Prioritizing face masks and respirators during shortages

A surgical mask and an N95 respirator. Science Photo Library–Getty Images


This is a work in progress. Feedback is welcome, appreciated, and valued.

Key points

In the setting of shortages,

  1. Respirators (N95 or higher) should be prioritized for health care workers in health care settings, or caregivers of confirmed cases, where the cumulative risk of acquiring an infection is highest. Airborne respiratory protection is the key component of airborne precautions.1
  2. Face masks should be prioritized for health care workers in health care settings, or caregivers of confirmed cases, where the cumulative risk of acquiring an infection is highest. Droplet respiratory protection is the key component of of droplet precautions.
  3. Face masks should be prioritized for symptomatic (infectious) patients when they are in close contact (within 6 feet) with susceptible persons, especially in a congregate settings such as a health care facility or crowded housing. This is called source control. Face masks do not provide reliable airborne respiratory protection.
  4. In the general population, N95 respirators and face masks should be limited to high risk situations based on principles discussed in this article.
  5. Respirators and face masks are not a substitute for basic infection prevention practices. CDC always recommends everyday preventive actions to help prevent the spread of respiratory viruses, including:
    • Avoid touching your eyes, nose, and mouth with unwashed hands.
    • Avoid close contact with people who are sick.
    • Stay home when you are sick.
    • Cover your cough or sneeze with a tissue, then throw the tissue in the trash.
    • Clean and disinfect frequently touched objects and surfaces using a regular household cleaning spray or wipe.
    • Wash your hands often with soap and water for at least 20 seconds, especially after going to the bathroom; before eating; and after blowing your nose, coughing, or sneezing.
    • If soap and water are not readily available, use an alcohol-based hand sanitizer with at least 60% alcohol. Always wash hands with soap and water if hands are visibly dirty.


Since December, 2019, we are experiencing an epidemic of a novel coronavirus virus (COVID-19) which originated in Wuhan, China, and with the potential to become a global pandemic. The World Health Organization (WHO) has declared a Public Health Emergency of International Concern, and the United States Department of Health and Human Services has declared a Public Health Emergency.

In affected areas we see the general public donning face masks, and we are learning about hoarding and shortages of face masks and respirators, even in non-affected areas such as the United States.

If shortages of face masks or respirators continue, as a society, we may need to prioritize and direct their use. Our general goals include

  1. prevent introduction into our community
  2. contain community transmission if/when it occurs,
  3. mitigate community transmission if infection spreads widely, and
  4. screen, diagnose, and treat patients to restore health.

Consequently, to achieve these goals, we focus on (a) mitigating the highest transmission risks, and (b) directing scarce resources to where we expect the greatest benefits.

Understanding transmission risk

Cumulative risk with pictures

Imagine two extreme scenarios. First, a health care worker seeing 10 infectious patients (Figure 1). In this scenario, we can see the importance of respiratory protection for the health care worker. For respiratory protection we use respirators (such as N95s or higher) to reduce the cumulative risk of infection.

Figure 1: A susceptible person (e.g., health care worker) in contact with ten infectious sources (e.g., patients)

Second, an infectious patient in contact with 10 susceptible patients in a health care setting (Figure 2). In this scenario, we can see the importance of source control for the susceptible, uninfected patients. In this case, the infectious patient is the source of infection to others and, in a health care setting, may infect many patients. For source control, in addition to isolating the infectious patient, we place a face mask over their nose and mouth.

Figure 2: An infectious source (e.g., infectious patient) in contact with ten susceptible, uninfected persons (e.g., patients)

In public health we consider both scenarios simultaneously as part of a comprehensive infection prevention program of which personal protective equipment (PPE) is only one component.2

Cumulative risk with equations

In general, for a susceptible person the cumulative risk of infection, \(R\), for a human-to-human transmissible respiratory agent can be represented by

\[ R(t) = e^{-\int_0^t I(u) du} \]

where \(I(t)\) is the infection rate among susceptibles:

\[ I(t) = c p P(t), \]

and depends on

  • \(c\), the contact rate and distance with potentially infectious sources,
  • \(P(t)\), the probability the source is infectious, and
  • \(p\), the transmission probabilitythe conditional probability of getting infected given contact to an infectious source.

For respiratory infections the definition of meaningful “contact” includes distance between an infectious source and a susceptible host. “Source” is the potential source of infection and often refers to people, but it could be a vector, animal, item, environment, etc.

The transmission probability, \(p\), depends on infectiousness of the source, susceptibility of the host, mode of transmission, and infection control measures.3

To summarize, getting infected—the cumulative risk—depends on

  1. contact rate to sources,
  2. distance from sources,
  3. probability source is infectious,
  4. infectiousness of the source (if infected),
  5. susceptibility of the host (e.g., no immunity),
  6. mode of transmission (droplet vs. airborne), and
  7. infection control measures.

