No. 528 October 2007 

Recirculation Filter Is Key to Improving Dust Control in Enclosed Cabs 

Objective 

Improve the dust control effectiveness of enclosed operator 

cabs on surface mining equipment. 

Background 

Overexposure to airborne respirable crystalline silica (or 

quartz) dust can cause debilitating or fatal respiratory disease. 

The workers most frequently overexposed to silica dust at surface mines are overburden drillers and operators of mobile 

excavating equipment, such as bulldozers, front-end loaders, and 

trucks. Enclosed cabs with heating, ventilation, and air conditioning (HVAC) systems are typically integrated into the drills 

and mobile equipment to protect the operator from the outside 

environment. Air filtration is often part of the HVAC system as 

an engineering control of airborne dusts. 

The basic HVAC system mainly recirculates cab interior air 

through the heat exchangers for effective heating and cooling of 

the cab, with some additional makeup air drawn by the recirculation fan(s) through an exterior inlet. Filtration is usually performed on the exterior intake air and sometimes on the interior 

recirculation air of the HVAC system. Cab air filtration performance depends on the efficiency of the air filters, airflow through 

the filters, dust loading on the filters, exterior air leakage around 

the intake filter, and wind penetration of outside air into the cab 

enclosure. Numerous filtration system designs have been 

observed on surface mine equipment cabs with varying degrees 

of success for limiting dust exposure. 

Approach 

The National Institute for Occupational Safety and Health 

(NIOSH) has investigated various cab filtration system factors 

on a basic HVAC system in the laboratory to evaluate their 

effects on overall cab dust protection performance. The factors 

experimentally investigated were intake filter efficiency, intake 

air leakage, intake filter loading (filter flow resistance), 

recirculation filter use, and wind penetration. A lower- and 

higher-efficiency intake filter were tested without and with a 

perforated plate on the filter exit to simulate each filter in an 

unloaded and a loaded condition. These intake filter test conditions were also conducted with a 1⁄2-inch inside-diameter hole 

closed or opened to examine leakage effects around the intake 

filter. All of the intake filter and leakage configurations were 

further tested in combination with and without an inside cab 

recirculation filter. These cab filtration system test factors 

(16 combinations) were conducted within a mine entry gallery 

under calm conditions and challenged under 10-mph wind conditions. Three 1-in-diam holes were located on the front door 

and similarly on the back of the cab to allow the intake air to exit 

the cab under positive pressure. Figure 1 illustrates the front 

view of a laboratory cab test apparatus (∼72-ft3

 volume) facing 

into the wind in the NIOSH Pittsburgh Research Laboratory’s 

longwall test gallery, which was capable of delivering a 10-mph 

wind velocity around the cab. 

Cab dust or particulate protection performance was determined by relative comparisons of portable HHPC–6 channel 

(ATRI or Met One) particle counter concentrations inside (C1) 

and outside (C3) the cab, which was challenged with ambient air 

particles. The largest fraction of ambient air particles was in the 

submicron size range (0.3–1.0 μm); these particle counter 

channels were summed to determine cab performance. Cab 

protection factors (C3 / C1) were determined over a 15-min 

period of stable interior cab concentrations. Cab intake air 

particle concentrations (C2) were also measured, allowing for the 

intake filter efficiency to be determined for the same size range 

under no-leak conditions around the filter. Cab differential pressure, filter differential pressure, intake leakage, intake airflow, 

recirculation airflow, and wind velocities were measured to 

quantify the cab conditions during airborne particulate performance testing. 

Results 

Table 1 shows the average cab protection factors (C3 / C1) 

and cab operating conditions achieved for the intake and recirculation filter combinations. The lower-efficiency intake filter used 

was a single-stage pleated round cartridge filter (7-in-diam by 

13-in-high) with an average measured efficiency of 38% in the 

0.3- to 1.0-μm size range. The higher-efficiency intake filter 

used was a multistage round cartridge filter (7-in-diam by 12-inhigh) with an average measured efficiency of greater than 99% 
in the 0.3- to 1.0-μm size range. A rectangular panel recirculation filter (12-in-high by 24-in-wide by 4-in-deep nominal size) 

was also used, which had an American Society of Heating, 

Refrigerating and Air-Conditioning Engineers (ASHRAE) minimum efficiency reporting value (MERV) of 15, or 85%–94.9% 

in the 0.3- to 1.0-μm size range. 

