Approvals: 1st Level 2nd Level 3rd Level 

National Institute for Occupational Safety and Health 

National Personal Protective Technology Laboratory 

P.O. Box 18070 

Pittsburgh, PA 15236 

Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005

DETERMINATION OF SILICA DUST LOADING TEST 

FOR POWERED AIR-PURIFYING RESPIRATOR FILTERS 

STANDARD TESTING PROCEDURE (STP) 

 

l. PURPOSE

This test establishes the procedure for ensuring that the level of protection provided by the silica 

dust test requirements on HEPA (also known as HE) filters designed for powered air-purifying 

respirators (PAPRs) submitted for Approval, Extension of Approval, or examined during 

Certified Product Audits, meet the minimum certification standards set forth in 42 CFR 84, 

Subpart G, Section 84.63(a)(c)(d) and Subpart KK, Section 84.1152; Volume 60, Number 110, 

June 8, 1995. These filters may be integral components of the PAPR; mounted individually; or 

used in conjunction with cartridges and canisters. 

2. GENERAL

This STP describes the Determination of Silica Dust Loading Test For Powered Air-purifying 

Respirator Filters test in sufficient detail that a person knowledgeable in the appropriate technical 

field can select equipment with the necessary resolution, conduct the test, and determine whether 

or not the product passes the test. 

3. EQUIPMENT/MATERIALS

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

3.1.1. Silica Dust Chamber: Tests are conducted in a specially built laminated 1⁄2 inch 

marine grade plywood chamber equipped with a door in the rear and safety glass 

windows in both front and rear. The chamber floor is 5 feet by 5 feet, and the 

 floor to ceiling distance is 7 feet. An explosion proof light fixture is installed 

inside the chamber, and a work shelf 24 inches wide extends across the front of 

the chamber. 

A control panel at the left side of the chamber front contains all electrical and air 

controls for the chamber operation. The following gauges and flow manometers 

are located on this panel: internal chamber pressure (in. H2O), sampling vacuum 

(in. Hg.), air supply pressure (psi), spiral air flow (cfm), venturi diluent (in. H2O), 

and concentration sampling flow meters (calibrated at 32 lpm).

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Dust is fed to the sizing equipment by means of a feed screw which lifts a glass 

holding tube at a constant rate. The feed tube is driven by an electronically 

controlled synchronous motor that allows rapid changes in feed rates for 

maintaining concentration. A step switch selects the feed rate; each step is 

synchronous and reproducible. Also, a fast forward and fast reverse clutch is 

installed for quickly setting up the proper conditions in the dust feed tube. 

Dust pick up is accomplished by a jet flow pickup tube which dries and disperses 

dust evenly in the air stream, thereby eliminating clumps of particles due to 

packing. All components of the dust pickup apparatus (except the tangential jets) 

are constructed of Pyrex so that operation can be monitored visually at all times. 

Sampling ports are located across the front of the chamber to allow the respirator 

to be connected to the breathing machine outside of the chamber and to allow for 

hourly concentrations to be taken. A 32 lpm vacuum source is readily available 

to take chamber concentrations. 

The source of negative pressure for the entire sampling system is a rotary carbon 

vane vacuum pump with a 12 gallon surge tank and vacuum regulator set to 

maintain a 6 inch of Hg vacuum. All air for the chamber is supplied by house 

air and is regulated to 14 psi. 

The test atmosphere is exhausted from the chamber at equal constant rates from 

the four vertical corner ports of the chamber through a high efficiency filter. A 

forge blower provides moving power for the exhaust system and discharges the 

filtered air back into the laboratory ventilation system. The entire chamber is 

maintained under negative pressure during operation to prevent laboratory 

contamination. 

A separate resistance measuring assembly is incorporated into the control panel 

for measuring positive and negative resistance at 85 lpm. Respirator resistance is 

measured on a slant tube manometer with a 6 in. water column scale. A flow 

meter calibrated for 85 lpm and valves to adjust flow are incorporated into the 

resistance assembly. 

 3.1.2. Analytical balance: A Mettler AK or equivalent balance is used to measure 

absolute filter weights and should have an accuracy of 10-4 gm. 

3.1.3. Particle sizing system: A variety of equipment - including laser, optical, and 

electron microscope systems - is available for particulate size determination. 

