Should We Force Children to Wear Masks Again When Schools Reopen? A Risk Analysis

Susan Miller
10 min readJun 29, 2021


COVID-19 has presented decision makers with a classic wicked problem, confronting them with difficult tradeoffs between competing goals in the context of a complex, constantly evolving situation.

Unfortunately, too many leaders have preferred to pass the buck, rather than having it stop at their desk.

Over the past 16 months, many parents have been shocked and angered as some adults seemed to put their interests above those of our children. This is an affront to both our evolutionary instincts and the many moral traditions that call upon adults to sacrifice in order to protect children, not the other way around.

In too many cases we’ve seen district superintendents and school boards defer decision making about school closure, masking, and quarantines to county health directors, who are usually responsible only for minimizing public health risks, not making difficult tradeoffs between multiple goals. Many have not hesitated to close schools, or impose onerous masking and quarantine requirements when schools reopened.

Too few elected leaders have publicly acknowledged the full cost of these decisions, including the long term personal and macroeconomic impact of student learning losses and mental health challenges, as well as the cascading negative effects of many mothers leaving the workforce in order to support their children’s remote learning at home.

Today teachers unions are calling for the continuation of mandatory student masking when school resumes. Does this make sense?

In October 2020, I wrote an analysis, “Are We Ready To Close Schools’ Windows? which warned of the need to improve schools’ indoor air quality to enable them to stay open during the pandemic. A lot has changed since then.

Let’s assume that students, parents, employers, and voters seek to achieve two overarching goals: Maximizing students’ in-person school days while keeping the risk of being infected with COVID while in school to an acceptable level.

Rather than a battle of angry tweets or an endless “my new study is better than your new study” argument, we should see the student masking question for what it really is: a classic risk management problem.

Let’s begin this risk analysis with some base rate data that puts the risk of death from COVID into a broader context.

The following tables show US Centers for Disease Control and Prevention (CDC) mortality rates for the top five causes of death per 100,000 people. The data are from 2017, before COVID arrived.

The last row shows US COVID mortality rates for 2020.

As you can see, COVID mortality risk — even before vaccinations became widely available — was small, relative to other mortality risks that we routinely live with.

Now lets move on to a look at how different interventions are reducing the risk of acquiring a COVID infection at school.

Vaccination has substantially reduced both COVID infection and hospitalization risks, even in the case of the new Delta (B.1.617.2) variant. Recent research by Public Health England (PHE) has found that after two doses, the Pfizer vaccine is still 88% effective in preventing symptomatic infections (“Effectiveness Of COVID-19 Vaccines Against The B.1.617.2 Variant” by Bernal et al).

PHE didn’t test the Moderna vaccine because it isn’t used in the UK. However, the US CDC has found them equally effective (“Interim Estimates Of Vaccine Effectiveness Of BNT162b2 And Mrna-1273 COVID-19 Vaccines In Preventing SARS-Cov-2 Infection Among Health Care Personnel, First Responders, And Other Essential And Frontline Workers — Eight U.S. Locations, December 2020–March 2021”, by Thompson et al).

PHE also found that, conditional on having a symptomatic COVID infection caused by the Delta variant despite being vaccinated, the Pfizer vaccine was 96% effective in avoiding the need for hospital admission (“Effectiveness Of COVID-19 Vaccines Against Hospital Admission With The Delta (B.1.617.2) Variant”, by Stowe et al).

In Jefferson County Colorado, where I sit on the school board, 74% of residents 12 years and older have received at least one vaccine dose.

If we assume that all of these people receive their second dose before school opens, and nobody else is vaccinated, the risk of becoming infected with COVID –- and transmitting it to others — will have been very substantially reduced. Already the rolling seven-day COVID prevalence rate in Jefferson County has fallen to just 19.7 cases per 100,000 people, far below our November 2020 peak of 590.8.

While vaccination has sharply reduced chances of an infected person coming into a school and infecting others, over the past 16 months, many districts have taken additional steps to significantly reduce this risk.

