Field Assessment of Enclosed Cab Filtration System

Performance Using Particle Counting Measurements

John A. Organiscak, Andrew B. Cecala, and James D. Noll

National Institute for Occupational Safety and Health, Office of Mine Safety and Health Research,

Pittsburgh, Pennsylvania

Address correspondence to: John A. Organiscak, National Institute for Occupational Safety and Health, Office of Mine Safety and

Health Research, 626 Cochrans Mill Rd., P.O. Box 18070, Pittsburgh,

PA 15236; e-mail: jorganiscak@cdc.gov

Enclosed cab filtration systems are typically used on mobile

mining equipment to reduce miners’ exposure to airborne dust

generated during mining operations. The National Institute for

Occupational Safety and Health (NIOSH) Office of Mine Safety

and Health Research (OMSHR) has recently worked with a

mining equipment manufacturer to examine a new cab filtration system design for underground industrial minerals equipment. This cab filtration system uses a combination of three

particulate filters to reduce equipment operators’ exposure to

dust and diesel particulates present in underground industrial

mineral mines. NIOSH initially examined this cab filtration

system using a two-instrument particle counting method at the

equipment company’s manufacturing shop facility to assess

several alternative filters. This cab filtration system design

was further studied on several pieces of equipment during

a two- to seven-month period at two underground limestone

mines. The two-instrument particle counting method was used

outside the underground mine at the end of the production

shifts to regularly test the cabs’ long-term protection factor

performance with particulates present in the ambient air. This

particle counting method showed that three of the four cabs

achieved protection factors greater than 1,000 during the field

studies. The fourth cab did not perform at this level because

it had a damaged filter in the system. The particle counting

measurements of submicron particles present in the ambient

air were shown to be a timely and useful quantification method

in assessing cab performance during these field studies.

INTRODUCTION

Enclosed cabs are an engineering control that can provide

a safe, comfortable, and healthy worker environment for

equipment operators. Most modern day enclosed cabs have

heating, ventilation, and air conditioning (HVAC) systems for

maintaining a comfortable temperature and a breathable quantity of air for their occupants. Various levels of filtration can be

incorporated into the HVAC system to improve the ventilation

quality of the air inside the cab by removing airborne pollutants

such as dusts, diesel, and other aerosols. Enclosed cab filtration

system effectiveness has been previously studied but it can be

difficult to measure in the field.

The American Society of Agricultural Engineers (ASAE)

previously devised a consensus standard for testing a cab’s

particulate reduction factor and specifying a cab’s performance

criteria for pesticide aerosol applications. These procedures

used an optical particle counter placed inside and outside the

cab to examine 2–4 μm ambient air particulate penetration

into the cab as it drives along on an end-use tractor.(1,2) The

procedure used two particle counters that were alternated

between four cab test replicates to negate or average out

measurement biases between instruments. Examination of this

test procedure indicated that inconsistent and low particle

count data for the 2–4 μm particles outside the cab can yield

unreliable cab performance results between replicated cab

tests.(2) It was observed that the cab filtration system needs to

be challenged by a high enough outside particle count concentration to make the unfiltered inside ventilation system aerosol

generation insignificant.(2) Another study showed that 200 to

1800 particles/L are generated from various cab manufacturer

blower fans, and that the outside cab test concentrations should

be sufficiently higher to make this internal particle source bias

insignificant.(3)

Alternative particle counting cab field test procedures have

been examined by NIOSH with submicron particles (0.3–

0.5 μm) and have been used to quantify and improve agricultural tractor cab protection performance for pesticide spray

applications.(4,5) One procedure used two optical particle counters alternated between the inside and outside locations of the

tractor cab during four mobile test replicates, similar to the

ASAE consensus standard test except that 0.3–0.4 μm particle

sizes were used as the test medium. This field study identified

and sealed several tractor cab filtration system leaks around a

high-efficiency particulate air (HEPA) filter, which effectively

reduced particle penetration into the cab from 11% to less

than 1%.(4) Another study measured 0.3–0.5 μm particles

inside and outside a stationary unoccupied cab parked inside a
temporary enclosure filled with incense smoke contaminants.

In this procedure a two-channel particle counting instrument

sampled alternate positions inside and outside an unoccupied

cab using equal lengths of sample hoses to these locations.

This procedure was used to quantify cab protection factors of

at least 50:1 (outside to inside particle concentrations) for a

pesticide spraying company after refurbishing and performing

maintenance on 13 tractor cabs.(5)

Particle counting measurements of submicron (0.3–1.0μm)

particulates in ambient air were further used by NIOSH during

laboratory experiments with multiple instruments to study the

key design factors of enclosed cab dust filtration systems.(6)

During these experiments two particle counting instruments

were alternated between test replicates to average out any

measurement biases between instruments, similar to the ASAE

procedure described above. The experimental tests were conducted in an unoccupied experimental cab test apparatus with

a 2.0-m3 (72-ft3

) inside enclosure volume, and at least four test

replicates were completed for each cab test condition studied.

