited.

Based on these results, the MicroVac provided the best 

estimate of Wollastonite mass available for resuspension. 

Therefore, these values will be used when calculating total 

mass emission factors. Size dependent emission factors can 

be calculated from the Fiber SEM estimate of mass available, 

but only for the new carpet tests.
13

Table 3-2. Experimental conditions recorded during each test. Asterisk (*) denotes invalid data collected.

Test # Carpet Resuspension

Method

Mass 

Deposited (mg) % RH Temp (oF) # steps Notes

1a New #1 Walk * 40 72 240 Deposition mass data invalid. 

Depletion @ 2 min

1b New #1 Walk 18.2 ± 7.3 45 70 250 Depletion @ 4 min

2a New #1 Walk 75.7 ± 13.2 37 73 230

2b New #1 Walk 65.0 ± 18.2 44 70 240

3a New #1 Walk 99.2 ± 17.2 45 70 260

3b New #1 Walk 101.3 ± 25.6 45 70 220

4a Old #1 Walk 22.7 ± 8.6 51 71 250

4b Old #1 Walk 29.1 ± 3.8 47 71 230

5a Old #1 Walk 69.2 ± 8.4 51 71 250

5b Old #1 Walk 71.4 ± 3.9 47 71 220

6a Old #1 Walk 101.6 ± 13.2 48 71 270

6b Old #1 Walk 108.3 ± 8.2 50 71 270

7a New #1 Walk * 80 72 250 Deposition mass & MicroVac 

data invalid.

7b New #1 Walk * 90 69 260 Deposition mass data invalid.

8a Old #2 Walk 63.7 ± 6.2 48 70 220

8b Old #2 Walk 60.9 ± 10.2 48 70 270

9a New #2 Vacuum 21.8 ± 1.0 41 71 – Depletion @ 2.5 min

9b New #2 Vacuum 20.5 ± 2.1 40 72 – Depletion @ 4 min

10a New #2 Vacuum 98.1 ± 9.7 41 71 –

10b New #2 Vacuum 100.5 ± 7.3 40 72 –

11a Old #2 Vacuum 20.3 ± 2.5 41 71 – Depletion @ 4.5 min

11b Old #2 Vacuum * 40 70 – Deposition mass invalid. 

Depletion @ 2.5 min

12a Old #2 Vacuum 86.0 ± 12.2 41 72 –

12b Old #2 Vacuum 91.2 ± 12.5 40 72 –

13a New #2 Vacuum 31.8 ± 3.0 90 70 –

13b New #2 Vacuum 33.9 ± 4.2 90 70 –

14a Old #2 Vacuum 41.2 ± 6.2 90 70 –

14b Old #2 Vacuum 33.6 ± 18.1 90 70 –

15a Old #1 Walk 21.7 ± 6.0 90 70 225

15b Old #1 Walk 22.1 ± 1.0 90 70 245
14

Table 3-3. General experimental results presented as particle mass. Asterisk (*) denotes invalid data. 

Dash (-) denotes sample collected, but not analyzed. “NS” denotes no sample collected.

Test #

Mass

Deposited

(mg)

Mass Available Mass Resuspended

MicroVac

Mass

(mg) 

Sonication

Grav Sonication Filter SEM Carpet Fiber SEM URG

Mass

(mg) 

APS Data

Mass

(mg)

Mass

(mg)

MMD

(μm)

GSD

(μm)

Mass

(mg)

MMD

(μm)

GSD

(μm)

Mass

(mg)

MMD

(μm)

GSD

(μm)

1a * 10.1 * * * * 107.6 5.7 1.9 0.2 0.2 4.0 1.6

1b 18.2 18.7 - * 4.1 1.7 53.3 3.1 1.8 1.6 0.3 3.3 1.5

2a 75.7 65.1 - * 5.8 1.9 69.8 3.6 1.8 2.9 0.5 4.0 1.6

2b 65.0 40.0 926 * - - 91.4 4.7 1.6 0.5 2.0 3.9 1.6

3a 99.2 74.7 587 * 4.0 1.7 101.5 2.9 1.8 3.5 1.7 3.6 1.5

3b 101.3 123.8 117 * - - 217.2 4.6 1.7 8.9 1.5 3.5 1.6

4a 22.7 72.1 4212 * 5.4 1.9 182.8 8.2 2.1 17.8 5.2 5.0 1.7

4b 29.1 48.6 2779 * 6.4 1.9 154.5 5.9 1.9 27.4 7.1 4.2 1.6

5a 69.2 218 2163 * 4.7 1.7 116.1 7.1 2.0 28.5 17.1 4.5 1.6

5b 71.4 166.7 4837 * 7.2 2.1 177.2 5.5 1.8 40.5 11.6 3.7 1.5

6a 101.6 262.9 6755 * 5.5 1.8 52.2 4.9 1.8 70.9 28.5 4.3 1.6

6b 108.3 250.3 1454 * 6.6 1.9 356.2 6.6 1.8 52.1 17.3 4.2 1.6

7a * * * * * * * * * 18.5 2.8 3.7 1.6

7b * 52.4 - * 4.8 1.8 45.7 3.2 1.8 21.2 1.8 3.9 1.5

8a 63.7 211.2 409 * 5.4 1.8 81.6 5.4 1.8 39.5 14.8 3.7 1.6

8b 60.9 159.7 4805 * 5.7 1.8 22.1 3.0 1.9 36.8 18.9 3.9 1.5

9a 21.7 21.7 244 * 4.5 1.7 48.5 4.6 1.9 0.3 0.2 4.8 1.9

9b 20.5 53.4 2953 * 4.8 1.8 104.3 6.0 1.8 3.0 0.1 3.9 1.7

10a 98.1 110.2 1148 * 4.0 1.7 189.0 8.6 2.1 0.9 1.6 3.8 2.1

10b 100.5 139.9 1236 * 5.2 1.8 101.0 5.1 1.7 5.8 0.6 2.5 1.8

11a 20.3 75.1 2860 * 7.0 1.9 * * * 5.2 0.3 3.2 2.4

11b * 64.9 1306 * 6.2 2.0 * * * 3.2 0.2 3.3 1.9

12a 86.0 242.4 2595 * 6.1 1.8 * * * 4.0 1.2 3.0 1.7

12b 91.2 307.2 3497 * 5.5 1.8 * * * 7.7 1.4 3.1 1.7

13a 131.8 73.9 ns ns ns ns ns ns ns 5.8 0.5 2.5 1.7

13b 133.9 101.8 ns ns ns ns ns ns ns 7.3 0.5 1.9 1.7

14a 141.2 190.8 ns ns ns ns ns ns ns 7.3 0.8 1.8 2.1

14b 133.6 389.9 ns ns ns ns ns ns ns 10.1 1.4 3.1 1.6

15a 71.7 99.2 ns ns ns ns ns ns ns 27.2 7.2 3.6 1.5

15b 72.1 168.2 ns ns ns ns ns ns ns 51.7 19.8 3.7 1.5
15

Table 3-4. General experimental results presented as particle count. MicroVac count data calculated from Fiber SEM or 

Filter SEM size distribution and MicroVac mass. URG count data calculated from APS size distribution and URG mass. 

CMD is the count median diameter. Asterisk (*) denotes invalid data. Dash (-) denotes sample collected, but not analyzed. 

“NS” denotes no sample collected.

