No. 550 September 2012 

 

A Test Method for Quantifying Unfiltered Air Leakage into Enclosed Cabs 

Objective 

To develop a simple, practical, quantitative, and reliable 

field test method for measuring air leakage into enclosed cab 

filtration systems with the goal of making enclosed cabs a 

healthy and safe work environment. 

Background 

Enclosed cabs are an engineering control that can provide a 

safe, comfortable, and healthy work 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 its 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 outside airborne 

pollutants such as dusts, aerosols, and vapors. However, outside 

air leakage around the intake filter into the HVAC system can 

notably diminish the effectiveness of the cab’s filtration system. 

A poorly sealed cab HVAC/filtration system can be difficult to 

recognize because of its concealed system components and the 

invisible nature of some airborne pollutants that can penetrate 

the cab. 

Researchers have previously measured airborne particulates 

in either ambient air or generated particulates inside a chamber 

surrounding the vehicle to quantify the particulate penetration 

into the enclosed cab. Several cab penetration studies have 

indicated that inconsistent and low particulate concentrations 

can yield unreliable cab performance results among replicated 

cab tests. Internal cab particulate generation such as dirty floors, 

interior surfaces, and abraded blower motor brushes can also 

interfere with measuring external particle penetration into the 

cab. 

Approach 

A desirable air leakage test method would use a relatively 

constant airborne tracer gas level around a vehicle’s cab which 

would pose minimal health and safety risks to the user. 

Atmospheric gases have these desirable attributes and were 

considered for the air leakage tracer gas. The technique involves 

replacing the HVAC intake particulate filter with a tracer gas 

filter, measuring tracer gas concentrations inside and outside the 

cab filtration system, and then applying a mathematical formula 

to calculate leakage. NIOSH partnered with the Clean Air Filter

(CAF) Company of Defiance, Iowa under a Cooperative 

Research and Development Agreement (CRADA) to develop 

this air leakage test method for evaluating the environmental 

integrity of enclosed cabs. This research focused on developing 

a timely stationary vehicle cab leak test inside an unoccupied 

cab. Cab leakage testing research was ultimately conducted 

using the carbon dioxide (CO2) in ambient air as the tracer gas 

given that CO2 instrumentation and gas-absorbent media were 

readily available for filtration and methodology development. 

Figure 1 shows the typical CO2 filter configurations 

constructed by CAF for leak testing enclosed cab filtration 

systems. 

Absorbent 

Media 

Gasket 

Material 

Block Style Round Style 

Filter 

Media 

L Filter 

T 

H 

T 

D 

H 

Figure 1. Carbon dioxide filter configurations (blue arrows depict 

airflow movement through carbon dioxide filter).

Initial laboratory testing indicated that the CO2

absorbent media bed thickness (T) needed to be greater than 2 

inches (5.1 cm) inside the filter for efficient CO2 removal. Three 

different types of CO2 instruments were also examined in the 

laboratory for their suitability for the proposed leak test. It was 

apparent that all the instrument types examined can drift and 

would need daily CO2 gas calibrations to ensure their accuracy 

for a multiple-instrument cab test method. 

Because maintaining instrument accuracy during field 

testing can be challenging, a single-instrument cab testing 

methodology was developed to eliminate instrument bias errors 

and field calibration. This method measures relative CO2

Hous

 
concentration differences inside and outside the cab filtration 

system using the same instrument at or near steady-state cab 

conditions. The equation shown below was developed to 

quantify air leakage around the CO2 intake test filter by 

measuring CO2 concentrations inside the cab, immediately after 

the intake filter, and outside the cab by using the same 

instrument. Air leakage is defined as the percentage of cab 

intake air that bypasses the filter media and is unfiltered. 

100

. .