The CDC and the American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) recommend four to six classroom air changes per hour to minimize infection risk.

To meet this target, districts have increased classroom ventilation rates, both by opening windows to bring virus free outside air into the building (and further boosting air flows with window fans) and by increasing the amount of outside air added to their HVAC systems’ recirculation cycle.

But on very hot and very cold days, windows have to close. Districts have therefore taken other steps to minimize infection risk when this occurs.

First, they have increased the filter quality in their HVAC systems.

The SARS-CoV2 virus itself is about 0.1 microns (um) in diameter. When exhaled by an infected person, the virus is enveloped in liquids of varying diameters. Droplets (which fall to the ground relatively quickly) are greater than 10um in size; aerosols that remain suspended in the air for longer are less than 10um (microns). Due to the partial evaporation of the liquid around them, viral aerosols have an average size of about 5um.

HVAC system filters are rated by their Minimum Efficiency Reporting Value (MERV), which is a measure of the percentage of particles of various sizes they capture.

Before COVID arrived, many school HVAC systems used MERV-8 filters, which only capture about 20% of particles from 1.0 to 3.0 microns in diameter. Since then, many districts have upgraded to MERV-10 filters, which capture 50% of particles in this size range.

CDC and ASHRAE have recommended further upgrading to MERV-13 filters if possible. These capture about 85% of 1.0 to 3.0um particles. So far, many districts have been unable to do this because of the pressure drop this causes in the airflow through their HVAC systems.

The good news, however, is that COVID has led to new innovations by filter manufacturers, a number of which are now marketing new MERV-13 filter designs that substantially reduce system pressure drop.

The second step some districts have taken is to supplement HVAC system filtration and air change rates with the placement of portable HEPA air cleaners in individual classrooms.

HEPA filters capture more than 99% of 1.0–3.0 micron particles, and are the effective equivalent of bringing fresh outside air into a classroom. For example, a widely available consumer product (the Honeywell 50250 HEPA Air Cleaner) provides about 1.67 air changes per hour for a typical 30 x 30 x 10 classroom.

The third step that some districts are investigating is the use of ultraviolet (UV) light either in fixtures mounted high on walls or in HVAC system ducts.

In contrast to increased ventilation (which reduces the number of viral particles within a volume of air) and filtration (which traps viral particles), UV light is used to inactivate a virus by destroying or damaging enough of its genetic material (RNA or DNA) to prevent it from replicating inside a person who inhales it.

At this stage of development, the use of UV radiation to reduce COVID infection risk is still subject to many uncertainties.

These include science-related uncertainties (e.g., the UV dose required to inactivate the virus, and the risks associated with only partial activation), engineering uncertainties (e.g., the delivery of the required dose in fast moving HVAC system airflow in the presence of varying levels of dust, humidity, temperature, and UV bulb output), and operational uncertainties (e.g., capital and operating costs, and the implementation of new safety policies and protocols).

As in the case of MERV-13 filters, UV virus inactivation is also an area in which innovation has accelerated.

All of these factors — vaccination, ventilation, filtration, and the possible use of UV to inactivate viruses — have sharply reduced the risk of classroom COVID infection.

A number of risk assessment models are available today that can be used to quantify how much different interventions reduce the risk of classroom COVID infection, with and without masking.

One of these is the model built by Khan, Bush, and Bazant from MIT, which can be found here:

The model is based on a very detailed quantitative risk analysis by Bazant and Bush (“A Guideline to Limit Indoor Air Transmission of COVID-19”).

I plugged the current conditions in Jefferson County into this model, including a county COVID prevalence rate of 19.7/100,000 and 74% of people 12 and over vaccinated, MERV-10 HVAC filters, four Air Changes per Hour, an unmasked classroom of 21 people, and that the Delta variant is the dominant strain of SARS-CoV-2 (note that some of these assumptions will vary across schools; you can use this model to do your own analysis).

Even at a low level of risk tolerance, these inputs led to the result that the estimated “Cumulative Exposure Time” required before a second person was infected (assuming there was one infected person in the classroom) is greater than 14 days of continuous (24 hour) occupancy of the classroom by these 21 people.

