How Employers Can Minimize Risk During the Pandemic: MERV 13 Filters and Portable Air Purifiers

As non-essential workers consider returning to the office, employers will need to answer one question above all else: Is it safe?  

In spite of rising vaccination rates, booster shots, and habituated social distancing, the risk of a COVID-19 infection continues to restrict our daily activities and influence our decisions.  Variants such as delta and omicron appear to be increasingly more contagious than previous strains and better equipped to evade the immune response gained by prior infection or vaccination. This has experts predicting a cat-and-mouse game between vaccine developers and a rapidly evolving virus—with no end in sight.

By offering a safe workspace, companies can provide a path for their employees to move forward, even as the pandemic draws out and takes unpredictable turns.  Improvements to air quality control in the office using HVAC filters or portable air cleaners can help minimize the risk of spreading COVID-19–regardless of what new form the virus takes.  

National surveys by Wakefield Research found that 85 percent of U.S. workers were looking forward to returning to the office and 95 percent expected their office to provide COVID-related protections. Of those who have returned to the office, 76 percent were concerned about being exposed to COVID-19 and its variants or interacting with unvaccinated coworkers.  Only 59 percent felt their companies were adequately protecting them.  

“Buildings are our first line of defense against COVID-19,” writes Dr. Joseph Allen, in an op-ed for the New York Times . “If managed poorly, they can spread disease. But if we get it right, we can enlist our schools, offices and homes in this fight.”

Dr. Allen, director of the Healthy Buildings program at the Harvard T.H. Chan School of Public Health, has spent over a decade researching how the engineering, design, and maintenance of buildings impact health. He led forensic investigations of indoor outbreaks even before the pandemic. In addition to proper social distancing and mask-wearing, Dr. Allen explains, effective air ventilation and filtration can reduce our exposure to viral particles by over 99 percent. 

The Case for Airborne Transmission

Following months of petitioning from aerosol scientists, virologists, and epidemiologists, the WHO and the CDC finally updated their guidelines in April and May of 2021, respectively, to acknowledge that COVID-19 can spread through airborne transmission. This means that someone can release infectious viral particles just by talking or breathing.  Unlike droplets from sneezing or coughing, these particles are so fine that they mix with the air (become aerosolized).  Aerosolized viral particles can accumulate and linger in a room for hours or circulate through the ventilation system into a different part of the building, leading to superspreader events.

Several case studies showed airborne transmission even before the CDC changed its recommendations. Dr. Allen and his colleagues came to this conclusion after using forensic modeling to study the Diamond Princess cruise ship, which he calls a “floating building,” where 712 out of 3,711 passengers and crew were infected in early 2020. 

In November 2021, there was a case in China where a traveler, who repeatedly tested negative while staying in a quarantine hotel, contracted the omicron variant from someone who was isolated across the hall, suggesting that the virus spread through the ventilation system. 

So how can we protect ourselves from airborne transmission inside the office?

The Basics vs Airborne COVID-19: Masking & Social Distancing, Plexiglass & Open Windows

While strict masking and social distancing practices still help prevent airborne transmission in the office, they can be difficult to enforce.  Coworkers may take off their masks to eat or drink. Some could wear masks made of poor filtering materials while others wear loosely fitted masks. Employers can help bridge the gap between ideal and realistic behaviors using engineering controls.

Physical barriers like cubicle walls and plexiglass can limit exposure between colleagues working from adjacent desks, but they essentially function as a sneeze-glass.  Several studies have shown that plexiglass can impede airflow and trap respiratory aerosols, causing higher infection rates. In other words, barriers are not effective against airborne COVID-19 and can actually make it worse.  Aerosols still build up in a room–regardless of physical barriers or spacing–depending on how many viral particles the infected person is breathing out and how many the ventilation and filtration systems are removing. 

Exchanging potentially infectious air inside the building with fresh air from the outside is ideal, but many offices do not have windows that open. Instead, they have HVAC systems that recirculate mostly indoor air. Recirculated air causes a higher risk of infection during outbreaks, explains Allen. The ventilation and filtration in office buildings have not been designed for infection control.  They tend to meet the bare minimum of standards set before the pandemic by The American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE)..  

