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Masks: How to choose, wear & dispose

Claire Chung
Jan 28 · 13 min read

[Updated on Jan 29 with WHO recommendation and more compliance test information to help choosing]

ASTM F2100? N95? Get the right surgical masks and respirators and use them properly to fight against the Wuhan 2019 Novel Coronavirus: (2019-nCoV).


  • 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.

Choosing the right mask

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) image.

Visualization of 2019-nCoV with Transmission Electron Microscopy. Source: The New England Journal of Medicine

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.

BFE/VFE testing experiment setting. Source:
BFE bacteria colony count test. Source:
VFE plaque count test. Source:

Surgical mask compliance standards

ASTM F2100
The American Society for Testing and Materials (ASTM) International provides a most commonly used standard ASTM F2100 recognized by the US FDA. The number behind the hyphen, e.g. 19 in ASTM F2101–19, is the version number, which usually is the year the standard being set or revised, i.e. 2019 in this case.

There are three levels of protection, generally depending on the amount of fluid, spray and/or aerosols to be protected against.

ASTM F2100–11 Standards for masks, Levels 1–3. Source: Infection Control Products

Level 3 differs from Level 2 by resisting higher fluid resistance, so a quicker spurt of fluid can be resisted.

All three levels must have Class I flammability, i.e. withstand exposure to a burning flame (within a specified distance) for three seconds.

According to WHO guidelines against 2019-nCoV, ASTM F2100 Level 2/3 or equivalent are recommended.

EN 14683

Here is a comparison between the European Standard and the US ASTM F2100 standard. WHO recommends EN14683 Type IIR or equivalent for a splash resistance of at least 120 mmHg. In addition, EN14683 place requirement on the the microbial cleanliness of the mask itself, i.e. the masks must be sterilized at production.

EN14683 & ASTM F2100 comparison. Source: Nelson Labs


KF94 is a Korean standard by the Korean Food and Drug Administration (KFDA) protocol similar to the European protocol. KF stands for “Korean Filter” and 94 means at least 94% of particles are filtered.

PM2.5 is *NOT a standard*, but refers to particles of 2.5 micron or smaller. Many face mask products with PM2.5-proof claim are designed against dust particles under heavy air pollution, in smoggy weather, dust storm, or during pollen seasons.

N95 / FFP2 respirators

For respirators, WHO recommends N95 (US NIOSH) or FFP2 (EU) standard or higher.

Respirators are often known as the “piggy nose” in Chinese for the common cone style model, but there is also the “duckbill” shape. However, not all cone-shaped respirators are N95-certified / FFP2-certified. There are many types of respirators to protect against different hazards.

N95 definition

N95 is a rating that describes the usable environment and ratio of particles filtered devised the National Institute of Occupational Safety and Health (NIOSH).

The initial letter describes the environment where the respirator is effective:

- N (Not resistant to oil): Used in particulate environments free of oil aerosols
- R (Resistant to oil): Used for oil and non-oil particles with time use limitations specified by NIOSH
- P (Oil-Proof): Used for oil and non-oil particles with time use limitations specified by manufacturer

NIOSH-approved filters are rated as N95/R95/P95/N99/P99/N100/P100. R and P series are usually used in industrial or construction settings. The number 95/99/100 (99.97%) indicates the filtration efficiency.

Why are N95 respirators often recommended?
The CDC Guidelines for Isolation Precautions in Hospitals written before SARS recommends protection against airborne diseases such as tuberculosis by wearing a respirator at least as protective as a fit-tested N95 respirator.

Types of N95 respirators
For N95, also notice that there are medical and industrial versions, where the former is FDA cleared. The 3M 8210 model recently famous in Hong Kong is actually not designed for healthcare use.

Source: 3M customer response for N95 respirator comparison

Here is a quick reference guide. The major differences are:

  • Medical vs Non-medical
  • Valved vs Unvalved
  • Flat-fold vs Cone Style
  • Normal size vs Small

Below is a comparison table for different healthcare use N95 respirators (1804, 1860, 1870+) with their small-sized counterpart with model number suffixed with an “S”.

Source: 3M Healthcare Respirators


The 3M Health Care Particulate Respirator and Surgical Mask models 1804/1804S, 1860/1860S, 1870, 1870+ have an established 5 year shelf life when respirators are stored in their original packaging within climatic conditions ranging from -4 °F (-20 °C) to +86 °F (+30 °C) and not exceeding 80% RH.

Test for N95 respirators

The certification test method is described in this excerpt from the study “Effectiveness of N95 respirators for nanoparticle exposure control (2000–2016): a systematic review and meta-analysis” by Ntlailane, et al. in 2019.

The certification test for N95 respirators is performed following ‘worst case’ scenarios including storage at 85% relative humidity and 25 °C temperature conditions for 25 h prior and charge neutralisation of the challenge aerosol. The sodium chloride (NaCl) test aerosol has a mass median diameter (MMD) of 238 nm, a mass median aerodynamic diameter (MMAD) of 347 nm and a count median diameter of 0.075 ± 0.020 μm and tested at a flow rate of 85 l/min. The respirator’s efficiency is assessed as either initial penetration or maximum penetration, depending on a loading curve obtained after it is challenged with 200 mg of the NaCl for the first 3 sample filters. This test is performed with a photometer.

