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How To Test Your Medical Face Mask For Bacterial Filtration Efficiency

What does ASTM F2101 cover?

ASTM F2101 covers how to measure the bacterial filtration efficiency (BFE) of medical face mask materials. BFE is a quantitative filtration efficiency test method that can determine a maximum filtration efficiency of 99.9 %. This test method does not apply to all biological aerosol exposure. This test method evaluates medical face mask materials as protective clothing but not as a respirator. If respiratory protection for the wearer is needed, a NIOSH-certified respirator, aka N19 mask, should be used. High bacterial filtration efficiency measurements for a particular medical face mask material will not ensure wearer protection from biological aerosols without further tests to confirm medical face mask design, fit, and facial-sealing properties. Combining values from the two systems may result in nonconformance of the standard. Thus, stick with one set of values when performing BFE testing. In addition to medical face mask design, this BFE test method does not address the breathability of the medical face mask materials, an important consideration for medical face mask factories. Nor does this test method define acceptable levels of bacterial filtration efficiency. Therefore, when using this test method, describe the specific condition(s) under which testing is conducted. ASTM F2101 BFE testing may be used to measure the bacterial filtration efficiency of other porous medical products such as surgical gowns, surgical drapes, and sterile barrier systems. Generally, colony-forming units are used for raw units of bacterial efficiency. Colony-forming units are then converted to percentages of bacterial efficiency for medical face mask factories to report.

How is bacterial filtration efficiency (BFE) testing performed?

As a quick overview of the BFE testing procedure, aerosolized bacterial is introduced into an aerosol chamber using a nebulized culture suspension of Staphylococcus aureus. The S. aureus aerosol is drawn through the medical face mask material within the aerosol chamber using a vacuum attached to the cascade impactor. The cascade impactor collects any aerosolized bacteria that make it through the mask material on agar plates. The collected bacteria from the agar plates are then quantified using traditional units of bacterial efficiency (CFU) and converted into a percentage of efficiency.

For bacterial challenge preparation, tryptic soy broth with S. aureus is incubated with mild shaking at 37 °C for 24 hours. The cultured S. aureus solution is then diluted to a concentration of approximately 5×105 colony-forming units per milliliter (CFU/mL). The bacterial challenge solution is nebulized at approximately 1700 to 3000 viable particles per test to determine the correct CFU units of bacterial efficiency. The challenge delivery rate is based on the results of the positive control plates when the aerosol is collected with no test material specimen clamped into the testing system.

For testing, a sample of the medical face mask material is clamped between a six-stage cascade impactor in an aerosol chamber. The medical face mask material is traditionally attached such that the inside of the medical face mask is in contact with the aerosolized bacterial challenge. However, BFE tests can be performed with the aerosol challenge directed through the side of the material hitting the wearer’s face or the side of the material hitting the external environment. Thus, filtration efficiencies for environment-generated aerosols and wearer-generated aerosols can be measured for mask materials.

New positive control plates are used for each BFE test. These positive controls determine the challenge delivery rate and the aerosol’s mean particle size (MPS).  The S. aureus challenge solution is added to the nebulizer using a peristaltic or syringe pump. The aerosol challenge begins after the air pressure and the pump connected to the nebulizer is turned on. The bacterial aerosol challenge will be delivered to the test specimens at a flow rate within the range of normal respiration. The aerosol challenge and cascade impactor sampling occur for the full minute the aerosol challenge is delivered. The cascade impactor will continue to sample for an additional minute after the aerosol challenge is delivered for a total of two minutes of impactor sampling. The challenge procedure is repeated for each test specimen with new agar plates added to the impactor for the positive controls and individual test specimens. Negative control samples are created by collecting a 2-min sample of air from the aerosol chamber. During negative control sampling, no bacterial challenge should be pumped into the nebulizer. The units of bacterial efficiency (CFU) for negative controls should be zero after nebulization.

