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National Institute for Occupational Safety and Health

National Personal Protective Technology Laboratory 

626 Cochrans Mill Road 

Pittsburgh, PA 15236

Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020

DETERMINATION OF PARTICULATE FILTER EFFICIENCYLEVEL FOR N99

SERIES FILTERS AGAINST SOLID PARTICULATES FOR NON-POWERED, AIRPURIFYING RESPIRATORS STANDARD TESTING PROCEDURE (STP)

1. PURPOSE

This procedure establishes the means for ensuring that the particulate filtering efficiency of N99

series filters used on non-powered respirators submitted for Approval, Extension of Approval, or

examined during Certified Product Audits, meets the minimum certification standards set forth in

42 CFR, Part 84, Subpart K, §84.181. These filters or filter cartridges may be integral to

respirator construction; mounted individually, or in sets of up to three; used in conjunction with

cartridges and canisters for chin-style, front-mounted, and back-mounted gas masks; or used in

combination with gas-and-vapor or atmosphere-supplying respirators.

2. GENERAL

This STP describes the test method to be used for the Determination of Particulate Filter

Efficiency Level for N99 Series Filters Against Solid Particulates for Non-Powered, AirPurifying Respirators test in enough detail sufficient to allow a person knowledgeable in the

appropriate technical field to conduct the test and determine whether or not the product meets the

established requirements.

3. EQUIPMENT/MATERIAL

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

3.1.1. TSI Model 8130 or 8130A Automated Filter Tester or equivalent instrument - Air 

flow control accuracy is 2% of full scale. Pressure measurement accuracy is 2% 

of full scale. Penetrations can be measured to 0.001%, efficiencies to 99.999%.

3.1.2. Microbalance accurate to 0.0001 grams (g)

3.1.3. Type A/E glass filters, 102 mm diameter, high efficiency filters with a 1 micron 

pore size
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Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020 Page 2 of 10

3.1.4. Timer (accurate to 0.01 percent) 

3.1.5. 2% sodium chloride solution in distilled water (NaCl)

3.1.6. Temperature and humidity chamber capable of maintaining 38 ±2.5°C and 85 ± 

5% relative humidity

3.1.7. Respirator filter holder supplied for specific manufacturer type which is 

compatible with TSI filter tester - NIOSH will not be obligated to use these 

holders for actual certification testing. All manufacturer test fixtures must be 

correlated with the NIOSH test method.

3.1.8. Thermal printer (supplied) or optional data acquisition system

3.1.9. TSI, Green Line Paper, part number 813010 - Lot number must be included on 

each box. Each lot must include the lot-specific “Penetration vs. Resistance 

graph”.

4. TESTING REQUIREMENTS AND CONDITIONS

4.1. Prior to beginning any testing, confirm that all measuring equipment employed has been 

calibrated in accordance with the testing laboratory’s calibration procedure and schedule. 

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

method traceable to recognized international standards when available.

4.2. Respirator filters and filter cartridges shall be tested as follows.

4.2.1. The filtering elements of the respirator, including the filter holders and gaskets 

will be tested for particle penetration.

4.2.2. When filters are not separable from the respirator body, the exhalation valves 

will be sealed to ensure that any leakage due to the exhalation valve is not 

included in the filter penetration measurement.

4.2.3. Filters used in conjunction with gas mask canisters, and odd or unusually shaped 

filters may be tested on a headform assembly or test fixture provided by the 

applicant. 

4.2.4. If a test fixture is supplied by the applicant, the test fixture shall have a serial 

number or other unique, easily referenced identifier permanently etched, 

engraved, or affixed.

5. PROCEDURE

Note: Where they have been developed to aid in the consistent operation of standard NIOSH 

test apparatus, work instructions are to be used.

5.1. Respirator filters will be challenged by a NaCl aerosol at 25 ±5 ̊C and a relative humidity 
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Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020 Page 3 of 10

of 30 ±10% that has been neutralized to the Boltzmann equilibrium state. The particle 

size distribution will be a count median diameter of 0.075 ± 0.020 micrometer and a 

geometric standard deviation not exceeding 1.86. Each respirator filter unit will be 

challenged with an aerosol concentration not exceeding 200 mg/m3

. 