3.1.4. Absolute filters: Filters used for gravimetric concentrations are MSA 1106B 

filters or equivalent. 

3.1.5. Silica dust: The silica dust must be ground to pass 99+ percent of the test 

suspension through a 270-mesh sieve. The test suspension will have a geometric 

mean of 0.4 to 0.6 micrometers with a geometric standard deviation not to exceed 

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Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005 Page 3 of 11 

2. The concentration of the test suspension that must be maintained must not be 

less than 50 mg/m3

 or more than 60 mg/m3. These parameters are specified in 

84.1144. 

3.1.6. Breathing machine: A breathing machine at a rate of 24 respirations per minute 

with a minute volume of 40 liters with a breathing machine cam that provides a 

work rate of 622 kg-m2

 as specified in 42 CFR 84. 1145. 

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 manufacturer's calibration procedure and schedule. At 

a minimum, all measuring equipment utilized for this testing must have been calibrated 

within the preceding 12 months using a method traceable to the National Institute of 

Standards and Technology (NIST). 

4.2. Normal laboratory safety practices must be observed. This includes safety precautions 

described in the current ALOSH Facility Laboratory Safety Manual. 

 4.2.1. Safety glasses, lab coats, and hard-toe shoes must be worn at all times. 

4.2.2. Work benches must be maintained free of clutter and non-essential test 

equipment. 

4.2.3. When handling any glass laboratory equipment, lab technicians and personnel 

must wear special gloves which protect against lacerations or punctures. 

5. PROCEDURE

 Note: Reference Section 3 for equipment, model numbers and manufacturers. For calibration 

purposes use those described in the manufacturer's operation and maintenance manuals. 

 5.1. Initial Airflow and Resistance: 

Three powered air purifying respirators (PAPRs) shall be tested in accordance with 42 

CFR 84.1152. The PAPRs shall be assembled according to the manufacturers 

instructions. Each unit shall be connected to an airflow measuring system and tested to 

ensure compliance with minimum airflow requirement as per NIOSH test procedure 

APRS-0012. Tight fitting respirators shall then be tested for initial resistance to airflow 

as per APRS -0003 and APRS-0007. The PAPR shall then be placed inside the silica 

dust chamber and connected to a breathing machine operated at a rate of 24 respirations 

per minute with a minute volume of 40 liters with a breathing machine cam that provides 

a work rate of 622 kg-m2

 as specified in 42 CFR 84. 1145. 

5.2. Chamber Preparation: 

 Before starting test, be sure that the door and all ports are closed. 

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Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005 Page 4 of 11 

5.3. Dust Tube Filling: 

To help maintain repeatability and uniformity in column packing, a known amount of 

dust (e.g. 200 gm) should be packed to a given column height (e.g. 20") each day. Using 

a funnel, pour about three or four inches of the preweighed dust into the tube and pack. 

Repeat filling and packing until all dust is added. Several techniques can be used to pack 

the tube after each three or four inch addition of silica dust. One technique is to attach a 

vacuum source to the top of the holding tube. (Note: A filter should be inserted into the 

vacuum line to prevent dust from entering the vacuum system). By tapping the bottom of 

the dust holding tube on a rubber pad, the dust bed is packed and any trapped air is 

removed by the vacuum source. 

The top surface of a properly packed column of dust should not bounce up and down 

more than 1/8 inch when the bottom of the holding tube is tapped on a rubber pad. 

 Excessive bouncing of the top surface is indicative of trapped air. 

 5.4. Chamber Start-Up: 

5.4.1. Place packed dust tube in the hoist carriage, taking care that the hoist carriage is 

firmly connected to the feed screw attachment. 

 5.4.2. Turn on main breaker switch. 

 5.4.3. (a) Turn stepping motor on-off switch to the on position and the up-down 

directional switch to the up (down if lowering the column) position. 

 (b) Set high-off-low speed switch, which provides manual control of stepping 

motor speed, in the high speed position. Tube will rise rapidly. Be sure the 

spiral tube enters the dust tube properly. 

 (c) When the top of the silica dust is approximately one inch from the 

bottom of the spiral tube, switch the motor speed control to off and then to low. 

 (d) Control low speed with the coarse and fine speed adjustments. Low speed 

 control is important because it directly helps control the chamber concentration 

 of silica dust. 

