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Why Masks Work

The Science Behind Masks and Face Coverings

At St. Luke's, we've receive many inquiries from community members and leaders around the effectiveness of masks and face coverings. There is a lot of misinformation circulating today, so it's important to review the latest scientific information. Please review our answers to the most frequently asked questions below. We include citations to the evidence-based science that informed our responses.

Frequently Asked Questions

Click each question below for its answer:

How do face coverings work to stop the spread of COVID-19?
SARS-CoV-2, the virus that causes COVID-19, is spread largely through respiratory droplets generated when an infected person coughs or sneezes. Research shows that viral shedding starts during a 2-to-3-day period before symptoms appear, when virus levels in the body are at their highest. Additional research shows regular speaking is sufficient to expel virus-carrying droplets. Though research is ongoing, it is increasingly clear that transmission of SARS-CoV-2 from pre-symptomatic (infected, but have not yet developed symptoms) and asymptomatic persons (infected, and will not develop symptoms) plays a significant role in the spread of this virus in the general population.[i] Wearing a face covering creates an effective barrier against these droplets. There is much scientific evidence to support this claim, and the evidence is growing weekly. See below under effectiveness for details. 

[i] He X, et al. Temporal dynamics in viral shedding and transmissibility of COVID-19. Nat Med 2020;26:672–5. 
Do face coverings protect those wearing them or those around them?
Probably both. Primarily, face coverings help to reduce the chance of droplets traveling from you to others.  Epidemiologically, this means they are an effective method of “source control.” In addition, there is evidence to suggest you may reduce your own risk of catching COVID-19 by wearing a face covering. Cloth and surgical masks should NOT be used to protect the wearer by filtering the air they breathe, though both do provide some partial protection against breathing in respiratory droplets. [i] [ii] In addition to mask-wearing potentially providing some protection against infection in the wearer, there is also emerging evidence that it could mitigate the severity of COVID-19 if a person acquires it by reducing the “dose” of virus inhaled. [iii]  

[i] Konda, et al. Aerosol Filtration Efficiency of Common Fabrics Used in Respiratory Cloth Masks. ACS Nano 2020;14: 6339−6347.  

[ii] Zhao, et al. Household Materials Selection for Homemade Cloth Face Coverings and Their Filtration Efficiency Enhancement with Triboelectric Charging. Nano Lett. 2020, 20, 7, 5544–5552. 

[iii] Gandhi, et al. Facial Masking for Covid-19 — Potential for “Variolation” as We Await a Vaccine. N Engl J Med 2020; 383:e101 DOI: 10.1056/NEJMp2026913

How effective are face coverings or other prevention methods in preventing the spread of COVID-19?
There really is no question that masks are safe and are helpful in containing COVID-19. Approximately 40% of COVID-19 infections are caused by asymptomatic people or those who have not yet experienced any symptoms. Since we don’t know who may be infected, or if we ourselves are infected, wearing a face covering and physical distancing are the top two prevention methods. Laboratory and real-world studies show that wearing face coverings reduces the risk of transmitting COVID-19 significantly, up to an 85% reduction in some studies. The lines of evidence come from basic science and real-world applications. 

In the basic science realm, a recent study demonstrated that individuals who had non-COVID-19 coronavirus infection (causes the common cold) demonstrated that wearing a surgical mask significantly reduced the amount of coronavirus emitted in droplets and aerosols into the air around the infected patient. This coronavirus virus (similar in size to the coronavirus that causes COVID-19) was detected in up to 40% of respiratory droplets in surrounding air samples from individuals NOT wearing face masks. No coronavirus was detected in air samples collected from participants wearing face masks.[i] In addition, homemade face coverings made from household fabrics may be less effective compared with commercially manufactured cloth and surgical masks, but they still can substantially limit dispersion of exhaled infectious respiratory particles, including respiratory droplet-sized particles containing COVID-19.[i] 

[i] Leung, et al. Respiratory virus shedding in exhaled breath and efficacy of face masks. Nature Medicine. 2020.26:676–680. 

[i] Verma, et al. Visualizing the effectiveness of face masks in obstructing respiratory jets. Phys Fluids (1994). 2020;32(6):061708. doi:10.1063/5.0016018 

Real world examples include case series as well as larger studies. 

Among the case series, a well-known case study describes a man who flew from China to Toronto and subsequently tested positive for COVID-19. He had a dry cough and wore a mask on the flight, and all 25 people closest to him on the flight tested negative for COVID-19.[i]  In another noteworthy case series, cotton facial coverings or masks were required in a county in Missouri. One hair stylist (A) developed COVID and transmitted it to a coworker (B) with whom she took non-mask wearing breaks with between clients. Otherwise both stylists wore masks while working with clients.  Even though these stylists worked for a combined total of 13 days and with 139 clients while infected with COVID, none of their clients or other stylists, who were also required to wear masks or cloth facial coverings, developed COVID. However, 4 of 4 household contacts of stylist A did get COVID infection (masks presumably were not worn when at home).[ii] 

[i] Schwartz, et al. Lack of COVID-19 transmission on an international flight. CMAJ 2020. 19(15);E410. 

[ii] MMWR / July 14, 2020 / Vol. 69 

Larger studies have also shown benefits in populations of people. A recent meta-analysis by Liang, et al estimated that masks could reduce viral infections by 80% in healthcare providers (HCP) and 47% in the community. However, they noted that studies from Asia suggest greater benefit from masks likely due to greater consistency in use compared to studies done in Western countries.[i] A meta-analysis by Chu, et al[ii] estimated a possible 85% reduction in transmission with the use of any N95, mask, or 12-16 layer cotton mask in HCP and about a 44% reduction for mask use in the community. 

