The Ultimate Guide to COVID-19 Face Masks

The Ultimate Guide to COVID-19 Face Masks

  • This page was last updated at July 14, 2024.

Respirators are masks that filter and/or purify particles in the air. They are used in many different fields that are exposed to dangerous air particulates like dust, allergens, bacteria, viruses, and chemical fumes.

Respirators that are explicitly air-purifying are commonly called face filtering respirators (FFR). There are two separate accrediting bodies +that certify these types of masks.

They are:

  • The National Institute for Occupational Safety and Health (NIOSH) and
  • European Norms (ENs).

NIOSH-approved respirators have their mark of approval, and the EN- certified marker is “CE” (Conformité Européen’).

  • NIOSH requires a minimum of 95 and 99.97% efficiencies for N95 and P100 FFR
  • EN requires 94 and 99% efficiencies for FFRs, class P2 (FFP2) and class P3 (FFP3)

    Testing and Certifying Face Filtering Respirator Masks

    Researchers Renegasamy, et al. did an independent laboratory assessment of eight models of NIOSH-approved N95 and P100 and CE-marked FFP2 and FFP3 respirators [6]. They used methods similar to NIOSH respirator certification test methods to mimic the procedures for control. The research team concluded, 

    “Particle penetration data obtained in this study showed that the eight models of NIOSH-approved N95 and P100 and CE-marked FFP2 and FFP3 respirators used in this study provided expected levels of laboratory filtration performance against nanoparticles [6].”

    That is to say, when respirators are properly certified and legitimately manufactured, they perform as they should. Not all classes of respirator masks work to the same degree of purification, but it is important that they are properly certified.

    Protection Levels for Surgical Masks

    The classification of respiratory face masks is important to understand. When people like healthcare workers cannot get adequate protection from a mask it can have major adverse effects.

    Surgical masks block large particles like droplets, splashes, and sprays that could contain chemicals, viruses, or bacteria. These masks cover both the nose and mouth but are mainly constructed of a three-ply fabric. They do not form a tight seal against the face. Surgical masks do not offer protection from airborne infectious diseases.

    It is still beneficial to block possible large droplets as a means of spreading bacteria and viruses, however, a surgical mask only blocks 0.04 to 1.3𝜇m sized particles. That is eight to twelve times less than an N95 mask. Lee, S. A., et al.  note that both surgical masks and N95 masks have been found to be equivalent in blocking low concentrations of the influenza virus [3]. The researchers note that. 

    “[Surgical masks] have been cleared by the Food and Drug Administration for sale in the US, while, in the UK, they must first comply with the Medical Devices Directive (MMD 93/42/EEC) and be CE marked. However, because N95/FFP2 respirators or above may be in short supply during a pandemic—or not available in many countries—it is important to know the protection efficiency of surgical masks [3].”

    How to Decode Mask Numbers and Letters

    According to Lee, S. A., et al. [3], “NIOSH-approved N95 filtering facepiece respirators or higher are recommended for healthcare workers against airborne infectious diseases such as Ebola.”  But how can you tell which type of mask provides that level of protection?

    Respirators and masks have special codes that are used for their names and are printed on the masks or packaging. NIOSH and the European Standard use different codes that correspond to different levels of protection. Here is a summary of those codes and what they mean:

    NIOSH Respirator Categories

    NIOSH classifies particulate filtering facepiece respirators into nine categories: N95, N99, N100, P95, P99, P100, R95, R99, and R100.

    • Masks with the prefix “N” are not resistant to oil, these respirators cannot protect from oil droplets.
    • Masks with the prefix “R” are somewhat resistant to oil.
    • Masks with the prefix “P” are strongly resistant to oil, these respirators can be used for protection against non-oily and oily aerosols.
    • The numerical designation 95 corresponds to a minimum filtration efficiency of 95%
    • The numerical designation 99 corresponds to a minimum filtration efficiency of 99%
    • The numerical designation 100 corresponds to a minimum filtration efficiency of 99.97%

      European Standard (EN 149:2001) CE Respirator Categories

      The European Standard classifies FFRs particulate filtering facepiece respirators into three categories: FFP1, FFP2, and FFP3. 