In other words, the risk of an individual becoming infected with COVID in this classroom is extremely low.

This also suggests a very low need to quarantine the other people in this classroom if an infected student or teacher is discovered.

This is the baseline against which we must examine the additional incremental risk reduction benefits and incremental costs of also requiring students to wear masks six hours a day when they return to school.

When assessing the incremental infection reduction benefits, it is critical to recognize that masks vary tremendously in their ability to block aerosols. This is known as their “fitted filtration efficiency” or “FFE”.

With respect to their FFEs, N95 masks block at least 95% of particles of 3um or greater in size (i.e., FFE = 95%). Surgical masks with ear loops block only 38%. Consumer masks of different materials and construction vary widely in FFE, from 27% to 79% (see, “Filtration Efficiency of Hospital Face Mask Alternatives Available for Use During the COVID-19 Pandemic”, by Sickbert-Bennett et al, and Evaluation of Cloth Masks and Modified Procedure Masks as Personal Protective Equipment for the Public During the COVID-19 Pandemic, by Clapp et al).

Regardless of the mask type, FFEs sharply decline when a mask is improperly fitted. Researchers have found that in groups of people, a minimum of 50% of masks are usually improperly fitted (see, “Comparing The Fit Of N95, KN95, Surgical, And Cloth Face Masks And Assessing The Accuracy Of Fit Checking”, by O’Kelly et al, and “Assessment of Use and Fit of Face Masks Among Individuals in Public During the COVID-19 Pandemic in China by Pan et al).

Given the wide variety of masks that students are likely to use, and the probability that half of them or more will be improperly fitted, the incremental infection risk reduction benefit of requiring students to wear masks — beyond the risk reduction already achieved by vaccination, ventilation, and filtration — is almost certainly minimal.

In contrast, the incremental costs of masking are very likely much larger.

For example, people have been found to speak more loudly when wearing a mask, which substantially increases the volume of aerosols they exhale. Masking also increases the fraction of emitted aerosols with small diameters, which are more difficult for masks to block. (see, “Efficacy Of Masks And Face Coverings In Controlling Outward Aerosol Particle Emission From Expiratory Activities”, by Asadi et al).

“Covering the lower half of the face also reduces the ability to communicate, interpret, and mimic the expressions of those with whom we interact. Positive emotions become less recognizable, and negative emotions are amplified. Emotional mimicry, contagion, and emotionality in general are reduced and (thereby) bonding between teachers and learners, group cohesion, and learning” (“Masked Education? The Benefits And Burdens Of Wearing Face Masks In Schools During The Current Corona Pandemic”, by Manfred Spitzer).

“The World Health Organization mentions several negative aspects of frequent / long-term use of facemasks, fueling the debate as to whether the benefits outweigh the drawbacks. Many people report claustrophobic experiences and difficulty getting sufficient oxygen due to the increased resistance to inhaling and exhaling. This can lead to an increased heart rate, nausea, dizziness and headaches and several other symptoms” (“Face Coverings For Covid-19: From Medical Intervention To Social Practice”, by Dr. Carla Peeters in the British Medical Journal).

Finally, requiring students to once again wear masks in school, despite the reduction in infection risk through vaccination, ventilation, and filtration, will almost certainly trigger very strong opposition from a large number of parents and community members, which will further erode their trust in and future support for the school district and its leaders.

To be sure, future risk assessments may lead to the conclusion that masking and perhaps even lockdowns may once again be needed if a new coronavirus or influenza variant emerges that is substantially more transmissible and/or virulent than the Delta strain.

But that is not the case today. Given the complex tradeoffs we face, and the assumptions I have used about school specific factors, this risk analysis concludes that current demands that students wear masks for six hours a day when they return to school are not warranted.

Susan Miller advises school districts around the country on enrollment, facilities, financing, and other issues. She is also a member of the Board of Education for Jefferson County (CO) Schools, the nation’s thirty-sixth largest district. These are her personal views. Learn more at