Test # Count

Deposited

Count Available Count Resuspended

MicroVac

Count

(#) 

Sonication Filter SEM Carpet Fiber SEM URG

Mass

(#) 

APS Data

Count

(#)

CMD

(μm)

GSD

(μm)

Count

(#)

CMD

(μm)

GSD

(μm)

Count

(#)

CMD

(μm)

GSD

(μm)

1a * 1.47E+09 * * * 1.56E+06 1.5 2.1 3.34E+06 4.80E+04 2.5 1.7

1b 2.37E+10 2.43E+10 * 1.7 1.9 3.70E+06 1.0 2.0 3.57E+07 2.49E+05 2.2 1.6

2a 2.13E+11 1.83E+11 * 1.6 2.2 3.02E+06 1.2 2.0 4.39E+07 5.45E+04 2.2 1.6

2b 3.45E+10 2.12E+10 * - - 1.21E+06 1.7 2.2 6.35E+06 1.84E+05 2.4 1.6

3a 5.78E+11 4.35E+11 * 1.5 1.9 7.92E+06 0.9 2.0 6.98E+07 2.23E+05 2.1 1.5

3b 1.91E+11 2.33E+11 * - - 3.30E+06 1.7 1.9 2.30E+08 5.75E+04 2.0 1.5

4a 1.63E+10 5.18E+10 * 1.4 2.1 1.00E+06 1.3 2.9 2.24E+08 4.78E+05 2.7 1.7

4b 2.09E+10 3.49E+10 * 1.8 2.3 1.50E+06 1.5 2.2 4.09E+08 7.66E+05 2.8 1.7

5a 1.61E+11 5.08E+11 * 1.8 1.9 4.23E+06 1.5 2.5 3.97E+08 1.71E+06 2.7 1.7

5b 1.27E+11 2.97E+11 * 1.6 2.2 7.53E+06 1.5 2.4 6.54E+08 1.36E+06 2.7 1.7

6a 5.22E+11 1.35E+12 * 1.9 2.0 2.73E+06 1.2 2.4 1.01E+09 2.92E+06 2.7 1.7

6b 2.22E+11 5.12E+11 * 1.9 2.2 2.08E+06 2.1 2.1 7.50E+08 1.80E+06 2.7 1.6

7a * * * * * * * * 3.18E+08 2.85E+05 2.4 1.6

7b * 1.56E+11 * 1.8 1.9 2.60E+06 1.1 2.0 3.37E+08 2.13E+05 2.5 1.6

8a 1.69E+11 5.59E+11 * 2.0 2.0 1.19E+06 16 2.2 5.22E+08 1.42E+06 2.7 1.6

8b 9.34E+11 2.45E+12 * 1.8 2.0 1.23E+06 1.0 1.8 4.64E+08 1.72E+06 2.8 1.6

9a 1.17E+10 1.17E+10 * 1.7 2.0 1.20E+06 1.4 2.1 4.13E+06 2.48E+04 1.7 2.1

9b 2.71E+10 7.07E+10 * 2.1 1.8 1.72E+06 1.8 1.9 6.39E+07 2.38E+04 1.6 2.2

10a 1.06E+11 1.19E+11 * 1.6 1.8 1.88E+06 1.2 3.2 2.91E+07 4.97E+05 0.7 1.9

10b 1.86E+11 2.59E+11 * 1.8 1.9 1.34E+06 1.5 2.3 2.90E+08 6.12E+05 0.9 1.9

11a 1.31E+10 4.84E+10 * 1.9 2.4 * * * 3.81E+08 3.37E+05 0.9 1.9

11b * 6.66E+10 * 2.0 2.2 * * * 6.81E+07 1.15E+05 0.9 2.6

12a 2.73E+10 7.70E+10 * 2.1 1.9 * * * 1.30E+08 5.33E+05 1.1 2.0

12b 3.18E+10 1.07E+11 * 1.8 5.0 * * * 2.65E+08 4.63E+05 1.4 1.8

13a - - ns ns ns ns ns ns 1.47E+09 3.90E+05 0.6 1.7

13b - - ns ns ns ns ns ns 1.10E+09 5.16E+05 0.8 1.7

14a - - ns ns ns ns ns ns 6.17E+08 4.64E+05 0.9 1.9

14b - - ns ns ns ns ns ns 2.98E+08 2.94E+05 1.2 2.0

15a - - ns ns ns ns ns ns 4.64E+08 9.01E+05 2.0 1.8

15b - - ns ns ns ns ns ns 1.52E+09 1.80E+06 2.2 1.7
16

Figure 3-1. Conversion of total Wollastonite mass to total number of Wollastonite 

fibers per 1296 in2 of carpet. Wollastonite mass data calculated from SEM images 

of carpet fibers.
17

Figure 3-2. Relationship between total Wollastonite mass resuspended during 

a 5 minute test and the average number concentration. Mass and number 

concentration data collected by APS.
18

Table 3-5. Correlation coefficients between different 

estimates of mass available for resuspension.

Statistically significant coefficient ( = 0.95) are italicized.

All Data Deposit MicroVac Sonic. Grav. Fiber SEM

Deposit 1

MicroVac 0.64 1

Sonic. Grav. -0.11 0.36 1

Fiber SEM 0.36 0.32 -0.19 1

New Deposit MicroVac Sonic. Grav. Fiber SEM

Deposit 1

MicroVac 0.85 1

Sonic. Grav. -0.42 -0.09 1

Fiber SEM 0.64 0.66 -0.14 1

Old Deposit MicroVac Sonic. Grav. Fiber SEM

Deposit 1

MicroVac 0.93 1

Sonic. Grav. 0.07 0.11 1

Fiber SEM 0.20 0.01 -0.39 1

Table 3-6. Correlation coefficients between different 

estimates of counts available for resuspension.

Statistically significant coefficient ( = 0.95) 

are italicized.

All Data Deposit MicroVac Fiber SEM

Deposit 1

MicroVac 0.55 1

Fiber SEM 0.31 0.25 1

New Deposit MicroVac Fiber SEM

Deposit 1

MicroVac 0.65 1

Fiber SEM 0.45 0.39 1

Old Deposit MicroVac Fiber SEM

Deposit 1

MicroVac 0.52 1

Fiber SEM 0.15 0.01 1
19

3.2.2 Count Comparison

Table 3-6 presents the same correlation coefficients 

calculated on a count basis. Because of the error associated 

with the ultrasonication gravimetric mass measurement, 

these data were not included in the analysis. Count based 

correlation coefficients followed the same trend as mass 

based coefficients. However, the magnitudes of the 

coefficients were somewhat lower, probably due to the error 

introduced by converting gravimetric data to count data.

3.3 Resuspended Wollastonite Measurement 

Method Comparison

3.3.1 Mass Comparison

Figure 3-3 shows the relationship between the resuspended 

Wollastonite mass as measured gravimetrically by the URG 

sampler and aerodynamically by the APS. The relationship 

between the two methods is statistically significant ( = 

0.95). In general, the APS measured about 30% of the 

total mass measured gravimetrically. The difference in 

resuspended mass was caused by the difference between 

aerodynamic and physical diameter resulting from how the 

asymmetric Wollastonite fibers align within the sensing 

volume of the APS. Appendix B provides a more thorough 

explanation. Therefore, both methods provided a valid 

estimate of mass resuspended allowing calculation of total 

mass and particle size dependent emission factors.

3.3.2 Count Comparison

Figure 3-4 illustrates the relationship between the 

resuspended Wollastonite particle counts as measured by 

the URG sampler and the APS. Again, there is a strong 

linear relationship between the two methods. However, the 

resuspended counts calculated from the URG data were 200 

times greater than the APS counts. This dramatic decrease in 

the regression slope most likely resulted from the conversion 

of gravimetric data to particle counts because of the cubic 

influence of particle diameter, especially at smaller sizes.

3.4 Wollastonite Size Distribution

The Wollastonite size distributions deposited on the carpet 

fibers and resuspended were analyzed to identify any 

differences between experimental conditions.

The size distribution of the Wollastonite deposited was 

measured via SEM of the ultrasonication filters and 

individual carpet fibers (Table 3-7). The resuspended 

Wollastonite size distribution was measured automatically 

by the APS. Statistical analysis yielded differences in mass 

median diameter (MMD) of Wollastonite deposited as a 

function of carpet age by both techniques. The Wollastonite 

MMD on old carpet was about 1.3 μm greater than that 

measured on new carpet. The larger size distribution more 

than likely was caused by the presence of other fibrous 

particles in the carpet not removed by the conditioning 

procedure. The geometric standard deviation (GSD) of the 

size distribution was consistent across all experimental 

conditions. There were not any differences in size distribution 

as measured by SEM analysis of ultrasonication filters 

andindividual carpet fibers when carpet age was controlled. 

The resuspended Wollastonite size distribution as measured 

by the APS varied between resuspension methods (Table 

3-8). Age of carpet did not influence the resuspended 

size distribution, although the median diameter from 

the older carpet was slightly larger (2.7 μm vs. 2.3 μm). 

Walking generated a larger size distribution of particles 

with a narrower range than vacuuming. The much smaller 

resuspended CMD from vacuuming indicates many more 

small particles were generated. The force imparted to the 

carpet fibers by the vacuum beater bar probably provided 

sufficient energy to dislodge more particles smaller than 

1 μm. This increase in resuspension of 1 μm particles 

was not evident from the mass data because of their small 

contribution to the total mass resuspended.