      • FFP1 corresponds to a minimum filtration efficiency of 80%
      • FFP2 corresponds to a minimum filtration efficiency of 94%
      • FFP3 corresponds to a minimum filtration efficiency of 99%

      FFP2 respirators are approximately equivalent to N95 respirators. They are recommended for use in preventing airborne infectious diseases in the US and some other countries.

      FFP3 respirators provide the highest level of protection and are the only respirator acceptable for protection against infectious aerosols in healthcare settings in the UK. 

      Currently, The Centers for Disease Control and Prevention (CDC) and the UK Health and Safety Executive (HSE) advise the use of N95 or higher respirators and FFP3 respirators against airborne infectious diseases in healthcare settings. Lee, S. A., et al. suggest that surgical masks may be an option when these respirators are scarce [3]. 

      Filtering Facepiece Respirator (FFR) Labels

      The CDC explains that individual filtering facepiece respirators are required to have the following markings:

      • Name of approval holder/manufacturer business name
      • NIOSH in block letters or the NIOSH logo.
      • NIOSH Testing and Certification approval number, e.g., TC-84A-XXXX.
      • NIOSH filter series and filter efficiency level, e.g., N95, N99, N100, R95, P95, P99, P100.
      • Model number or part number: The approval holder’s respirator model number or part number
      • NIOSH recommends the lot number and/or date of manufacture, however, this is optional

        Sample of a generic filtering facepiece respirator with appropriate markings. Image from NIOSH, 2018 [5].


        CDC Emergency Use Authorization (EUA) for Respirators

        The COVID-19 pandemic has taken the world by storm. High rates of hospitalization mean that protective equipment like masks is in short supply for healthcare workers. On April 3, 2020, the CDC added language to the FDA’s Emergency Use Authorization (EUA) to respond to the crisis.

        In this language, the CDC authorized additional N95 respirator alternatives. This applies to certain NIOSH-approved respirator models to be used in healthcare settings. They also temporarily suspended the need to perform fit tests when issuing employees' respirator masks.

        The CDC’s EUA plant breaks down responses by capacities. There are three levels of response: surge capacity, conventional capacity, contingency capacity, and crisis capacity. 

        Surge Capacity

        There are no distinguished criteria for “surge capacity,” however, it is generally marked by an approaching decrease in N95 mask supplies [4].

        “The ability to manage a sudden, unexpected increase in patient volume that would otherwise severely challenge or exceed the present capacity of a facility [4].”

        During surge capacity, steps are taken to conserve the use of N95 respirators and prioritize their use to the people that need them most [4].

        Conventional Capacity

        When there is not any additional need for N95 masks, they are said to be at conventional capacity [4]. During times of conventional capacity, all of the normal rules for mask use and types of masks that can be used apply without exception. In times of conventional capacity, there is no strain on the N95 mask supply chain and they are readily available for use and purchase.

        Contingency Capacity

        When daily standard practices need to be changed but there will be no impact on patient care or worker safety, contingency capacity measures are applied [4]. These practices are temporary measures for periods of expected N95 respirator supply shortages. 

        Crisis Capacity

        When supplies of N95 respirators are known to be low, crisis capacity measures come into play [4]. During crisis capacity, strategies of protection may be taken that would not normally be acceptable with US care standards. Such is the case with the COVID-19 crisis and the need to use alternatives to the N95 respirator.

        N95 Respirator Alternatives

        Healthcare facilities are authorized to use NIOSH-approved alternatives during the COVID-19 pandemic [4]. These alternatives include other classes of filtering facepiece respirators, elastomeric half-mask and full facepiece air-purifying respirators, powered air-purifying respirators (PAPRs). Certified NIOSH equipment lists can be viewed online.

        “Every other NIOSH approved filtering facepiece respirators is at least as protective as the N95. These include N99, N100, P95, P99, P100, R95, R99, and R100. Many filtering facepiece respirators have exhalation valves and should not be used in surgical settings as unfiltered exhaled breath would compromise the sterile field.  On March 2, 2020, FDA issued an Emergency Use Authorization (EUA) authorizing the use of certain NIOSH-approved respirator models in healthcare settings [4].” 

