What's the #1 cause of COVID-19 cases? Indoor gatherings. Infection risk peaks when virus particles concentrate over time in poorly ventilated indoor air. Let's unpack what causes that risk and how to mitigate it.
Many buildings where people gather—such as homes, schools, restaurants, and offices—are not equipped with the ventilation and filtration systems needed to prevent the spread of COVID-19, experts say.
“People are familiar with masks, handwashing, and social distancing, but fewer people are aware that ventilation and filtration really matter.” -Dr. Joseph Allen, Director of Harvard Health’s Building Program in the Wall Street Journal.
How COVID-19 spreads through the air
Why are indoor gatherings so dangerous? Virus particle accumulation.
A high school student in Korea became infected indoors after five minutes of exposure from more than 20 feet away, according to a published medical study.
Airborne virus particles or aerosols are released every time we breathe, talk, or cough. Indoors, each room we inhabit is like a box where particles accumulate over time. In each “box,” studies show that virus particles:
- Float up to 16 feet—well past ‘social distance’
- Remain airborne for hours
- Stay infectious for up to 3 hours
According to Dr. Erin Bromage, a Dartmouth Immunology and Infectious Disease professor, the formula for COVID-19 spread is (EXPOSURE TO VIRUS) X (TIME) = SUCCESSFUL INFECTION. Indoor spaces account for 90% of all transmission events, Bromage wrote. So, any enclosed environment with poor air circulation and a high density of people spells trouble.
An open letter from 239 international scientists to the World Health Organization stated:
“Viruses in aerosols can remain suspended in air for many seconds to hours, like smoke, and be inhaled. They are highly concentrated near an infected person, so they can infect people most easily in close proximity. But aerosols containing infectious virus can also travel more than [two meters] and accumulate in poorly ventilated indoor air, leading to superspreading events."
Three things we know about COVID-19 infection:
- It’s airborne. While handwashing and surface disinfection were the early focus, we now know the virus is primarily transmitted by breathing air shared with those who are infected.
- Masks and distancing, while reducing risk, will not fully protect against virus transmission indoors because the longer a group is together in a shared space, the greater the risk.
- Removing virus particles quickly from indoor shared air spaces, without relying upon constant mask-wearing, is one reason why the CDC and EPA recommend in-room air purifiers.
What is air exchange and how does it reduce COVID-19 risk?
During winter, even small indoor gatherings can quickly become super-spreaders. Why? Poor air exchange. Air exchange, measured by Air Changes Per Hour (ACH), is the total amount of “clean” air entering a room. We know that COVID risks are lower outdoors because natural air exchange disperses virus particles.
Here’s the problem when things move indoors: Most home heating, ventilation, and air conditioning (HVAC) systems recirculate indoor air mixed with outdoor air. When it's very cold or hot outside, HVAC systems recycle far more indoor air because they're not powerful enough to quickly heat or cool enough outdoor air.
“Many people are beginning to realize that their buildings can’t respond," Allen said. “Those air exchange systems are not dynamic or resilient, and they can’t increase capacity.”
The proven way to mitigate virus concentration is by pumping in clean air. But what if you can't open windows or rely on HVAC?
"Consider supplemental air cleaning with a portable air cleaner with a HEPA filter,” Allen said. “If you size these correctly for the room, for a restaurant, a school classroom, a bedroom at home, you can get 4, 5, or 6 air changes per hour of clean air, and it can be cost-effective.”
How to calculate Air Exchanges or Air Changes per Hour (ACH)
Air Changes per Hour (ACH) determines how many times an air purifier can exchange the entire air volume from a specific room in an hour.
Harvard Health recommends 4-6 full air changes per hour (ACH) to reduce COVID-19 risk.
To calculate the ACH, you only need two things:
- The dimensions of the room you want to purify
- The Clean Air Delivery Rate (CADR) of an air purifier (more info on that below)
The average living room is about 400 SqFt, according to the National Association of Homebuilders. Let’s say you have 8’ ceilings.
The Alen BreatheSmart 75i, our bestselling True HEPA purifier for large spaces, has a CADR of 400 CFM.
That means an Alen BreatheSmart 75i can provide 7.5 ACH in a typical living room, which exceeds Harvard Health’s recommendation of 4-6 ACH.
“For me, the priority is to increase the air exchange rate," said Harvard's Dr. Joseph Allen. "Consider supplemental air cleaning with a portable air cleaner with a HEPA filter.”
What is the meaning of CADR and CFM? Why do they matter?
Clean Air Delivery Rate (CADR) measures a purifier's ability to clean and circulate air throughout a room effectively. CADR is measured in Cubic Feet per Minute (or CFM). You can think of a CADR rating like horsepower in a car.
- CADR rating is the #1 metric for determining if a purifier is powerful enough to safely and effectively clean a room.
- Other important considerations include filter performance and noise levels.
Does HEPA really capture COVID? Yes, not all HEPA performs the same
Despite widespread confusion about why HEPA filter efficiency is based on 0.3 microns, HEPA is extremely effective at capturing particles in the virus size range (0.06 - 0.125 microns).
“Filters are rated for the size they perform worst at—0.3 microns,” Allen said. “As you get to larger and smaller particles, HEPA filters will capture closer to 100%."
The key differentiator is speed. How quickly a purifier can clean a room’s air. And that brings us back to airflow and CADR.
"Simply put, a car with more horsepower will do 0-60 faster," said Alen CEO Andy Graham. "A purifier with higher CADR will clear a room of virus particles more rapidly and consistently."
According to independent microbiology tests by LMS Labs, the Alen BreatheSmart 75i captured over 99% of virus particles from a 1,050 ft3 test chamber within 20 minutes.
The 3rd-party lab testing utilized a sealed test chamber and an FDA-approved live COVID-19 proxy (MS-2 bacteriophage).
“Any HEPA H13 filter in such a chamber will eventually clear it depending on its CADR airflow, said Alen CEO Andy Graham. “Alen purifiers are unique in the efficiency (time) in which we clear the chamber.”
The testing also demonstrated that Alen’s True HEPA H13 filters captured over 99% of particles throughout the entire virus range (0.06 - 0.125 microns).
Alen sought 3rd party virus testing prior to helping over 30,000 school classrooms nationwide reopen safely. Because we aren't just air quality experts, we're parents, too.
For over 15 years, we've built customer relationships based on pure air protection—with powerful, quiet, cost-effective purifiers guaranteed for life.
Five Ways to Reduce COVID Risk with Airflow
Health officials advise against gathering with those outside your household. If a small gathering is necessary or you choose to gather with people in your quarantine bubble, here are some tips to minimize risk:
- Eat and socialize outdoors if the weather permits.
- Open doors and windows while indoors when possible.
- Run your HVAC system's fan continuously.
- Be sure to use a HEPA purifier that’s right for the size of your indoor space.
- Run your air purifier at full speed before and during the gathering.
Let’s Stay Connected
Have questions about how to make your indoor environment safer? Need help selecting the best purifiers for your home, business, or school? We’d love to hear from you!
Contact our in-house team of Customer Experience experts here. Or call 1-800-630-2396 8am–6pm CST.