Synopsis of Literature to Date on Masks and Reducing the Spread of COVID-19

Since March 2020, over fifty countries around the world have made masks mandatory in public spaces.  In combination with other public health measures such as physical distancing, regular handwashing and disinfecting, wearing a mask in enclosed public spaces is critical to getting COVID-19 under control and eradicated.

Below is a synopsis of literature related to common concerns and questions. Consult the reference list to review the literature that was reviewed.

How does wearing a mask protect others?

There are three types of people who can spread COVID-19: those with symptoms, those without symptoms but who will develop them (pre-symptomatic) and those without symptoms who will never develop the disease but carry the virus and are able to spread it (asymptomatic). 

We know that individuals who do become ill with COVID-19 are at their most contagious 1-3 days prior to developing symptoms.1,2 Also, studies seem to find that up to 45% of any given population may carry the virus but are asymptomatic at the time of transmission.3   Thus, the need to protect others.

Research shows that in non-healthcare setting, wearing a mask helps to protect others but it does not protect the person wearing a mask.  Virus transmission occurs either when larger droplets are secreted  directly by speaking, eating, coughing, and sneezing and then drop to a surface or  when airborne droplets (aerosols) are created as water evaporates from smaller virus droplets, which can remain in the air for an extended period. Every virus-laden particle retained in a mask is not available to hang in the air as an aerosol or fall to a surface to be later picked up by touch. Regardless of the type or the size of the virus, a mask’s filtration efficiency depends on its ability to physically trap it.  Homemade cloth masks (especially those made with multiple layers) can block droplets and aerosols and prevent them from spreading further.4

The use of cloth masks for outward protection (protecting others from the person wearing the mask) has been extensively studied decades ago and the findings are still relevant to this day.  Compared with unmasked volunteers, a mask made of muslin and flannel blocked bacteria by 99.3% to 99.9%, total airborne microorganisms by 99.5% to 99.8%, and bacteria recovered from aerosols (<4 µm) by 88% to 99%. A similar experiment in 1975, compared 4 medical masks and 1 commercially produced reusable mask made of four layers of cotton muslin, for filtration efficiency of airborne droplets (<3.3 µm). The study showed reduced transmission of 72% to 89%.4 The effect occurs even when it is assumed that facemasks are only 50% effective at capturing exhaled virus droplets.5

An ecological study from Germany released in June 2020, evaluated the impact of mandatory mask use on public transportation and in sales shops in the city of Jena. It showed that mandatory masking reduced the daily growth rate of COVID-19 in Jena by 40%.6

Modelling studies have estimated variable but substantial population level effects from mask use by the general public. Even when risks associated with public mask use were incorporated into their modelling study, Stutt et al. (2020) were able to demonstrate that overall population level benefits persisted.5  When facemasks are used by the public all the time (not just from when symptoms first appear), the effective reproduction number (Re) can be more easily decreased to ideal levels (below 1). This means that the virus will not easily transmit thus leading to a ‘flattening of the curve’. Even a partial reduction of transmission may be sufficient to drive reproduction numbers below one, especially when combined with other measures such as physical distancing and good hand hygiene

Under certain conditions, when lock-down periods are implemented in combination with 100% facemask use, there is much less disease spread, secondary and tertiary waves are flattened and the epidemic is brought under control.7 This may explain why some regions, where adoption of facemask use by the public is around 100%, have experienced significantly lower rates of COVID-19 spread and associated deaths. 5,7

Why does wearing a mask protect others but not the person wearing it?

Research shows that in non-healthcare settings, wearing a mask helps to protect others but it does not protect the person wearing the mask. The virus spreads by entering the body primarily through the mouth, the nose, or the eyes.  If other people who are not wearing a mask cough or sneeze around you, they may send viral droplets into the air that can infect you by entering your eyes or through the sides of your mask.  To reduce the spread of the virus more effectively, all those who can wear a mask should when physical distancing is difficult or impossible.  You wear a mask to protect others and others wear a mask to protect you.5

Does masking replace other practices such as physical distancing or handwashing?