[i] Liang M, et al. Travel Med and Infect Dis 2020  

[ii] Chu, et al. Physical distancing, face masks, and eye protection to prevent person-to-person transmission of SARS-CoV-2 and COVID-19: a systematic review and meta-analysis. Lancet 2020; 395: 1973–87. 

Finally, studies have been conducted on the effectiveness of policies or mandates around mask use. In a study of the largest health care system in Massachusetts, a leadership-mandated policy of universal masking for all health care workers (HCWs) and patients on rates of SARS-CoV-2 infection was evaluated. At the time universal masking was started (late March 2020), new infections in HCWs was increasing exponentially, reaching 21.3% of overall staff (increase of 1.2% per day). Following universal masking, positive test results began to drop after two weeks to 14.7%, and 11.5% by four weeks (about 0.5% per day). The authors rightly note that other community-wide and hospital-specific interventions (distancing, washing hands, testing, etc.) also played a role, but only after universal masking was in effect for one week did infections in HCWs start to decline.[i] And in an additional recent study, COVID-19 growth rates before and after face mask mandates were compared in 15 states and the District of Columbia, demonstrating that required mask use led to a slowdown in daily COVID-19 infections. In the first five days after a mandate, the daily growth rate slowed by 0.9% compared to before mandate. At three weeks, the daily growth rate had slowed by 2% (keeping in mind that overall infection rates were 10-15%).[i] 

[i] Lyu, et al. Community Use of Face Masks And COVID-19: Evidence from A Natural Experiment of State Mandates in the US. Health Affairs 2020. 39;8: 1–7.

[i] Wang, et al. Association between universal masking in a health care system and SARS-CoV-2 positivity among health care workers. JAMA. Published online July 14, 2020. doi:10.1001/jama.2020.12897
If we are practicing physical distancing do we still need to wear face coverings?
There are the “Three Ws” to help prevent the spread of COVID-19: Wearing a face covering, Watching your distance, and Washing your hands. While all are important, the first two are the most important. In addition, you can think about mask wearing as a way of creating physical distancing even when you are on the move. The droplets you breathe out as you move about in spaces are largely caught by the mask, creating the effect of physical distancing even when that might not be possible. 
Does everyone need to wear a face covering to stop transmission in the community and region?
When using cloth face coverings, the personal protection is derived from their use by all members of the community. With wearing a mask, the concept is risk reduction rather than absolute prevention.[i] 

[i] Mueller, et al. Assessment of Fabric Masks as Alternatives to Standard Surgical Masks in Terms of Particle Filtration Efficiency. Found at:

Recent modeling from the Institute of Health Metrics and Evaluation has suggested that if 95% of people wore coverings, we could avoid 33,000 deaths by October 1. In addition, wearing face coverings is more impactful in reducing the spread than Stay-at-Home Orders or strict lockdowns.
How do you wear a face covering?

A face covering should be worn over both the mouth and nose, covering below the chin. It should have a close fit to your face, not too snug and not too loose. Before you put on your face covering, ensure it and your hands are clean. An excellent video on mask wearing is available from PBS: How Well Do Masks Work.

View more info on how to wear a mask

Does it matter what type of face covering I am wearing?

For masks to be effective, they must be worn consistently and correctly, fitting closely to the face, and be made of fabrics that are effective in filtering respiratory droplets made by breathing, talking and coughing. A good double-layered cloth face mask is very effective. If available, a surgical mask will achieve similar results. The most important factor is if they fit comfortably covering the nose and mouth. A covering that is not worn properly is ineffective. N95s are not necessary unless used in medical situations. Any face masks with valves, such as some N95s, are not recommended due to the ability to transmit from the carrier out through the valves, thus not protecting those around you. If you wear a valved mask, you must also wear a surgical or cloth mask to protect those around you.

I am not a high-risk or vulnerable person, why do I have to wear face coverings?
First, serious illness and even death are not limited to vulnerable individuals, they are just more common in that group. We have had life-threatening cases requiring mechanical ventilation in people without underlying conditions and even people younger than 18 years old. Second, it is important because you may be infected and not know it, thus shedding the virus during critical periods to the people around you, and some of these people will become infected and continue to spread the virus. Our best option for stopping the virus, saving lives, keeping kids in school, and keeping businesses open is for all of us to do our part.
Do I need to wear a face covering when I am in my car, walking in my neighborhood or riding my bike?

In your car, no, unless there is a vulnerable person in the car with you that you do not generally have close contact with (e.g., someone you do not live with). Any time you are in public and physical distancing becomes a challenge, face coverings should be worn.

Should I clean my face covering or change to a new one?

Face coverings should be washed regularly if they made of reusable materials (e.g., cloth), depending on the frequency of use. Cloth face coverings can be effectively cleaned in a washing machine on a normal setting. One-time use masks should be changed out frequently.  

Why don't young children have to wear face coverings?
Children are half as likely to become infected from exposure as adults. Additionally, they are less likely to have a severe case or even have symptoms. Finally, they appear to be less likely to infect others. Since young children (2 and under) are at risk of airway issues with face coverings and because they have such a low risk of contracting or spreading the disease, it is not recommended they wear face coverings. Older children should wear them when around others.

Why We Wear Masks

Related Information

COVID-19 Resources

COVID-19 Resources

Navigating COVID-19 Together

St. Luke's is prepared to support our patients and community as we enter the fall flu season with COVID-19 still spreading across Idaho. Our commitment to safe care includes coronavirus testing, visitor screening, enhanced safety practices and more.