HVAC and MERV-13 Filters

Adding better filters to HVAC systems can help prevent COVID-19 from spreading through a ventilation system.  Although the CDC says more studies are needed to determine whether COVID-19 can be spread through HVAC systems, it recommends “using the highest efficiency ventilation filters possible, without having detrimental effects on overall HVAC system performance.”

The ASHRAE and Dr. Allen both recommend using a MERV filter with a level 13 rating or higher.  MERV stands for Minimum Efficiency Reporting Value and represents the filter’s ability to capture different sized particles.   Ratings range from 1 to 16; before the pandemic, most commercial buildings used MERV 8 filters, which offered a compromise among health, comfort, and energy concerns.

A MERV 13 filter captures 50% of particles in the 0.3 µm to 1.0 µm size range and 85% of particles in the 1 µm to 3 µm size range, according to the CDC.  Although the COVID-19 virus is only 0.1 µm, it does not float in the air by itself: its sticky lipid envelope causes the viral particles to clump together inside respiratory droplets. Using a 2012 study on the influenza virus, which has a similar size to COVID-19, the ASHRAE calculates that MERV 13 filters effectively capture 90% of COVID-19 particles.  Dr. Allen puts this efficiency closer to 80%.

Variants like delta and omicron have mutations to the spike protein that help it spread more easily or evade our immune system’s defenses.  However, these mutations do not change the virus’s basic physical structure or size, meaning that these filters will be equally effective in capturing any variant. 

The caveat to installing MERV 13 filters is that many HVAC systems, especially older ones, cannot accommodate them. Higher rated filters cause a higher resistance to airflow, which puts strain on the HVAC system’s fan.  This can reduce the system’s ability to circulate outdoor air or filtered indoor air, reduce occupant comfort, raise utility bills, cut its longevity in half, and cause mechanical failures like cracked heat exchangers and damaged compressors. Overhauling or replacing a commercial HVAC system can be prohibitively expensive, especially for larger spaces.  Installing an HVAC system in an office building can cost between $15 and $23 per square foot, and a commercial HVAC system averages between $6,000 and $12,000 for a 1,000 sq ft building.  

Portable Air Purifiers

For buildings with HVAC systems that cannot accommodate a MERV 13 filter, Allen recommends using portable air purifiers.  Even if an employer upgrades the HVAC system to replace a MERV 8 filter with a MERV 13, that still only raises the capturing efficiency from 20 to about 85 percent.  Air purifiers can fill the holes in an office’s anti-air defenses, especially in high traffic areas.

Quality air purifiers are usually equipped with high-powered fans and HEPA (High Efficiency Particulate Air) filters, which are “at least 99.97% efficient at capturing human-generated viral particles associated with SARS-CoV-2,” according to the CDC.  HEPA filters are even better at capturing COVID-19 than MERV 16 filters, but they are almost never used in HVAC systems, even in medical settings, because they would overload them.  The CDC describes HEPA-equipped portable air purifiers as the best option for auxiliary air cleaning in high risk environments like health clinics, vaccination and medical testing locations. 

A study at the Royal Melbourne Hospital found that two HEPA-equipped portable purifiers removed over 99 percent of simulated respiratory aerosols from a single-patient hospital room in just 5 minutes.  A separate study found that two HEPA equipped portable air cleaners reduced aerosol exposure between a simulated infected speaker and two uninfected participants by 65 percent.  A combination of masking and air purifiers reduced exposure by 90 percent.

In considering a work environment that is also comfortable for employees, portable air purifiers offer a safe solution.  HVAC systems equipped with high rated MERV filters not only consume more energy, but can cause pressure to drop inside buildings, which can cause general discomfort, sinus headaches, migraines, or joint pain.    

How do you choose a portable air purifier and how many do you need?