As compared to the size distribution of Wollastonite 

deposited on the carpet fibers, the resuspended particle 

size distribution was smaller and narrower. This finding 

is expected considering only a fraction of the deposited 

Wollastonite becomes resuspended.
20

Figure 3-3. Relationship between mass resuspended estimates measured by URG 

and APS. APS measured approximately 30% of the mass measured gravimetrically.
21

Figure 3-4. Relationship between resuspended Wollastonite counts measured by 

URG and APS.

3.5 Emission Factors

3.5.1 Total Mass

Emission factors calculated on a total mass basis are 

presented in Table 3-9. The average plus standard deviation 

for each experimental condition is presented. Emission 

factors in the APS/Fiber SEM and URG/Fiber SEM ratios 

for tests using old carpet were not calculated. As discussed 

earlier, the Fiber SEM mass available data for tests with 

old carpet could not be related to the Wollastonite mass 

deposited because of the presence of other fibrous particles. 

Any emission factors calculated using this data would be 

incorrect. Therefore, emission factors using Fiber SEM data 

for old carpets were not analyzed.

The emission factors in Table 3-9 indicate differences 

between experimental conditions. Table 3-10 shows the 

p-values ( = 95%) for each experimental variable calculated 

using linear regression models as described in Section 2.1.14. 

However, mixing within the exposure chamber prevented 

calculation of emission factors as function of height.

3.5.2 Total Counts

Emission factors calculated on a total count basis are 

presented in Table 3-11. The magnitudes of the count 

emission factors vary greatly between calculation methods 

and in comparison with the mass emission factors. The 

APS/SEM and URG/MicroVac emission factors probably 

are the most representative of reality because these factors 

were calculated from count:count or mass:mass raw data. 

Calculation of emission factors where the numerator and 

denominator were measured on a different basis (e.g., 

count:mass or mass:count) introduced significant error into 

the values. As a result, extremely small emission factors were 

calculated for the APS/MicroVac. In the other extreme, URG/

Fiber SEM emission factors greater than 1 were calculated 

These emission factors are impossible to achieve given the 

low background Wollastonite fiber concentrations in the 

carpet. 

Statistical analysis of the APS/SEM and URG/MicroVac 

count emission factors showed the same influence of the 

independent variables as the mass emission factors. Table 

3-12 shows the p-values ( = 95%) for each experimental 

variable calculated using linear regression models as 

described in Section 2.1.12.
22

Table 3-7. Size distribution of Wollastonite deposited on new 

and old carpet as measured from ultrasonication filters and 

individual carpet fibers. Size distribution of bulk Wollastonite 

shown for comparison.

Ultrasonication 

Filter

Carpet 

Fiber

New Carpeta

MMD 4.65 ± 0.64 4.74 ± 1.65

GSD 1.76 ± 0.07 1.81 ± 0.13

CMD 1.70 ± 0.19 1.37 ± 0.30

GSD 1.94 ± 0.13 2.15 ± 0.37

Old Carpeta

MMD 5.98 ± 0.74 5.83 ± 1.56

GSD 1.87 ± 0.11 1.89 ± 0.11

CMD 1.80 ± 0.21 1.50 ± 0.29

GSD 2.10 ± 0.18 2.30 ± 0.26

Bulk Wollastoniteb MMD 2.5

GSD 2.2

a

Size distribution presented as aerodynamic diameters

b

Size distribution measured optically. Data provided by manufacturer.

Table 3-8. Size distribution of Wollastonite 

resuspended via walking and vacuuming as measured 

by the APS. Size distribution of bulk Wollastonite

shown for comparison.

Walkinga

MMD 3.93 ± 0.40

GSD 1.57 ± 0.06

CMD 2.53 ± 0.28

GSD 1.62 ± 0.06

Vacuuminga

MMD 2.98 ± 1.06

GSD 1.90 ± 0.30

CMD 1.15 ± 0.36

GSD 2.05 ± 0.26

Bulk Wollastoniteb MMD 2.5

GSD 2.2

a

Size distribution presented as aerodynamic diameters

b

Size distribution measured optically. Data provided by manufacturer.
23

Table 3-9. Emission factors calculated on total mass resuspended for each 

test condition.

Test # APS/MicroVac APS/ Fiber SEM URG/MicroVac URG/Fiber SEM

1 0.016 ± 0.006 0.003 ± 0.002 0.059 ± 0.040 0.021 ± 0.025

2 0.042 ± 0.040 0.018 ± 0.006 0.034 ±0.031 0.024 ± 0.025

3 0.018 ± 0.009 0.017 ± 0.007 0.072 ± 0.029 0.038 ± 0.017

4 0.124 ± 0.084 * 0.337 ± 0.165 *

5 0.077 ± 0.019 * 0.244 ± 0.120 *

6 0.073 ± 0.039 * 0.203 ± 0.153 *

7 0.022 ± 0.015 0.040 0.233 ± 0.186 0.46 ± 0.05

8 0.117 ± 0.084 * 0.256 ± 0.153 *

9 0.006 ± 0.005 0.002 ± 0.002 0.037 ± 0.035 0.017 ± 0.019

10 0.009 ± 0.007 0.007 ± 0.002 0.023 ± 0.021 0.040 ± 0.031

11 0.004 ± 0.002 * 0.052 ± 0.041 *

12 0.005 ± 0.001 * 0.022 ± 0.014 *

13 0.006 ± 0.001 ns 0.079 ± 0.003 ns

14 0.003 ± 0.001 ns 0.040 ± 0.025 ns

15 0.105 ± 0.044 ns 0.317 ± 0.054 ns

* Emission factors for old carpet were not useable because mass available estimated by SEM 

 could not be related to mass deposited.

ns: No sample collected for analysis

Table 3-10. Table of p-values for each method of calculating mass

emission factors (EmFa).

EmFa Independent Variables

Load Age Method Rh βO R2

URG/MicroVac 0.254 0.0001 0.0002 0.0001 0.240 0.46

APS/MicroVac 0.600 0.0005 0.0002 0.656 0.089 0.44

URG/Fiber SEM 0.520 – 0.007 0.0001 0.011 0.94

APS/Fiber SEM 0.160 – 0.027 0.002 0.010 0.81
24

3.5.3 Interpretation of Emission Factors

Data from the tables show consistency in parameters 

influencing emission factors between the different calculation 

methods. Carpet age (when included as a variable), 

resuspension method, and chamber relative humidity all 

influenced the total mass emission factor. Interestingly, the 

Wollastonite loading did not influence the emission factors. 

A similar result was discovered for generic Arizona Test Dust 

(ATD) (Rodes and Thornburg, 2004).

Based on these data, three different emission factor scenarios 

were statistically different from each other.

1. New carpet versus old carpet at low Rh: With the 

relative humidity constant at about 40%, emission 

factors from new carpet were significantly lower than 

those from old carpet. The lower emission factors 

possibly were caused by greater adhesion of the 

particles to the fibers due to electrostatic charging of 

the fibers. This relationship was consistent with the 

one between new-old carpet seeded with Arizona Test 

Dust developed during earlier research.

2. Influence of relative humidity on new carpet: Similar 

to the relative humidity emission factor influence 

discovered using ATD, raising the relative humidity 

within the chamber possibly weakens the static charge 

on the fibers. As a result, the Wollastonite emission 

factors from walking and vacuuming are on new 

carpet are 2-4 times greater at relative humidity levels 

approaching 90%. Relative humidity did not have 

a statistically significant influence on walking and 

vacuuming emission factors from old carpet.

3. Influence of resuspension method: Vacuuming new or 

old carpet seeded with Wollastonite fibers resuspended 

significantly less mass than walking. Even with a 

beater bar, the vacuum efficiently sucked a majority 

of the particles into the machine for collection in the 

HEPA type bag and prevented the fibers from reaching 

the surrounding air. Using a less efficient vacuum 

bag probably will increase the emission factors as 

more particles penetrate through and escape into the 

room air. Also, a different vacuum with less efficient 

collection of particles by the vacuum head probably 

will yield higher emission factors.

3.5.4 Size Dependent Emission Factors

Size dependent, mass based emission factors were calculated 

using the APS/Fiber SEM data for new carpet tests (Table 

3-13). Emission factors for 0.5 μm fibers of ~10-9 were 

obtained. However, these values were not statistically 

different from zero. The size distribution and mass of 

Wollastonite available for Test 13 was not determined. 

A statistical analysis of these size dependent emission factors 

did not yield any statistically significant relationships with 

the test conditions (Table 3-14). However as shown in Figure 

3-5, there seems to be a difference in emission factors as 

function of particle size and relative humidity. At low relative 

humidity, the emission factor generally decreases as the fiber 

aerodynamic diameter increases. At high relative humidity, 

the emission factor peaks at a larger fiber diameter and 

remains elevated. Although statistically insignificant, these 

general trends agree with the relationship between particle 

diameter and emission factor found with ATD. Additionally, 

the trends may support the validity of the influence of relative 

humidity on emission factors discussed earlier. High relative 

humidity possibly decreases influence of electrostatic forces 

as well as increases the surface tension bond. As a result, 

smaller particles will have lower emission factors at high 

relative humidity because the dominant surface tension force 

is increased by the presence of water. Conversely, the larger 

particles which adhere to carpet fibers due to electrostatics 

have lower emission factors because the humidity appears to 

neutralize some or all of the charge on the carpet fibers.

A similar analysis was conducted for count based emission 

factors (Table 3-15). Interestingly, vacuuming generated 

statistically significant count-based emission factors. The 

vacuum beater bar probably provides sufficient energy to 

release a large number of sub-micrometer particles that do 

not contribute significantly to the total mass resuspended. 

Although the statistical analysis did not indicate any 

statistically significant variables (Table 3-16), Figure 3-6 

does indicate a relationship between count based emission 

factor and particle diameter may exist due to the resuspension 

of the submicrometer particles.

3.6 K-Factors

K-factors, ratio of resuspended Wollastonite number 

concentration and Wollastonite surface loading, were 

calculated for each test with valid APS and Fiber SEM 

image data. K-factors provide a widely recognized means for 

calculating airborne concentrations from surface loading data 

and the type of activity being performed. In this research, 

K-factors are a different method for expressing emission 

factors. For these tests, K-factors were calculated as:

Aggregate K-factors across all fiber aerodynamic diameters 

and all test conditions are shown in Table 3-17. Average 

size-dependent K-factors for new carpet only (Table 3-18) 

were calculated from data presented in Table 3-15 and the 

corresponding volumetric airflow and carpet area.
25

Table 3-11. Emission factors calculated on total count resuspended for each test condition.

Test # APS/MicroVac APS/ Fiber SEM URG/MicroVac URG/Fiber SEM

1 2.1E-05 ± 1.6E-05 4.9E-02 ± 2.6E-02 1.9E-03 ± 5.7E-04 > 1

2 4.5E-06 ± 5.9E-06 8.5E-02 ± 9.5E-02 2.7E-04 ± 4.2E-05 > 1

3 3.8E-07 ± 1.9E-07 2.3E-02 ± 7.6E-03 5.7E-04 ± 5.9E-04 > 1

4 1.6E-06 ± 8.9E-06 * 8.0E-03 ± 5.2E-03 *

5 4.0E-06 ± 8.6E-07 * 1.5E-03 ± 1.0E-03 *

6 2.8E-06 ± 9.6E-07 * 1.1E-03 ± 5.1E-04 *

7 1.4E-06 8.2E-02 2.2E-03 > 1

8 1.6E-06 ± 1.3E-06 * 5.6E-04 ± 5.2E-04 *

9 1.2E-06 ± 1.3E-06 1.7E-02 ± 4.8E-03 6.3E-04 ± 3.9E-04 > 1

10 3.3E-06 ± 1.3E-06 3.6E-01 ± 1.4E-01 6.8E-04 ± 6.2E-04 > 1

11 4.3E-06 ± 3.7E-06 * 4.5E-03 ± 4.8E-03 *

12 5.6E-06 ± 1.8E-06 * 2.1E-03 ± 5.6E-04 *

13 9.5E-07 ± 2.4E-07 ns 2.9E-03 ± 1.8E-03 ns

14 1.1E-07 ± 1.2E-07 ns 1.5 E-04 ± 1.7E-04 ns

15 1.2E-06 ± 3.1E-07 ns 7.9E-04 ± 7.1E-05 ns

* Emission factors for old carpet were not useable because mass available estimated by SEM could not be

 related to mass deposited.

ns: No sample collected for analysis

Table 3-12. Table of p-values for each method of calculating count 

emission factors (EmFa).

EmFa Independent Variables

Load Age Method Rh βO R2

URG/MicroVac 0.254 0.0001 0.0002 0.0001 0.240 0.46

APS/Fiber SEM 0.604 – 0.0002 0.007 0.089 0.77

Table 3-13. Table of size dependent APS/SEM Fiber mass emission factors for each test condition using new carpet.

Test #

1 2 3a 7a 9a 10a 13

Aerodynamic

Diameter

(μm)

10.0 0 – – – – 0.001 ± 0.001 ns

8.0 – – – – 0.001 ± 0.002 0.003 ns

7.0 – 0.012 ± 0.002 0.001 – 0.001 ± 0.001 0.001 ns

6.0 0.011 ± 0.013 0.020 ± 0.019 0.008 ± 0.010 0.055 – 0.007 ± 0.004 ns

5.0 0.016 ± 0.021 0.009 ± 0.001 0.021 ± 0.029 0.034 0.001 ± 0.001 0.012 ± 0.009 ns

4.0 0.054 ± 0.073 0.039 ± 0.028 0.010 ± 0.012 0.045 0.004 ± 0.001 0.016 ± 0.002 ns

3.5 0.007 ± 0.008 0.029 ± 0.019 0.008 ± 0.009 0.080 0.002 ± 0.001 0.015 ± 0.011 ns

3.0 0.022 ± 0.028 0.018 ± 0.009 0.007 ± 0.006 0.025 0.002 ± 0.000 0.016 ± 0.006 ns

2.5 0.019 ± 0.019 0.055 ± 0.067 0.008 ± 0.006 0.028 0.004 ± 0.003 0.044 ± 0.024 ns

2.0 0.012 ± 0.010 0.015 ± 0.013 0.005 ± 0.001 0.019 0.001 ± 0.000 0.046 ± 0.019 ns

1.5 0.006 ± 0.004 0.080 ± 0.105 0.007 ± 0.001 0.007 0.007 ± 0.007 0.092 ± 0.033 ns

1.0 0.001 ± 0.000 0.005 ± 0.006 0.001 ± 0.000 0.001 0.003 ± 0.003 0.168 ± 0.018 ns

0.5b 0 0 0 0 0 0 ns

– = No particles measured by SEM

ns = No SEM fiber sample collected

a

Emission factors without standard deviations only had one valid measurement per test condition

b

Emission factors for 0.5 mm particles were not significantly different from zero
26

Figure 3-5. Average size dependent, mass based emission factors calculated from 

APS and SEM Fiber data for high and low relative humidity tests. Curves fitting 

size distribution are shown for illustrative purposes only. Curves are not statistical 

regressions.

Table 3-14. Table of p-values for APS/SEM Fiber mass emission factors 

including particle diameter as an independent variable.

EmFa Independent Variables

Load Age Method Rh Diameter βO R2

APS/Fiber SEM 0.060 – 0.282 0.167 0.122 0.742 0.09
27

Table 3-15. Table of size dependent APS/SEM Fiber count emission factors for each test condition using new carpet.

Test #

1 2 3a 7a 9a 10a 13

Aerodynamic

Diameter

(μm)

10.0 0 – – – – 0.0001 ± 0.0002 ns

8.0 – – – – 0.001 ± 0.001 0.004 ns

7.0 – 0.010 ± 0.010 0.001 – 0.001 ± 0.001 0.002 ns

6.0 0.031 ± 0.004 0.013 ± 0.002 0.005 ± 0.006 0.097 – 0.003 ± 0.002 ns

5.0 0.032 ± 0.021 0.006 ± 0.003 0.015 ± 0.020 0.040 0.002 ± 0.001 0.006 ± 0.002 ns

4.0 0.054 ± 0.050 0.030 ± 0.034 0.007 ± 0.007 0.031 0.001 ± 0.001 0.009 ± 0.009 ns

3.5 0.012 ± 0.004 0.018 ± 0.017 0.005 ± 0.006 0.060 0.001 ± 0.001 0.006 ± 0.002 ns

3.0 0.023 ± 0.012 0.010 ± 0.008 0.004 ± 0.004 0.015 0.001 ± 0.001 0.009 ± 0.007 ns

2.5 0.028 ± 0.012 0.031 ± 0.040 0.005 ± 0.003 0.014 0.0005 ± 0.0004 0.016 ± 0.003 ns

2.0 0.025 ± 0.019 0.008 ± 0.008 0.003 ± 0.001 0.010 0.001 ± 0.001 0.020 ± 0.002 ns

1.5 0.026 ± 0.028 0.049 ± 0.067 0.005 ± 0.001 0.005 0.001 ± 0.001 0.067 ± 0.064 ns

1.0 0.002 ± 0.002 0.002 ± 0.003 0.000 ± 0.000 0.001 0.004 ± 0.003 0.057 ± 0.030 ns

0.5 0 0 0 0 0.011 ± 0.012 0.080 ± 0.040 ns

– = No particles measured by SEM

ns = No SEM fiber sample collected

a

Emission factors without standard deviations only had one valid measurement per test condition

Table 3-16. Table of p-values for APS/SEM Fiber count emission factors 

including particle diameter as an independent variable.

EmFa Independent Variables

Load Age Method Rh Diameter βO R2

APS/Fiber SEM 0.130 – 0.141 0.684 0.079 0.005 0.06
28

Figure 3-6. Average size dependent, count based emission factors calculated from 

APS and SEM Fiber data for vacuuming and walking resuspension method tests. 

Curves fitting size distributions are shown for illustrative purposes only. Curves are 

not statistical regressions.
29

Table 3-17. Aggregate K-factors for each test.

Test Average Std Dev

1 6.19E-04 3.26E-04

2 2 1.07E-03 1.20E-03

3 2.88E-04 9.59E-05

4 * *

5 * *

6 * *

7a 1.03E-03

8 * *

9 2.18E-04 6.10E-05

10 4.55E-04 1.72E-03

11 * *

12 * *

13 ns

14 ns

15 ns

a

 only 1 valid sample collected

* Emission factors for old carpet were not useable because 

mass available estimated by SEM could not be related to 

mass deposited.

ns: No SEM Fiber sample collected for analysis

Table 3-18. Size specific K-factors for each test using new carpet. K-factors calculated from 

emission factors in Table 3-15.

Test #

1 2 3 7 9 10 13

Aerodynamic

Diameter

(μm)

10.0 0 – – – – 1.71E-06 ns

8.0 – – – – 1.22E-05 4.89E-05 ns

7.0 – 1.30E-04 1.27E-05 – 1.21E-05 2.43E-05 ns

6.0 3.94E-04 1.58E-04 6.27E-05 1.23E-03 – 3.71E-05 ns

5.0 4.07E-04 7.92E-05 1.89E-04 5.11E-04 1.90E-05 7.36E-05 ns

4.0 6.77E-04 3.82E-04 8.21E-05 3.91E-04 1.45E-05 1.19E-04 ns

3.5 1.55E-04 2.27E-04 6.19E-05 7.64E-04 1.24E-05 7.02E-05 ns

3.0 2.89E-04 1.22E-04 5.29E-05 1.95E-04 1.26E-05 1.12E-04 ns

2.5 3.51E-04 3.95E-04 6.27E-05 1.74E-04 6.11E-06 2.06E-04 ns

2.0 3.14E-04 9.76E-05 4.34E-05 1.23E-04 9.33E-06 2.50E-04 ns

1.5 3.34E-04 6.23E-04 6.26E-05 6.33E-05 1.29E-05 8.52E-04 ns

1.0 2.79E-05 3.15E-05 3.47E-06 8.21E-06 5.22E-05 7.19E-04 ns

0.5 0 0 0 0 1.35E-04 1.01E-03 ns
30

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31

4.0

Quality Assurance

Quality assurance and quality control measures for the 

project were outlined in the Quality Assurance Project Plan 

for basic research projects - “Resuspension of Fibers from 

Indoor Surfaces Due to Human Activity.” Tables 4-1 and 

4-2 summarize the QA/QC measures for the project. The 

QA/QC results for each metric are presented in subsequent 

sections. Note the detection limits for the URG samplers, 

MicroVac, and deposition chamber are the same because 

all rely on gravimetric analysis of the filters collected to 

determine whether the data quality objectives were achieved. 

The Mettler Toledo MT2 balance was used for all gravimetric 

analyses.

4.1 URG Samples

Gravimetric analysis of URG filters determined the 

mass resuspended from the carpet during experiments. 

The data quality indicators used to determine whether the 

data quality objectives for this metric were achieved are 

listed in Table 4-3.

The number of URG samples attempted and successfully 

collected determined the completeness percentage. The 

cumulative completeness percentage, including all blank and 

collocated samples, was 100% and exceeded the data quality 

objective. Field and lab filter blanks determined whether 

handling the filters caused inadvertent contamination. Field 

blanks were loaded and unloaded with the experimental URG 

samples. Lab blanks were kept in the gravimetric analysis 

chamber and were weighed with the experimental and field 

blank filters. The mean masses collected on the filter blanks 

were used as correction factors for the experimental samples.

Precision and accuracy were other quality assurance criteria. 

Precision in the sample collection and gravimetric analysis 

was assessed by collecting 4 sets of collocated URG 

samples. Precision was calculated as the percent relative 

standard deviation (%RSD). Collocated URG samples during 

experiments could not be collected. Therefore, collocated 

samples were collected by sampling laboratory air at height 

of 36 inches for 8 hours. Accuracy of the gravimetric 

analysis was assessed every session (both pre-weighing 

and post-weighing) by weighing a 100 μg standard weight. 

A gravimetric analysis session was not started until the 

measured weight was within 5% of the stated value.

4.2 MicroVac Samples

Gravimetric analysis of MicroVac filters determined the 

mass available for resuspension from the carpet. The 

data quality indicators used to determine whether the 

data quality objectives for this metric were achieved are 

listed in Table 4-4.

The number of MicroVac samples attempted and successfully 

collected determined the completeness percentage. The 

cumulative completeness percentage, including all blank and 

collocated samples, was 97.2% and exceeded the data quality 

objective. Both MicroVac filters from Test 7a were invalid 

because the samples were dropped during unloading from the 

cassette after sample collection.

Field and lab filter blanks determined whether handling 

the filters caused inadvertent contamination. Field blanks 

were loaded and unloaded with the experimental MicroVac 

samples. Lab blanks were kept in the gravimetric analysis 

chamber and were weighed with the experimental and field 

blank filters. The mean masses collected on the filter blanks 

were used as correction factors for the experimental samples.

Precision and accuracy were other quality assurance criteria. 

Precision in the sample collection and gravimetric analysis 

was assessed by collecting duplicate MicroVac samples for 

each carpet piece. The MicroVac mass collected on each 

sample was normalized by the mass loaded in that section of 

carpet to eliminate the variability caused by the Wollastonite 

loading procedure. Precision was calculated as the percent 

relative standard deviation (%RSD. Accuracy of the 

gravimetric analysis was described in Section 4.1. The data 

listed in Tables 4-3 and 4-4 are identical. The accuracy data 

in Table 4-4 are included for completeness.

Table 4-1. QA/QC criteria for measurements collected.

Metric Precision Accuracy Completeness IDL MDL MQL

URG ± 20% ± 5% > 95% 0.1 μg 0.5 μg 1.5 μg

MicroVac ± 20% ± 5% > 95% 0.1 μg 0.5 μg 1.5 μg

Deposition Chamber ± 30% ± 5% > 90% 0.1 μg 0.5 μg 1.5 μg

Ultrasonication > 90%

SEM Image > 90%

APS ± 5% a

± 15% > 95% 0.5 μm NA NA

T NA ± 5% > 95% 1°C NA NA

RH NA ± 10% > 95% 1% NA NA

NA = not applicable

a Determined from manufacturer’s calibration certificate.
32

Table 4-2. Quality control measures to implement during testing.

Metric Quality Control Evaluation

URG Precision: Collocated samples 5% of filters collected

Accuracy: Asses for =5% filters

Field Blanks: =5% of filters collected

Lab Blanks: =5% of filters collected

Background: Collected prior to each test

MicroVac Precision: Duplicate samples for each carpet

Accuracy: Asses for =5% filters

Field Blanks: =5% of filters collected

Lab Blanks: =5% of filters collected

Deposition Chamber Precision: 9 filters for each carpet

Accuracy: Asses for =5% filters

Field Blanks: =5% of filters collected

Lab Blanks: =5% of filters collected

Ultrasonication Precision: Duplicate aliquots of =5% of extracts

Field Blanks: =5% of filters collected

Lab Blanks: =5% of filters collected

SEM Image Analysis Precision: Duplicate analysis of =5% of images

Blanks: NA

APS Precision: Collocated instruments once/week

Zero: HEPA filter installed on inlet daily

Background: Collected prior to each test

Temp/Rh Precision: NA

Accuracy: Temp = 5%, Rh = 10%

4.3 Deposition Chamber Samples

Gravimetric analysis of deposition chamber filters 

determined the Wollastonite mass per unit area deposited and 

the variability in the mass loading for each carpet sample. 

The data quality indicators used to determine whether the 

data quality objectives for this metric were achieved are 

listed in Table 4-5. The number of deposition chamber 

filters attempted and successfully collected determined the 

completeness percentage. Nine filters were collected for each 

carpet sample. The cumulative completeness percentage, 

including all blank and collocated samples, was 91.3%. 

Three sets of filters (27 total) were invalid. Two sets were 

not attached properly to the carpet and moved during the 

Wollastonite loading. The third set of filters was placed in the 

wrong locations on the carpet sample.

Field and lab filter blanks determined whether handling 

the filters caused inadvertent contamination. Field blanks 

were loaded onto the carpet for five minutes then returned 

to the filter storage container. Lab blanks were kept in 

the gravimetric analysis chamber and were weighed with 

the experimental and field blank filters. The mean masses 

collected on the filter blanks were used as correction factors 

for the experimental samples.

Precision and accuracy were other quality assurance criteria. 

Precision in the sample collection and gravimetric analysis 

was assessed from the 9 filters deployed per carpet piece. 

Precision was calculated as the percent relative standard 

deviation (%RSD). The precision determined the variability 

in the Wollastonite mass loaded across the surface area of 

the carpet. The average variability in the mass deposited was 

17.4%.Accuracy of the gravimetric analysis was described in 

Section 4.1.

4.4 Ultrasonication

The ultrasonic extraction of particles was one method for 

quantifying the amount and size distribution of Wollastonite 

available for resuspension. The data quality indicators 

for measurement of particle loading via gravimetric mass 

using this method are listed in Table 4-6. The data quality 

indicators for measurement of particle size distribution via 

SEM image analysis are presented in Table 4-7. As described 

in Section 3.1, the particle loading (mass or count) could 

not be determined from SEM analysis of the ultrasonication 

samples because of the large number of particles collected 

on the filter. Therefore, QA/QC data for this metric are not 

presented.

The completeness percentage was 93% for gravimetric 

and size distribution analyses. Four ultrasonication filters 

were invalid because filter handling errors. Note that 

ultrasonication samples were not planned for Tests 13 thru 15 

because of budget limitations.

Field blanks were fresh polycarbonate filters that were 

extracted according to the procedure described in Section 

2.1.8. Lab blanks were filters removed directly from the 

package and either weighed gravimetrically or imaged via 

SEM to determine the particle size distribution. Both sets of 

blanks did not show a significant increase in mass because 

particles were not present on the filter surface. This finding 

was verified by the SEM image analysis.

Variation in the paired ultrasonication filter particle masses 

and size distributions for the two samples removed from 

a single 36” by 36” piece of carpet was measured for each 

test. The mass extracted from carpet sections removed from 
33

different areas varied greatly (RSD = 33%) because of the 

large amount of (backing material, glue, etc) collected on 

the filter along with the Wollastonite particles. However, the 

%RSD is still within the limit set by the author of the method 

(Millette et al., 1993). Precision in the size distribution 

measurements was better (RSD = 17.4%).

Variation in the mass and size distribution within a single 

sample also was measured. The procedure specified removal 

of one 50 ml aliquot. For three randomly selected samples, a 

second 50 ml aliquot was collected and filtered. The variation 

in mass and size distribution within a sample was much 

smaller, indicating a single aliquot was representative of the 

carpet extract.

Table 4-3. URG sample completeness, filter blank, precision, and accuracy statistics.

Completeness Field Blanks Lab Blanks Precision Accuracy

Valid Samples 72 Number 4 Number 4 Number of Pairs 4 Number

Mean (μg)

20

Planned Samples 72 Mean (μg) 0.2 ± 0.2 Mean (μg) 0.1 ± 0.3 Mean %RSD 6.1% 99.6

% Completed 100% % Difference 0.4%

a

Includes 5% field blanks, 5% lab blanks, and 5% collocated samples

Table 4-4. MicroVac sample completeness, filter blank, precision, and accuracy statistics.

Completeness Field Blanks Lab Blanks Precision Accuracy

Valid Samples 70 Number 4 Number 4 Number of Pairs 30 Number

Mean (μg)

20

Planned Samplesa 72 Mean (μg) 0.1 ± 0.2 Mean (μg) 0.1 ± 0.1 Mean %RSD 5.7% 99.6

% Completed 97.2% % Difference 0.4%

a

Includes 5% field blanks, 5% lab blanks, and 5% collocated samples

Table 4-5. Deposition chamber filter sample completeness, blank, precision, and accuracy statistics.

Completeness Field Blanks Lab Blanks Precision Accuracy

Valid Samples 283 Number 13 Number 13 Number of Sets 22 Number

Mean (μg)

20

Planned Samplesa 310 Mean (μg) -0.8 ± 0.5 Mean (μg) -0.1 ± 0.6 Mean %RSD 17.4% 99.6

% Completed 91.3% % Difference 0.4%

a

Includes 5% field blanks, 5% lab blanks, and 5% collocated samples

Table 4-6. Gravimetric mass as measured by ultrasonication: sample completeness, 

blank, precision, and accuracy statistics.

Completeness Field Blanks Lab Blanks Precision

(different section)

Precision

(same section)

Valid Samples 53 Number 3 Number 3 Number of Sets 22 Number of Sets

Mean %RSD

3

Planned Samplesa 57 Mean (μg) -0.3 ± 0.4 Mean (μg) 0.0 ± 0.3 Mean %RSD 33% 8.4%

% Completed 93.0%

a

Includes 5% field blanks, 5% lab blanks, and 5% collocated samples

Table 4-7. Size distribution from SEM images of ultrasonication filters: sample completeness, 

blank, precision, and accuracy statistics.

Completeness Field Blanks Lab Blanks Precision Accuracy

Valid Samples 53 Number 3 Number 3 Number of Sets 22 Number of Sets

Mean %RSD

3

Planned Samplesa 57 Mean (μm) no particles Mean (μm) no particles Mean %RSD 17.4% 7.3%

% Completed 93.0%

a

Includes 5% field blanks, 5% lab blanks, and 5% collocated samples
34

4.5 SEM Image Analysis

Completeness, precision, and accuracy were the QA/QC 

metrics applicable to the SEM image analysis. Blanks were 

not needed because particles were not present on clean filters 

or fibers. Completeness was 100% (416 images) for the SEM 

image analysis. The high quality of all photographs allowed 

SEM image analysis to be performed on all. Precision was 

assessed by repeating the image analysis for particle number 

and size measurements for 5% (21 images) of the images 

collected. The %RSD in the particle counts and diameters 

were 1.3% and 2.8%, respectively. The difference in particle 

concentration between the SEM image analysis used in this 

project and the computer controlled SEM measurements 

(selected as the reference standard) was less than 5%. There 

was bias in the accuracy of the particle size measured by the 

analysis of the SEM images. This issue is described in

Appendix A.

4.6 Aerodynamic Particle Sizer

The APS measured resuspended particle number and mass 

concentration at the resuspension area boundary. The QA/QC 

results for this metric are summarized below.

The number of APS samples attempted and successfully 

collected is outlined in Table 4-8. The cumulative 

completeness percentage (100%) exceeded the data quality 

objective.

Precision and accuracy of the APS measurements were 

other quality assurance criteria. Precision could not be 

assessed because only one APS was available. Accuracy 

was determined to be within specifications because the APS 

manufacturer’s calibration was still valid. The APS was 

calibrated on December 15, 2004.

The quality control assessment was to make sure the APS 

measured a concentration of 0 particles per cm3

. A HEPA 

filter was installed on the inlet to the APS once per day, for a 

total of 24 samples. The APS always measured 0 particles per 

cm3 when the HEPA filter was installed.

4.7 Temperature & Relative Humidity

The HOBO H8 measured the temperature and relative 

humidity within each house during resuspension and tracking 

data collection. Only one HOBO was used, so precision was 

not assessed. Accuracy was measured by placing the unit in 

a temperature and humidity controlled chamber for 2 hours. 

The QA/QC results for this metric are summarized in Table 

4-9. All data quality objectives were achieved.

Table 4-8. Aerodynamic Particle Sizer sample 

completeness statistics.

Metric Background Test Cumulative

Valid Samples 30 30 60

Planned Samples 30 30 60

% Completed 100% 100% 100%

Table 4-9. QA/QC results for the HOBO H8 

temperature and relative humidity data

Completeness Accuracy

# %

Valid Samples 24 Temperature 98.8

Planned Samples 24 Rh 97.1

% Completed 100%
35

5.0

Conclusions

Asbestos Fiber Simulant Available Estimates

• Neither the MicroVac, Fiber SEM, nor Ultrasonication 

SEM methods are sufficiently robust to provide 

accurate mass available measurements for calculation 

of emission factors. Additional work is needed to 

develop methods to accurately quantify the mass 

available.

• The MicroVac method (the RTI-modified version of 

ASTM D5755-95) had the highest correlations of 

collections with the Wollastonite mass deposited. 

Typically, the MicroVac method underestimated the 

quantity deposited on new carpet. Conversely, the 

MicroVac method overestimated the amount deposited 

on older carpet because additional non-Wollastonite 

particles were collected. This suggests that any more 

aggressive surface vacuuming method (e.g. HVS3) 

would exhibit the same or more pronounced problems.

• The Fiber SEM method had a statistically significant 

correlation with the amount deposited only for new 

carpet. Fibrous particles originally present that were 

not removed during the cleaning positively skewed the 

mass estimate such that a significant correlation could 

not be obtained. This method generally overestimated 

the mass deposited.

• The Ultrasonication SEM method had the poorest 

correlations with the Wollastonite mass deposited. 

This destructive technique generated many additional 

particles (fibrous and general). As a result, the method 

created a positive artifact in the gravimetric and SEM 

mass estimates that could not be distinguished from 

Wollastonite mass.

Emission Factors

• Emission factors based on total mass varied from 0.005 

to 0.45, depending on the experimental conditions 

and calculation method. Count based emission factors 

covered a similar range: 0.0001 to 0.30

• Size dependent emission factors, for new carpet 

only, varied from 0.0 to 0.17 for fiber aerodynamic 

diameters from 0.5 to 10 μm.

• Emission factor estimates can be improved by 

determining the amount available for resuspension as 

a function of the force applied to the fibers. Additional 

research is required to account for the influence of 

electrostatic and surface tension adhesion forces on 

emission factors.

• Wollastonite emission factors increase with carpet 

age, consistent with the trend observed for Arizona 

Test Dust emission factors. Emission factors from 

old carpet were 2 to 3 times greater than those from 

new carpet, possibly due to the decreased electrostatic 

adhesion of large Wollastonite particles (> 3 μm) to 

the fibers.

• Increasing the relative humidity from ~45% to ~90% 

increased the Wollastonite emission factors from 

new carpet 3 to 4 times. It is suspected that the extra 

moisture neutralizes the electrostatic charge and 

particles larger than 3.5 μm became resuspended 

easier.

• Walking generated larger emission factors than 

vacuuming, especially for older carpet. The vacuum 

suction efficiently collected particles resuspended by 

the vacuum beater bar and prevented these particles 

from becoming airborne.

• Quantifiable count based emission factors for submicrometer particles were obtained from vacuuming 

experiments but not from walking. The energy 

imparted to the carpet fibers by the vacuum beater bar 

was sufficient to dislodge these particles. Mass based 

emission factors for sub-micrometer particles were 

zero because the mass resuspended was insignificant 

and difficult to quantify.

• Emission factors for all test conditions were constant at 

all levels of Wollastonite loading. This finding suggests 

emission factors can be used to estimate airborne 

concentrations from the quantity available 

for resuspension found on the carpet 

via the MicroVac technique.

• Size dependent emission factors were not influenced 

by the experimental variables. General trends indicate 

that emission factors for particles less than 2 μm 

decrease as relative humidity increases, and emission 

factors for particles greater than 3.5 μm increase with 

increasing relative humidity.

• Emission factors as function of height were not 

calculated because of mixing within the exposure 

chamber and the contraction of the chamber plenum 

did not yield the expected concentration gradient.

General Findings

• Wollastonite was a suitable simulant for asbestos 

fibers. The Wollastonite fibers had a consistent aspect 

ratio of 1:3 over the range of cross sectional diameters 

from 1 to 5 μm. Aerodynamically, the Wollastonite 

fibers covered the size ranges (1-10 μm) that penetrate 

into the lung during respiration. The Wollastonite 

dispersed easily and agglomeration was not an issue 

during seeding of carpets. The only issue may be that 

Wollastonite aspect ratio was not as large as typical 

asbestos fibers (> 1:5).
36

• K-factors ranged from 10-3 to 10-6, depending on 

particle diameter and experimental conditions.

• Depletion of the Wollastonite fibers resuspended 

during an experiment was noticed, but did not affect 

the validity of the experiments. The elapsed time 

before depletion occurred was used as input into Eq. 

2-2 to calculate the total mass or counts resuspended.

• New and old carpets could be cleaned sufficiently 

to minimize the mass resuspended from unseeded 

carpets to almost the minimum detection limits of the 

instrumentation. However, the cleaning process did 

not completely remove all particles from the carpet 

fibers, as evidenced by the ultrasonication and fiber 

SEM data.

• Background characterization of the new and old carpet 

for fibrous particles on the fibers and resuspended 

during walking/vacuuming significantly improved 

data quality. Additional characterization samples from 

old carpet are recommended for future studies to check 

for spatial inhomogeneity in the background particle 

loading.

• Theoretically determined correction factors could be 

applied to the APS mass concentration data to make 

the values equivalent to the gravimetric data.
37

6.0

References

Brockman, J.E., “Sampling and Transport of Aerosols”, Chapter 6 in Aerosol Measurement: 

Principles, Techniques, and Applications, 2nd edition, edited by P. Baron and K. Willeke, John Wiley 

and Son, New York, September, 2001.

Leith, D., “Drag on Nonspherical Objects.” Aerosol Sci. & Technol., 6:153–162 (1987).

Hinds, W.C., Aerosol Technology: Properties, Behavior, and Measurement of Airborne Particles. 

Wiley Interscience, New York, New York. 1982.

Millette, J.R., Clark, P.J., Brackett, K.A., and Wheeles, R.K., “Methods for the Analysis of Carpet 

Samples for Asbestos.” Environmental Choices: Technical Supplement. 1:21–24 (1993).

Peters, T.M., Leith, D., “Concentration Measurement and Counting Efficiency of the Aerodynamic 

Particle Sizer 3321.” J. Aerosol Sci., 34:627–634 (2003).

Ranade, M.B. “Adhesion and Removal of Fine Particles on Surfaces.” Aerosol Sci. & Technol., 

7:161–176, 1987.

Rodes, C.E., and Thornburg, J. 2004. “Study of Resuspension of Particulate Matter on Flooring 

Surfaces Due to Human Activity.” Final Report, RTI Project 08886. U.S. EPA Contract 3C-R185-

NALX.

Rodes, C. E. and R. W. Wiener, “Indoor Aerosols and Exposure Assessment”, Chapter 29 in Aerosol 

Measurement: Principles, Techniques, and Applications, 2nd edition, edited by P. Baron and K. 

Willeke, John Wiley and Son, New York, September, 2001.

Thornburg, J., Ensor, D.S., Rodes, C., Lawless, P.A., Sparks, L.E., and Mosley; “Penetration of 

Particles into Buildings and the Influence of Associated Physical Factors, Part I: Sensitivity Model.” 

Aerosol Sci. & Technol., 34:284–296, 2001.

Thornburg, J., and Rodes, C.E. 2004b. “Indoor PM Resuspension Testing for Metals.” Final Report, 

RTI Project 08931. U.S. EPA Contract 3C- R340-NALX.

Thornburg, J., Rodes, C.E., Lamvik, M., Willis, R., and Rosati, J., Image Analysis Method (IAM) for 

Measurement of Particle Size Distribution and Mass Availability on Carpet Fibers. Aerosol Sci. & 

Technol. 40:274–281, 2006.
38

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A-1

Appendix A

SEM Image Analysis

Introduction

Determination of the total number of particles available 

for resuspension from carpet fibers as a function of particle 

size is important for accurate determination of resuspension 

emission factors. As part of previous research for Department 

of Housing and Urban Development, RTI developed a 

procedure for counting and sizing particles from SEM 

images. The HUD research focus and optimization of 

the procedure was for Pb (lead) particles. The developed 

procedure was modified for EPA funded projects for general 

particulate matter (PM), fibers, and metals. The modified 

procedure for general PM is presented here. SEM images 

and particle size distribution were provided by ManTech 

Environmental (METI) under contract to EPA. The RTI 

method for size distribution analysis was compared with the 

particle size distribution measured automatically during the 

computer controlled SEM imaging by METI. The results of 

this comparison are presented in this report.

Procedure

The RTI procedure for measuring the particle size from 

SEM images is attached. The image analysis procedure to 

highlight particles from the background was optimized for 6 

SEM photographs (500x) provided by METI in August 2004. 

Several assumptions are required to convert project area 

measurements from the RTI image analysis to aerodynamic 

diameters (Hinds, 1982). A volume shape factor of 0.25 was 

assumed to convert the projected area diameter (dpa) to an 

equivalent diameter (de). A dynamic shape factor of 1.12 was 

used to convert the de to an aerodynamic diameter (dae). RTI 

used a constant particle density of 2.5 g/cm3.

The procedure used by METI to calculate aerodynamic 

diameter is unknown, but RTI assumed the actual particle 

density measured by elemental analysis was used.

Results

One interesting finding between the RTI and METI 

procedures was immediately evident. The RTI procedure 

counted 2 to 3 times more particles per image than the METI 

procedure. The extra particles counted by RTI always were 

smaller than 2 μm. These extra particles were ignored by 

RTI. Only the largest RTI particles corresponding to the same 

number of particles measured by METI were compared.

The RTI and METI aerodynamic diameters calculated from 

the six 500x images are compared in Figure 1. The linear 

regression results are shown as well. The slope less than 1 

and the intercept greater than 1 indicate a difference in the 

aerodynamic diameters. However, the high coefficient of 

determination (R2

) shows the bias is repeatable.

The RTI aerodynamic diameters showed a positive bias for 

the upper end of the size distribution, and a negative bias 

for the lower end of the size distribution. This bias is an 

unavoidable artifact because of the type of particles being 

measured from the SEM images. Particles containing silicon, 

calcium, and potassium are more difficult to distinguish 

from the image background in a backscatter image. Higher 

molecular weight elements, like lead, are much easier to 

distinguish and this bias is less pronounced. As a result, the 

thresholding of the image to separate the particles from the 

background tends to lose area for smaller particles composed 

of lower molecular weight elements. Increasing the number 

of pixels associated with a smaller particle to offset the 

negative bias leads to the positive bias for the larger particles. 

Additional pixels cannot be applied solely to the smallest 

particles. The size of all particles is increased during the 

procedure. The influence of differences in the RTI and METI 

procedures for calculating aerodynamic diameters from 

project areas on the bias needs to be determined too.

Even with the bias in particle size measurements, the 

calculated size distributions were quite similar. There was 

a 5% difference in count median diameter. This bias did 

cause a larger difference in the geometric standard deviation, 

about 15%.

Conclusions

1. METI should not change their procedure for obtaining 

SEM images. The RTI image analysis procedure has 

been optimized for the images being obtained by 

METI. Changing the SEM image quality would require 

substantial effort to recalibrate the RTI procedure.

2. Confirm METI and RTI are using the same conversion 

factors for calculating aerodynamic diameters from 

projected area measurements.

3. METI used individually calculated particle densities 

based on the elemental composition of the particles. 

The METI densities averaged 2.4 g/cm3 ± 0.2. The 

difference in RTI and METI densities had a negligible 

effect on calculated aerodynamic diameter.

4. It is unknown why the RTI procedure was able to 

detect particles less than 2 μm from 500x images 

whereas METI did not. Is it possible METI was not 

looking for particles less than 2 mm during their 

particle measurements?
A-2

Particle Size Distribution Measurement 

Using SEM Image Analysis

Jonathan Thornburg, PhD

RTI International

August 19, 2004

This procedure was modified specifically for analysis of 

METI generated SEM photographs of particles on carpet 

fibers. Original procedure is located in RTI\CAT Standard 

Operating Procedure 07061.001, SEM-EDS Analyses, 

Version 1, 2000. This modified procedure is applicable to 

RTI projects 08886, 08931, and 08924.

Procedure

1. Use NIH Image Beta V. 4.02, available from Scion 

Corp. http://www.scioncorp.com/frames/fr_scion_

products.htm

2. Open desired photograph in NIH image

3. Select ANALYZE\SET SCALE from menu bar

a. Select units of scale (micrometers)

b. Specify # pixels covered by image in known 

distance box. Move cursor to far right of image and 

read # pixels in “Info” box

c. Enter corresponding scale of image in measured 

distance box.

i. 500x = 177.8 μm

ii. 880x = 101 μm

iii. 1000x = 88.9 μm

4. Select PROCESS\ENHANCE CONTRAST to 

highlight particles

5. Select OPTIONS\THRESHOLD to highlight particles. 

Grab threshold intensity bar in LUT window. Move 

cursor up until threshold value in “Info” window reads 

148. Threshold value of 130 is for METI produced 

photographs. Other photographs may need different 

threshold. Trial and error process to get best

6. Covert photo to binary image by selecting PROCESS\

BINARY\MAKE BINARY

7. Remove holes in particles by selecting PROCESS\

BINARY\ERODE

8. Remove phantom particles by selecting PROCESS\

BINARY\DILATE

9. Remove new holes in particles by selecting PROCESS\

BINARY\ERODE

10. Select EDIT\INVERT to make particles black, 

background white

11. Select ANALYZE\OPTIONS. Check boxes for:

a. Area

b. Ellipse major axis

c. Ellipse minor axis

d. Include interior holes

e. Heading

f. Max Measurements = 1000

g. Field Width = 9

h. Digits = 3

Figure A-1. Comparison of RTI and METI aerodynamic diameters
A-3

12. Select ANALYZE\ANALYZE PARTICLES. Check 

boxes for:

a. Label particles

b. Ignore particles touching edge

c. Include interior holes

d. Reset measurement counter

13. Save image of labeled particles

14. Select ANALYZE\SHOW RESULTS. Projected area 

data for each particle opens in new window.

15. Select EDIT\COPY RESULTS to copy data to 

clipboard.

16. Copy results into Excel spreadsheet “SizeDist.xls”. 

Paste into Cell A2.

17. Spreadsheet uses Hinds equations to automatically 

convert projected area measurements to:

a. Projected area diameter

b. Equivalent diameter

i. Use volume shape factor = 0.25

c. Aerodynamic diameter

i. Shape factor = 1.12

ii. Density = 2.5 g/cm3 for ATD. Use appropriate 

density for other materials.

User specified size intervals can be used to calculate CMD, 

GSD, and R2. Upper size of each interval needs entered in 

Column
A-4

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B-1

Appendix B

Relationship between Aerodynamic 

and Fiber Diameters

Introduction

Particles with high aspect ratios, like the Wollastonite 

used in these experiments, present a challenge when trying 

to interpret mass data based on different measurement 

techniques. The gravimetric mass measurements depend 

on the physical volume and true density of the collected 

particles. On the other hand, the mass measurements from the 

APS are estimated from aerodynamic diameters and assume 

unit density. APS mass measurements of fibrous aerosols 

typically are underestimated because the crosssectional 

diameter of the fiber is measured due to how the fibers align 

in the flow field through the APS sensing volume.

To check the accuracy of the APS mass concentrations 

measured, a theoretical exercise, presented below, provided 

a correlation between the APS mass measurement (based on 

aerodynamic diameter) URG mass measurement (based on 

physical volume).

Procedure

1. Convert cross sectional fiber diameter to 

aerodynamic diameter using equations from 

Leith (1987). See Figure B-1.

(B-1)

2. Relate mass measurements based on aerodynamic 

(APS) and physical (Gravimetric) data

(B-2)

Results

Using Figure B-1 and Eq. B-2, the mass measured by the 

APS will be about 25% of the mass measured gravimetrically 

(See Eq. B-3). This result agrees with the slope of the linear 

regression equation shown in Figure 3-1.

(B-3)

Figure B-1. Comparison of Fiber Aerodynamic (from APS) and Fiber Cross Sectional Diameters.
B-2

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B-3

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