These laboratory experiments studied the effects of intake

filter efficiency, intake filter loading, intake air leakage around

the filter, recirculation filter use, and wind on cab filtration

system performance. Laboratory measurements showed that

the largest number particles in the ambient air were submicron (0.3–1.0 μm) and that concentrations remained relatively

constant during the testing on a particular day, but varied

noticeably from day to day.(6) These experimental cab test

results determined that intake filter efficiency and recirculation filter use were the two most important influential factors

on cab filtration system performance.(6) The addition of a

recirculation filter to the cab’s filtration system significantly

reduced its particulate penetration performance by an order

of magnitude and reduced the time to reach its lowest stable

concentration after the entrance door is closed by about 60%.(6)

NIOSH most recently used this multiple-instrument particle

counting method to study and quantify cab filtration system

performance in the field. In cooperation with a mining equipment manufacturer, NIOSH examined the performance of a

new cab filtration system design for the underground industrial

minerals industry. This cab filtration system used a combination of three particulate filters (intake, recirculation, and final

filter) to reduce the equipment operator’s exposure to dust and

diesel particulates present in underground industrial mineral

mines. Particle count testing and airflow measurements were

initially conducted at the company’s manufacturing shop to

examine the cab filtration system performance with several

types of final filters and intake filter configurations.

The performance of this cab filtration system design was

further studied in the field on several pieces of underground

mining equipment. Four pieces of equipment with the new

cab filtration system design were studied intermittently over a

two- to seven-month period at two different underground limestone mines. The underground field dust and diesel sampling

results from the seven-month study are described elsewhere

by Cecala et al.(7) and Noll et al.(8) and showed inconsistent

cab protection factors among production shifts because of

unpredictable equipment operator practices, such as the frequency and duration of opening cab doors and the variability in

HVAC system operation. Particle counting and HVAC airflow

measurements were also made on these cabs in ambient air

outside the mine between production shifts to better quantify

their protection factor performance under more consistent cab

operating test procedures. This article describes the particle

counting method, measurement uncertainty analysis, and results obtained from studying the new cab filtration system at

the manufacturing facility and during the field surveys.

METHODS AND APPARATUS

Cab Filtration Systems Tested

Figure 1 illustrates the new filtration system that was initially tested on a face drill cab at the manufacturing facility

and was later tested in the field studies. The face drill’s cab

at the manufacturing facility had an inside volume of at least

4.5 m3 (160 ft3

). The cabs at the mines had inside volumes that

ranged from 2.1 to 4.5 m3 (75 to 160 ft3

). This filtration system

uses the HVAC fan to jointly pull outside cab air through an

external intake filter unit and interior cab air through a wallmounted recirculation filter before routing this total airflow

mixture through the HVAC heat exchangers, final filter, and

cab interior. The final air filter was initially specified as a

HEPA (efficiency ≥99.97 down to 0.3 μm particle sizes) with

rectangular dimensions of 29.0-cm wide by 44.4-cm long

by 9.7-cm deep (11.375-in. wide by 17.5-in long by 3.75-

in. deep). An alternative final filter considered for the cab

HVAC system was a similarly sized panel filter with an American Society of Heating, Refrigerating and Air-Conditioning

Engineers (ASHRAE) minimum efficiency reporting value of

16 (MERV 16, efficiency ≥95% down to 0.3 μm particle

sizes).(9) A pleated cellulose cylindrical intake cartridge prefilter (20.3-cm diameter by 25.4-cm length or 8-in. diameter

FIGURE 1. New HVAC and filtration system design with an

intake, recirculation, and final filter.
TABLE I. Filtration System Components Tested at the Manufacturing Shop and in the Field

Standard Pressurizer Recirculation HEPA Final MERV 16-A MERV 16-B

Cab Testing Intake A Intake UnitB Filter A Filter Final Filter Final Filter

Drill Shop Test 1 X X X

Drill Shop Test 2 X X X

Drill Shop Test 3 X X X

Drill Shop Test 4 X X X

Mine A, Face Drill X X X

Mine A, Scaler X X X

Mine B, Face Drill X X X

Mine B, Bolter X XC X

AUnrated pre-filters.

BContaining MERV 16 rated intake filter.

CRecirculation filter removed in bolter after 526 hours of operation.

by 10-in. length) and rectangular recirculation panel pre-filter

(7.6-cm width by 40.6-cm length by 5.1-cm depth or 3-in.

width by 16-in. length by 2-in. depth) were both located

upstream of the final filter to reduce dust loading of the higherefficiency final filter. The intake filter was located in an external filter housing unit (referred to as a standard intake unit

throughout the article) connected into the HVAC system with

a flexible 7.62-cm (3-in.) diameter spiral tubing.

An optional fan-powered pre-cleaner intake filtering unit

(referred to as a pressurizer intake unit throughout the article)

with a MERV16 cylindrical intake filter (20.3-cm diameter

by 33.0-cm length or 8-in. diameter by 13-in. length) was also

tested to assist the HVAC system with pressurizing the cab. The

cyclonic pre-cleaner fan centrifugally removed dust particles

greater than 5 μm from the air upstream of the intake filter,

thereby extending its operating life. Table I shows the various

filtration system components that were tested on cabs at the

manufacturing shop and in the two mine field studies.

Cab Testing Procedures

Optical particle counters were used to measure airborne

particles at the outside and inside locations of an unoccupied

cab enclosure with the HVAC system running on maximum

airflow speed settings while the mining vehicle was idling.

Intake and recirculation airflows were also measured at the

maximum airflow speed settings using hot wire and vane

anemometers. The interior to exterior differential cab pressures

were also measured.

