Do Air Purifiers in Classrooms Reduce Illness?

Ahh, this is certainly one time when I’m glad to be working remotely!  As my co-workers send their children back to school, the illnesses (from the common cold to COVID-19) ramp up again in their families as germs get passed back and forth in classrooms.

In 2021, there was a lot of discussion about how to keep students and teachers safe from COVID-19.  Many school districts rushed out to purchase and install air purifiers, with ensuing debate on which purifiers were effective, or in fact, which were dangerous.  It can be a bit confusing, so I headed online to find studies on what works.  I found that across a wide spectrum of experts, the following three solutions to reducing illness and increasing classroom performance are, in order, 

  1. Fresh air ventilation

  2. HVAC system filter maintenance

  3. Air purifiers

This list really is in order of importance.  First of all, air purification technology is great, but we at HypoAir are always in favor of the most natural option first, one that replicates the outdoors, and that will be fresh air VENTILATION.  That’s right, you can put an air purifier in a classroom, but without a continual supply of fresh air to increase oxygen and dilute rising CO2 and virus and bacteria levels, the air purifier can only do so much.  Fresh air can be supplied through an open window if the weather or outdoor air quality is nice, but there should be fresh air ventilation built into every HVAC system so that air quality outside doesn’t limit the quality of air indoors.  Many buildings in the U.S., especially schools, do not meet recommended ventilation rates. The quantity of ventilation depends on how many people are in the room; it should be 15 cubic feet per minute per person. In one study coauthored by Rengie Chan, a research scientist at Lawrence Berkeley National Labs, 85 percent of the California classrooms included failed to meet the minimum standard of 15 CFM per person. Other studies show many American classrooms have an average ventilation rate of only 6 to 11 cfm per person.( 

The problem with estimating actual fresh-air ventilation to a room by HVAC is that unless there is an intake from the outside in the system, air blown into the classroom is just being recirculated through the ducts and maybe only filtered once, not adding any fresh air at all.  This type of estimation requires a person knowledgeable about the building’s HVAC system.  If fresh air intake is included, then one expert (Joseph Allen, a professor at Harvard’s School of Public Health) estimates that for reducing Covid-19 risk, the air in the room should be completely replaced at least five times an hour. In a Boston school, the sensor registered about 400 cfm of fresh air coming in through the unit ventilator in one classroom. The room measured 1,010 square feet and had 9.5 foot ceilings: It had 9,595 cubic feet of air. Multiply 400 cubic feet per minute by 60 minutes, divide it by the volume, and you find that the air only gets turned over 2.5 times an hour, which was not sufficient. (  However, when conducting another experiment in the same school, they found they could boost air changes to 17 to 20 air changes per hour by opening windows and doors.

An easier way to determine if ventilation is sufficient is by monitoring the CO2 level.  To demonstrate the effect of reducing CO2 levels on disease transmission,  researchers in Taiwan reported on the effect of ventilation on a tuberculosis outbreak at Taipei University. Many of the rooms in the school were underventilated and had CO2 levels above 3,000 ppm. When engineers improved air circulation and got CO2 levels under 600 ppm, the outbreak completely stopped. According to the research, the increase in ventilation was responsible for 97% of the decrease in transmission.(

Since the coronavirus is spread through the air, higher CO2 levels in a room likely mean there is a higher chance of transmission if an infected person is inside. Based on the study above, experts recommend trying to keep the CO2 levels below 600 ppm. You can buy good CO2 meters for less than $100 online (check out our post including them); just make sure that they are accurate to within 50 ppm. ( What can it hurt to donate (or get together with other parents to donate) several CO2 sensors with remote readings, in order to check the levels of CO2 in your childrens’ classrooms?  

After fresh-air ventilation has been established, let’s look at air cleaning.  In any HVAC system, there should be at least filters in the air returns and they should be changed regularly.  By increasing the MERV rating on these filters to MERV 13, smaller particles like viruses can be filtered out to reduce illness transmission rates.  Changing the rating of the filters should be done in cooperation with the facilities manager in order not to overload the HVAC system, but it is quite possible even if filter box sizes need to be enlarged (see our post on Air Filter Thickness for how to increase MERV rating without increasing pressure drop). 

Third, air purifiers can be considered.  If the school has done what they can to provide adequate ventilation and HVAC filter maintenance, then air purifiers can add another layer of protection by filtering or killing the germs that get by these first two conditions.  In order to be effective, an air purifier must either:

  • pull all of the air in the room through a filter unit several times an hour, OR

  • Send out a non-toxic disinfectant that disperses to all areas of the room.

The first of these can be accomplished with units that include high-powered fans, but these can be noisy.  Noise in a classroom, just like in your home, can be distracting and debilitating for the teacher and students!  For this reason, air purifiers that depend upon air throughput for efficacy need to be evaluated for noise when running at the optimum fan speed for the size of classroom considered.  Also, replacement parts such as filters need to be considered in the total cost.  The cost of a HEPA filter (and possibly UV lamp) for every purifier, for example, can quickly add up to thousands of dollars a year when changes are needed in a school with dozens of classrooms.  Maintenance of these units will fall on the school’s facilities staff, who are likely already over-burdened with an increased cleaning schedule.  

The second option is one that HypoAir promotes because it really is akin to what goes on naturally outdoors.  Ions are one of nature’s cleaning devices, because positive and negative ions are continually floating through the air and reacting with allergens, viruses and bacteria, deactivating them.  These ions are naturally produced by natural phenomena in the air such as sunshine, lightning, crashing water like at the seashore or a waterfall, and plants.  Indoors, we produce them by passing a small electrical charge through stainless steel “needles” to produce positive and negative ions, which get distributed through the air to every part of the room (like adding drops of dye to clean water, soon every part of the water is changed!).  This is done nearly silently, because powerful fans are not required for distribution (any fans already in use in the room will boost circulation of the unit’s small fan).  In addition, maintenance on HypoAir ionizers is virtually nil, because no filters are required and there are no replacement parts.  The cost of running our ionizers is very small, as they use minimal electricity. 

So what about real world testing of these methods?  The CDC released a study on 123 elementary schools in Georgia in 2021.  The schools included did one of three things:

  1. Nothing

  2. Increased ventilation by opening doors, windows or using fans 

  3. Added HEPA filters to classrooms.

In schools that improved ventilation through dilution methods alone, COVID-19 incidence was 35% lower than the schools that did nothing, whereas in schools that combined dilution methods with filtration, incidence was 48% lower than the schools that did nothing.  The takeaway here is that ventilation and HEPA filtration work, even with some added cost for ventilation modification or filter replacements!  Doing nothing, on the other hand, increases the cost of lost school days, makeup time and medical costs for students and teachers substantially.   

It’s a new world with viruses and allergens challenging young and old alike everywhere, but the wisdom of fresh-air ventilation combined with the technology of purification can make it significantly easier to bear!

Photo by CDC on Unsplash