        Elastomeric Respirators

        • Half-facepiece, tight-fitting respirators
        • Made of synthetic or rubber material
        • Can be repeatedly disinfected, cleaned, and reused
        • Equipped with replaceable filter cartridges
        • Should not be used in surgical settings

          Powered Air-Purifying Respirators (PAPRs)

          • Reusable respirators that are typically loose-fitting hoods or helmets
          • Battery-powered with a blower that pulls air through attached HEPA filters or cartridges
          • PAPRs should not be used in surgical settings 

            What is a KN95 Mask?

            N95 masks filter at least 95 percent of particles that are 0.3 microns or larger [2]. NIOSH, under the CDC, approves N95 masks. When it comes to KN95 masks, they are regulated by the Chinese government. Their performance is almost identical to that of the N95 mask with minor differences in their specifications.

            The FDA has issued an emergency use authorization (EUA) for KN95 masks, and the CDC lists KN95 masks as a suitable alternative when N95 respirators are not available [2]. The FDA is willing to authorize KN95 masks as long as they meet the criteria for documentation and authenticity.

            Still, Chang, K. [2] for the New York Times reports that hospitals are reluctant to order KN95 masks amid uncertainty that they will be successfully delivered. Hospitals are also disinclined to take on the liability of masks potentially being counterfeit. While the FDA is taking steps to help guarantee authenticity, it is still a risk that hospitals are not comfortable taking on.

            According to a January 2020 3M Technical Bulletin [1],

            “China KN95, AS/NZ P2, Korea 1st Class, and Japan DS FFRs should be considered as ‘equivalent’ to US NIOSH N95 and European FFP2 respirators, for filtering non-oil-based particles such as those resulting from wildfires, PM 2.5 air pollution, volcanic eruptions, or bioaerosols (e.g. viruses).”

            KN95 Respirator Approval and Importation

            If a Chinese KN95 manufacturer has submitted all necessary documentation, the FDA may issue their certification. This FDA certificate means that the company’s KN95 mask complies with the emergency use authorization (EUA). Importers can request this certificate from the Chinese manufacturer. While there is concern over counterfeit documentation, there is little that can be done to provide certainty.

            Final Thoughts on COVID-19 Respirators

            Ultimately, the consumers of KN95 masks and non-NIOSH-approved alternatives must be willing to accept a degree of uncertainty as a lesser risk than not having the respirators. Importers can do their best to verify mask markings and certifications but with many counterfeit foreign products, it can be almost impossible for retailers to guarantee legitimacy.

            Finally, it is important for the public to understand their duty to reserve N95 masks for the healthcare industry. It has been advised that the general public wears masks, such as surgical masks and homemade face coverings to help prevent the spread of the COVID-19 virus.  

            Those with cases of COVID-19 in their homes face a greater challenge. Not all cases are being hospitalized or tested, so those who are not in need of hospitalization are waiting out the course of the illness in-home quarantine. N95 respirator alternatives could be especially helpful to these individuals who are trying to prevent the spread of COVID-19 within their homes and families.


            1. 3M Personal Safety Division Comparison of FFP2, KN95, and N95 and Other Filtering Facepiece Respirator Classes (Revision 2). (Jan 2020). 3M Personal Safety Division, 3M Technical Bulletin. Retrieved from:
            2. Chang, K. (2020, April 3). F.D.A. to Allow Use of KN95 Masks Approved by China. Retrieved from:
            3. Lee, S. A., Hwang, D. C., Li, H. Y., Tsai, C. F., Chen, C. W., & Chen, J. K. (2016). Particle size-selective assessment of protection of European standard FFP respirators and surgical masks against particles-tested with human subjects. Journal of healthcare engineering, 2016. Retrieved from:
            4. National Center for Immunization and Respiratory Diseases (NCIRD), Division of Viral Diseases. (04 Apr 2020). Strategies for Optimizing the Supply of N95 Respirators. Center for Disease Control, Coronavirus Disease 2019 (COVID-19). Retrieved from:
            5. National Institute for Occupational Safety and Health (NIOSH). (06 Dec 2018). NIOSH-Approved Particulate Filtering Facepiece Respirators. Center for Disease Control, The National Personal Protective Technology Laboratory (NPPTL). Retrieved from:
            6. Rengasamy, S., Eimer, B. C., & Shaffer, R. E. (2009). Comparison of nanoparticle filtration performance of NIOSH-approved and CE-marked particulate filtering facepiece respirators. Annals of Occupational Hygiene, 53(2), 117-128. Retrieved from: