National Institute for Occupational Safety and Health 

National Personal Protective Technology Laboratory 

Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023

DETERMINATION OF SULFUR DIOXIDE SERVICE LIFE TEST,

AIR-PURIFYING RESPIRATORS AND POWERED AIR-PURIFYING RESPIRATORS

WITH CARTRIDGE(S) OR CANISTER(S) 

STANDARD TESTING PROCEDURE

1. PURPOSE

This document establishes the procedure for ensuring that the level of protection provided by airpurifying respirators (APR) with cartridge(s) or canister(s) and powered air-purifying respirators 

(PAPR) with cartridge(s) or canister(s) submitted for Approval, Extension of Approval, or 

examined during Certified Product Audits meet the minimum sulfur dioxide (SO2) service life 

test requirements set forth in 42 CFR Part 84 Subpart I, Sections 84.110 and 84.126, Subpart K, 

Section 84.175(b), and Subpart L, Section 84.207. 

2. GENERAL

This standard testing procedure (STP) describes the determination of sulfur dioxide service life 

test for APR or PAPR with cartridge(s) or canister(s) in sufficient detail that a person 

knowledgeable in the appropriate technical field can select equipment with the necessary 

accuracy and resolution, conduct the test, and determine whether or not the product passes the 

test.

3. EQUIPMENT/MATERIAL

3.1. The list of necessary test equipment and materials follows:

3.1.1. Flow-Temperature-Humidity control system: Miller Nelson Research (MNR), 

model HCS-501-250, or equivalent. 

3.1.2. Chilled mirror hygrometer: Edgetech Instruments, DewMaster, or equivalent. 

Accuracy is ± 0.2 degrees Celsius (°C) for temperature and dew point. 

3.1.3. Challenge sulfur dioxide detector: Interscan Corporation, model RM-24-0, or 

equivalent. Detector range: 0 – 1999 parts per million (ppm), resolution: 1 ppm. 

3.1.4. Breakthrough sulfur dioxide detector: Interscan Corporation, model RM-24-2, or 

equivalent. Detector range: 0 – 19.99 ppm, resolution: 0.01 ppm.

3.1.5. Dilution system: Interscan Corporation, model DS-20/50, or equivalent.

3.1.6. Mass flow controller: Brooks Instrument, model SLA5850S, or equivalent. With 

read out and control electronics: Brooks Instrument, model 0254, or equivalent. 

Accuracy is ± 0.9 percent (%) set point or ± 0.18% full scale, whichever is 

greater.

3.1.7. Dry test meter: American Meter Co., Model DTM-325, or equivalent. Accuracy 

is ± 1% of reading.
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 2 of 13

3.1.8. Sulfur dioxide gas cylinders, 5 ppm and 500 ppm, balance nitrogen. Primary 

standard, or better. NOTE: For APR or PAPR with cartridge(s) only. 

3.1.9. Sulfur dioxide gas cylinders, 5 ppm and 5,000 ppm, balance nitrogen. Primary 

standard, or better. NOTE: For chin style or escape APR with canister(s), or 

PAPR with canister(s) only. 

3.1.10. Sulfur dioxide gas cylinders, 5 ppm and 20,000 ppm, balance nitrogen. Primary 

standard, or better. NOTE: For front or back mounted APR with canister(s) only. 

3.1.11. Sulfur dioxide gas cylinder, 99% purity, or better. 

3.1.12. Electronic balance with a readability of 0.01 grams (g) and capacity of 1000 g, or 

better.

3.1.13. Thermocouple: Type J or better.

3.1.14. Test chamber: The test chamber has interior dimensions of approximately 8 

inches (in.) x 21 in. x 21 in. and is constructed of PVC. Within the chamber, 

there are four evenly spaced canister ports with 40 millimeter (mm) female 

threads and EDPM rubber gaskets. The chamber front panel is clear acrylic, 

attaches with eight 1⁄2 in. bolts and wing nuts, and seals air-tight with a rubber 

gasket. The gas enters through a 1-1⁄2 in. PVC pipe, mixes in a rear chamber of 

approximately 3 in. x 21 in. x 21 in. with baffles and enters the main test 

chamber. The gas is pushed through the respirator cartridge(s) or canister(s) and 

exits the chamber via a 1-1⁄2 in. PVC pipe. Sampling ports are 1⁄4 in. female pipe 

thread and are located on the inlet and outlet pipes. This fixture is not 

commercially available.

3.1.15. Test fixture for mounting cartridge(s) or canister(s): The test fixture used is 

specific to each manufacturer and respirator model. When possible, respirators 

using a 40 mm male thread design are directly connected to a 40 mm port in the 

test chamber. When necessary, a custom test fixture is constructed. The canister 

or cartridge interface is removed from the respirator, while keeping the sealing 

surface and locking interface intact. The interface is affixed by hot melt glue to a 

PVC pipe fitting of appropriate size, sealed with beeswax, and adapted to a 40 

mm male thread fitting with standard pipe fittings. For respirators using a 

multiple cartridge or canister configuration, the canisters or cartridges are tested 

as a set and a test fixture is constructed using a PVC tee. Respirators with 

canister(s) and breathing tube shall include the breathing tube as part of the test 

fixture. When possible, PAPR systems with cartridge(s) or canister(s) shall use 

the respirator body and breathing tube as the test fixture, with a 40 mm male 

thread adapter affixed to the breathing tube by hot melt glue and sealed with 

beeswax.

4. TESTING REQUIREMENTS AND CONDITIONS

4.1. Prior to beginning any testing, all measuring equipment to be used must have been 

calibrated in accordance with the testing laboratory’s calibration procedure and schedule. 
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 3 of 13

All measuring equipment utilized for this testing must have been calibrated using a 

method traceable to the National Institute of Standards and Technology (NIST) when 

available.

4.2. Normal laboratory safety practices must be observed. Refer to Safety Data Sheets (SDS) 

for the proper protection and care in handling, storing, and disposing of the chemicals and 

gases used in this procedure. All applicable local, state, and federal regulations for the 

safe handling and use of hazardous substances shall be followed.

4.3. The cylinder of 99% sulfur dioxide, as well as the calibration gas cylinders, are used 

inside the laboratory fume hood. If there is a release of 99% sulfur dioxide outside the 

hood, sound an alarm, and any personnel in the laboratory should immediately exit from 

the building. Sulfur dioxide is nonflammable.

4.4. SULFUR DIOXIDE BENCH TEST FOR CARTRIDGE(S) OR CANISTER(S)

4.4.1. For any APR with cartridge(s) or canister(s): 

For the complete APR, exhalation resistance to airflow will be measured before 

each test, and inhalation resistance to airflow will be measured before and after 

each test. The STPs are described in TEB-APR-STP-0003 and TEB-APR-STP0007.

4.4.2. For any PAPR with cartridge(s) or canister(s): 

For the complete tight-fitting PAPR, exhalation resistance to airflow will be 

measured before each test, and airflow and inhalation resistance to airflow will 

be measured before and after each test. For the complete loose-fitting PAPR, 

airflow will be measured before and after each test. The STPs are described in 

TEB-APR-STP-0003, TEB-APR-STP-0007, and TEB-APR-STP-0012.

4.4.3. For respirators designed for a multiple cartridge or canister configuration, the 

canisters or cartridges are tested as a set. Each set will be tested together on a 

single test fixture and will be treated as a single sample.

4.4.4. Test conditions as required by 42 CFR 84.110, 84.126, 84.175(b), and 84.207 are 

described in Tables 1 through 5.

4.4.4.1. The breakthrough concentration is the maximum allowable penetration 

for the respirator.

4.4.4.2. A respirator cartridge or canister For One Type provides respiratory 

protection against SO2 and may also include only other acid gases or 

vapors.

4.4.4.3. A respirator cartridge or canister For More Than One Type provides 

respiratory protection against SO2, may also include other acid gases or 

vapors, and must also include at least one other gas or vapor of another 

type.
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 4 of 13

Table 1: Test Conditions and Minimum Service Life Criteria for Air-Purifying Respirator with 

Cartridge(s)

Table 2: Test Conditions and Pass/Fail Criteria for Air-Purifying Respirator with Canister(s)

Table 3: Test Conditions and Minimum Service Life Criteria for Tight- or Loose-Fitting Powered AirPurifying Respirator with Cartridge(s)

Number 

of 

Samples

Condition Equilibration 

Conditions

For 6 hours

Test Conditions Concentration Minimum Service Life

Temp. Airflow RH Temp. Airflow RH Challenge Breakthrough For One Type For More Than One Type