Aside from a HEPA filter, portable air cleaners should have a fan capable of moving a large volume of air.  The fan’s power is measured in cubic feet per minute (CFM).  Look for the Air Delivery or Clean Air Delivery Rate (CADR) on the label or specifications.  Simply put, the higher the number, the more powerful the fan, the quicker the air is sanitized. 

Calculating how many units you need in a room is relatively easy.  If you convert CFM to CFH (cubic feet per hour) and divide it by the volume of the room, you get air changes per hour (ACH).  ACH measures how often polluted air is replaced with clean air.

For example, an air cleaner with a CADR rating of 300 CFM in a 500-square-foot room with 8-foot ceilings generates 4.5 ACH. ( 18,000 CFH / 4000 cu ft  = 4.5 ACH).  For a 1000-square-foot room on the same floor, two of these units would be just as effective.

To prevent the spread of COVID-19 in indoor public spaces like office buildings and schools, the Harvard Healthy Buildings program recommends anywhere between 4 and 6 ACH.  This range is on par with standards used in health care facilities, where airborne infections could be life-threatening even before the pandemic.  The CDC guidelines set a very high bar for areas in a hospital where patients have open wounds: operating rooms and trauma rooms should have a minimum of 15 ACH.  Recovery rooms, as well as critical and intensive care units, should have a minimum of 6 ACH. 

Using the same size room in the previous example, A HEPA-equipped portable air purifier rated at 400 CFM could generate 6 ACH.  In other words, a 100 CFM difference in fan motor power can determine whether or not a room meets healthcare standards for controlling airborne infections.

  Measurements like CADR and ACH can also be used to calculate how long it takes for a portable air purifier to replace contaminated air with clean air.  The CDC provides tables to help calculate room clearance wait times for airborne pathogens, but these require estimating of a mixing factor (k value between 1 and 10), which represents how well the ventilation system mixes and dilutes the concentration of airborne particles within the room. 

Some portable air cleaners, however, automatically monitor the level of pollutants in the room using air quality sensors.  Once the sensors cannot detect contaminants, the unit will indicate to the user, usually with a light on the control panel, that the air is clean.  These systems can provide workers peace of mind that their workspace is safe, making them less anxious and more productive.  One study found that these smart portable air cleaners can reduce peak exposures to particles under 2.5 microns compared to continuous filtration, making them best suited for areas with high-traffic or acute emitting events.

Noise is another factor that HVAC experts and consumers consider when comparing portable air purifiers.  A unit’s noise or sound level is measured in decibels and listed under its specifications as dB(A). Decibel levels, like earthquake magnitudes, scale logarithmically, meaning a sound level of 60dB(A) is ten times louder than 50db(A).  A normal conversation is about 60db(A), the same level attributed to air conditioners.  Approaching subway trains, car horn at 16 feet, and sporting events are measured around 100db(A). 

In an office, a purifier with sound level above 60db(A) could drown out a conversation, making meetings or collaboration difficult.  In most cases, portable air cleaners will be placed in each room, and unlike a centralized HVAC system, employees will have control of the units.  If they are routinely frustrated by loud motor impeding communication, they could periodically turn it off or stop using it altogether.  In most cases, quality portable air cleaners are equipped with better motors, which can run quieter, even at higher speeds.

In concert, these technologies can help companies add several lines of defense against COVID-19.  Although zero-risk is not possible in the context of airborne infectious diseases, minimizing risk is.  In the context of the workplace, these measures help employers keep employees safe. In the context of the global pandemic, they can help alleviate the pressure on the healthcare system by reducing infections outside the office.  

In addition to protecting against COVID-19, Dr. Allen and his team found that better ventilation and filtration can lead to improved cognitive function.  He thinks that improvements made to buildings during the pandemic will continue to provide health benefits and raise productivity after it is over.

“We are an indoor species. We spend 90 percent of our time indoors,” says Allen in an interview with MedCram. “My hope with a silver lining coming out of COVID is that people recognize how much buildings impact our health and that it leads to a fundamental shift in terms of how we prioritize these places where we live, work, play, pray, and heal.”