Reducing dust exposure from contaminated work clothing with a 

stand-alone cleaning system 

A.B. Cecala, D.E. Pollock, & J.A. Zimmer 

National Institute for Occupational Safety and Health, Pittsburgh Research Laboratory, Pittsburgh, PA, 

USA 

A.D. O’Brien & W.R. Fox 

Unimin Corporation, Winchester, VA, USA 

ABSTRACT: A few years ago, the National Institute for Occupational Safety and Health and Unimin Corporation 

developed a clothes cleaning system that is able to quickly, effectively, and safely remove dust from a worker’s 

clothing without dust exposure to the worker, the work environment, or co-workers during the cleaning process. 

To perform the process, a worker enters a cleaning booth and activates an air spray manifold which blows the dust 

from the worker’s clothing. Since the cleaning booth is under negative pressure, all the dust and product removed 

are contained. In the original design, this dust-laden air was exhausted to a dust collector system. However, since 

most operations do not have excess dust collector capacity, it was decided to determine an alternative method to 

provide for a stand-alone cleaning system. This was achieved by ducting the exhaust outside the plant and into the 

atmosphere. This technique was recently tested and shown to be a viable method to provide for a stand-alone 

clothes cleaning system. 

1 Introduction 

New methods are constantly being investigated to lower 

respirable dust exposures to workers in the mining 

industry. When the ore being processed contains quartz or 

silica-bearing material, the need to achieve low respirable 

dust levels is especially critical. One area of known high 

exposure can come from contaminated work clothing 

(Cohen and Positano, 1986; Fogh et al., 1999). A U.S. 

Bureau of Mines study documented a number of cases of 

high worker exposure from background dust sources in the 

minerals processing industry. This report highlighted two 

cases of a 10-fold increase in respirable dust from 

contaminated work clothing (Cecala and Thimons, 1986). 

Once the individual’s clothing was contaminated, dust was 

continuously emitted from it and the only way to eliminate 

the dust source was for the worker to clean or change their 

clothing. Contaminated work clothing is not only a hazard 

to the worker themselves, but also contaminates coworkers and even family members when it is deposited in 

personal vehicles and at home when work clothes are not 

changed or cleaned at the end of the work shift (Chiaradia 

et al., 1997; NIOSH 97-125, 1997; Chan et al., 2000). 

There are two federal regulations that affect the 

cleaning of work clothing for the US mining industry. The 

first regulation is a Mine Safety and Health Administration 

(MSHA) standard that states: “At no time shall compressed 

air be directed toward a person. When compressed air is 

used, all necessary precautions shall be taken to protect 

persons from injury” – 30 CFR Part 56.13020. A second 

regulation that is a general industry standard established by 

the Occupational Safety and Health Administration 

(OSHA) in 29 CFR 1910.242 9 (b), states that: 

“Compressed air shall not be used for cleaning purposes 

except where reduced to less than 206.8 kPa (30 psi) and 

then only with effective chip guard and personal protective 

equipment.” Because of these regulations, the approved 

method for the US mining industry is to use a HEPA-filter 

vacuuming system. To perform this technique, a worker 

uses the vacuum hose and manually moves the nozzle over 

his/her soiled clothing in an attempt to remove the 

contamination. This is a very difficult, ineffective, and 

time-consuming task; therefore, few workers actually use 

this technique. In practice, the most common technique is 

to use a single compressed air hose to blow dust from the 

workers clothing, even though this is a prohibited method 

of cleaning. Normally this is performed at full line 

pressure, 552 to 689 kPa (80 to 100 psi), in an open work 

area which not only exposes the worker but also creates a 

significant dust cloud in the work environment and 

ultimately exposes co-workers. 

