“In the pandemic economy, face masks are like bars of gold. Hoarders are hoarding them. Governors are bartering for them. Hospital workers desperately need them.“ — Michael Schulman, New Yorker
The N 95 Mask
An N-95 respirator (mask) is a respiratory protective device designed to achieve a very close facial fit and efficient filtration of airborne particles. In contrast to surgical face masks, which provide only one‑way protection (by capturing their own droplets and protecting those around), N‑95 respirators are tightly fitting masks that seal off the wearer’s face and work in a bidirectional sense, in particular for the protection of the wearer. When properly fitted, selected, used, and maintained, particulate‑removing respirators have been demonstrated to reduce the amount of aerosols that are inhaled.
The N-95 mask (American classification system) is so‑called because it can remove 95% of all particles with a diameter >0.3 μm, and it is comparable to a Filtering Face piece Respirator2 (FFP 2) mask (European classification system.)
Type of mask Filtration efficiency (%)
American classification system
N 99 ≥99
European classification system
FFP 1 ≥80
FFP 2 ≥94
FFP 3 ≥99
Source: Handbook of Respiratory Protection by Craig E. Colton.
A properly fitted N-95 will block 95% of aerosolized particles, down to three‑tenths of a micron in diameter, from reaching the wearer’s face. In contrast, surgical masks, designed to protect patients from a surgeon’s respiratory droplets, are not effective at blocking particles smaller than 100 μ.
An older, pre‑SARS‑CoV‑2 Chinese study in 2013 half found that twice as many health workers (17%), contracted a viral respiratory illness (like influenza or rhinovirus), when wearing a surgical mask while treating sick patients, compared with 7% of those who continuously used an N-95.
Researchers from 2 hospitals in Seoul, Korea, instructed 4 COVID‑19 patients admitted in negative pressure isolation rooms to cough five times while wearing cloth and surgical masks. Measuring the median viral load, they found that swabs from the outer mask surfaces were positive for SARS–CoV‑2 in both cloth and surgical masks. They concluded that surgical masks were ineffective in preventing the dissemination of SARS–CoV‑2 from the coughs of patients with COVID‑19 to the environment and external mask surface.
In contrast to inhaled droplets that get deposited in the upper regions of the respiratory tract and subsequently get expelled via nasal secretions or the muco-ciliary escalator, inhaled aerosols can penetrate into the depths of the lung and may be deposited in the alveoli.
WHY IS QUALITY CONTROL SO IMPORTANT?
The US regulators recently reported that significant numbers of imported and Chinese manufactured N-95 masks were being pushed into the market without meeting National Institute for Occupational Safety and Health (NIOSH) standards. Recent tests performed by
NIOSH on a batch of 67 different imported masks found that as many as 60% did not meet standards. Some filtered out as few as 35% of particles only, far short of the 95% it advertised. Many were found to have ear loops to secure them to the head instead of the headbands, which are essential to ensure a tighter fit.
WHAT IS A FIT TEST AND HOW IS IT PERFORMED?
The protection offered by a respirator or a mask is dependent not just on its filter capacity but even more crucially on the fit factor. A fit test is a way to assess if an N‑95 has been correctly applied and fitted. Those individuals who passed a fit test have been shown to have a better protection factor.
Fit tests are of two types:
Qualitative fit testing
It is a pass/fail test method that uses one’s sense of taste or smell or reaction to an irritant to detect leakage into the respirator facepiece. Qualitative fit testing does not measure the actual amount of leakage. Whether the respirator passes or fails, the test is based simply on us detecting leakage of the test substance into the face‑piece.
There are four qualitative fit test methods accepted by the Occupational Safety and Health Act. These include
isoamyl acetate (banana smell),
saccharin (sweet taste),
bitrex (bitter taste), and
irritant smoke, which can cause coughing.
Quantitative fit tests
This uses a machine to measure the actual amount of leakage into the facepiece and does not rely upon an individual’s response to stimuli. The respirators used during this type of fit testing will have a probe attached to the facepiece that is connected to the machine by a hose. Quantitative fit testing can be used for any type of tight‑fitting respirator.
WHAT ARE THE MEASURES FOR PROLONGING USE DURING THE CRISIS?
Because of shortages of N‑95 masks experienced by almost all hospitals, every attempt to prolong their life and use without compromising safety should be explored. Three measures that allow more prolonged use of N‑95 respirators have been looked at.
a. Extended use refers to the concept of wearing the same respirator for repeated close contact encounters with several patients, without removing the respirator between patient encounters. WHO recommends extended use of PPE, if necessary in the current crisis of acute shortage of PPE
b. Reuse refers to the practice of using the same N-95 respirator for multiple encounters with patients but removing it (“doffing”) after each encounter. The respirator is stored in between encounters to be later put on again (“donned”) prior to the next patient encounter.