°C LPM % °C LPM % ppm ppm min min

3 As-received NA NA NA 25 64 50 500 5 30 15

2 Equilibrated 

25 %RH 25 25 25 25 32 50 500 5 30 15

2 Equilibrated 

85 %RH 25 25 85 25 32 50 500 5 30 15

Number 

of 

Samples

Condition Equilibration 

Conditions

For 6 hours

Test Conditions Concentration Minimum Service Life

Temp. Airflow RH Temp. Airflow RH Challenge Breakthrough For One Type For More Than One Type

Front or

Back 

Mounted

Chin 

Style or

Escape

Front or 

Back 

Mounted, or 

Chin Style

Escape Front or 

Back 

Mounted, or 

Chin Style

Escape

°C LPM % °C LPM % ppm ppm ppm min min min min

3 As-received NA NA NA 25 64 50 20,000 5,000 5 12 12 6 12

2 Equilibrated 

25 %RH 25 64 25 25 32 50 20,000 5,000 5 12 12 6 12

2 Equilibrated 

85 %RH 25 64 85 25 32 50 20,000 5,000 5 12 12 6 12

Number 

of

Samples

Condition Equilibration Conditions

For 6 hours

Test Conditions Concentration Minimum 

Service Life

Temp. Airflow RH Temp. Airflow RH Challenge Breakthrough For 

One 

Type

For More 

Than 

One Type TightFitting 

LooseFitting TightFitting LooseFitting 

°C LPM LPM % °C LPM LPM % ppm ppm min min

3 As received NA NA NA NA 25 115 170 50 500 5 30 15

2 Equilibrated 

25 %RH 25 115 170 25 25 115 170 50 500 5 15 7.5

2 Equilibrated 

85 %RH 25 115 170 85 25 115 170 50 500 5 15 7.5
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 5 of 13

Table 4: Test Conditions and Minimum Service Life Criteria for Tight-Fitting Powered Air-Purifying 

Respirator with Canister(s)

Table 5: Tolerances for Test Conditions 

1 Temperature specification is for upstream temperature only. There is no requirement or 

specification for downstream temperature.

2 Tolerance on accuracy of airflow rates exceed specification on the MNR because flow 

rates are calibrated for every test. This improves the precision of the measurement and 

allows for the tighter tolerance on short-term drift. 3 RH levels greater than 85% are difficult to maintain and may cause rapid degradation of 

service life.

4.4.5. All as-received cartridges or canisters will be stored in containers same as or 

similar to the original packaging prior to testing. 

4.4.6. All equilibrated cartridges or canisters will be stored in sealed containers, kept in 

a position such that the direction of airflow would be horizontal, and at room 

temperature, and testing shall begin within 18 hours after equilibration. 

5. PROCEDURE

NOTE: Reference Section 3 for equipment, model numbers and manufacturers. 

Equipment should be operated and calibrated in accordance with the manufacturer’s 

operation and maintenance manual(s), or the laboratory’s quality management system. 

5.1. Set up the test equipment as shown in Attachment A: Test Schematic. 

5.2. For equilibration of samples prior to testing against SO2, follow sections 5.5 through 5.9, 

5.12 through 5.16, and 5.19 below. Repeat as necessary for additional samples.

Number 

of 

Samples

Condition Equilibration 

Conditions

For 6 hours

Test Conditions Concentration Minimum Service Life

Temp. Airflow RH Temp. Airflow RH Challenge Breakthrough For One Type or More 

Than One Type

°C LPM % °C LPM % ppm ppm min

3 As-received NA NA NA 25 115 50 5,000 5 12

2 Equilibrated 

25 %RH 25 115 25 25 115 50 5,000 5 6 

2 Equilibrated 

85 %RH 25 115 85 25 115 50 5,000 5 6 

Parameter Tolerance

25 °C1 ± 2.5 °C

115 LPM2 ± 1.0 LPM

170 LPM2 ± 1.0 LPM

25% RH ± 3 %RH

50% RH ± 3 %RH

85% RH3 +0/-5 %RH

500 ppm ± 10%

5,000 ppm ± 10%

20,000 ppm ± 10%
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 6 of 13

5.3. For testing against SO2, follow sections 5.4 through 5.14, and 5.17 through 5.19 below.

Repeat as necessary for additional samples.