5.5. Turn on Venturi vibrator. This chamber modification was made to prevent excessive 

accumulations of silica dust within the Venturi body, which would break loose and 

suddenly alter the chamber concentration of silica dust. 

5.6. Turn on the house air supply and adjust to 14 psig with the in-line pressure regulator. 

5.7. Turn on the stirrer, exhaust, and inject switches. 

5.8. Chamber controls determined by operator should be set and held at their respective 

values, e.g.: 

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Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005 Page 5 of 11 

 Air Pressure ............................................ 14 psig 

 Chamber Pressure .................................... 0.50 + 0.02 in H20* 

 Cyclone Flow ........................................... 7.0 + 0.5 CFM 

 Cyclone Pressure ...................................... 2.1 to 2.2 in H20 

 Spiral Flow ............................................... 2.0 CFM 

 Venturi Flow ............................................. 3.0 CFM 

 Venturi Diluent .......................................... 1.0 CFM 

 Venturi Pressure ......................................... 1.0 to 1.2 in H2O 

 Chamber Diluent ......................................... 0 CFM 

 * Make certain all ports are tightly closed.

5.9. Check exhaust filter resistance. If the resistance exceeds three inches of water, remove 

the prefilter and replace with new prefilter. 

5.10. Let the chamber run for a minimum of 30 minutes to reach equilibrium. 

 5.11. Using a psychrometer, measure chamber relative humidity and record. 

 (a) A battery-operated psychrometer is placed in the chamber for approximately 1 

minute to determine relative humidity. 

(b) Be sure that the port is tightly closed. Leakage can be checked by observing the 

chamber pressure reading. If the chamber pressure reading returns to approximately 

0.50, no leakage is present. 

5.12. Check Chamber Concentration: 

(a) Weigh and record initial weight of absolute filter. 

(b) Place absolute filter in filter holder. 

(c) Remove blank port and install sampling port. 

(d) Attach vacuum line to absolute filter holder. 

(e) Turn on vacuum switch and timer. Adjust the flow rate to 32 Lpm and the 

 vacuum pressure to 6.2 in. 

(f) After 10 minutes, stop vacuum and timer. 

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(g) Remove sampling port, and replace blank port. 

(h) Carefully remove and weigh filter. Record weight. 

(i) Calculate chamber concentration (see analysis and recording of data, item # 2). 

 5.13. If concentration is between 50 and 60 mg/m3

, begin test. If the concentration is not 

within this range, run an additional concentration test for 10 minutes. If this test is within 

expected levels, start respirator test. If this test is still outside the acceptable range, 

adjustments must be made to bring chamber concentrations into proper level. To increase 

or decrease concentration, increase or decrease the speed of dust hoist drive motor by 

changing setting on dust hoist speed controls. Wait at least 15 minutes before the next 

chamber concentration check. Once the test concentration is maintained at the correct 

level, then a chamber dust concentration is taken once each hour. 

 5.14. Testing Respirator:

 The PAPR face piece is mounted on an anthropometric head which in turn is connected 

with a ground glass joint that is attached to the silica dust chamber door port. The outside 

portion of the door port contains the absolute filter holder which is used to collect the 

leakage sample during the test. The filter holder is connected to the breathing machine 

with a three foot piece of 2-inch vinyl tubing to the inhale/exhale port of the breathing 

machine. The machine is operated at a rate of 24 respirations per minute with a minute 

volume of 40 liters with a breathing machine cam that provides a work rate of 622 kg-m2

as specified in 42 CFR 84. 1145. The respirator shall be exposed to the test suspension 

for 4 hours . 

 5.15. Final Airflow and Resistance: 

After the silica dust test, the PAPR shall be connected to an airflow test system and tested 

to ensure compliance with minimum airflows as per APRS-0012. Final inhalation and 

exhalation resistance to airflow shall be taken on all tight fitting PAPRs as per APRS0003 and APRS-0007. 

5.16. Final Leakage: 

After the silica dust test, the leakage filter sample is weighed to determine the total 

amount of leakage that was collected during the 4 hour silicca dust loading test. 

6. PASS/FAIL CRITERIA

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

84.63(a)(c)(d) and Subpart KK, Section 84.1152; Volume 60, Number 110, June 8, 1995. 

