Types of Masks

When a pandemic begins difficulties arise due to the lack of vaccines and ideal treatments for the disease, and, therefore, protection barriers play an important role in controlling the spread and prevention of the disease. ^{Reference Sigua-Rodriguez, Bernal and Lanata18} Taking this into account, 2 types of masks have been described: surgical and conventional (or respiratory). The efficacy of these masks depends on their structure and filtering capacity. In this sense, respiratory masks guarantee better protection compared with surgical masks, both of which are disposable. ^{Reference Hirschmann, Hart and Henckel19}

Under the European standard, surgical masks are considered a medical device with an official nomenclature of the EN 14683 standard that classifies these masks as Type I, Type II, and IIR. The latter is classified as the most effective for presenting a microbial barrier and resistance to splashes, offering a filtration rate of around 80%. ^{Reference Sigua-Rodriguez, Bernal and Lanata18,Reference Lepelletier, Grandbastien and Romano-Bertrand20} They are designed for protection in only 1 direction to avoid the transmission of infectious agents carried by the user. They prevent the passage of microorganisms present from the inside out; therefore, the use of these masks is recommended for COVID-19 patients. ^{Reference Hirschmann, Hart and Henckel19,Reference Azap and Erdinç21,Reference Long, Hu and Liu22} However, these surgical masks do not ensure a good hermetic seal, and thereby allow particles to enter around the edges.

The classification of respiratory masks is independently certified by 2 major entities: the European Committee for Standardization (EN) and the National Institute for Occupational Safety and Health (NIOSH). Both entities guarantee a percentage of filtering capacity of particles that measure 0.3 microns in diameter. Respiratory masks must have multiple layers of polypropylene and electrostatic charge, providing adequate protection in 2 directions; that is, they are able to filter both incoming and outgoing air and must be resistant to liquid spray, blood splatter, and of other bodily fluids. Likewise, masks are considered to be effectively adjusted when a hermetic seal is achieved on contact with the skin. ^{Reference Liao, Xiao and Zhao17,Reference Hirschmann, Hart and Henckel19,Reference Azap and Erdinç21,23,Reference Boškoski, Gallo and Wallace24}

The European standard EN 149: 2001 establishes 3 categories or levels of protection for respiratory masks according to their filtering facepiece (FFP) parts, and these are divided into FFP1, FFP2, and FFP3, with a particle filtration capacity of 0.3 microns of 80%, 95%, and 99%, respectively. ^{Reference Hirschmann, Hart and Henckel19,Reference Lepelletier, Grandbastien and Romano-Bertrand20} On the other hand, the NIOSH establishes 9 respirator classifications, all with a particulate filtering capacity combining the respirator series (N, R, or P) and the level of efficacy (95%, 99%, 100%). The first part of the respirator classification indicates the resistance of the filter to degradation when exposed to oil-based aerosols, whereas the N series is for use in particulate environments and oil-free aerosols, and the R and P series are for use in particulate environments with and without oil. The number determines the filtering capacity of 0.3-micron particles, being measured in percentages of 95%, 99%, and 100%. ^{Reference Liao, Xiao and Zhao17,23,Reference Howard25}

The use of N95 masks has been considered a US standard administered by the NIOSH. These masks are designed to protect users from air particles, including aerosols, ^{Reference Offeddu, Fu and Fong26} with a particle filtration capacity of 0.3 microns of 95% and have less leakage in the face seal due to the tight fit to the user’s face ^{Reference Hirschmann, Hart and Henckel19,Reference Long, Hu and Liu22,Reference Boškoski, Gallo and Wallace24,Reference Smith, MacDougall and Johnstone27,Reference Ma, Shan and Zhang28} N99 masks have a 0.3-micron particle filtration capacity of 99%, while N100 masks provide 99.7% filtration protection. ²³

On the other hand, the use of KN95 respirators of Chinese origin are available in the dental market and comply with GB 2626-2006 regulations. These masks have a filtration capacity of 94-95% of particles with 4 overlapping layers, which are fused together to avoid the exit of particles from the carrier and the aspiration of aerosols or drops that may contain the virus. These respirators are considered to be functionally similar to the NIOSH-certified N series. ^{Reference Umer, Haji and Zafar12,23}

