Masks: How to choose, wear & dispose

  • Wear a surgical mask whenever going out from home
  • Wear an N95 respirator in high risk areas, e.g. hospitals
  • Purchase masks from trustable sources
  • Check mask specifications:
    Surgical mask: ASTM F2100 Level 2/3 or EN14683 IIR
    Filter efficiency: BFE: 3 micron vs PFE: 0.1 micron
    Respirator: N95 (US NIOSH) or FFP2 (EU)
  • Sanitize hands before and after wearing masks
  • Make sure the mask is on the right side and fits tightly
    Carry out fit test for respirators
  • Replace face masks timely
  • Do not touch the outside of used mask
  • Discard mask into covered trashcan per use. DO NOT REUSE.
  • Hand hygiene is also important, but DOES NOT REPLACE face mask protection

Here, let’s first look at how to choose, wear and dispose the face mask as an important piece of personal protection equipment.

Why wear a mask

The answer is obvious: to ward off pathogen and prevent infection.

We are now under the threat of an infectious respiratory disease outbreak caused by nanometer-sized virus particle . The HKU team projected that without any public health intervention, a pandemic can happen soon. Wearing masks is among the most effective ways to prevent transmission and communal outbreak.

Public health intervention examples include strengthened hygiene measures and population movement control.

Common mask types in market

There are several common types of commercially available masks, namely cotton masks, dust filter masks, active carbon masks, surgical masks, and respirators. The former two are mainly used for warmth, protection against large dust and pollen particles, and removes unpleasant odours respectively, with little protection effect against diseases.

Surgical masks and respirators are valid protectors which both block large-particles droplets and bacteria. Unfortunately, coronaviruses are more likely airborne than carried by large droplets, which can be filtered effectively by N95 but not surgical masks. That said, since respirators will circulate CO2 breathed out and hence cause dizziness with extended use. Therefore, it is NOT recommended to wear an N95 for long hours in general, but ALWAYS put on the right mask when necessary.

Also make sure to get the right size for proper fitting and check for the shelf-life or expiry date.

Surgical masks

There are a number of commonly tested aspects for surgical masks: fluid resistance, breathing resistance, filter efficiency and flammability.

ASTM F2100, a well-recognized de facto standard that we will introduce later, tests for the following four aspects

  • Fluid Resistance, mmHg (ASTM F1862)
  • Bacterial Filtration Efficiency (BFE), 3 micrometer (“micron”, µm)(ASTM F2101)
  • Particle Filtration Efficiency (PFE), 0.1 micrometer (ASTM F2299)
  • Differential Pressure (”Delta P”; measures breathing resistance)

Note that many masks specify only the more lenient BFE or ASTM F2101, but not PFE (ASTM . We should look for ASTM F2100 for better protection.

Another less tested but probably much concerned criteria is Virus Filtration Efficiency (VFE). It tests for the retention rate of E. coli-infecting bacteriophage virus. While the size of an ΦX174 bacteriophage is only 32 nm (0.0032 micron), VFE method also generates 3-micron aerosol as BFE, but not testing for freely viral particles in air. Surgical masks cannot filter out free nanoparticles anyway, so the presence of VFE value is not an absolute requirement.

The 2019 novel coronavirus (2019-nCoV) has a diameter of 60 to 140 nm (Zhu, et al.). Below is a Transmission Electron Microscopy (TEM).

How is filtration efficiency tested?

For PFE ASTM F2299 standard, the filtration rate of challenge particle size of 0.1µm is tested at a flow rate of 1 cubic foot per minute (CFM) or 28.3 liters per minute (LPM).

For biological tests of BFE or VFE, bacteria or virus aerosol sprayed with specified flow rate across tested filter are collected in the 6-tier Anderson sampler. In each tier, an agar plate is set as medium to grow any bacteria not captured by the filter for BFE test, which will multiply and form visible “colonies”. For virus, the agar plate will contain bacteria to start with. Bacteriophage virus landed on the plate will lyse the bacteria and form clear zones called “plaques”. By comparing the colony/plaque counts of sample and control (no filter) experiments, we get the filtration efficiency rates.