After cascade impactor sampling, the agar plates are incubated for 48 hours. Each of the six-stage plates of the cascade impactor is counted after incubation for the number of viable particles collected. The ratio of the upstream counts to the downstream counts (the mask filtration count to positive control count) collected for the test specimen is calculated. The raw CFU units for bacterial efficiency are reported as a percent bacterial filtration efficiency, which medical face mask factories report.

Why is S. aureus used in bacterial filtration efficiency testing?

S. aureus is used for BFE testing since it is the leading cause of infections contracted within the hospital. While Staphylococcus aureus is a standard part of our normal flora of the body, it can also be a human pathogen.

What material wear stresses should you consider when performing a BFE test?

Physical, chemical, and thermal stresses can degrade mask materials and impact the performance of the medical face mask factories choice of material. Physical and chemical stresses could result from material flexing, abrasion between the material and facial hair, or wetting of the mask with contaminants such as alcohol or sweat. BFE testing without these stresses (if applicable to the wearer) could lead to a false sense of security based on traditional BFE testing alone. If these conditions are of concern, it is recommended to evaluate the performance of the medical face mask material for bacterial filtration efficiency following an appropriate pretreatment technique representative of the expected conditions of use. Preconditioning conditions also include storage conditions and the effects of laundering and sterilization for reusable products. BFE testing with preconditioning can be used for quality control if proper statistical design and analysis of larger data sets are performed.

Close picture of a face mask next to a syringe. How to test your medical face mask. Bacterial filtration efficiency test. BFE testing. N19 masks testing. Staphylococcus aureus

How are filtration efficiencies for bacterial filtration efficiency testing calculated?

Total the counts from each of the six plates (test specimens and positive controls) are made as detailed below.

The filtration efficiency percentages are calculated using the following equation:

[(CT)/ (C)] *100 = % BFE

Where:

C = average plate count total for test controls

T = plate count total for test sample

% BFE = bacterial filtration efficiency percentage

The manufacturer’s specification calculates the mean particle size for the cascade impactor used. The mean particle size of the bacterial aerosol shall be maintained at 3.0 micrometers.

Why is reported for ASTM F2101 bacterial filtration efficiency testing?

ASTM F2101 testing reports contain information on the test specimen area evaluated. These include:

 *The flow rate during testing

 * The mean particle size of the challenge aerosol

 * The percent bacterial filtration efficiency for each test specimen

* The average plate count results for the positive controls

* The average plate count results of the negative controls

* The plate count total for each stage

* The side of the specimen was oriented toward the challenge aerosol.

Summary

Overall, ASTM F2101 covers how to perform BFE testing for medical face masks and what requirements must be met for BFE testing. Bacterial filtration efficiency testing is a simple procedure that involves aerosolized bacterial being introduced into an aerosol chamber using a nebulized culture suspension of Staphylococcus aureus. The S. aureus aerosol is drawn through the medical face mask material within the aerosol chamber using a vacuum attached to the cascade impactor. The cascade impactor collects any aerosolized bacteria that make it through the mask material on agar plates. The collected bacteria from the agar plates are then quantified. While BFE testing is a simple procedure, ensure you choose a contract testing organization that can support you with appropriate bacterial filtration efficiency testing or other testing for your unique medical device or product needs.

MycoScience is a contract manufacturing organization that specializes in filling sterile syringes and vials for parenteral products. MycoScience also offers Bacterial Endotoxin Testing, Preservative Efficacy Testing, Sterilization Validations, Bioburden Testing, Cleaning Validations, Microbial Aerosol Challenge Testing, Accelerated Aging, Microbiology Testing, Cytotoxicity Testing, EO Residual Testing, Package Integrity Testing & Environmental Monitoring services for medical device companies, and allied industries. MycoScience is an ISO 13485 certified facility.

References

American Society for Testing and Materials. Standard Test Method for Evaluating the Bacterial Filtration Efficiency (BFE) of Medical Face Mask Materials, Using a Biological Aerosol of Staphylococcus aureus. West Conshohocken, PA, United States. (ASTM F2101-19).

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