Mean Life of the Masks

An important approach to moderating mask wear and avoiding scarcity is to adopt strategies for mask reuse. ^{Reference Liao, Xiao and Zhao17,Reference Sigua-Rodriguez, Bernal and Lanata18} The CDC recommends 2 conservation strategies for respirators: extended use and limited reuse. ^{Reference Prakash, Rao and Nair29,Reference Kobayashi, Marins and dos Santos Costa30}

Surgical and respiratory masks are for single use per patient. However, conservation of these resources is imperative in this crisis. Thus, an alternative is use with multiple patients, ^{Reference Umer, Haji and Zafar12,Reference Lepelletier, Grandbastien and Romano-Bertrand20} but with strict biosecurity conditions, involving safeguards, evaluation of sealing, and mask integrity during use. A second surgical mask can be used on the respiratory mask to serve as direct protection against patient fluids, being discarded after use.

In the case of using only 1 respiratory mask while under great exposure to infectious droplets due to the aerosols, it is not recommended to reuse the same mask between patients because of a higher risk of contamination. However, this condition can be ameliorated with the use of a face protection mask or the use of 2 masks, which will allow the second protective mask to be discarded and the facial mask disinfected, thereby preserving the respiratory mask. ^{Reference Umer, Haji and Zafar12,Reference Lepelletier, Grandbastien and Romano-Bertrand20}

The second form of conservation of respiratory masks is limited reuse, which is the removal of the mask after each patient with restrictions limiting the number of uses. However, this requires strict validation regarding cleaning, sterilization, and functional performance. ^{Reference Prakash, Rao and Nair29} It is important to consider that the duration of use for surgical masks should not exceed 4 h and 8 h for FFP masks. ^{Reference Hirschmann, Hart and Henckel19,Reference Lepelletier, Grandbastien and Romano-Bertrand20,Reference Bein, Bachmann and Huggett31}

In the literature, few studies have evaluated the reuse of masks. One study evaluated health policies of 27 countries (in Europe and the Americas), finding that widespread use and limited reuse of masks is allowed in 10 countries. On the other hand, more than 60% of the countries do not recommend 1 of these 2 strategies. ^{Reference Kobayashi, Marins and dos Santos Costa30}

How to Use

In 1 of its publications, the CDC recommends how to use and dispose of respiratory masks. ^{Reference Ather, Patel and Ruparel32}

Sterilization

Given the shortage of personal protective equipment (PPE), sterilization and disinfection methods have been determined to prolong the effectiveness of the masks for the prevention of virus transmission. It is important that the sterilization treatment does not deteriorate the material of the respiratory mask, which would decrease the filtering power against infectious pathogens. The CDC has recommended different chemical, radioactive, and physical sterilization methods. ^{Reference Boškoski, Gallo and Wallace24}

Different decontamination strategies, such as sterilization by exposure to ultraviolet germicidal irradiation (UVGI), ethylene oxide, or vaporized hydrogen peroxide, have been shown to be effective in maintaining adequate protective function. ^{Reference Kobayashi, Marins and dos Santos Costa30,Reference Perkins, Villescas and Wu33-Reference Friese, Veenema and Johnson36}

In 1 study, different common sterilization methods for masks were performed including heat treatment, cabinet UVGI sterilizer, steam, alcohol solutions, and chlorine-based solutions. The last 2 methods caused degradation of the static charge required by the FFR. On the other hand, dry heat in the presence of humidity at 100°C preserved the characteristics of the N95 respirator, limiting temperature and humidity for 30 min. This study determined that the best sterilization method was UVGI (254 nm, 8W) with 3.6 J/cm². ^{Reference Liao, Xiao and Zhao17}

UVGI uses UV-C radiation to inactivate microorganisms causing damage to deoxyribonucleic acid (DNA), preventing replication. Several studies have demonstrated the efficacy of UVGI in reducing contamination by drops applied to the N95 respirator, with effective destruction of SARS-CoV and Middle East respiratory syndrome coronavirus (MERS-CoV) ^{Reference Boškoski, Gallo and Wallace24,Reference Hamzavi, Lyons and Kohli37,Reference Nogee and Tomassoni38} However, the use of UVGI for the destruction of this new virus requires further studies to establish the exact exposure or light intensity required, as 3M reports have shown that UVGI treatments damage the mask, while other studies show that this technique had no impact on the masks. ^{Reference Liao, Xiao and Zhao17}