The particle count instruments used during this study were

hand-held MetOne Model HHPC-6 (six-channel) instruments

(Hach Ultra Analytics, Grants Pass, Ore.). These instruments

count airborne particles within the six channel size ranges

of 0.3–0.5 μm, 0.5–0.7 μm, 0.7–1.0 μm, 1.0–3.0 μm, 3.0–

5.0 μm, and ≥5.0 μm. The instruments operate at an airflow

rate of 2.83 L/min and have selectable air sampling periods that

are stored in a 500-sample rotating buffer (sample memory).

These instruments provide particle count data in either concentrations or total number of particles measured in differential- or

cumulative-sized sample statistics. The instruments’ specified

coincidence error or error of undercounting particles hidden

behind other particles is 5% for 70,670 particles/L or 2,000,000

particles/ft3

.

Cab testing was conducted using two model HHPC-6 particle counters to simultaneously sample and record the inside

and outside cab particle size concentrations for one-minute

periods over a 30-minute-long test, as previously discussed in

NIOSH’s laboratory cab experiments when using a recirculation filter.(6) Inside and outside instruments were alternately

switched during the subsequent 30-minute test replicates. Instrument rotations between the inside and outside cab locations

for an equal number of test replicates are intended to average

out any instrument biases. The last 15 minutes of data from

each test replicate were used to calculate the average outside

and inside cab concentrations at the lowest steady-state particle

count conditions inside the cab. A cab protection factor for each

test replicate was determined by dividing the average outside

particle concentration by the average inside particle concentration. It represents a reduction ratio of all the exterior and interior particles removed by the cab filtration and HVAC system.

At the mining machine manufacturing shop facility, four

particle counting test replicates were performed on the cab for

each filter combination studied. During the mine field studies,

only two particle counting test replicates were conducted due

to the time constraints encountered with testing two cabs after

the production shift. Each cab could be particle counted in one

hour for the two test replicates. Both cabs could be particle

counted and have their airflows measured in less than two

hours when using four HHPC-6 particle counters.

In order to examine the cab operating effects of different

filter combinations at the manufacturer’s facility and the filter

loading effects in the field, airflow readings were measured

for the intake and recirculation circuits of the cab’s HVAC and

filtration system. VELOCICALC hotwire anemometer models

8346 or 9555 (TSI Incorporated, Shoreview, Minn.) were used

to measure the centerline air velocity inside the middle of a

122-cm (4-ft)-long section of smooth 10.2-cm (4-in.) diameter

PVC pipe which was added to the inlet of the intake filtering

unit. Both these hot wire instruments have the capacity to
input the duct shape and dimensions to directly provide the air

quantity measurements. For the recirculation component, oneminute moving traverse velocity measurements were made

with a vane anemometer (Davis Instruments, Vernon Hills,

Ill.) over the recirculation filter inlet area. The average air

velocity value was then multiplied by the recirculation filter

inlet area (307 cm2 or 0.33 ft2

) to determine the recirculated

airflow quantity.

The cab’s inside to outside differential static pressure was

also measured to insure that cab pressurization was achieved.

Cab pressure measurements were taken with either a 0.0–

0.5 in. Magnehelic differential pressure water gauge (Dwyer

Instruments, Inc., Michigan City, Ind.) or DP-CALC Model

5825 Micromanometers (TSI Incorporated). The magnehelic

pressure gauge was used to spot check the cab pressure at

the manufacturer’s facility and the DP-CALC was used inside

the cab during production shifts of the field studies. The DPCALC time recorded one-minute cab pressure averages during

the production shifts.

Finally, engine operating hours were recorded on each

vehicle cab studied in the field to document timelines for cab

operation. The differential engine hours of operating time from

the beginning of the field tests through the follow-up field tests

were used to quantify the cabs’ operating performance over

time.

Cab Protection Factor Measurement

Uncertainty Analysis

The propagation of error analysis approach was used on

the outside and inside particle count test data to estimate the

uncertainty of the cab protection factors (PF = outside/inside,

particle concentrations) measured for the shop and field test

replicates. The 15 one-minute particle count samples collected

outside and inside the cab during the last half of the test were

averaged and used to calculate the cab protection factor (reduction ratio) for a test. The cab’s protection factor during a particular test configuration was determined from the average of the

test replicates. Since the two-replicate cab test procedure used

in the field is generally considered too small a sample size for

determining the measurement uncertainty from the standard

deviation of the two tests, an alternative propagation of error

analysis approach (for a two-variable ratio) was used to determine the standard error of each test replicate and combine the

test replicate standard errors by using Satterthwaite’s standard

error approximation.(10,11) This approach uses the variation of

the one-minute particle count samples collected from each

test replicate to estimate the measurement uncertainty of the

cab protection factor. The equations used for this uncertainty

analysis and an example of their application in determining

confidence intervals are shown in the Appendix of this article.

RESULTS

Shop Testing

Figure 2 shows a graph of the minimum, median, and the ma

maximum particle count concentrations measured in the various size ranges outside the cab with the MetOne HHPC6 during all the shop testing at the manufacturer’s facility.

As can be seen in Figure 2, the largest number of particles

measured at the outside cab sampling location in the shop

facilities was in the 0.3–0.5 μm size range with the smallest number of particles above 5.0 μm in size. Welding and

grinding were taking place in other areas of the shop while

cab testing was being conducted, and this contributed to the

particle count concentration variations measured outside the

cab while testing. The cumulative-sized particle concentrations (0.3–1.0 μm, 0.3–3.0 μm, and ≥0.3 μm) were similar,

given that most of the particles counted were less than 1.0 μm.