Infection control measures include the appropriate use of face masks and respirators. Their benefit is highest when

  • they are effective in reducing transmission,
  • the cumulative infection risk is high,
  • the non-infection control risk factors cannot be controlled easily (e.g., many symptomatic patients seeking health care), and
  • the mode of transmission is known.

For these reasons the cumulative infection risk is highest for

  1. health care workers, and
  2. close contacts4 of infected persons.

The risk is lowest for the general public that are not having close contact with potential infectious sources.

Table 1 summarizes the prioritization of face masks and respirators based on risk and benefit.

Table 1: Prioritizing face mask and respirator use based on risk and benefit
User Goal Priority Face mask or respirator priority
Infectious patient Source control very high face mask
Person Under Investigation (PUI)5 Source control high face mask
General public (symptoms) Source control medium face mask6
Health care worker Respiratory protection very high respirator (N95 or higher)
Close contact of infectious person(s) Respiratory protection high respirator (N95 or higher)
General public (no symptoms) Respiratory protection low generally not required7

For these reasons, when there is shortage of face masks or respirators, we should prioritize supplies for use by health care workers, caregivers, and patients.

By far, health care workers and health care settings have the highest risk for infection. Health care workers must wear respirators and symptomatic patients must wear a face mask.

Health care workers play a critical role in screening, diagnosing, and treating symptomatic (likely more infectious) patients. Consequently, they are at the highest risk of becoming infected.

Figure 1 summarizes the key differences between face (“surgical”) masks and N95 respirators.

Figure 3: Comparison of face masks to N95 respirators


Infection prevention

Infection control measures for health care workers include

  1. placing face mask on symptomatic patient, and
  2. implementing infection control precautions (Table 2).
Table 2: Infection control precautions in health care setting
Precaution level Description HCW Patient
Standard assumes blood and body substances are infectious Y
Droplet face mask (source control) Y
Airborne respirator (respiratory protection) Y
Contact gown and gloves Y
Eye protection goggles or face shield Y

Note: For airborne infections (e.g., tuberculosis), N95 respirators are required. For respiratory infections transmitted by large droplets requiring close contact, a face mask may be sufficient in the right setting [1]. However, for novel respiratory agents with increased pathogenicity, even if transmitted primarily by respiratory droplets, a N95 is highly recommended in the health care setting [2].8 In respiratory transmissable infections the size spectrum from large droplets to fine particle aerosols is a continuum (Figure 2). In health care settings where procedures may produce fine particle aerosols, these respiratory infections results in opportunistic aerosol transmission [3].

Figure 4: The Aerobiologic Pathway for the transmission of communicable respiratory eisease

Transmission dynamics

To learn more about epidemiologic concepts for controlling microbial threats see


1. Radonovich LJ Jr, Simberkoff MS, Bessesen MT, Brown AC, Cummings DAT, Gaydos CA, et al. N95 respirators vs medical masks for preventing influenza among health care personnel: A randomized clinical trial. JAMA. 2019 Sep;322(9):824–33.

2. MacIntyre CR, Chughtai AA, Rahman B, Peng Y, Zhang Y, Seale H, et al. The efficacy of medical masks and respirators against respiratory infection in healthcare workers. Influenza Other Respir Viruses. 2017 Nov;11(6):511–7.

3. Roy CJ, Milton DK. Airborne transmission of communicable infection–the elusive pathway. N Engl J Med. 2004 Apr;350(17):1710–2.

  1. See transmission-based precautions at↩︎

  2. To learn more about the “Hierarch of Controls” in infection prevention visit↩︎

  3. To learn more about concepts for controlling microbial threats see↩︎

  4. Close contact is defined as (a) being within approximately 6 feet (2 meters) of a 2019-nCoV case for a prolonged period of time; close contact can occur while caring for, living with, visiting, or sharing a health care waiting area or room with a 2019-nCoV case; OR (b) having direct contact with infectious secretions of a 2019-nCoV case (e.g., being coughed on). Source:↩︎

  5. A person under investigation (PUI) has symptoms with a risk factor for COVID-19, and is considered infectious.↩︎

  6. A face mask is a barrier to contain secretions from spreading when one is symptomatic, or to protect against large droplets (coughs, sneezes) when one is within 6 feet of a symptomatic person. In resouce poor environments, or during shortages, as we witnessed with SARS, individuals will improvise and develop alternative barrier methods.↩︎

  7. Face masks provide some protection against large droplets (coughs, sneezes) when one is within 6 feet of a symptomatic person↩︎

  8. Certain regulations, such as California’s Aerosol Transmissible Disease Standard, legally require the use of N95 respirators in cases of novel respiratory viruses.↩︎

Tomás Aragón
Tomás Aragón
Health Officer, City & County of San Francisco; Director, Population Health Division

I exercise legal authority to protect and promote equity and health, and I direct core public health services.

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