Also note that the values of tests for surgical masks and respirator filters like N95 cannot be directly compared. Below is quoted from the study “A Comparison of Facemask and Respirator Filtration Test Methods” (Rengasamy, et al., 2017).

PFE was measured using 0.1 µm size polystyrene latex particles and BFE with ∼3.0 µm size particles containing Staphylococcus aureus bacteria. VFE was obtained using ∼3.0 µm size particles containing phiX 174 as the challenge virus and Escherichia coli as the host. Results showed that the efficiencies measured by the NIOSH NaCl method for “N95 FFRs” were from 98.15–99.68% compared to 99.74–99.99% for PFE, 99.62–99.9% for BFE, and 99.8–99.9% for VFE methods. Efficiencies by the NIOSH NaCl method were significantly (p = <0.05) lower than the other methods. SMs [surgical masks] showed lower efficiencies (54.72–88.40%) than “N95 FFRs” measured by the NIOSH NaCl method, while PFE, BFE, and VFE methods produced no significant difference.

Can I use a respirator with an exhalation valve?
Yes and No, and personally not recommending.

According to the US Centers for Disease Control and Prevention (CDC), “usually yes”, for self-protection if you are sure you are not infected.

An exhalation valve reduces excessive dampness and warmth in the mask from exhaled breath.

The valve opens to release exhaled breath and closes during inhalation so that inhaled air comes through the filter. Health care workers may wear respirators with exhalation valves unless the patient has a medical condition (such as an open wound) for which a health care worker would normally wear a surgical mask to protect the patient. Similarly, respirators with exhalation valves should not be placed on a patient to contain droplets and prevent spread of infectious particles; surgical masks are adequate for this purpose.

However, the novel coronavirus is known to have a symptomless incubation period as long as 14 days. In high risk area where surgical mask is not sufficient for protection, I would personally recommend a respirator without exhalation valve. In fact, all healthcare versions of the 3M N95 products are unvalved.


The European Standard (EN 149:2001) classifies FFRs into three classes: FFP1, FFP2, and FFP3 with corresponding minimum filtration efficiencies of 80%, 94%, and 99%.

Using a face mask properly

Surgical masks

Surgical mask is the default choice recommended for public daily use. Most qualified surgical masks contain three layers.

Surgical mask layers and functions. Source: Centre for Health Protection, HKSARG

Structure of 3-layer surgical masks:
- Outer layer: Waterproof against droplet spray (usually colored)
- Middle layer: Particle filter to block pathogens
- Inner layer: Water absorbent to absorb breathed out moisture

The stiff edge with a metal strip is on top so the mask can be moulded to fit against the nose.

Note that while many surgical mask are coloured on the outside to aid recognition, the colour does not guarantee the type and quality.
There exist white surgical masks with good specifications and poor unqualified masks can be coloured. We should check the structure and specifications, and purchase only from trusted sources.


  • Sanitize hands before wearing
  • Take a mask and check that it is intact
  • Determine the right side of the mask: Waterproof on the outside (usually coloured)
  • Hold the mask by earloops. Place an earloop around each ear. (Check this for tie- or band-type, not recommended if unsure about the usage though)
  • Pinch the stiff edge to mould the metal strip on top against the nose to fit the mask tightly
  • Pull the bottom of the mask over your mouth and chin


  • Hold both of earloops. Gently lift and remove the face mask


  • Throw the mask into covered trash
  • Only touch the earloops and avoid the outside of the mask which should be considered as contaminated
  • Sanitize hands

Demonstration videos

How to wear an N95 respirator

How to wear and remove N95 mask. Source: SingHealth (Singapore General Hospital)

Fit test for respirators

N95 respirators should be fit-tested. Here is a video training provided by 3M Healthcare.

Can I use multiple layers of surgical masks?

A team in the Chinese University of Hong Kong (CUHK) conducted a study to predict whether wearing multiple surgical masks at a time can better protect healthcare staff from SARS, and concluded that “no combination of multiple surgical masks was able to meet the requirements for a respirator” and that “multiple surgical masks will reduce the number of viruses inhaled, but whether the degree of reduction is sufficient to produce significant protection is unknown”.

Let’s look at the result presented below in Figure 1 of the literature, wearing multiple masks at a time only slightly increases the median filtration capacity from 2.7 to 3.8 or 5.5 for wearing two or five masks, where a minimum of 100 should be achieved to compare with a half face respirator.

Statistical prediction of filtration capacity wearing different numbers of masks. Source:

In my humble opinion, wearing multiple masks at a time have the following disadvantages:

  1. Disrupts fitting especially with body movement, which may cause leakage
  2. Makes breathing more difficult. The wearer may inadvertently wants to adjust the mask often and cause leakage or contamination
  3. Traps moisture which may also trap dirt and incubate pathogens like bacteria
  4. The extra cost may deter wearers from replacing the mask timely.
  5. Increases difficulty in removal and hence increasing risk of contamination

These factors together may in turn increase risk for the general public.

In scenarios with increased risk of infection, a proper N95 respirator is highly recommended.

Other hygiene measures

I also created a website, now in progress of updating content, hoping to help each other stay informed. Coming posts intended include HKU press release on Jan 27 (of evident self-sustaining human-to-human transmission, epidemic projection and call for public health intervention measures), hand hygiene methods, and bioinformatics 101: genome sequencing and how to understand the frequently appearing terms in news.

Stay healthy!


Claire Chung

Written by

I admit I should be writing my thesis instead =w=

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