Combination strategies have been studied, including the use a solution of sodium hypochlorite with microwave-generated steam UVGI, demonstrating effective sterilization with a reduction of multiple registrations of MS2 bacteriophage virus from masks. This virus is 7-10 times more resistant to aerosolization and ultraviolet light than the coronavirus. ^{Reference Turgeon, Toulouse and Moineau39}

The susceptibility of the virus to gamma irradiation has shown good disinfecting ability by penetration of all the layers of the respirators. However, the use of ionizing radiation is limited because gamma radiation cannot be performed in a health-care center, requiring the need for transportation to another location entailing a risk for the personnel transporting the masks. ^{Reference Prakash, Rao and Nair29}

On the other hand, the CDC reports that some autoclave methods at 160°C in dry heat, 70-75% isopropyl alcohol and soapy water can deteriorate the filter of the respirators, and consequently allow the access of particles through the mask. ^{Reference Boškoski, Gallo and Wallace24}

For mask sterilization, certain requirements must be taken into account, such as the efficacy against the SARS-CoV2 organism, avoiding damage to the respirator filtration changes in the physical characteristics of the respirators, and ensuring biosafety to the persons who must wear the respirator mask. ^{Reference Prakash, Rao and Nair29}

Proposed Alternatives of Masks for Dentists

According to the information obtained from COVID-19, the transmission routes are direct or indirect contact with contaminated patients, saliva drops, ^{Reference Umer, Haji and Zafar12} and large aerosol particles suspended in the air (up to 1 m away), which remain present for a short period of time. ^{Reference Liao, Xiao and Zhao17,Reference Azap and Erdinç21,Reference Ferioli, Cisternino and Leo40} In dentistry, there is direct contact with the patient through fluids, such as blood or saliva, and many dental treatments generate aerosols. It should also be taken into account that the SARS-CoV2 virus has affinity for the angiotensin 2 converting enzyme receptor (ACE-2), which is found in the respiratory tract and the salivary gland ducts, producing a high viral load in saliva. ^{Reference Umer, Haji and Zafar12,Reference Liao, Xiao and Zhao17}

Different models of masks are available, making it difficult to choose the most suitable type of respirator for dental care. Therefore, according to past recommendations, it is recommended to perform fit tests of masks for health personnel to determine the best fit and hermetic seal according to facial dimensions, ethnic origin, and appearance of the fit.

The literature recommends that surgical masks should not be used as a substitute for respiratory masks. ^{Reference Hirschmann, Hart and Henckel19,Reference Smith, MacDougall and Johnstone27,Reference Coulthard41} The use of an N95, FFP2, and FFP3 mask is recommended for personnel working with aerosols. Filtration is achieved by combining a polypropylene network and electrostatic charge, ^{Reference Azap and Erdinç21} thereby explaining their good protective effect against aerosols, and the reason why respirators should not be used by the general public. ^{Reference Long, Hu and Liu22,Reference Bein, Bachmann and Huggett31,Reference Zhou, Yue and Mu42}

On analyzing the filtering capacity according to the mask classification, the N95 has shown a similar filtration efficacy to that of FFP2 or KN95. ^{Reference Boškoski, Gallo and Wallace24} Regarding mask sterilization, although it is necessary to determine the exact UVGI doses for mask sterilization against the SARS-CoV-2 virus, this strategy provides a possibility to extend the use of the limited supply of respirator masks against COVID-19, being both profitable and accessible. ^{Reference Perkins, Villescas and Wu33}

A major problem with respirator masks is discomfort and the generation of humidity inside, decreasing air permeability. Therefore, a super absorbent polymer (SAP) has been designed for use in respirator masks. This harmless material absorbs large amounts of liquid is used in baby diapers, sanitary napkins, and incontinence pads. An absorbent layer or SAP pad is cut according to the shape and size of the respirator and finally placed inside the respirator. The SAP pad helps to quickly absorb exhaled moisture, leading to a longer mask life cycle and providing greater comfort to the professional. ^{Reference Song, Xie and Zhu43}