In 2001, Unimin Corporation approached the National 

Institute for Occupational Safety and Health (NIOSH) to 

work in a cooperative effort with the goal of developing a 

safe and effective method for removing dust from soiled 

work clothes without exposing the worker, co-workers, or 

the work environment. Through work performed in 2002 

and 2003 at Unimin’s Marston facility in North Carolina 

and NIOSH’s Pittsburgh Research Laboratory in 

Pennsylvania, a successful clothes cleaning system was 

developed. In 2004, a petition for modification was 

submitted to MSHA, which was approved and allowed for 

the use of the clothes cleaning system at the Marston 

facility. This approved system was composed of a 

“cleaning booth” that captured the product and dust 
 

removed from the worker’s clothes during the cleaning 

process and then exhausted it to a Local Exhaust 

Ventilation (LEV) system. During numerous tests 

performed at this facility in the development of this 

system, it was verified that no dust escaped from the 

cleaning booth to contaminate co-workers or the work 

environment. It was also shown there was very minimal to 

no respirable dust exposure to the worker wearing a fittested 1⁄2-mask respirator while performing the cleaning 

process. In a matrix of tests that were performed during 

this field analysis, it was shown that the clothes cleaning 

technique was 40.8 pct more effective than vacuuming and 

50.6 pct than the commonly used method of using a single 

compressed air hose to remove dust from the worker’s 

clothing. The clothes cleaning process was also superior in 

its ability to uniformly remove dust from all areas of the 

coveralls used for testing (Pollock et al., 2006; Cecala et 

al., 2007). 

After the completion of this successful effort, both 

Unimin and NIOSH realized that the vast majority of 

operations would not have the LEV capacity to keep the 

cleaning booth under negative pressure. Because of this, it 

was decided to perform a second phase of research to 

modify the design to develop a stand-alone system. After 

considering a number of different options, it was believed 

that the optimal design would be to exhaust the air from 

the cleaning booth to outside the plant and discharge it into 

the atmosphere through an exhaust stack. A similar 

technique that has been used for many years called a total 

mill ventilation system (TMV) also exhausts dust-laden air 

from within a structure to the outside atmosphere. For this 

system, clean air is brought in at the base of the facility and 

then sweeps through the building before being discharged 

at or near the roof of the structure. This TMV system is a 

common practice for most mineral processing plants today. 

Realizing that exhausting dust-laden air into the 

atmosphere will have environmental implications, the 

Environmental Protection Agency (EPA) was contacted to 

determine if there were any new regulations that would be 

relevant to this intended practice. Based on feedback from 

the EPA, the only regulation that addressed releasing the 

dust removed from a worker’s clothing into the atmosphere 

would be the opacity standard, which is a subjective visual 

determination of the degree of a dust cloud exiting any 

exhaust duct or stack. When one considers the possible 

quantity of dust on a worker’s clothing relative to the 

exhaust volume of air being drawn from the booth, any 

dust cloud viewed coming from the exhaust stack would be 

for a very short duration and should not be a concern from 

an environmental standpoint for this system. 

Phase II of this research was performed at Unimin’s 

Elco Facility, located in Elco (southern), Illinois which 

funded all aspects of this clothes cleaning system. In 

addition, all plant personnel involved in this research effort 

were extremely insightful and cooperative which allowed 

for various modifications to the design of the system to be 

tried in an effort to optimize the system. During the second 

phase of this research effort, SK Bowling Company1 

offered to fabricate the design developed during this 

research with the ultimate goal of being able to 

manufacture and sell the clothes cleaning booth system 

once the research was completed. SK Bowling Company 

built the unit that was tested during phase II of this 

research project and is currently offering this design for 

sale (http://www.cleanclothbooth.com/). 

1

 Mention of any company name or product does not 

constitute endorsement by the National Institute for 

Occupational Safety and Health. 

2 Clothes Cleaning System Design 

The clothes cleaning system developed under this 

cooperative research effort consists of four major 

components: a cleaning booth, an air reservoir, an air spray 

manifold, and an exhaust ventilation system. The following 

discussion is the final optimized design determined during 

phase II of this research effort to develop a stand-alone 

system where the product cleaned from a worker’s clothing 

is discharged through an exhaust stack outside the plant. 

The cleaning booth is the first component to this system 

and has a base dimension of 122 by 107 cm (48 by 42 in). 

A standard size door is located on the front to allow ingress 

and egress from the cleaning booth. All intake air enters 

the cleaning booth through a 61-cm (24-in) cutout on the 

roof directly over where the worker performs the clothes 

cleaning process. The air flows directly down over the 

worker in the booth before flowing through an expanded 

metal grating on the floor. At this point, the air exits 

through a return air plenum at the bottom and back of the 

booth, transitioning to a 30.5-cm (12-in) duct. Once in the 

30.5-cm duct, the dust-laden air is drawn outside the plant 

through a blower before being discharged from an exhaust 

stack into the atmosphere. 

The second major component to this system, the air 

reservoir, supplies the required air volume necessary for 

the air nozzles used in the spray manifold. The size 

requirement for the air reservoir was calculated based upon 

the design of the air spray manifold. Although a 0.45-m3 

(120-gallon) reservoir was used during phase I of this 

research, it was decided to use a 0.90-m3

 (240-gallon) unit 

for this second phase. Knowing that the smaller reservoir 

was sufficient for only one cleaning cycle, it was felt that 

the benefit of multiple cleanings one after another far 

outweighed the additional cost and space requirement of 

the unit. The air reservoir was located next to the cleaning 

booth and hard-piped to the air spray manifold located 

inside the booth. Figure 1 shows the layout of the booth 

and the air reservoir. 

The next main component of this system, the air spray 

manifold, was constantly modified and improved during 

various laboratory and field tests. During the approval 

process with MSHA during phase I, MSHA required that 

the manifold be designed for the average height of a US 

worker. The final design for the manifold is composed of 
 

 

 

 

 

26 spray nozzles spaced at 5.1 cm (2 in) apart, as shown in 

Figure 2. These spray nozzles are designed to operate at 

206.8 kPa (30 psi) and a regulator is used to ensure that the 

pressure is not exceeded. The top 25 spray nozzles are flat 

fan nozzles; the bottom nozzle is a circular design and was 

used for cleaning the individual’s work boots. 

Figure 1. Clothes cleaning booth and air reservoir. 

Figure 2. Air spray manifold design. 

The fourth and final component is the exhaust 

ventilation system to keep the cleaning booth under 

negative pressure throughout the entire clothes cleaning 

process. As previously stated, the goal of the phase II effort 

was to design a stand-alone unit that would not need to be 

tied into an LEV system. After considering a number of 

different options, both Unimin and NIOSH believed that 

the optimal design would be to exhaust the dust-laden air 

from the cleaning booth to outside the plant where it would 

not contaminate other workers nor be entrained back into 

the plant. In the design stage of phase II of this research, 

Unimin’s corporate engineering department evaluated all 

the requirements for the clothes cleaning process and the 

booth and determined that an exhaust volume of 

approximately 0.94 m3

/sec (2,000 cfm), at 20o C and 101 

kPa would be a targeted goal. It was believed that this 

exhaust volume would provide an adequate degree of 

safety to keep the booth under negative pressure at all 

times. During the testing of the system in August 2006, it 

was verified that 0.94 m3

/sec exhaust volume was adequate 

to maintain negative pressure throughout the entire clothes 

cleaning cycle. 

3 Testing 

3.1 Dust Sampling Instrumentation 

Gravimetric dust sampling consisted of two sampling 

pumps, flexible Tygon tubing, the 10-mm Dorr-Oliver 

cyclone, and the dust filters. All dust samples were 

collected with the 10-mm cyclone, which classifies the 

respirable portion of dust. Each gravimetric sampler was 

calibrated to a flow rate of 1.7 L/min, the required flow 

rate established by the MSHA for the metal/nonmetal 

mining industry for respirable dust sampling. The 

respirable dust classified by the cyclone was deposited on a 

37-mm dust filter cassette, which were pre- and postweighed to the nearest 0.001 mg on a microbalance at the 

Pittsburgh Research Laboratory. For all sample locations 

except the one inside the PPE, the two gravimetric filter 

dust concentrations were averaged together to provide a 

single concentration for each location. 

The instantaneous monitor used at all eight sample 

locations was the Personal Data RAM (pDR, Thermo 

Electron Corp , Franklin, MA). This is a real-time aerosol 

sampler that measures the respirable dust concentration 

based upon the light scatter of particles in an internal 

sensing chamber. The respirable dust levels were recorded 

on an internal data logger and were downloaded to a laptop 

computer at the end of each sampling period. Since the 

clothes cleaning process had such a short cleaning 

duration, the sample log time was set to every two seconds, 

which required the data to be downloaded every 4 hours. 

All of these instruments, except for the unit monitoring 

inside the respirator, were operated in the passive mode; 

for these instruments, the dust particles travel through the 

sampling chamber naturally without any mechanical 

assistance. 