The average outside cab particle count concentrations for

about half of the test replicates exceeded the instruments’

coincidence error specifications of 5% for 70,670 particles/L

or 2,000,000 particles/ft3

, thereby undercounting the outside

particle concentrations and somewhat reducing the protection

factors for these tests. Previous laboratory experiments with

outside cab concentrations above and below the coincidence

error specifications have suggested that these errors can be

inconsequential when determining the cab’s relative protection

performance.(6)

Figures 3 and 4 illustrate the dynamics of outside and inside

submicron (0.3–1.0 μm) particle concentrations measured in

the shop during two MERV 16-B final filter cab tests with

their associated cab protection factors. These two cab test

replicates show that the outside cab particle count conditions

can be steady or variable during cab testing around active

industrial sites. The steady test conditions shown in Figure 3

had outside cab concentrations typically less than coincidence

error specifications with an average protection factor of 51.5,

while the variable conditions shown in Figure 4 had outside cab

concentrations typically exceeding the coincidence error specifications with an average protection factor of 50.1. Exceeding

the outside sampling instrument’s coincidence error specifications undercounts outside particles and appears to somewhat

reduce the cab’s protection factor. This is further illustrated in

Figure 4, where the corresponding increase in particle count

concentrations at the end of the test corresponded to a decrease

FIGURE 2. Optical particle sizes measured outside the cab at

nufacturer’s facility.
FIGURE 3. Test replicate with steady outside cab test concentrations and minimal coincidence errors.

in the cab protection factor. Therefore, it is reasonably expected

that the cab protection factor would be greater than measured

when outside coincidence errors are present during cab testing.

Figure 5 shows the average cab protection factors and 95%

confidence levels for the various shop tests (shown in Table I)

determined at several cumulative particle size ranges. As can

be seen in this figure, the protection factors were not significantly different between the smallest size range (0.3–0.5 μm)

and the larger cumulative size ranges (0.3–1.0μm, 0.3–3.0μm,

and ≥0.3 μm). Since the largest number of ambient particles

outside the cab tends to be in the respirable size range of

0.3–1.0 μm, the cab protection factor results presented thereafter in this report will be based on this respirable size range.

The new system design with the HEPA final filter, nonpressurizing intake filtration unit, and recirculation filter provided the best cab protection factor of 81 as compared to

the two MERV 16 final filter types tested (from different

manufacturers) or by adding the pressurizer intake unit. The

first MERV16-A final filter tested provided a cab protection

factor of 5 as compared to 53 for the second MERV16-B

final filter tested. The MERV16-B filter was selected as an

alternative final filter type for field testing because of its

superior performance in the shop tests. Adding the optional

FIGURE 4. Test replicate with unsteady outside cab test conditions and coincidence errors.

FIGURE 5. Final filter and pressurizer test results at the manufacturer’s facility.

pressurizer intake unit to the original cab system design with

the HEPA final filter provided a cab protection factor of 60.

The cab pressure was increased from 0.15 to 0.18 in water

gauge by adding the pressurizer intake unit.

Mine A Field Testing

Protection factors (with 95% confidence levels) and cab

airflow field measurements for a face drill cab and scaling

machine cab at the first underground limestone mine site are

shown in Figures 6 and 7, respectively. Figure 6 is missing

several data points because of an occasional time limitation

to test the drill. Table I shows the cab filtration system components tested at Mine A. The face drill had a cab size of at

least 4.5 m3 (160 ft3

) and included the initial filtration system

configuration tested at the manufacturer’s shop (HEPA final

filter with standard intake filter unit and recirculation prefilter). The scaling machine had a smaller inside cab volume

of 2.1 m3 (75 ft3

) with a filtration system identical to the

face drill’s system, except that it used the optional pressurizer

intake unit with a MERV 16 intake filter. Particle counting and

airflow measurements were made outside the limestone mine

between shifts in ambient air.

FIGURE 6. Field test results for face drill cab at Mine A.
FIGURE 7. Field test results for scaling machine cab at Mine A.

As can be seen in Figure 6, the face drill initially had a

protection factor of about 400 with new filters (0 hours) and

increased to nearly 1,800 at 65 hours of operation with a drop

off in airflow due to initial filter loading. After 240 hours of

operation the cab protection factor decreased to 100 for the

lowest intake and recirculation airflows measured. These protection factor changes were significant at the 95% confidence

levels, because their confidence intervals did not coincide

over the 240-hour testing period. This is an indication that

some air started to bypass the filtration system after the filters

additionally loaded up with dust and diesel particles during

the production shifts in the mine. The drill cab pressure ranged

from 32 Pa (0.13 inH2O) at the beginning of field testing to 10

Pa (0.04 inH2O) at the end of field testing. The cab pressure

decrease from the start to the end of the testing was primarily

attributed to a decrease in cab intake airflow from 137 m3

/h

(81 ft3

/min) to 93 m3/h (55 ft3/min) caused by dust and diesel

loading of the filters. Cab pressure is directly related to the

amount of intake airflow quantity delivered into the cab.(6)

Figure 7 shows that the scaling machine had a protection factor of about 150 with new filters (0 hours) and that

the protection factor trended down to 20 after 283 hours

of operation, indicating that some of the cab airflow was

also bypassing the filtration system. These protection factor

changes were significant at the 95% confidence levels, because

their confidence intervals did not coincide over the 283-hour

testing period. After these tests visual examination of the final

filter on the scaling machine indicated a few small sections of

the filter media were damaged or breached during installation

or by mine personnel removing the filter housing cover during

interim periods when NIOSH was not present for cab testing.

