Masks Help, But Not Entirely Foolproof

Do masks work?


In the last three months, the 7-day moving average for daily COVID-19 deaths has nearly quintupled, from 550 in October to 2,500 deaths per day now. This number will soon surpass the daily rate observed at the height of the spring surge (2,856 deaths per day on April 21, 2020). 

COVID-19 has become the leading cause of death in the United States. Daily mortality rates for heart disease and cancer, which have been the two leading causes of death for decades, are approximately 1,700 and 1,600 deaths per day, respectively. 

Below we'll discuss some key factors in mask-wearing, and what the recent studies indicate in terms of efficacy.

Are PPE Masks Safe?

Wearing a mask in public is now widely accepted as one of the best things that the average person can do to reduce infection spread and save lives during the pandemic. Early skepticism about wearing masks was due to a lack of understanding that both pre-symptomatic and asymptomatic transmission are possible with COVID-19. 

Studies in biology, epidemiology, and physics can now offer a better understanding of the virus' behavior. In particular, we now know that viral load peaks in the days before symptoms begin and that talking is a common mode of transmission. 

With the realization that there is no way to discern who should be wearing a mask in a crowd of people, it now makes sense to ask everyone to wear one. This understanding eventually led public health groups to change their stance on wearing masks. In April of 2020, the CDC recommended that masks be worn when physical distancing isn't an option; the WHO followed suit in June. Previously, both had recommended mask-wearing for only those who presented symptoms.

Epidemiology Studies About Masks

Epidemiological data also supports universal mask-wearing. One study of 200 countries found weekly increases in per-capita mortality to be four times lower in places where masks were recommended by the government or were the norm. 

For example, Mongolia adopted PPE mask use in January 2020. One year later, it has still not recorded a single death related to COVID-19. Another study estimates that U.S. state-government mandates for mask use in April and May decreased the growth of COVID-19 cases by up to 2 percentage points per day

Many different variables impact epidemiological analysis based on existing human populations. These factors include whether mask mandates are enforced, proper usage, and the effect of simultaneous interventions like social distancing, hand washing, and limits on gatherings. 

Animal studies, on the other hand, are much easier to control. One study puts infected and healthy hamsters in adjoining cages with a surgical-mask partition separating them. They find that two-thirds of the uninfected animals caught SARS-CoV-2 when no barrier was in place. With the barrier, only 25% of the animals get infected. Those who do are less sick than they otherwise would have been since exposure to more a virus leads to a worse infection. This study suggests that wearing a mask may protect from infection or decrease illness severity.

Where Masks Fall Short

While masks are now almost universally recommended, new, physics-based research indicates that masks alone are insufficient to guarantee safety. 

A study published in Physics of Fluids on December 2, 2020, reveals that masks are not entirely foolproof in blocking airborne droplets even if they fit snugly. The study is motivated by a concern about transmission among face mask wearers who are not social distancing. 

Disregard for social distancing due to its real or perceived impracticality is highly prevalent, especially in "closed spaces, such as hospitals, homes, gymnasiums, public transportation, and schools, or physically close interactions in indoor and outdoor spaces, such as in crowded gatherings at organized events and political campaigns.” 

There appears to be a commonly held misconception that susceptible people wearing a mask do not need to social distance. Hence, the study asks the following question:

Will a face mask offer the wearer protection from foreign airborne droplets in the close vicinity of a sneeze or a cough, thereby reducing the chances of catching COVID-19?

In brief, the answer is, only sometimes. The key variables here include what type of mask is involved and whether the infection threshold is reached. 

Understanding Transmission

The volume of expelled aerosol droplets in order from most to least is as follows: 

  • sneeze
  • cough
  • speaking for 20 minutes
  • breathing for 20 minutes

Given the higher risk associated with coughs and sneezes, researchers in the Physics of Fluids study use a machine to simulate coughs and sneezes. They also test five types of masks (and ensure no leakage around the edges). 

In order from most to least protection, the masks tested include: 

  • N95 mask
  • two-layer cloth mask with a wet PM 2.5 filter (Cloth PM 2.5 Wet)
  • surgical mask
  • dry two-layer cloth mask with a PM 2.5 filter (Cloth PM 2.5)
  • dry two-layer cloth mask with no filter (Cloth). 

The term PM 2.5 filter, or Particulate Matter 2.5 filter, refers to a filter capable of filtering out tiny particles or droplets in the air that are two and one-half microns or less in width. 

