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What’s in the Air: All About Filters and Airborne Virus Control

I'm an accredited journalist working at the intersections of science, food and public health. I am also a certified nutritionist.

The lifting of restrictions has led businesses to review their control strategies to limit the risks from airborne viruses.


Air Filtration Effectiveness in Reducing Aerosol Virus Spread

A fraction of the population is about to spend more time inside as we gradually return to offices, even if only a couple of times a week, and indoor spaces are where nearly all transmission of COVID-19 occurs.

We know from recent experiments that the virus spreads primarily through the air in large and small droplets, which can remain aloft longer. Studies have also shown that the virus remains viable in the air with a one-hour half-life.

Healthy building strategies, such as improved ventilation and filtration, are among the great tools that can help slow the spread of this and future viruses.

Defining Airborne Viruses

Viruses can collect in airborne particles, known as bioaerosols. These aerosols form a cloud of particles smaller than 100 µm that can travel beyond 6 feet (up to 16 feet away) when an infected person coughs, sneezes, sings, talks or breathes.

The SARS-CoV-2 aerosol contains particles 5 or 10 microns in size and takes about 8-30 minutes to disperse and fall onto the ground. These aerosols rapidly accumulate in poorly ventilated indoor air.

The virus particles stay suspended in the air until they are pushed outdoors by ventilation or trapped on a filter.

The conditions that affect how long they stay in the air and can be inhaled include lack of sufficient ventilation, closeness to people and the crowd’s density, which produces more aerosols simultaneously.


Factors Impacting Airborne Transmission

Research tools for studying SARS-CoV-2 particles, such as light scattering, speed of travel in a chamber and the amount of virus load in the respiratory fluid, have revealed the impact of temperature, solar irradiation and humidity on the size of the particles.

This is important because particles smaller than 5 microns carry more viruses than larger ones among those that stay in the air long enough to be inhaled.

Many viruses survive well in dry conditions, so adding humidifiers to the home or workplace is one strategy to reduce the size of droplets and particles.

Challenges of Studying Airborne Viruses

Due to technical challenges and unrecognized importance, the study of airborne viruses has long been a vastly underexplored research area. Now it has become one of the hottest topics around.

Studies that have looked at how airborne viruses spread through the air are few and far between, although there have been studies targeting particular viruses, including the influenza A virus.

Yet studying the structure of viral aerosols — often through metagenomics — can tell us a lot about airborne transmission trends so we can develop better countermeasures that promote more effective air-cleaning methods.

Such studies can inform us about how particles of a given virus strain are transported through the air, considering factors like settling velocity, particle density and size, virus transport distances, wind speed, and more.

In plain English, these tell us how long the virus stays aloft, how far it travels, how quickly it falls to surfaces and how efficiently filters remove it.

Metagenomic studies of viral aerosols are the most accurate to answer these questions. They use reference databases for sequenced viruses, but these databases are limited.

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Only about 1% of all viruses known to man have been cultured, sequenced and uploaded to one of the widely used databases.


General Strategies to Prevent Airborne Transmission

Some general guidelines can be followed as we return to the office to minimize risks. One is to increase ventilation with fresh air, not recycled air, as much as possible.

Your litmus test is to imagine a cigarette smoker standing in the room. In this room, will I take in any smoke? If yes, then the space is likely to be poorly ventilated. Some precautions include:

1) Opening windows and doors to regularly bring in more outdoor air.

2) Upgrading air-filtering systems. The air-change-per-hour rate should be as high as possible (equal to or superior to 12 cycles per hour).

3) Using surfactant-containing cleaning detergents to inactivate the virus on impact when particles hit the ground.

Control Strategies in High-Risk Environments or Situations

Schools require a layering of strategies for lowering risk transmission: good ventilation in the room, distance and, ideally, everyone wearing masks. There should be no mixing of recycled air.

Group sports activities provide benefits — of being active — which arguably outweigh risks. However, it is recommended to prioritize individual skill drills whenever possible indoors.