The scaling machine cab pressure ranged from 25 Pa (0.10

inH2O) at the beginning of field testing to 7 Pa (0.03 inH2O) at

the end of field testing. This cab pressure decrease was more

likely attributed to an increase in enclosure seal leakage to

the outside given the increase in intake airflow from 77 m3

/h

(46 ft3

/min) to 95 m3/h (56 ft3/min) at the start to the end

of testing, respectively. Cab pressure is directly related to the

amount of intake airflow quantity delivered into the cab.(6)

FIGURE 8. Field test results for face drill cab at Mine B (the

symbol ∗ marks the point in time that the recirculation filters were

changed or removed).

A second round of tests was planned using MERV 16-B

final filters in both cab systems. A final filter modification of

placing a protective screen over the exposed filter media was

suggested by NIOSH to the equipment manufacturer before

any more testing was conducted. Before the second round of

tests could be initiated the mine was closed and testing was

conducted at the second mine site (Mine B).

Mine B Field Testing

Protection factors (with 95% confidence levels) and cab

airflow field measurements for a face drill cab and roof bolting

machine cab at the second underground limestone mine are

shown in Figures 8 and 9, respectively. Figures 8 and 9 are

missing a few data points because of the occasional limited

access time to the drill and bolting machine. The particular

time that the recirculation filters were changed or removed

is illustrated by a symbol ∗ next to the operating time in the

figures. Table I shows the cab filtration system components

tested at Mine B. The face drill had a cab size of at least

4.5 m3 (160 ft3

) and included the alternative filtration system

FIGURE 9. Field test results for roof bolting machine at Mine B

(the symbol ∗ marks the point in time that the recirculation filters

were changed or removed).
configuration tested at the manufacturer’s shop (MERV 16-

B final filter with standard intake unit and recirculation prefilter). The MERV 16-B final filter had the protective screen

over the filter media to protect it from damage during installation and operation. The roof bolter machine had a smaller

inside cab volume of 2.3 m3 (80 ft3

), and its filtration system

was identical to the face drill’s system except that a pressurizer

intake unit with a MERV 16 filter was installed in place of the

standard intake unit. Particle counting and airflow measurements were made outside the limestone mine between shifts

in ambient air.

As can be seen in Figure 8, the face drill initially had a protection factor of about 73 with new filters (0 hours) and steadily

increased to over 1,000 after 174 hours of operation. These protection factor changes were significant at the 95% confidence

levels due to the separation between their confidence intervals

and were attributed to initial dust and diesel loading of the cab

system’s filters in the mine. At the start of testing (0 hours), the

recirculation airflow was noticeably higher (427 m3

/h or 251

ft3

/min) with the MERV16-B final filter as compared to the face

drill cab filtration system using the HEPA final filter (245 m3

/h

or 144 ft3

/min, see Figure 6) at Mine A. The recirculation

airflow decreased significantly after the initial 174 hours of

operation and was nearly restored to its initial airflow rate with

recirculation filter replacements at 174 hr and 487 hr of operation. Protection factors remained above 1,000 for 717 hours

of operation after the two recirculation filter changes.

At 726 hours of operation all the filters were removed and

inspected. The used intake and final filter were placed back into

the face drill to measure the HVAC system airflows without the

recirculation filter to examine if this filtration configuration is

a viable HVAC system option when using the standard intake

unit. Since only a nominal 14 m3

/h (8 ft3/min) of intake air flow

could be achieved without the recirculation filter, the used

recirculation filter was placed back into the cab to increase

the intake airflow above 85 m3

/h (50 ft3/min) and to continue

with the long-term testing. As can be seen in Figure 8, the

protection factor decreased to 680 after the filter inspections

and increased back to over 1,000 before cab testing stopped at

1,132 hours of operation. The drill cab pressure ranged from

40 to 10 Pa (0.16 to 0.04 inH2O) during field testing, with the

lower part of this range attributed to the decrease in cab intake

airflow from 117 m3

/h (69 ft3/min) to 83 m3/h (49 ft3/min)

caused by the dust and diesel loading of the filters throughout

the testing period.

Figure 9 shows that the roof bolter cab initially had a

protection factor of about 133 with the new filters (0 hours) and

the protection factor increased to over 1,000 after 106 hours of

operation. These protection factor changes were significant at

the 95% confidence levels because of the separation between

their confidence intervals and were attributed to initial dust

and diesel loading of the cab system’s filters in the mine. At

the start of testing (0 hours) the recirculation airflow was also

noticeably higher 381 m3

/h (224 ft3/min) with the MERV16-

B final filter compared to the scaling machine cab filtration

system using the HEPA final filter (208 m3

/h or 123 ft3/min,

see Figure 7) at Mine A. The recirculation airflow decreased

after the initial 106 hours of operation and was nearly restored

to its initial airflow rate with recirculation filter replacements

at 106 hr and 337 hr of operation. The cab’s protection factor

decreased to 515 before the second filter replacement and

increased back to above 1,000 after the recirculation filter was

replaced for the second time at 337 hours.

