Doing the Math

THE EFFECTIVENESS OF 

ENCLOSED-CAB AIR-CLEANING 

METHODS CAN BE SPELLED OUT 

IN MATHEMATICAL EQUATIONS.

BY JOHN A. ORGANISCAK 

AND ANDREW B. CECALA

Enclosed cabs are a primary means of reducing 

equipment operators’ silica dust exposure at surface mines. The National Institute of Occupational 

Safety and Health recently performed a laboratory study to 

evaluate which factors on an enclosed-cab filtration system 

are most significant. The various factors evaluated were 

intake filter efficiency, intake air leakage, intake filter loading, wind infiltration, use of a recirculation filter, and the 

use of an intake pressurization fan.

The result of this laboratory testing has shown that the 

two most important factors for an effective filtration system on an enclosed cab were the efficiency of the intake filter and the use of a recirculation filter. A higher-efficiency 

intake filter considerably increased the quality of the intake 

air that was delivered into the enclosed cab. It also was 

determined that air leakage around the intake filter noticeably reduced its air cleaning effectiveness. 

The second key factor is the use of a recirculation filter, 

which was shown to improve the air quality in the enclosed 

cab by six to 12.7 times more than the intake filter alone. 

The reason for the significant improvement was that the 

cab air was constantly drawn through the recirculation filter, thus continually filtering the dust out of the air. 

The outcome of this laboratory study was the development of a mathematical equation to calculate enclosed-cab 

air-cleaning effectiveness with respect to specified filtration 

system components and operating parameters. This equation is useful to mine operators for examining cab performance specifications and allowing them to achieve a desired 

cab dust reduction effectiveness or protection factor.

Overexposure to airborne respirable dust, especially 

dust containing crystalline silica, is a serious health concern to the mining industry. The most frequent silica dust 

overexposures at surface mines are commonly overburden 

drillers and operators of mobile excavating equipment such 

as bulldozers, loaders and haul trucks. Enclosed cabs with 

heating, ventilation and air conditioning systems are typically integrated into surface mining equipment to protect 

the operator from the outside environment. Air filtration 

often is part of the HVAC system as an engineering control 

of airborne dusts. 

The basic HVAC system mainly recirculates cab air 

through the heat exchangers for effective heating and cooling with some additional intake air drawn by recirculation 

fans through an exterior inlet. Dust filtration is usually performed on the intake air and sometimes on the recirculation air. Field studies of filtration systems on surface mine 

equipment cabs have shown varying degrees of success for 

lowering dust exposure to the operator. 

Enclosed-cab air-cleaning effectiveness can be described 

by three relative measures that compare outside with inside 

dust concentrations: protection factor, efficiency and penetration. For this research, cab-air cleaning effectiveness 

is described by protection factor, or PF. This measure is 

most widely known for describing respirator effectiveness. Higher protection factor values indicate better cab-air 

cleaning effectiveness.

For this study, NIOSH researchers built a 72-cubic-foot 

plywood enclosure with a mock-up HVAC system to replicate a typical operator cab found on mining equipment. Cab 

protection factors were determined from respirable particle 

count concentrations measured outside (Co) and inside (Ci) 

the cab. Respirable particle count concentrations also were 

measured in the cab intake airflow for determining intake 

filter efficiencies. The recirculation filter efficiency was measured separately from the cab experiments at comparable filter airflows. Cab pressure, intake air leakage, intake airflow, 

recirculation airflow and wind velocities also were measured 

to quantify the experimental cab operating conditions. 
Table 1 shows average cab operating conditions and 

protection factors for the intake and recirculation filter test 

combinations without and with the intake pressurizer fan. 

TABLE 1: AVERAGE CAB PERFORMANCE VALUES FOR FILTER COMBINATIONS TESTED

Intake 

Filter 

(%Efficiency) 

Recirculation 

Filter 

(%Efficiency) 

Intake 

Airflow 

ft3/min 

Intake Air 

Leakage 

% 

Recirculation 

Airflow 

ft3/min 

Cab 

Pressure 

inches w.g. 

Decay 

Time 

min 

Protection

Factor

Co/Ci

 Low (38%) None 37.3 2.0 366 0.17 17 1.7

P Low (34%) None 55.1 2.3 363 0.27 20 1.5

High (>99%) None 18.1 3.6 386 0.07 29 13.3

P High (>99%) None 30.3 3.5 379 0.11 22 14.4

Low (38%) Yes (72%) 41.0 2.6 328 0.19 8 13.4

P Low (34%) Yes (72%) 65.8 2.2 322 0.31 9 9.0

High (>99%) Yes (72%) 23.2 4.9 338 0.08 8 168.4

P High (>99%) Yes (72%) 35.6 3.8 338 0.14 7 162.2

P Cab test stand operated with intake pressurizer fan.