The use of the recirculation filter remarkably improved the 

average cab protection factor by an order of magnitude over 

what was provided by the intake filter itself because of 

multiplicative filtration of the cab interior air. The cab protection 

factor increased from 1.7 with the low-efficiency intake air filter 

to 13.4 with the addition of the recirculation filter. The cab 

protection factor also increased from 13.3 with the highefficiency intake filter to 168.4 with the addition of the 

recirculation filter. It should be noted that recirculation filter use 

also decreased recirculation airflow somewhat, with a corresponding increase in the intake airflow and cab differential pressure. Although the increase in intake filter airflow was met with 

a higher filter differential pressure and higher percentage of 

intake leakage, the recirculation filter yielded the better cab 

protection factors because of the much higher airflow filtration 

rate of the cab interior. 

Further, the recirculation filter noticeably decreased the time 

needed for the cab interior concentrations to go down and stabilize after the cab door was closed. The average stabilization time 

decreased from 17 min and 29 min, respectively, with the lowand high-efficiency intake filters to 8 min with the addition of 

the recirculation filter. This decrease in stabilization time was 

also achieved at a higher cab protection factor. Thus, a cab 

recirculation filter subsequently reduces respirable dust concentrations and time of exposure inside the cab. 

For More Information 

For more information about filtration in enclosed operator cabs, contact John A. Organiscak (412–386–6675, 

JOrganiscak@cdc.gov) or Andrew B. Cecala (412–386–6677, 

ACecala@cdc.gov), NIOSH Pittsburgh Research Laboratory, 

P.O. Box 18070, Pittsburgh, PA 15236–0070. 

 

 

 

 

Shield Canopy 

vw 

Curtain 

HVAC 

C C 3 1 

Shield Pontoon 

ER QI 

L 

QR 

Δpc 

EI 

C2 

Δpf 

C1 – Inside cab concentration, counts/liter 

C2 – Intake air concentration, counts/liter 

C3 – Outside air concentration, counts/liter 

PF – Protection factor, C3 /C1 

QI – Intake air quantity or 

cab discharge air quantity, ft3

/min 

QR – Recirculation air quantity, ft3

/min 

EI – Intake filter efficiency, % reduction 

ER – Recirculation filter efficiency, % reduction 

L – % intake air leakage, % of QI 

Δpf – Intake filter differential pressure, 

inches w.g. 

Δpc – Cab differential pressure, inches w.g. 

vw – Wind velocity 

Figure 1.—Laboratory cab test apparatus used in longwall test gallery. 

 

Table 1.—Average cab performance values for filter combinations tested 

Filters Δpf Δp Stabilization PF Q c I L Q Recircu- (inches R (inches time Intake (C3 / C1) (cfm) (% of Q ) (cfm) lation? w.g.) I w.g.) (min) 

Lower EI No 1.7 37.3 0.30 2.0 366 0.17 17 

Lower EI Yes 13.4 41.0 0.47 2.6 328 0.19 8 

Higher EI No 13.3 18.1 0.52 3.6 386 0.07 29 

Higher EI Yes 168.4 23.2 0.70 4.9 338 0.08 8 

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occupational safety and health topics, contact: 1–800–CDC– 

INFO (1–800–232–4636), 1–888–232–6348 (TTY), e-mail: 

cdcinfo@cdc.gov, or visit the NIOSH Web site at http:// 

www.cdc.gov/niosh 

Mention of any company name or product does not constitute 

endorsement by the National Institute for Occupational 

Safety and Health. 

DHHS (NIOSH) Publication No. 2008–100