CDC recommends that even if it is disposable, the same respirator can be reused by the same HCW provided, it maintains its fit and function and used in accordance with the local infection control policies. Limited reuse may also be considered as an option for conserving respirators in hospitals where they are in short supply during this pandemic. Extended use is preferred over reuse as it involves less risk of contact transmission
c. Decontamination to increase the life may compromise the fit, filtration efficiency, and breathability of disposable respirators as a result of changes to the filtering material, straps, nose bridge material, or strap attachments.
Decontamination and reuse are not recommended as the standard of care but may be considered at times of crisis. An ideal decontamination method must remove the viral threat, not affect laboratory performance (filter aerosol penetration and filter airflow resistance), should not compromise the fit or integrity of the various elements of the respirator, be harmless to the user. There is a lack of current data supporting the effectiveness of these decontamination methods specifically against SARS‑CoV‑2; hence, these respirators must be carefully handled even after decontamination.
The three most promising decontamination methods are:
Ultraviolet germicidal irradiation (UVGI)
Has a promising role in inactivating human respiratory viruses, including coronaviruses, on various models of N-95s. It has been shown to keep the antimicrobial efficacy >99.99% (for influenza, SARS‑CoV‑1 and 2 and MERS‑CoV) without compromising on fit even after 3 cycles of decontamination with UVGI. The levels of UVGI used to inactivate the virus is well below that which adversely affects the fit and filtration characteristics of N-95 masks; UVGI exposures of 1 J/cm2 are effective in decontaminating influenza virus on these masks and exposures as low as of 2–5 mJ/cm2 are capable of inactivating coronaviruses on surfaces. One study by Lowe et al. has used UVGI at a dose of 300 mJ/cm2, the dose being several folds the amount of exposure needed to inactivate SARS-CoV-2, thus providing a wide margin of safety against the virus. UVGI can be safely administered when appropriate safeguards are in place. There is uncertainty regarding how long a UVGI treated N-95 maintains properties to achieve an adequate fit, requiring health care providers to carefully inspect them before and after each reuse for any visible damage.
Vaporized hydrogen peroxide (VHP)
VHP decontamination for a single cycle does not significantly affect filter aerosol penetration and airflow resistance. The filtration efficacy was found to be more than 99.99% for the tested fungal spores. The fit was unaffected even after 20 cycles of treatment. The time for decontamination is less than an hour. It may cause tarnishing of metallic nosebands and straps of the respirator after about 20 cycles. Because of the acute shortages of N‑95s in this pandemic, the FDA, has recently approved and fast‑tracked a Columbus‑based company Battelle’s new technology, which uses vapor phase hydrogen peroxide to disinfect N-95 masks, conceding to the company’s request for permission to clean up to 80,000 masks a day. Their technology allows the masks to be reused about 20 times without significant compromise on their filtration capacity. The company has developed this technology years ago, but the present COVID 19 pandemic has forced the company to ramp up their machines to disinfect masks on a larger scale, which is truly the need of the hour.
Moist heat (60°C and 80% relative humidity) caused minimal degradation in the filtration and fit performance of the tested respirators against two strains of influenza viruses. Heimbuch disinfected FFPs contaminated with H1N1 using moist heat and achieved a 99.99% reduction in virus load. An important limitation of the moist heat method is the uncertainty of the disinfection efficacy for various pathogens, including SARS‑CoV‑2.
Currently, what is being used in some centers due to shortages, but cannot be recommended is the use of ethylene oxide (EtO) and autoclaving. While EtO does not compromise the filtration efficacy, it may be carcinogenic and teratogenic and hence is not recommended. Autoclaving and the use of disinfectant wipes are not recommended as they may alter respirator performance.
Practical points to be considered during extended use/limited reuse of (N-95)
Discard N-95 respirators following aerosol generating procedures
Discard N-95 respirators contaminated with blood or respiratory secretions
Consider use of a cleanable face shield over an N-95 respirator
Hang used respirators in a designated storage area or keep them in a clean, breathable properly labeled container such as a paper bag between uses
Follow the employer’s maximum number of donning (or up to five if the manufacturer does not provide a recommendation)
Discard any respirator that is obviously damaged or becomes hard to breathe through
Another practical strategy is to issue five respirators to each HCW caring for COVID-19 patients with advice to wear 1 respirator each day and store it in a breathable paper bag at the end of each shift. The order of respirator use should be repeated with a minimum of 5 days between each respirator use