Wearing a mask in enclosed indoor public spaces is part of a multi-pronged approach to reducing the spread of the virus.  Regular hand washing and physical distancing will protect you from others who are COVID-19 positive but asymptomatic. However physical distancing and masking are critical to protecting others from yourself. Community-wide benefits are likely to be greatest when face masks, used along with other practices, are adopted universally (community-wide) and compliance is high.7 A recent study has shown that mask wearers tend to mix more readily with other mask wearers rather than non-mask users and therefore the impact of mask wearing is reduced when there is a lower compliance rate.8

What does the evidence say about breathing in expired carbon dioxide (CO2) from wearing a mask?

Hypercapnia – an excess of carbon dioxide in the body – is generally caused by hypoventilation (respiratory depression), lung disease, or diminished consciousness. It may also be caused by exposure to environments containing abnormally high concentrations of carbon dioxide (greater than 10%), such as from volcanic or geothermal activity, or by deep sea divers using a rebreather apparatus.9 Normally, the carbon dioxide level present in the atmosphere is about 0.04%. 

It is highly unlikely that wearing a face mask would cause a lack of oxygen or an increase in your body’s carbon dioxide levels. The extent to which a mask can affect carbon dioxide levels would depend on what it is made of, and how tightly it fits. In general, a cloth mask is not fitted tightly to your face and allows the passage of air around the mask and through the pores in the material. Therefore, a cloth mask does not absolutely protect you from inhaling the virus, but by disturbing your exhalation flow it tends to protect those around you from droplets in your breath.10

For comparison, an auditorium with poor fresh air ventilation filled with people for several hours can represent a 1% concentration of carbon dioxide9 and may cause drowsiness in some people.   This is not the same as wearing a mask for short periods of time while out grocery shopping or in line waiting to be served at the bank.

Studies show that prolonged use of any face mask, including the N95 respirator, has not been shown to cause carbon dioxide toxicity in healthy people. The N95 respirator fits more tightly to the face and therefore could cause discomfort when breathing. However, the general public should not wear an N95 respirator (unless otherwise advised by their primary care provider) as these types of masks are part of the personal protective equipment (PPE) used in the healthcare sector.


  1. MIT Medical/Covid-19 update. Available from:
  2. Wei WE, Li Z, Chiew CJ, Yong SE, Toh MP, Lee VJ. Presymptomatic Transmission of SARS-CoV-2 Singapore, MMWR Morb Mortal Wkly Rep 2020;69:411–415. January 23–March 16, 2020. Available from: icon
  3. Daniel P. Oran, Eric J. Topol. Prevalence of Asymptomatic SARS-CoV-2 Infection. Annals of Internal Medicine, 2020; Available from:
  4. Clase CM, Fue El, Joseph M et al. Cloth masks may prevent transmission of COVID-19: An evidence-based, risk-based approach. Ann Intern Med. 2020 May 22.
  5. Stutt R, Retkute R, Bradley M, Gilligan CA, Colvin J. A modelling framework to assess the likely effectiveness of facemasks in combination with ‘lock-down’ in managing the COVID-19 pandemic. Proc R Soc Lond A Math Phys Sci. 2020 Jun 10 [Epub ahead of print]. Available from:
  6. Mitze T, Kosfeld R, Rode J, Walde K. Face masks considerably reduce COVID-19 cases in Germany: a synthetic control method approach. Discussion paper series Institute of Labor Economics; 2020. Available from:
  7. Steffen E. Eikenberry, Marina Mancuso et al. To mask or not to mask: Modeling the potential for face mask use by the general public to curtail the COVID-19 pandemic, Infectious Disease Modelling Volume 5, 2020, Pages 293-308. Available from: 
  8. David N. Fisman , Amy L. Greer , Ashleigh R. Tuite, Bidirectional impact of imperfect mask use on reproduction number of COVID-19: A next generation matrix approach, Infectious Disease Modelling 5 (2020) 405-408.
  9. Inspectapedia, The Effects of CO2 at Various Levels or Concentrations in Air.  Available from: 
  10. Claire Gillespie Does Wearing a Face Mask Reduce Oxygen—and Can It Increase CO2 Levels? Here's What Experts Say, May 13, 2020. Avaialble from: 
  11. Christian J. Kähler, Rainer Hain, Fundamental protective mechanisms of face masks against droplet infections, Journal of Aerosol Science, Volume 148October 2020.  Available from:
  12. PHO, A Synopsis Covid-19 – what we know so far about…wearing masks in public, 06/17/2020. Available from:
Eastern Ontatio Health Unit / Bureau de santé de l'Ontario