. . %,

Outside Conc ConcFilter

ConcCab ConcFilter

LeakageAir

Laboratory tests were conducted with controlled air leakages 

of 0%, 5.5%, and 11.5% into an unoccupied enclosed cab test 

stand. The leakage opening was downstream of two stacked 

block-style intake filters, which were 98% effective for 

removing CO2. A mass airflow meter was used to measure the 

quantity of air leakage into the filtration system, and a hot wire 

anemometer was used to measure the intake airflow into the cab. 

The CO2 instruments were located outside the unoccupied cab 

enclosure and sequentially sampled the air inside the cab, 

immediately after the intake filter, and outside the cab through 

sampling hoses routed to these locations. A Sable CA-10a CO2

instrument (Sable Systems International, Las Vegas, NV) used 

during the laboratory testing was an active sampling instrument 

with an internal air pump and was suitable for this type of cab 

sampling procedure. Several Vaisala GM70 CO2 instruments 

(Helsinki, Finland) used during the laboratory testing were 

passive handheld sampling instruments and had to be air 

aspirated by separate air sampling pumps to move the air from 

the various cab locations to the instrument’s sensing chamber 

via calibration hood. 

The best cab leak test procedure devised in the laboratory 

was the measurement of the inside cab concentrations for 3 

minutes after they stabilize from the filtration system startup. 

Then, after 2 minutes of stabilization time between the sampling 

location changes, the CO2 concentrations were measured for 3 

minutes at the other two sampling locations. This sampling 

procedure was sequentially repeated three times at all the 

sampling locations for each leakage condition. This procedure 

was replicated twice on two different days in the laboratory. 

Finally, this CO2 leak test method was performed on a John 

Deere 7820 series tractor cab to demonstrate its application on 

an end-use vehicle cab. A round-style cartridge filter, as shown 

in Figure 1, was used for the test. 

Results

 Laboratory test results showed that the CO2 leak test 

method similarly quantified the cab leakage as compared to the 

directly measured airflow leakage into the filtration system. 

Figure 2 shows the measured CO2 leakages as compared to the 

airflow leakage measurements into the cab enclosure. 

Figure 2. Air leakage test measurements made on the Clean Air 

Filter (CAF) Company’s test stand. 

-2%

0%

2%

4%

6%

8%

10%

12%

14%

0% 2% 4% 6% 8% 10% 12% 14%

CO2 Measured Leakage, % 

Air Flow Leakage, % 

Sable Vaisala 1 Vaisala 2

Unity Line, y =x

The unity 

line on the figure illustrates good correlation of the CO2 leakage 

test method with the airflow measurements using the Sable and 

Vaisala instruments. 

 Table 1 shows the data collected on the John Deere 

tractor cab for both the Sable and the Vaisala 1 instruments. 

Table 1. CO2 measurements on the John Deere tractor cab 

Instrument Cab 

Conc. 

(ppm) 

Filter 

Conc. 

(ppm) 

Outside 

Conc. 

(ppm) 

Cab 

Leakage 

(%) 

Sable 19 14 420 1.2

18 13 405 1.2

17 13 429 1.0

Vaisala 1 47 43 441 0.8

50 46 432 1.1

52 42 438 2.5

ppm – parts per million 

The round-style CO2 intake test filter provided better than 96% 

removal efficiency. This table illustrates the consistent 

measurement biases between the two instruments at the various 

sampling locations. However, each instrument measured 

comparable air leakages because relative CO2 concentration 

differences in the cab filtration system were used to calculate 

leakage. This test method provides cab manufacturers, cab 

service personnel, and industrial hygienists with a measurement 

tool to ensure environmental cab integrity and minimize worker 

exposure to outside airborne substances. 

For More Information 

For more information on this patented cab leakage method, 

contact John Organiscak, NIOSH (412-386-6675, 

jorganiscak@cdc.gov), Michael Schmitz, Clean Air Filter (800-

488-4828, www.cleanairfilter.com) or the Health 

Communications Coordinator (OMSHR@cdc.gov), NIOSH 

Office of Mine Safety and Health Research, P.O. Box 18070, 

Pittsburgh, PA 15236-0070. 

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about 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. 2012–180