At 526 hours of operation, the recirculation filter was removed to measure the HVAC system airflows without the

recirculation filter and to examine if this filtration configuration

is a viable HVAC system option when using the pressurizer

intake unit. The recirculation and intake airflows were measured at 399 m3

/h (235 ft3/min) and 87 m3/h (51 ft3/min),

respectively, with the recirculation filter removed while using the pressurizer intake unit. This configuration initially

provided a protection factor of 247 which increased to back

over 1,000 after 707 hours of operation. The recirculation and

intake airflows started to decrease after 707 hours of operation

from the additional final filter loading (with no recirculation

filter) while the cab protection factor remained above 1,000

throughout the remainder of 842 hours of testing. The bolter

cab pressure was the lowest measured during the field studies

at 12 to 5 Pa (0.05 to 0.02 inH2O) and was primarily attributed

to a poorly sealed resin cartridge supply access door into the

cab. This access door allows resin cartridges to be placed

into the cab’s interior storage bin for pneumatic injection, by

the operator, into a roof bolt hole during the bolt installation

process. The access door had visual unsealed gaps around its

perimeter.

DISCUSSION

The ambient air particle count concentrations at the manufacturing shop and in the field for many of the cab

tests were higher than the specified coincidence errors of the

instruments. Previous laboratory experiments with outside cab

concentrations above and below the coincidence error specifications have indicated that these errors can be inconsequential

when determining the cab’s relative protection performance.(6)

Figure 10 shows the protection factors measured in the laboratory experiments for four particular cab test configurations

conducted over a wide range of particle count concentrations

in the ambient air. Two of these cab test configurations shown

used a high-efficiency intake filter (>99% on particle sizes

≥0.3 μm) under calm and 16 km/h (10 mph) wind conditions.

The two other test conditions shown used the same highefficiency intake filter with the addition of a MERV15 recirculation filter under calm and 16 km/h (10 mph) wind conditions.

These four particular test configurations were conducted with

cab enclosure static pressures (≤12 Pa or 0.05 inH2O with

no wind) that were typically below the equivalent air velocity

pressure of the 16 km/h (10 mph) wind condition tested in the

laboratory.(6)

These laboratory tests clearly illustrate that the cab protection factors remained relatively the same or slightly decreased
FIGURE 10. Replicated laboratory cab tests conducted with a

wide range of outside particle count concentrations.

when outside cab concentrations exceeded the instrument’s

coincidence error specification of 70,670 Particles/L. They

also show that a recirculation filter improves the cab protection

factor by about an order of magnitude and mitigates particle

penetration into the cab from the wind as compared to using

just an intake filter. A decrease in the cab protection factor was

further observed when outside particle count concentrations

were less than 20,000 Counts/L for the test condition without

a recirculation filter and with no wind. This decrease suggests

that the protection factor measurements of a cab without a

recirculation filter may be subjected to an additional overcounting error of particulates generated inside the cab at the

lower outside particle count test concentrations. Any internally

generated cab particles will be counted as particles that penetrated the cab’s filtration system from the outside, thereby

lowering its measured protection factor. Other studies have

measured notable internal particulate generation from blower

motors (200 to 1,800 Particles/L) and have used incense smoke

generated around a vehicle cab tested inside a temporary constructed field enclosure when outside cab concentrations were

less than 42,000 Particles/L to make this error insignificant.(3,5)

The minimum outside particle count concentrations needed for

cab testing is currently open to discussion, but the laboratory

test results discussed here indicate that the minimum may be

somewhere around 20,000 Counts/L. However, internal cab

particulate generation and instrument coincidence errors both

tend to err conservatively with respect to cab protection factor

measurements.

Ideal cab particle counting testing would be performed at

steady outside particle concentrations that are sufficiently high

to reduce internal cab particulate generation biases and that

are within the instrument’s coincidence error specifications.

However, the laboratory, shop, and field results presented in

this article show that cab testing with less than ideal particle

counting conditions can adequately measure changes to and

deficiencies in cab performance.

CONCLUSION

Particle counting inside and outside environmental cabs

using ambient air with identical HVAC fan settings was

found to be a reliable method for measuring cab filtration

system performance in the field. Conducting these tests with

an unoccupied cab in a nonproduction environment removes

any operator impacts on cab performance while measuring the

optimum cab protection factor performance. The propagation

of error analysis approach for the cab protection factor ratio

can be used to determine the standard error of each test, and

the Satterthwaite’s standard error approximation can be used to

estimate the combined standard error of the cab test replicates.

This procedure utilized all the data from each test replicate to

estimate the measurement uncertainty of the cab protection

factor determined from a small number of test replicates.

Although particle count concentrations varied during field testing, the propagation of error analysis accounted for both random and covariance errors during cab protection factor testing

with the two-instrument test method. The propagation of error

analysis does not account for coincidence errors present when

outside particle count concentrations exceed the instrument’s

specifications, but these errors tend to conservatively measure

cab protection factors because of undercounting the outside

cab particles.