The top and bottom half of the table shows the test results 

without and with a recirculation filter, respectively. The 

higher-efficiency intake filter significantly improved cab 

protection factors throughout all the test conditions as compared to the lower-efficiency filter. Also, the recirculation 

filter significantly increased the cab protection factor by 6 

to 12.7 times more than provided by the intake filter itself, 

due to continual filtration of the cab air. The cab protection 

factor for the lower efficiency intake filter ranged from 1.5 

to 1.7 without the recirculation filter, increased to a range 

of 9 to 13.4 with the recirculation filter. The cab protection 

factor for the higher efficiency intake filter ranged from 

13.3 to 14.4 without the recirculation filter, increased to a 

range of 162.2 to 168.4 with the recirculation filter. 

The recirculation filter also noticeably decreased the 

time needed for the cab interior concentrations to decrease 

and stabilize after the door was closed; this is known as 

decay time. The average decay time ranged from 17 to 29 

minutes without the recirculation filter and from 7 to 9 

minutes with the recirculation filter. This decrease in decay 

time also was achieved with a higher cab protection factor. Thus, a cab recirculation filter subsequently reduced 

respirable dust concentrations and time of exposure inside 

the cab.

Intake-filter loading and wind infiltration were the two 

least-important factors found in this study. However, a 

loaded intake filter reduces both intake airflow and cab 

pressure, making wind infiltration more likely. Using 

the intake-pressurizer fan increased the cab’s intake 

airflow quantity and pressurization, thus decreasing the 

potential for wind blowing outside contaminants into the 

enclosed cab. 

One of the experiments’ objectives was to develop a 

practical mathematical equation of these test results to be 

used by mine operators in achieving desired cab performance specifications. The equation describes cab protection factor in terms of intake filter efficiency, intake-air 

quantity, intake-air leakage, recirculation-filter efficiency, 

recirculation-air quantity and wind-quantity infiltration. 
It determines the cab protection factor after it reaches its 

lowest inside concentration under equilibrium conditions, 

with doors and windows closed. 

Where:

Protection Factor, ratio

unit time

Intake air quantity into the cab (Q > 0), volume per I

Intake filter efficiency ( N < 1), fractional I

quantity

Intake air leakage, fractional portion of intake air 

Recirculation filter airflow, volume per unit time

Recirculation filter efficiency, fractional

unit time

Wind quantity infiltration into the cab, volume per 

This equation requires that air quantities be expressed 

in equivalent units and filter efficiencies and air leakage 

be expressed as fractions, not percentages. Wind-quantity 

infiltration can be assumed to be zero when the cab pres

sure exceeds the wind velocity pressure. 

In the following example, a cab-filtration system has an 

intake-air filter, which is 95% efficient on respirable-size 

dust and is operating at 40 cubic feet per minute of airflow. 

This system recirculates 200 cubic feet per minute of cab 

air without a recirculation filter. The mathematical equa

tion can be used to determine the baseline PF of this cab 

and evaluate several cab performance changes. 

This 72-cubic-foot enclosed cab test stand was used in 

the filtration system experiments.

1) Baseline design.

2) A 5% intake air leak around the intake filter gasket 

3) Adding a 75% efficient respirable dust recirculation 

filter with the 5% intake leak. 

4) A 75% efficient respirable dust recirculation filter 

with no leak. 

These example calculations show that the cab protection 

factor increased from 20 to 95 by using a 75% efficient 

recirculation filter in a system with no leaks. They also 

illustrate how the cab protection factor can be noticeably 

diminished by an intake air leak. A 5% intake air leak 

reduced the cab protection factor from 20 to 10 without the 

recirculation filter and from 95 to 49 with the recirculation 

filter. Other enclosed cab filtration system specifications 

that can be examined with this equation are various intake 

and recirculation airflows. 

These cab protection factor calculations represent 

operating conditions at steady-state conditions within 

a sealed, pressurized cab (doors and windows closed). 

Actual cab protection factors over a working shift will vary 

below this calculated value, depending on the frequency 

and time that the operator opens the cab door and win

dows. Therefore, keeping the cab tightly sealed and pres

surized is a key aspect in achieving the highest protection 

factor for an operator. The higher the protection factor 

achieved on a cab reduces the operator’s exposure to the 

outside dust. Finally, an effective cab filtration system 

reduces the dust and dirt that infiltrates the HVAC system, 

increasing its thermal effectiveness and reducing wear on 

its internal components.