Facemasks – challenges during pandemics

Behnam Pourdeyhimi PhD
Executive Director, The Nonwovens Institute

Clearly, we are all being impacted by the reality of the coronavirus (COVID-19) global pandemic.  Everyone receives minute by minute accounts of how quickly the virus is spreading throughout the World.  Of course, this is not the first time we have faced such challenges.

The natural reaction has been to panic, stockpile food, hand sanitizers, and the like and also facemasks.

There are various types of masks available on the market.  The N95 or the N99 are the most well known as are surgical masks.  These masks are quite different from one another.

N95 respirators and surgical masks are considered as personal protective equipment (PPE) that are used to protect the wearer from airborne particles, and from contaminating liquid in the case of surgical masks.  The N95 and N99 respirators are regulated by the Center for Disease Control and Prevention (CDC), National Institute for Occupational Safety and Health (NIOSH) and Occupational Safety and Health Administration (OSHA), and must adhere to the strict performance guideline established by these organizations.

CDC does NOT recommend the use of N95 masks by the general public.  There are good reasons for this recommendation.  CDC states that:

“For the general American public, there is no added health benefit to wear a respiratory protective device (such as an N95 respirator), and the immediate health risk from COVID-19 is considered low.”

An N95 respirator is a respiratory protective device designed to achieve a very close facial fit and very efficient filtration of airborne particles. The edges of the respirator are designed to form a seal around the nose and mouth.

Surgical N95 Respirators are commonly used in healthcare settings and are a subset of N95 Filtering Facepiece Respirators (FFRs), often referred to as N95s.

The similarities between surgical masks and surgical N95s are:

  • They are tested for fluid resistance, filtration efficiency (particulate filtration efficiency and bacterial filtration efficiency), flammability, and biocompatibility.
  • They should not be shared or reused.

The N95 fitted masks may have a higher pressure drop than surgical masks.  Therefore, CDC recommends that people with chronic respiratory, cardiac, or other medical conditions that make breathing difficult should check with their health care provider before using an N95 respirator because the N95 respirator can make it more difficult for the wearer to breathe.

Some models have exhalation valves that can make breathing out easier and help reduce heat build-up. Note that N95 respirators with exhalation valves should not be used when sterile conditions are needed.

Also, N95 respirators are not designed for children or people with facial hair, i.e., a proper fit cannot be achieved on them.  Consequently, the N95 respirator may not provide full protection for children and people with facial hair.

The single-use, disposable respiratory protective devices are used and worn by health care personnel during procedures to protect both the patient and health care personnel from the transfer of microorganisms, body fluids, and particulate material. These surgical N95 respirators are class II devices regulated by the FDA, under 21 CFR 878.4040, and CDC NIOSH under 42 CFR Part 84.

N95s respirators regulated under product code MSH are intended to prevent specific diseases or infections or filtering surgical smoke or plumes.

N95 Respirator filters must meet stringent certification tests (42 CFR Part 84) established by NIOSH. The NIOSH tests use what are considered “worst case” parameters, including [CDC]:

  • A sodium chloride (for N-series filters) or a dioctyl phthalate oil (for R- and P-series filters) test aerosol with a mass median aerodynamic diameter particle of ~ 0.3 µm, which is in the MPPS-range for most filters
  • Airflow rate of 85 L/min, which represents a moderately-high work rate
  • Conditioning at 85% relative humidity and 38°C for 24 hours prior to testing
  • An initial breathing resistance (resistance to airflow) not exceeding 35 mm water column (~343 Pa) height pressure and initial exhalation resistance not exceeding 25 mm water column (~245 Pa) height pressure
  • A charge-neutralized aerosol
  • Aerosol loading conducted to a minimum of 200 mg, which represents a very high workplace exposure
  • The filtering efficiency cannot fall below the certification class level at any time during the NIOSH certification tests

To learn more about Disposable N95 durability, follow this link.

surgical mask is a loose-fitting, disposable device that creates a physical barrier between the mouth and nose of the wearer and potential contaminants in the immediate environment. These are often referred to as face masks, although not all face masks are regulated as surgical masks. Unlike the N95 respirators, the edges of the mask are not designed to form a seal around the nose and mouth.