Initial shop testing of a new cab filtration system design

showed that most of the ambient air particles outside the cab

tended to be below 1.0 μm in size and that the cumulative

respirable size range of 0.3–1.0 μm provided a relevant particulate medium for testing unoccupied cabs while using ambient

air. Cab particle count testing of multiple final filter alternatives

at the manufacturer’s shop facility showed that the initial

design with the HEPA final filter had a significantly higher cab

protection factor of 81 compared to the cab protection factors

of 5 and 51 achieved with the MERV 16-A and MERV16-

B final filters, respectively. Adding a pressurizer intake unit

increased the cab’s air pressure from 37 to 45 Pa (0.15 to 0.18

inH2O) while achieving a protection factor of 60.

Field testing showed that after the initial dust and diesel

particulate loading of the filters on three of the four cab

filtration systems, the protection factors significantly improved

to greater than 1,000 with either of the HEPA and MERV 16-B

final filters used. The one cab that showed a consistent decline

in protection factor performance from 150 to 20 had visible

damage to the HEPA final filter, allowing air to bypass the filter.

The MERV 16-B final filters initially provided an additional

100 cfm of recirculation airflow on both cabs at Mine B

compared to the HEPA final filters used at Mine A. However,

the cabs at both mines experienced a significant drop off in

recirculation airflow within 100 hours of operation. Recirculation filters were not replaced while using the HEPA final filters

at Mine A, which diminished the cabs’ protection factors in

less than 283 hours of operation. New recirculation filters were

changed at Mine B in about 200-hour replacement intervals,

extending the cabs’ effective operating time of the MERV 16

final filter by about three times over the HEPA filters at Mine A.
After an extended time of usage all the cab pressures were

found to diminish because of intake airflow reductions and

enclosure seal leakages to the outside. Therefore, the twoinstrument particle counting method with intake and recirculation airflow measurements can provide beneficial cab

performance benchmarking and servicing information for optimizing cab particulate protection performance in the field.

Cab pressure measurements only indicate intake airflow and

seal integrity changes to cab enclosures and do not necessarily

provide information about the air quality that the cab filtration

system is providing.

DISCLAIMER

The findings and conclusions in this report are those of

the authors and do not necessarily represent the views

of the National Institute for Occupational Safety and Health

(NIOSH). Mention of any company name, product, or software

does not constitute endorsement by NIOSH. This article not

subject to U.S. copyright law.

REFERENCES

1. American Society of Agricultural Engineers (ASAE): Agricultural

Cabs—Environmental Air Quantity, Part 1: Definitions, Test Methods,

and Safety Practices([Standard S525-1.1] St. Joseph Mich.: ASAE, 1997.

2. Heitbrink, W.A., R.M. Hall, L.D. Reed, and D. Gibbons: Review of

ambient aerosol test procedures in ASAE Standard 525. J. Agric. Saf.

Health 4(4):255–266 (1998).

3. Heitbrink, W.A., and S. Collingwood: Aerosol generation by blower

motors as a bias in assessing aerosol penetration into cab filtration

systems. J. Occup. Env. Hyg. 2(1):45–53 (2005).

4. Heitbrink, W.A., E.S. Moyer, P.A. Jensen, D.S. Watkins, and S.B.

Martin: Environmental agricultural tractor cab filter efficiency and field

evaluation. AIHA J. 64:394–400 (2003).

5. Moyer, E.S., W.A. Heitbrink, and P.A. Jensen: Test for the integrity

of environmental tractor cab filtration systems. J. Occup. Env. Hyg.

2:516–523 (2005).

6. Organiscak, J.A., and A.B. Cecala: Key design factors of enclosed

cab dust filtration systems (Report of Investigations No. 9677, DHHS

(NIOSH) Publication No. 2009-103). Pittsburgh, PA: NIOSH. 2008.

7. Cecala, A.B., J.A. Organiscak, and J.D. Noll: Long-Term Evaluation

of Cab Particulate Filtration and Pressurization Performance. Presented

at the SME Annual Meeting, Seattle, Wash., Feb. 19–22, 2012.

8. Noll, J.D., A.B. Cecala, J.A. Organiscak, and J.P. Rider: Field study

on the effects of MERV 16 filters on enclosed cabs and routine work

practices for reducing dust and DPM exposures. Min. Eng. (in press).

9. American Society of Heating, Refrigerating and Air-Conditioning

Engineers (ASHRAE): Method of Testing General Ventilation AirCleaning Devices for Removal Efficiency by Particle Size[Standard 52.2].

Atlanta, GA: ASHRAE, 1999.

10. Bevington, P.R.: Data Reduction and Error Analysis for the Physical

Sciences. New York: McGraw-Hill Book Company1969. pp. 60–64.

11. Satterthwaite, F.E.: An approximate distribution of estimates of variance

components. Biometrics Bulletin, 2(6):110–114 (1946).

APPENDIX

Equation 1 provides a method to determine the standard

error of estimate for the protection factor of a particle

counting test.(10) This equation assumes that there are an equal

number of outside and inside samples (n) collected during

the test.(10) Covariance can be ignored in the equation if the

variables are independent or there are insufficient pairs of

data for a good estimate. Since outside and inside cab sample

data are not independent, the covariance was included in the

standard error of estimate calculations for the 15 pairs of

concurrent sampling data collected per test replicate.

sPF

√n = PF√n



so

o

2

+

si

i

2

− 2

s2

oi

oi

= PF

√n RSD2

o + RSD2

i − 2roi

sosi

oi 

(1)

Where:

o = mean of outside particle concentrations,

so = standard deviation of outside particle concentrations,

RSDo = relative standard deviation of outside particle concentrations, so/o,

i = mean of inside particle concentrations,

si = standard deviation of inside particle concentrations,

RSDi = relative standard deviation of inside particle concentrations, si/i,

s2

oi = covariance between outside and inside particle concentrations,

PF = protection factor, o/i

sPF = standard deviation of protection factor,

roi = correlation coefficient between outside and inside

particle concentrations,

n = equal number of particle count samples taken inside

and outside the cab.