5.4. Calibrate the challenge and breakthrough SO2 detectors.

5.4.1. For all respirators, calibrate the breakthrough SO2 detector using the gas cylinder 

containing 5 ppm SO2. 

5.4.2. For APR and PAPR cartridges, calibrate the challenge SO2 detector using the gas 

cylinder containing 500 ppm SO2. 

5.4.3. For APR chin style canisters, calibrate the challenge SO2 detector using the gas 

cylinder containing 5,000 ppm SO2. 

5.4.4. For APR front or back mounted canisters, calibrate the challenge SO2 detector 

using the gas cylinder containing 20,000 ppm SO2. 

5.4.5. For PAPR canisters, calibrate the challenge SO2 detector using the gas cylinder 

containing 5,000 ppm SO2. 

NOTE: For challenge concentrations greater than 1,999 ppm, a gas dilution system must 

be used. The challenge SO2 detector will be damaged at concentrations greater than 1,999 

ppm. Adjust the dilution system to an appropriate dilution ratio such that the detector will 

receive an adequate gas flow (typically greater than 500 milliliters per minute (mL/min)) 

and will not be damaged. The true challenge concentration is determined by multiplying 

the displayed SO2 concentration by the dilution ratio. Example: If the dilution ratio is 

10:1 and the displayed concentration is 500 ppm, the true challenge concentration is 

5,000 ppm.

5.5. Establish the testing or equilibration conditions to the required values described in section 

4.4 above.

5.5.1. Airflow is controlled and adjusted using the MNR, and measured at the test 

chamber using the dry test meter prior to each test. Use caution as to account for 

any flow losses from the hygrometer or gas detector, and do not expose the dry 

test meter to SO2.

5.5.2. Relative humidity is controlled and adjusted using the MNR, and continuously 

measured using the chilled mirror hygrometer. A stream of clean air is fed to the 

hygrometer from near the outlet of the MNR, and is controlled to approximately 

0.8 LPM using a mass flow controller. The temperature probe is mounted in the 

inlet pipe of the test chamber.

5.5.3. Temperature is typically determined by the laboratory environmental conditions. 

It is possible to control and adjust heating of the test system using the MNR, but 

may result in condensation within the test system if the laboratory temperature is 

not adequate. Upstream temperature is continuously measured using the chilled 

mirror hygrometer, with the temperature probe mounted in the inlet pipe of the 

test chamber. Downstream temperature is continuously measured using a

thermocouple probe mounted in the outlet pipe of the test chamber. NOTE: 
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 7 of 13

Temperature differentials in and around the test system may cause condensation 

and inaccurate test conditions.

5.6. Weight of the cartridge(s) or canister(s) is measured in the initial as-received state, 

following equilibration, and following test completion. 

5.7. Measure airflow and resistance to airflow of the respirator before and after each test as 

described in section 4.4 above. The measurements before each test are taken prior to 

equilibration.

5.8. Ensure that the airflow has been diverted and is not entering the test chamber.

5.9. Mount cartridge(s) or canister(s) onto an appropriate test fixture and place in test

chamber.

5.10. Ensure there is adequate flow to the challenge and breakthrough detectors. Typically, 500 

mL is required for proper detector operation. NOTE: Over- or under-pressurization of the 

detector will result in inaccurate concentration measurement.

5.11. Establish the challenge concentration to the required values described in section 4.4 

above.

5.11.1. Set the challenge agent mass flow controller to the necessary flow rate setting to 

achieve the challenge concentration in the air stream. Theoretical flow rates of 

pure SO2 are shown in Table 6.

Table 6: Theoretical Flow Rate of Pure Sulfur Dioxide to Achieve Challenge Concentration

5.11.2. Open the 99% SO2 cylinder to begin challenge agent flow.

5.11.3. Adjust the challenge agent mass flow controller as necessary to achieve the 

required challenge concentration of SO2.

5.12. Testing or equilibration may begin once all parameters have stabilized at the required 

values.

Filter Type Flow Rate 

for Test

Challenge 

Concentration

Pure SO2

Flow Rate

LPM ppm mL/min

APR Cartridge 32 500 16

APR Cartridge 64 500 32

APR Canister

(Chin Style & Escape) 32 5,000 160

APR Canister

(Chin Style & Escape) 64 5,000 320

APR Canister

(Front & Back Mounted) 32 20,000 640

APR Canister

(Front & Back Mounted) 64 20,000 1,280

PAPR Canister 115 5,000 575

PAPR Cartridge 115 500 57.5

PAPR Cartridge 170 500 85
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 8 of 13

5.13. Testing or equilibration begins when the airflow is directed into the test chamber.

5.14. Continuously monitor and record the challenge concentration, breakthrough 

concentration, RH, upstream temperature, and downstream temperature throughout 

testing or equilibration. Make adjustments as necessary to maintain the required 

conditions. 

5.15. Equilibration is complete when six hours has elapsed.

5.16. When equilibration is complete, divert airflow away from the test chamber. Remove the 

cartridge(s) or canister(s) from the test chamber. Measure the equilibrated weight as 

described above. The equilibrated sample is now ready to be used as a test sample.

5.17. A test is complete when the breakthrough concentration limit has been exceeded or the 

minimum service life has been surpassed by 10%, whichever comes first. 

5.18. When testing is complete, divert airflow away from the test chamber and close the SO2

gas cylinder. Remove the cartridge(s) or canister(s) from the test chamber. Measure the 

final weight, airflow, and resistance to airflow as described above. 

5.19. At the completion of all testing or at the end of a work shift, purge the entire test system 

with clean, dry air. Prior to powering off equipment, purge the system for a minimum of 

30 minutes, or until the gas detectors show no measured concentration. 

6. PASS/FAIL CRITERIA

6.1. The requirement for passing this test is set forth in 42 CFR Part 84 Subpart I, Sections 

84.110 and 84.126, Subpart K, Section 84.175(b), and Subpart L, Section 84.207. 

6.2. Minimum service life requirements are shown in Tables 1 - 4 above. 

6.3. A failure occurs when the measured test breakthrough concentration exceeds the stated 

breakthrough concentration limit prior to the test reaching the stated minimum service 

life time.

6.4. A failure occurs when the airflow or resistance to airflow before or after a test fails to 

meet the requirements set forth in 42 CFR Part 84 Subpart I, Section 84.122, Subpart K, 

84.175(b), and Subpart L, 84.203, and described in TEB-APR-STP-0003, TEB-APRSTP-0007, and TEB-APR-STP-0012. 

7. RECORD/TEST SHEETS

7.1. Record the test data in a format that shall be stored and retrievable. Data shall be reported 

as shown in Attachment B: Sample Test Data Sheet for Sulfur Dioxide Service Life Test.
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 9 of 13

8. LIST OF ABBREVIATIONS AND ACRONYMS

Table 7: List of Abbreviations and Acronyms Used Within This Document 

9. ATTACHMENTS

9.1. Attachment A: Test Schematic

9.2. Attachment B: Sample Test Data Sheet for Sulfur Dioxide Service Life Test

Abbreviation 

or Acronym

Definition

°C degrees Celsius

APR air-purifying respirator

CFR Code of Federal Regulations

g gram(s)

in. inch

LPM liters per minute

min minute(s)

mL/min milliliters per minute

mm millimeter

MNR Miller Nelson Research Flow-Temperature-Humidity control system

NIOSH National Institute for Occupational Safety and Health

NIST National Institute of Standard and Technology

NPPTL National Personal Protective Technology Laboratory

PAPR powered air-purifying respirator

ppm parts per million

RH relative humidity

SDS safety data sheet

SO2 sulfur dioxide

STP standard testing procedure
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 10 of 13

Attachment A: Test Schematic
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 11 of 13

Attachment B: Sample Test Data Sheet for Sulfur Dioxide Service Life Test
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 12 of 13
Procedure No. NPPTL-APR-STP-0048 Revision: 3.0 Date: Sept. 22, 2023 Page 13 of 13

Revision History

Revision Date Reason for Revision

1.0 14 March 2002 Historic document

1.1 30 June 2005 Update header and format to reflect lab move from Morgantown, WV

No changes to method

2.0 1 December 2008 Significant rewrite of RCT-APR-STP-0048. Changes affect form and 

provide clarification of technical content.

3.0 22 September

2023

Significant rewrite to combine TEB-APR-STP-0048A, -0048B, 

-0048C, and -0048D. Changes affect document format and header. 

Minor revisions to test equipment and method to match current lab 

practices. Clarification of tolerances for all measured values.