The music industry has had to pivot significantly in the past two years. Singing is more like coughing than talking, which increases exposure tremendously in confined indoor settings. Hosting gigs outdoors and spaced apart is preferable.

Travel is on everyone’s mind as countries reopen frontiers and reduce testing requirements, especially the risks associated with long-haul flights.

Airline executives have argued that the air-filtration systems on planes effectively eliminate virtually all airborne pathogens.

A 2020 study conducted for the U.S. Department of Defense supports this view and found that aircraft ventilation and filtration systems reduced the risk of airborne SARS-CoV-2 exposure by more than 99%.

However, at best, these studies represent only a baseline understanding of how aircraft air-handling systems impact the transport of virus aerosols throughout planes. They do not consider the short-range type of aerosol transmission when sitting a few rows away from an infected person.

Of course, traveling aboard a half-full plane is safer, but most airlines no longer sell tickets with reduced-capacity seating.

The risks in Ubers and taxis mainly stem from contaminated surfaces. In addition to not touching our faces, it is best to keep windows open by at least 3 inches and wipe our hands when exiting.


Improving Indoor Ventilation and Air Quality

Enhanced ventilation acts to dilute and remove any airborne virus, using fresh air or filtered-recirculated air to reduce the airborne concentration and, therefore, the exposure/transmission risk.

Stale inside air should be replaced by outside air several times per hour, aiming for an air-exchange rate of six per hour. That recommendation comes from studies of tuberculosis transmission, which spreads farther and stays longer in the air.

It is essential to monitor the airflow and know the air-exchange rate of commercial HVAC systems. It is possible to adjust the amount of fresh air pumped into a building’s ventilation system. Many HVAC systems do not run on 100% outside air by default, which is considered too energy intensive.

Older buildings may have faulty or deficient exhaust fans that prevent optimal ventilation. Different structures from different eras will potentially need other solutions too.

Newer systems, including air-cleaning and -filtration technologies for different purposes, are becoming ever more efficient. But there are limitations. For example, plane ventilation systems will lessen the build-up of airborne viruses in the passenger cabin to reduce or prevent longer-range airborne transmission; they won’t prevent short-aerosol transmission.


Ventilation and Filtration Upgrades

One of the significant challenges in diluting airborne viruses is humidification. Humidity determines the final size of the viral particle.

The lower the relative humidity, the faster the viral particles get inactivated and the less distance they travel.

Maintaining relative humidity in the 40-60% range can decrease the risk of transmission with many viruses. One study shows that the COVID-19 virus decays faster at close to 60% relative humidity than at other levels.

Our modern HVAC systems in most public or commercial buildings, including hospitals, don’t have built-in humidification — unless we pair them with portable humidifiers.

What Makes a Good Air Filter?

HVAC filters vary significantly in their efficiency at trapping bioaerosol particles, depending on the air filter’s minimum efficiency reporting value (MERV) rating.

MERVs rate a filter’s performance in filtering out particles between 0.3 and 10 microns. The higher the MERV number, the higher the filter’s probability of removing these particles. Filters with the highest MERV ratings on the market require greater power to move air through them.

For reference, the MERV ranges from 1 to 20, and studies of virus bioaerosols typically use sampling filters with a MERV rating greater than 12 to guarantee that 90% of bioaerosols are collected.

The American Society of Heating, Refrigerating and Air-Conditioning Engineers suggests MERV-13 filters or higher or a portable HEPA filter. HEPA filters filter out at least 99.97% of particles that are 0.3 microns.

Building operators should look out for improperly sealed filters that allow unfiltered air to sneak past them and recirculate in the building, which substantially degrades filtering ability. These higher-MERV and higher-quality filters need to be serviced more frequently.


Putting It All Together

These new research insights on the kinetics of COVID-19 virus aerosols and air transmission pathways help further our understanding of better mitigation strategies for this and future outbreaks.

Adopting some of these strategies can appreciably reduce exposure to airborne viruses and vastly improve indoor air quality.

This content is accurate and true to the best of the author’s knowledge and is not meant to substitute for formal and individualized advice from a qualified professional.

© 2022 Camille Bienvenu

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