Equation 2 illustrates the method for Satterthwaite’s standard error approximation for multiple test replicates, assuming

unequal variances between test replicates.(11) Equation 3 shows

how to determine the degrees of freedom for Satterthwaite’s

approximation.(11)

sep =



s2

1

n1

+

s2

2

n2

+···+

s2

k

nk

(2)

df =

 s2

1

n1 + s2

2

n2 +···+ s2

k

nk

2

(n1 − 1)

−1 s2

1

n1

2

+ (n2 − 1)

−1 s2

2

n2

2

+···+ (nk − 1)

−1 s2

k

nk

     2

(3)

Where:

sep = Satterthwaite’s standard error approximation,

sk = standard deviation of the kth test replicate,

nk = number of samples in the kth test replicate,

k = number of test replicates,

df = degrees of freedom.

The confidence levels for the protection factor of similar

test replicates can be determined by multiplying the Satterthwaite’s standard error approximation (Eq. 2) by the student’s
TABLE II. Initial Cab Filtration System Particle Counting Test Results Using the HEPA Final Filter

Test Information Test Replicate 1 Test Replicate 2 Test Replicate 3 Test Replicate 4

AOutside Average 99,266 128,649 91,065 94,022

AOutside Std. Dev. 19,192 17,478 6314 10,129

AInside Average 1,301 1,668 1,114 1,074

AInside Std. Dev. 230 211 93 83

Covariance 4,142,231 3,162,488 498,549 758,239

No. of Samples 15 15 15 15

BProtection Factor 76.3 77.1 81.7 87.5

CStandard Error 1.32 1.41 0.93 1.14

A0.3–1.0 μm sized particle count per liter. BOutside average ÷ inside average. CStandard error determined from Eq. 1.

t-statistic for the calculated degrees of freedom (Eq. 3). This

uncertainty analysis uses all the particle count sample data

during each test replicate to estimate the confidence level of

the average protection factor for the replicates. Table II in the

text illustrates the particle count information and standard error

calculations for four test replicates conducted with two HHPC6 instruments on the initial drill cab filtration system design

with a HEPA final filter at the manufacturing shop facility. The

test information in Table II in the text shows the cumulative

particle count concentrations measured for the 0.3–1.0 μm

size range. The protection factor (PF) for replicate 1 would be

calculated as follows:

PF = 99,266

1,301 = 76.3

The protection factor standard error of estimate for test

replicate 1 would be calculated as follows:

sPF

√n = 76.3√

15

×

19,192

99,2662

+

 230

1,3012

− 2 ×

 4,142,231

99,266×1,301

= 1.32

The protection factors and standard error of estimates for

the other test replicates would be similarly calculated. The

Satterthwaite’s standard error approximation for all the test

replicates would be the square root of the summation of the

standard errors squared by using Eq. 2 as shown below for the

four test replicates in Table II in the text.

sep = 1.322 + 1.412 + 0.932 + 1.142 = 2.43 

The degrees of freedom for the Satterthwaite’s standard

error approximation would be calculated by Eq. 3 and is shown

below for the four test replicates. To be conservative, the degree

of freedom calculation was rounded down to the nearest whole

number which would be 51.

df = (1.322 + 1.412 + 0.932 + 1.142)

2

1.324

(15−1) + 1.414(15−1) + 0.934(15−1) + 1.144(15−1)

= 51.6

The average cab protection factor would be calculated by

averaging the test replicates. When there are a balanced number of samples in each test replicate the average can be simply

computed by the numeric average of the test replicates as

shown below.

Average PF

= (15 × 76.3) + (15 × 77.1) + (15 × 81.7) + (15 × 87.5)

15 + 15 + 15 + 15

= 76.3 + 77.1 + 81.7 + 87.5

4 = 80.6

The 95% confidence level for the average protection factor

of the test replicates is then determined by multiplying the

Satterthwaite’s standard error approximation by the student’s

t-statistic for the calculated 51 degrees of freedom. As determined from the above calculations using Table II in the text,

the estimated 95% confidence level for the average protection

factor of 80.6 would be (2.43 × 1.96) or ± 4.8.

By comparison, the calculation of the standard deviation,

standard error, and confidence level using the protection factor

for each test replicate would yield the following result:

PF Std.Dev.

=



(76.3−80.6)

2 + (77.1 − 80.6)2 + (81.7 − 80.6)2 + (87.5−80.6)

2

4 − 1

= 5.15

PF std.error = 5.15

√4 = 2.58

Although the 2.58 standard error is similar to the Satterthwaite’s standard error approximation of 2.43, its 95%

confidence level would be larger at ± 8.2 (2.58 × 3.182)

because of the greater uncertainty in the student’s t-statistic

for three degrees of freedom. Given the additional measurement uncertainty expected with conducting only two cab test

replicates during the field studies, the alternate propagation of

error analysis and Satterthwaite’s error approximation procedures were used to estimate the confidence levels from all the

available sampling data collected during the two test replicates.