For measuring dust inside the respirator, the pDR 

instrument was operated in the active sampling mode. In 

this case, the air sample was mechanically pulled into the 

sensing chamber of the instrument and then deposited on a 

filter cassette, identical to that used with the gravimetric 

sampler. A new filter cassette was used for each sampling 

period. 

The average gravimetric dust value at each sampling 

location was used to determine a correction factor for the 

pDR instantaneous dust monitor. This was performed by 
 

 

 

 

 

using the respirable dust concentration measured by the 

gravimetric samplers and determining the average 

respirable dust concentration for the pDR monitor for the 

exact sampling time period. A correction factor was then 

calculated by dividing the pDR average concentration 

value into the gravimetric value. This calculated correction 

factor was then multiplied by all the individual dust 

measurements taken with the pDR device in an Excel 

spreadsheet. Using both types of respirable dust monitoring 

equipment provided a good profile of the dust 

concentrations throughout the sampling period, as well as 

variations and changes in respirable dust concentrations 

during the evaluation of the clothes cleaning process. 

3.2 Setup 

Field testing was performed on phase II of this research 

effort during August 28-31, 2006. In order to validate a 

number of issues with the new system, a detailed test plan 

was established. This test plan was composed of eight 

different dust sampling locations in and around the clothes 

cleaning system. The following is a brief description of 

these various dust sampling locations. 

The first sample location was to determine the 

respirable dust levels inside the half-mask respirator for 

both test subjects performing the clothes cleaning testing at 

this facility. Both of these test subjects were NIOSH 

employees and were fit-tested with their own North 7700 

Silicone Half Mask Respirator prior to the field testing to 

ensure that the personal protective equipment (PPE) 

provided an acceptable level of protection (fit-factor). For 

the field tests, fit-testing adaptors were purchased from 

North and were used to determine respirable dust levels 

inside the face piece of the respirator during the clothes 

cleaning process. A special sampling chamber was also 

fabricated of clear Plexiglas and used for this testing. 

Tygon tubing connected from the respirator to this 

sampling chamber and was attached to the inside wall of 

the cleaning booth. A 10-mm Dorr-Oliver cyclone was 

located inside this sealed sampling chamber and was then 

connected to the pDR, also using Tygon tubing. Since the 

pDR instrument was being used in the active sampling 

mode, a sampling pump and filter were used to draw an air 

sample from inside the respirator to the sampling chamber, 

through the 10-mm cyclone, and finally into the pDR 

instrument (Figure 3). This allowed the respirable dust 

concentration inside the half-mask respirator to be 

determined and logged on the pDR’s internal data logger 

during testing. 

Figure 3. Sampling setup to obtain respirable dust sample 

inside half mask-respirator. 

All of the remaining seven dust locations consisted of a 

pDR instantaneous dust monitor and two gravimetric 

sampling units on a sampling rack. The second dust 

sampling location was inside the clothes cleaning booth 

and was also attached to the wall. This provided the 

respirable dust concentration inside the cleaning booth 

during the clothes cleaning process, as well as a 

comparison to the respirable dust concentration inside the 

half-mask respirator for the two test subjects. The third 

dust sampling location was above the cleaning booth to 

monitor the intake air flowing into the unit. This sample 

location could also detect leakage from the cleaning booth 

if the air spray manifold pressurized the booth to a degree 

that it caused respirable dust to be pushed out from the 

intake opening. The next three sample locations were 

positioned around the outside of the clothes cleaning booth 

to measure the potential of any dust leakage from the 

booth. These three sample locations were called: main 

right, main center, and main left. These locations were 

designated based on a person standing outside looking 

toward the booth. 

The last two dust sampling locations were background 

A and B and were located inside an open garage door at the 

entrance to the bag loading mill area, as shown in Figure 4. 

After viewing the discharge duct for the exhaust fan, these 

two sample locations were chosen as the most likely 

locations where respirable dust could be re-entrained back 

into the mill building. 

