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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-0059 Revision: 3.2 Date: 13 December 2019

DETERMINATION OF PARTICULATE FILTER EFFICIENCY LEVEL FOR N95 

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 N95

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 N95 Series Filters Against Solid Particulates for Non-Powered, AirPurifying Respirators test in sufficient detail 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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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 “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 

of 30 ±10% that has been neutralized to the Boltzmann equilibrium state. The particle 
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Procedure No. TEB-APR-STP-0059 Revision: 3.2 Date: 13 December 2019 Page 3 of 9 

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, then 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 then 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.

5.1.5. The NaCl particle size distribution shall be verified using “green line” filter discs 
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Procedure No. TEB-APR-STP-0059 Revision: 3.2 Date: 13 December 2019 Page 4 of 9 

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 3), 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 3), 

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 3), load the remaining 17 filters with 

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

penetration reading.

5.4.4. Type 4. If filter testing of all three initial test filters consistently results in 
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Procedure No. TEB-APR-STP-0059 Revision: 3.2 Date: 13 December 2019 Page 5 of 9 

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

the remainder of the complete run (Figure 3), 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 has a penetration greater than 5.0%, 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.

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

shall be equal to or greater than 95 %.

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

8.4. Close up of respirator test fixture in the closed chuck
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Procedure No. TEB-APR-STP-0059 Revision: 3.2 Date: 13 December 2019 Page 6 of 9 

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-0059 Revision: 3.2 Date: 13 December 2019 Page 8 of 9 

8.3. Photograph of TSI 8130 CertiTester with chuck open

8.4. Close up of respirator test fixture in the closed chuck
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Procedure No. TEB-APR-STP-0059 Revision: 3.2 Date: 13 December 2019 Page 9 of 9 

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 31 October 2019 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.

3.2 13 December 2019 Correction to Section 2.0, the test name was inadvertently changed 

to the wrong test in the General desctiption.