There are numerous standards for testing surgical masks.  Their 0.3 micron efficiency is significantly lower than that of the N95 but they offer fluid barrier properties.  While the ASTM F2299 specifies various charge-neutralized particle sizes, the FDA guidance recommends unneutralized 0.1 micron particles.  The flow rate and the test area are undefined while the face velocity is given in range from 0.5 to 25 cm/s.  This is problematic in that one can choose the lowest face velocity which will yield a higher capture efficiency and a lower pressure drop.

A summary of the test methods is shown below.

USA: ASTM F2100-19 STANDARD SPECIFICATION FOR PERFORMANCE OF MATERIALS USED IN MEDICAL FACE MASKS

EUROPE: EN 14683:2019 MEDICAL FACE MASKS – REQUIREMENTS AND TEST METHODS

ASTM F2100-19 EN 14683:2019 Barrier Level

Level 1

Level 2

Level 3

Type I

Type II

Type IIR

Barrier testing

BFE%
ASTM F2101, EN 14683

≥95

≥98

≥98

≥98

PFE%
ASTM F2299

≥95

≥98

Not Required

Synthetic Blood
ASTM F1862, ISO 22609

Pass at
80 mmHg

Pass at
120 mmHg

Pass at
160 mmHg

Not Required

Pass at
≥16.0 kPa
(≥120 mmHg)

Physical Testing

Differential Pressure
EN 14683

<5.0 mmH2O/cm2

<6.0 mmH2O/cm2

<40 Pa/cm2

<60 Pa/cm2

Safety Testing

Flammability

16 CFR Part 1610

Class 1 (≥3.5 seconds)

See European Medical Directive
(2007/47/EC, MDD 93/42/EEC)

Microbial Cleanliness

ISO 11737-1

Not Required

≤30 cfu/g

Biocompatibility

ISO 10993

510 K Guidance recommends testing to ISO 10993

Complete an evaluation according to ISO 10993

Sampling

ANSI/ASQC Z1.4

ISO 2859-1

  • AQL 4% for BFE, PFE, Delta P
  • 32 masks for Synthetic Blood
    (Pass= ≥29 passing, Fail= ≤28 passing)
  • 14 masks for Flammability
  • Minimum of 5 masks up to an AQL of 4% for BFE, Delta P and Microbial Cleanliness
  • 32 masks for Synthetic Blood
    (Pass= ≥29 passing, Fail= ≤28 passing)

The technology used in almost all masks for filtration is a meltblown fabric that is electrostatically charged.  A nonwoven filter media that uses a combination of mechanical structure and electret charge provides a means of achieving high initial efficiency (due mostly to the charge) and sustained high efficiency (mostly due to the structure).

The electrostatic charge is a critical feature of the mask in that it boosts the filtration efficiency (mechanical) from about 30 to 35% to over 95%.  While some media may exhibit efficiencies higher than 35% when tested as received, the higher efficiency is likely due to process-induced charging.

Almost all meltblown fabrics are made from polypropylene and are rather fragile and cannot be reused, laundered or re-sterilized.  They are often protected by layers of spunbond nonwovens made up of larger fibers that provide protection for the meltblown filter layer.  Most meltblown filters weigh about 20 to 30 g/m2 and the spunbond may also be similar in weight.

Any post-treatment like washing and sterilization may lead to a potential loss of electrostatic charge and reduces the ability of the filter to protect the wearer.

The supply chain is quite simple.  These include:

  1. Meltblown fabric manufacturers
  2. Spunbond fabric manufacturers
  3. Toll converters who convert the meltblown and spunbond fabrics into N95 molded masks or surgical masks.

Very few companies are vertically integrated to produce the base materials and the masks.

The challenge faced in the US and globally today remains the shortage of both meltblown fabrics (the most critical component of the mask) and the converting capacity.