ASTM F2101

• Bacterial aerosol challenge
  • The testing begins with the introduction of aerosolized bacteria into the aerosol chamber using a nebulized culture suspension containing Staphylococcus aureus with 5 x 10⁵ CFU/mL.
  • The aerosolized bacteria are then drawn through the medical face mask material using a vacuum connected to a six-stage viable particle cascade impactor.
• Control runs and aerosol particle size
  • Before testing the mask material, a positive control run is performed without a test specimen to determine the number of viable aerosol particles generated. 
  • The mean particle size (MPS) of the aerosol is calculated based on the results of these control plates. 
  • The mean particle size is critical as it influences the filtration performance of the material and for the test, it is maintained at 3.0 µm ± 0.3 µm.
• Testing process
  • During the test, the face mask material sample is clamped between the six-stage cascade impactor in the aerosol chamber. 
  • The aerosol challenge is introduced and drawn through the mask material. After the challenge, agar plates in the cascade impactor collect the bacteria that penetrate the mask. 
  • These agar plates are then incubated at 37 ± 2 °C for 48 ± 4 hours.
• Viable bacteria count
  • After incubation, the number of viable bacteria collected on each stage of the cascade impactor is counted. 
  • These counts are expressed as colony-forming units (CFUs) and converted into percentages to determine the bacterial filtration efficiency (BFE) of the material. 
  • The bacterial challenge solution is nebulized at a rate of approximately 1700 to 3000 viable particles per test.
• Challenge delivery and negative control: 
  • The challenge delivery rate is based on the results of positive control plates, where no test specimen is clamped into the test system. 
  • Additionally, a negative control is performed by collecting a 2-minute air sample from the aerosol chamber without introducing a bacterial challenge. 
  • Incubate agar plates at 37± 2 °C for 48 ± 4 h.
  • This ensures that no contamination or unintended aerosol introduction affects the results.

Passing criteria

Face masks are categorized into the following levels based on their barrier performance:

• Level 1 Barrier: Lightweight and breathable, ideal for low-risk settings like doctor visits or public transport. 
• Level 2 Barrier: Thicker materials for moderate-risk scenarios, such as dental procedures. 
• Level 3 Barrier: Maximum protection for high-risk situations, like surgery. 

Depending on these levels, the performance criteria of medical face masks vary as:

For Level 1 masks:  ≥ 95% BFE

For Level 2 and 3 masks: ≥ 98% BFE.

Precautionary measures to undertake while testing

• Staphylococcus aureus used in the test is a human pathogen, and is a common cause of nosocomial infections; therefore, microbiological training is required for technicians conducting the testing.

• Gloves and all other personal protective equipment shall be worn at all times during the execution of the test in order to prevent contamination and exposure.
• Any aerosol produced in the course of testing shall be contained to prevent exposure of personnel and reduce the risk of laboratory contamination. Proper containment techniques and equipment shall be employed.