3.3 Weighing Procedures for Coveralls 

As previously mentioned, two NIOSH test subjects were 

used for evaluating the clothes cleaning system. Both test 

subjects used their own sets of coveralls for the entire 

study which were a polyester-cotton blend material. The 

coveralls were manually soiled with dust by the researchers 

to a level which would be considered a worst case 

scenario. A detailed coverall weighing procedure was 

performed with the goal of quantifying the effectiveness of 

the clothes cleaning technique at removing dust soiled on 

the coveralls for both test subjects. 

Through this pre- and post-weighing procedure, the 

amount of product added to the coverall was determined 

and the amount of product removed during the cleaning 

process was also calculated. This detailed weighing 

process was performed as accurately as possible to 

determine the amount of dust that was added to each 

coverall, and then the amount of dust removed by the air 
 

nozzles during the cleaning process in an effort to 

determine the system’s effectiveness. 

For each day of testing, all coveralls were laundered for 

a clean baseline measurement. Each pair of coveralls was 

used three times during the course of the day, and then 

laundered again for the next day of testing. 

Figure 4 Dust sampling locations for testing and exhaust 

stack layout. 

3.4 Test Plan 

There were three areas of concern to be tested for this 

research effort: 1) ensure that test subjects were not overexposed during the clothes cleaning process, 2) ensure that 

respirable dust was not escaping from the cleaning booth 

during the cleaning process, and 3) ensure that respirable 

dust being blown from the exhaust stack was not being 

entrained back into any buildings at the facility. All three 

of these areas were evaluated while performing clothes 

cleaning tests on the subjects in the cleaning booth. These 

tests were performed over a three-day period on August 

28-31, 2006. The actual run time for each test was 18 

seconds, being the time that the air spray manifold would 

be actuated with compressed air for the clothes cleaning 

process. 

4 Results 

There were 96 clothes cleaning tests performed during the 

study, 48 tests for each test subject. Table 1 provides the 

mean and standard deviation for the respirable dust 

concentration measured inside the respirator and inside the 

cleaning booth for the 18 seconds while the air spray 

manifold was activated during testing. This table also 

indicates an identical and extremely low respirable dust 

exposure inside the respirator for both test subjects. An 

interesting aspect to consider is the difference in respirable 

dust concentrations between the inside respirator and the 

inside cleaning booth location. This confirms the first 

objective of this research, which was to ensure that the test 

subjects were not over-exposed during the clothes cleaning 

process. When one compares the respirable dust 

concentrations inside the cleaning booth relative to those 

inside the respirators, it verifies that both test subjects 

remained at very low and acceptable dust levels for all 

tests. 

Table 1. Mean and standard deviation of 18-second test for 

48 tests for both test subjects. 

Inside Respirator Inside Booth 

Subject 1 0.01 752.33 

Std Dev Sub 1 0.01 634.29 

Subject 2 0.01 715.94 

Std Dev Sub 2 0.02 609.92 

It must be noted that the respirable dust concentration 

inside the cleaning booth exceeded the pDR instrument’s 

range of detection (maximum of 400 mg/m3

). The values 

varied significantly from test to test as seen in the high 

standard deviation for both test subjects. Because of this, 

the dust concentration at the inside booth location should 

only be viewed as a relative value. 

When considering the amount of dust added to the 

coveralls and the amount remaining after cleaning, the 

results of the 96 tests indicate that the system removed 

83.1 pct of the dust on the worker’s clothing during the 

cleaning process. Figure 5 shows the effectiveness of this 

cleaning process for one of the test subjects. 

Table 2 presents the results for the other six sample 

locations. This table not only includes the 18 seconds for 

each clothes cleaning test, it also considers a time period of 

one minute after the test is completed. This should provide 

enough time to determine the impact of any dust leakage 

from the clothes cleaning booth or exhaust stack at the 

various sample locations. To verify the possibility of any 

impact, the results of this table, along with the individual 

traces of each of the tests (96) were examined to determine 

if any spikes occurred immediately following a clothes 

cleaning cycle. This was not the case from the first four 

locations evaluated, verifying that there was no respirable 

dust escaping from the cleaning booth. 

The last area being evaluated was to ensure that 

respirable dust being blown from the exhaust stack of the 

cleaning booth was not being entrained back into any 

buildings at the test facility. From the traces of different 
 

 

 

 

 

pDR dust monitors, spikes shown at the inside booth 

location corresponded with spikes seen at the two 

background locations A and B. These background sample 

locations were inside a garage door to a packaging mill 

structure, which was the closest structure to the exhaust 

stack from the clothes cleaning system. The data in Table 2 

for Background A and B also clearly demonstrates the 

problem. Once noting entrainment of dust back into a 

structure, it became necessary to redesign the exhaust 

stack. 