 6.2. This test establishes the standard procedure for ensuring that: 

 84.63. Test requirements; general. 

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Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005 Page 7 of 11 

(a) Each respirator and respirator component shall when tested by the applicant and by 

the Institute, meet the applicable requirements set forth in subparts H through L of this 

part. 

(c) In addition to the minimum requirements set forth in subparts H through L of this 

part, the Institute reserves the right to require, as a further condition of approval, any 

additional requirements deemed necessary to establish the quality, effectiveness, and 

safety of any respirator used as protection against hazardous atmospheres. 

(d) Where it is determined after receipt of an application that additional requirements will 

be required for approval, the Institute will notify the applicant in writing of these 

additional requirements, and necessary examinations, inspections, or tests, stating 

generally the reasons for such requirements, examinations, inspections, or tests. 

84.1152. Silica dust loading test; respirators designed as protection against dusts, fumes, 

and mists having an air contamination level less than 0.05 milligram per cubic meter and 

against radionuclides; minimum requirements. 

(a) Three non-powered respirators will be tested in accordance with the provisions of 

84.1144, or equivalent, and shall meet the minimum requirements of 84.1144 and 

84.1149. 

(b) Three powered air-purifying respirators will be tested in accordance with the 

provisions of 84.1144 except they will be tested for a period of 4 hours each at a flowrate 

not less than 115 liters per minute to tight-fitting facepieces, and not less than 170 liters 

per minute to loose-fitting hoods and helmets. The total amount of unretained test 

suspension in samples taken during testing shall not exceed 14.4 milligrams for a 

powered air-purifying respirator with tight-fitting facepiece, and 21.3 milligrams for a 

powered air-purifying respirator with loose-fitting hood or helmet. They shall meet the 

minimum requirements of 84.1149. 

6.3. All tight fitting facepieces must not exceed the following inhalation resistance 

requirements both before (initial) and after (final) the silica dust test when mounted on a 

head form at a flow rate of 85 lpm with the blower on and off as per APRS-0007: 

 Tight fit filter PAPRS with chemical cartridges: Initial: 50 mmH2O Final: 70 mmH2O. 

Tight fit filter PAPRS without chemical cartridges: Initial: 20 mmH2O Final: 40mh2o . 

 Tight fit filter PAPRS with FM or BM canisters: Initial: 70 mmH2O Final: 85 mmH2O. 

 Tight fit filter PAPRS chin style canisters: Initial: 65 mmH2O Final: 80 mmH2O. 

Note: For those also containing chemical cartridges or canisters, the “Final” criteria 

should be measured after silica dust testing and all gas and vapor tests. 

7. RECORDS/TEST SHEETS

7.1. All test data will be recorded on the DETERMINATION OF SILICA DUST TEST 

PROCEDURE FOR POWERED AIR-PURIFYING RESPIRATORS test data sheet. 

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Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005 Page 8 of 11 

7.2. All videotapes and photographs of the actual test being performed, or of the tested 

equipment shall be maintained in the task file as part of the permanent record. 

7.3. All equipment failing any portion of this test will be handled as follows: 

7.3.1. If the failure occurs on a new certification application, or extension of approval 

application, send a test report to the RCT Leader and prepare the hardware for 

return to the manufacturer. 

 7.3.2. If the failure occurs on hardware examined under an Off-the-Shelf Audit the 

hardware will be examined by a technician and the RCT Leader for cause. All 

equipment failing any portion of this test may be sent to the manufacturer for 

examination and then returned to NIOSH. However, the hardware tested shall be 

held at the testing laboratory until authorized for release by the RCT Leader, or 

his designee, following the standard operating procedures outlined in Procedure 

for Scheduling, and Processing Post-Certification Product Audits, RB-SOP0005-00. 

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Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005 Page 9 of 11 

8. LIST OF ABBREVIATIONS

BM – back mounted 

cfm – cubic feet per minute 

FM – front mounted 

Hg – mercury 

H2O – water 

in. – inches 

lpm – liters per minute 

mg/m3

 – milligrams per cubic meter 

mm – millimeter 

psi – pounds per square inch 

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Procedure No. RCT-APR-STP-0025 Revision: 1.1 Date: 14 June 2005 Page 11 of 11 

Revision History 

Revision Date Reason for Revision 

1.0 11 July 2001 Historic document 

1.1 14 June 2005 Update header and format to reflect lab move from Morgantown, WV 

No changes to method 

 

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