5.1.1. The NaCl aerosol concentration will be determined on the days that initial 

penetration testing is performed by the following gravimetric method and 

calculated as milligrams per cubic meter (mg/m3

). 

5.1.2. Weigh a 102 mm filter to the nearest 0.1 mg., mount in the gravimetric filter 

holder, subject it to the generated aerosol at 30 Lpm for 40 minutes and reweigh 

the filter. Use a timer to monitor the duration of the test. Record the pre- and 

post-weights, time, and average flow rate on the data sheet and calculate the 

aerosol concentration in mg/m3 by the following formula:

Concentration in mg/m3 = W2 – W1

 (Q / 1000) (T)

Where:

W1 = Initial filter weight in mgs.

W2 = Final filter weight in mgs.

Q = Flowrate in liters per minute

T = Elapsed time in minutes

With a flowrate of 30 Lpm for 40 minutes, the above formula simplifies 

to:

C = W2 – W1

1.2

5.1.3. Use the following formula to calculate the test duration:

T in minutes = (mg load) (1000 L / m3) 

(C) (Q)

Where:

C = Concentration in mg/m3 from 5.1.2.

Q = Flow rate for test in Lpm

5.1.4. The upstream and downstream photometer readings are used for monitoring 

stability and for calculating a photometer correlation factor (CF). The correlation 

factor is determined with an empty filter holder and is calculated internally as 

shown below:

CF = Downstream Photometer Voltage – Downstream Background Voltage

Upstream Photometer Voltage – Downstream Background Voltage

The correlation factor is used by the software to express the upstream photometer 

signal in terms of the downstream photometer signal.
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5.1.5. The NaCl particle size distribution shall be verified using “green line” filter discs 

supplied by TSI with a known penetration range. Graphs of penetration vs.

resistance for two sheets and five sheets of stacked filter discs are supplied with 

each lot of the standard filters, with a central line and upper and lower lines 

representing the expected penetration range at a given resistance. The test data 

should fall within an acceptance zone having boundaries defined by the upper 

and lower curves on the graphs. The standard filter test using both 2 sheets and 5 

sheets will be run at least once in each 8-hour test period to verify that the aerosol 

distribution is within the acceptance zone.

5.2. Respirator filters will be pre-conditioned at 85 ± 5% relative humidity and 38 ±2. 5 °C 

for 25 ±1 hours. After conditioning, the filters shall be sealed in a gas tight container and 

tested within 10 hours.

5.3. Filters will be mounted and sealed on holders to prevent leakage around the filter holder. 

Single air purifying respirator filters will be tested at a challenge flow rate of 85± 4 Lpm. 

Filters used as pairs on a respirator are tested using a single filter of the pair at 42.5 ± 2 

Lpm challenge flow rate. Filters used in threes are tested using a single filter of the set at 

28.3 ± 1 Lpm challenge flow rate.

5.3.1. The challenge flow rate must be checked for stability for at least 30 seconds prior 

to testing.

5.4. A sample of 20 filter units will be tested against the NaCl aerosol. Three filters will be 

loaded until the aerosol mass loading levels as shown in the table below are reached and 

evaluated to determine the method for the remaining 17 filters using one of the five type 

characterizations shown in 5.4.1., through 5.4.5. The mass loading values in the table 

represent the mass of NaCl aerosol that has contacted the filter.

 

Respirator Filter Configuration

(Inlet Airflow Split)

Aerosol Mass 

Loading Level

Single 200 ± 5 mg.

Double 100 ± 5 mg.

Triple 66.7 ± 5 mg.

 

5.4.1. Type 1. If preliminary testing of all three initial test filters consistently results in 

a straight line (Figure 1), for the remaining 17 filters, record the initial 

penetration reading.

5.4.2. Type 2. If filter testing of all three initial test filters consistently results in a 

curve which indicates increased efficiency during the complete run (Figure 1), 

for the remaining 17 filters, record the initial penetration reading.