The results of the study are usefully summed up in two figures. In Figure 1, Escape Percentage (E.P.) denotes the percentage of droplets from a sneeze or cough that escape or travel through the mask material. As given in the figure, 

  • N95 EP = 0%
  • Cloth PM 2.5 Wet EP = 0.03%
  • Surgical EP = 0.06%
  • Cloth PM 2.5 EP = 1.70%
  • Cloth EP = 3.61% EP


Figure 1. "Results of the study. E.P. denotes the Escape Percentage, i.e., the percentage of droplets from a sneeze or cough that could escape or travel through a snugly fit mask...A typical sneeze and a cough are assumed to contain 40,000 and 3000 droplets, respectively".

The E.P. values for source control, i.e. the percentage of droplets that escape the mask of the person coughing/sneezing, are as follows: 


  • N95 EP = 0.98%
  • Cloth PM 2.5 Wet EP = 0.07%
  • Surgical EP = 0.19%
  • Cloth PM 2.5 = 0.19%
  • and Cloth EP = 2.7%

viral filter protectiveness per mask type

One expects the source control E.P. values to be smaller than the protection E.P. values given the widespread consensus that masks are more effective at source control. We see this for Cloth PM 2.5 masks and Cloth masks. Strangely, N95 masks, Cloth PM 2.5 Wet masks, and Cloth PM 2.5 Wet masks show the opposite relation. This relation may be associated with the higher velocity of expelled droplets at the source mask than at the protection mask. 

The study also attributes these outliers "to a difference in the arrangement, material, and the stretching of the clothing layers that could generate a different porous path (e.g., tortuosity) for the flow when the mask is reversed.”

Just knowing an Escape Percentage is only partly useful. It is also important to know the number of particles expelled from the infection source to determine whether infection will occur.

According to previous studies, the viral infection threshold for COVID-19 for a susceptible person is 1000 virus particles, inhaled either at once or in batches. The literature also finds that a single sneeze can contain a wide range of 10 - 200 × 10^6 virus particles (depending on the carrier's virus concentration). 

Viral Infection Threshold graph

Figure 2 ultimately shows whether each type of mask will protect from infection for incoming sneezes or coughs for varying numbers of expelled virus particles. It shows that the range of guaranteed protection across all mask types is 100,000 virus particles or less. Thus, there exists an extensive range of coughs and sneezes for which the presence of a mask makes no difference.

Figure 2: "The graph shows the average number of virus particles that can pass through the mask of a susceptible person when exposed to a single cough or sneeze from a virus carrier in a close (<6 ft) face-to-face interaction. Studies have shown that the infection threshold for a susceptible person to catch a virus is 1000 virus particles, inhaled either at once or in batches. 

Since the N95 mask has statistically zero particles escaping through it in the protection configuration, it was excluded from this figure. However, in the source control configuration, the cutoff for the N95 mask could be as low as 100,000 virus particles based on its average E.P. value.”

This data simultaneously emphasizes the protection afforded by face masks while quantitatively demonstrating their imperfections. 

Given that masks do not offer complete protection to a susceptible person from a viral infection in close (e.g., <6 ft) face-to-face interaction, the take-home lesson is simple: 


Masks work, but they are not infallible. So, keep your distance.

The Risks of Not Wearing Masks

Beyond biology, epidemiology, and physics, human behavior is critical to the efficacy of mask-wearing in the real world. According to data from the Institute for Health Metrics and Evaluation (IHME) at the University of Washington, mask use in the U.S. has slowly increased from 50% in May to 74% today. This is slightly higher than the global usage, which has stayed steady at 60% since May. 

Johns Hopkins University's Coronavirus Resource Center reported 4,085 coronavirus-related deaths on January 7, bringing the total U.S. death toll since the beginning of the pandemic to 365,882. This report is likely underestimated due to reporting delays, miscoding of COVID-19 deaths, and an increase in non–COVID-19 deaths caused by the pandemic's disruptions. 

Excess deaths are estimated to be 50% higher than publicly reported COVID-19 death counts. The IHME model's latest update estimates 201,313 more deaths will occur by April 1. That is an increase of 55% over current deaths.  

The IHME model predicts deaths by April 1 will reach 567,195, which can be reduced to 518,464 by 95% mask-wearing instead of the present 74%, saving 48,731 lives. 

U.S Projection of Deaths With and Without Universal Mask Use

USA projection of deaths with and without universal mask use

The prospect of a vaccine offers consolation for 2021. Still, that solution will not come soon enough to avoid catastrophic increases in COVID-19–related hospitalizations and deaths. 

The need for the entire population to take the disease seriously—notably to wear masks and maintain social distance—could not be more urgent.

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