Figure 5. Before cleaning (top) and after cleaning (bottom) 

using clothes cleaning system. 

Table 2. Mean and standard deviation of 18-second test 

and 1 minute afterward for 48 tests for both test subjects. 

Main 

Rgt 

Main 

Cent 

Main 

Left Intake 

Back 

A 

Back 

B 

Sub 1 0.02 0.01 0.02 0.01 0.60 0.17 

StDev 

Sub 1 0.02 0.02 0.03 0.01 0.46 0.14 

Sub 2 0.02 0.03 0.02 0.01 0.55 0.24 

StDev 

Sub 2 0.02 0.03 0.04 0.01 0.45 0.19 

After evaluating a number of different options, it was 

decided to extend the exhaust stack significantly higher in 

an effort to eliminate the entrainment of dust back into any 

structure. The original design had a 6.1-m (20- ft) section 

of vertical duct extending from the fan, as shown in Figure 

4. After entrainment was found at the two background 

locations, the exhaust system was modified. A 45-degree 

elbow was connected to the end of the stack and then run 

over to a mill structure. Once reaching the structure, 

another 45-degree elbow was used to run 12.2 m (40 ft) 

vertically up the outside of the structure, and then 

discharge at the top of the structure. This modification 

extended the stack an additional 18.3 m (60 ft) in the air. 

Testing of the new exhaust stack design was performed 

for three consecutive days being March 31 through April 2, 

2007, in an effort to achieve different variations in weather 

and wind conditions for three consecutive days. For each 

day, four clothes cleaning cycles were performed over 

approximately a one-hour time period. In an effort to 

ensure that there was no entrainment into any structure, 

two additional sample locations were added for this testing 

(Doors 2 and 3, Figure 4). These twelve clothes cleaning 

tests did not indicate any increase in respirable dust at any 

of the sampling locations, Figure 6. Although there were 

fluctuations in respirable dust concentrations at all 

locations from day to day, there was no indication of 

elevated respirable dust concentrations immediately 

following a clothes cleaning cycle at any sample locations. 

This verifies that the redesign of the exhaust stack was 

effective and did not allow any entrainment of respirable 

dust back into any structures at this facility. 

5 Conclusion 

The clothes cleaning system designed in phase II of this 

research effort was a stand-alone system that proved to be 

very effective. One aspect that remained unchanged from 

the original design was the air spray manifold, which again 

was shown to be very effective at removing dust soiled on 

an individual’s work clothes in a very short time period (18 

seconds). 

During phase I, this clothes cleaning system was shown 

to be much more effective than either the vacuuming or the 

single air hose method of cleaning soiled work clothes 

(Cecala et al., 2007). The cleaning booth was also 

redesigned during phase II to provide a more laminar flow 

which was also shown to be very effective. Field testing 

also showed that when the two test subjects were fit-tested 

with half-mask N-100 respirators, there was very minimal 

to no respirable dust exposure to the individual during the 

cleaning cycle. 

The last area of this research effort was to ensure that 

the exhaust stack from the cleaning booth was not causing 

respirable dust to be entrained back into any of the 

structures at this facility. During the initial test, it was 

found that respirable dust was flowing through an open 

garage door into the packaging mill. After the exhaust 

stack was modified by extending it an additional 18.3 m in 

height, this situation was rectified. 

Both Unimin and NIOSH believe that this clothes 

cleaning system significantly improves the health of 

workers by providing a quick and effective method for 

workers to clean dusty clothes during the workday without 

risk to worker safety. This phase II design should be 

applicable to any operation wishing to implement a clothes 

cleaning system. Although this system was originally 

designed to assist in meeting the stringent respirable dust 

standards for the silica sand industry, it would be 

applicable to all types of mining, as well as other 

industries. 
 

 

 

 

 

643

671

Figure 6. Dotted lines represent four clothes cleaning 

cycles for three test days to determine the possibility of 

recirculation of dust into any structures from the system’s 

exhaust stack. 

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