5.4.3. Type 3. If filter testing of all three initial test filters consistently results in 

decreased efficiency over time (Figure 1), load the remaining 17 filters with 

NaCl to the level specified in the table above and record the maximum 

penetration reading.
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Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020 Page 5 of 10

5.4.4. Type 4. If filter testing of all three initial test filters consistently results in 

increased efficiency, then a decrease in efficiency, and then flattens out during 

the remainder of the complete run (Figure 1), for the remaining 17 filters, record 

the maximum penetration reading after reaching and maintaining a flat line for a

period of 20 minutes following the decreasing segment in efficiency.

5.4.5. For any other filter type, determine loading at which maximum penetration 

consistently occurs and test at that loading value for the remaining 17 filters.

5.4.6. If any one of the 20 filters exhibit a penetration greater than 1.0%, further testing 

of that filter will be terminated. Any filter that exceeds the specified limit shall 

be remounted and retested to ensure that leakage was not caused by a mounting 

leak. If retesting eliminates the excessive leakage and testing has gone beyond 

the initial penetration, that sample will be considered an invalid sample, and 

another tested in its place.

5.5. The penetration of the first three filters will be measured, recorded, and printed at 

approximately 1-minute intervals during the test period. The highest penetration 

observed throughout the test of each filter will be recorded as the maximum penetration 

of that filter.

5.6. Determine and record on the data sheet the maximum filter penetration for each of the 20 

filters.

6. PASS/FAIL CRITERIA

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

84.181 (i). 

6.2. The minimum efficiency for each of the 20 filters shall be determined and recorded and 

shall be equal to or greater than 99 %.

6.3. For the sample of 20 filters or filter cartridges to demonstrate acceptable performance, 

each filter shall meet or exceed the specified minimum efficiency level at the end point of 

the test.

7. RECORDS/TEST SHEETS

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

8. ATTACHMENTS

8.1. Figure 1, Graph of Typical Filtration Efficiency versus Time Curves

8.2. Example Data Sheet

8.3. Photograph of TSI 8130 CertiTester with chuck open
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Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020 Page 6 of 10

8.4. Photograph - Close up of respirator test fixture in the closed chuck
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Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020 Page 7 of 10

8.1. Figure 1, Graph of Typical Filtration Efficiency versus Time Curves
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8.2. Example Data Sheet
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Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020 Page 9 of 10

8.3. Photograph of TSI 8130 CertiTester with chuck open

8.4. Photograph - Close up of respirator test fixture in the closed chuck
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Procedure No. TEB-APR-STP-0058 Revision: 3.1 Date: 15 January 2020 Page 10 of 10

Revision History

Revision Date Reason for Revision

1.0 7 March 2002 Historic document

1.1 24 August 2005 Update header and format to reflect lab move from Morgantown, WV

No changes to method

2.0 05 October 

2007

Significant rewrite of RCT-APR-STP-0051-56. Changes affect form 

and provide clarification of technical content.

2.1 27 August 2012 Removed reference to SMPS, removed product-specific reference for 

Type A/E glass filter, added information on green-line media, 

removed references to 6 work instructions, removed photographs of 

outdated equipment.

3.0 13 April 2016 Updated facility address in main title block, removed reference to 

work instruction, changed the accuracy of the timer specified in 

3.4.1., now expressed as percent, edited Section 4.4.1., to allow for 

three unused sheets of media to be added to the two used sheets in 

lieu of five new (unused) sheets of media, and edited section 5.2.1., to 

clarify the need to determine NaCl concentration on the days that 

initial penetration evaluations are performed. Removed section 

5.2.6., to comply with practice, this step is not performed. Added a 

statement to section 5.5.6 about retesting respirators. In addition to 

an editorial change in 6.1., the graph that was listed as Figure 1, now 

Figure 3, was also relabeled, with no changes to the graph itself. 

Corrected the P&A statement in section 4.3.

3.1 15 January 2020 With the greatest impact occurring in Section 4, this revision 

represents an update to current content and editorial standards, with 

no change to method or criteria. Figure 3 reverted back to Figure 1.