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Experts: Air Cleaners Still Allow Some COVID-19 Risk

[CREDIT: Professor Shelly Miller] Engineering Professor Shelly Miller's video tutorial on COVID-19 & mitigation methods, including ventilation and air cleaners.


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WARWICK, RI — The 1,306 room Rabbitt Air (models SPA-700A and SPA-780A) air cleaners Warwick Schools spent $659,940 to filter classroom air are among the best at filtering pathogens like COVID-19 out of the air, but leave some risk of infection, experts say.

According to the manufacturer, the filters trap allergens and particles 0.3 microns in size at 99.97% efficiency and particles less than 0.1 microns in size at higher than 99% efficiency, sufficient to filter out particles carrying COVID-19, according to the EPA.

The RI Department of Health measures ventilation of potentially virus-laden air by how often the air in a space is completely replaced. “This is called Air Changes per Hour (ACH). In a 30-foot by 30-foot classroom that has 25 students in it, the air should be replaced at least every 15 minutes, which equals an ACH of 4. If the air is replaced at least every 10 minutes, there is an ACH of 6, which is better. There is not a standard for ACH, but we do know that a higher ACH lowers the risk of disease spreading through the air,” according to the agency.

“As we are following RIDOH guidelines, the district is striving to achieve the most air exchange with the least impact on our older electrical infrastructure,” said Steven Gothberg, director of school buildings and maintenance at Warwick Public Schools. “The Rabbitt units have 5 filters, 1 prefilter, 1 medium filter, 1 BioGS* True HEPA filter, 1 “Germ Defense filter”, 1 Charcoal Based Activated Carbon filter and also ionization,” he said.

“The Rabbitt units draw a very minimal .5 amps per unit and still move approximately 200 cfm (cubic feet per minute), with very low DBa (noise level). With 2 units per classroom, along with our existing exhaust fans, unit ventilators and the ability to crack a window or 2, gives most classrooms the 4-6 air exchanges per hour we were trying to achieve. Any interior classrooms without windows or unit ventilators have either 3 or 4 Rabbitt units installed, depending on the size of the classrooms, achieving the required air exchange,” Gothberg said.

The air cleaners will be used when Warwick Public School students return to in-person schooling Jan. 7.

While effective in limiting the spread of the novel coronavirus that causes COVID-19, air cleaners are not, by themselves, sufficient protection against infection, according to the EPA. “However, by itself, a portable air cleaner is not enough to protect people from COVID-19,” the agency warns.

“Air cleaners (a better term) reduce the risk of long-range transmission, which can be understood by watching this video that I made. It would help to reduce risk in a classroom from a kid who might be shedding virus and is sitting in the back of the room while you are teaching in the front of the room,” said Shelly Miller, Professor of Mechanical Engineering at the University of Colorado, whose research focuses on urban and indoor air quality, control technology, health effects, particulate air pollution, and development and evaluation of indoor air quality control measures.

Richard L. Corsi, Dean of the Maseeh College of Engineering and Computer Science at Portland State University, agreed.

“Centralized filtration systems (in building mechanicals systems) and portable HEPA air filters both reduce levels of virus-laden particles in the far field but not in close contact. For the latter the best protection is universal mask wearing and distancing.  The only highly-effective and proven portable (stand alone) air cleaners for indoor spaces are portable HEPA air cleaners,” Corsi said.

In Miller’s video tutorial on COVID-19 and various mitigation methods, including ventilation and air cleaners, also embedded at the top of this article, she charts the methods of COVID-19 transmission and multiple precautions:

Indoor risk of COVID-19 infection methods

  • Contact transmission There isn’t strong evidence that contact transmission (touching objects with the virus, then your nose or eyes) is how people get COVID-19.
  • Sprays of large particles from an infected person’s speech, sneeze or cough into your eyes, nose or mouth can transmit COVID-19. This requires close contact.
  • Airborne/Aerosol transmission or particles emitted when people speak, sneeze or cough, floating in the air, can spread COVID-19. This can happen near or far from infected people.

Multiple precautions required to reduce COVID-19 infection risk

  • You cannot ventilate your way out of short-range COVID-19 infection risk
  • Ventilation is effective in reducing long-range COVID-19 infection
  • Masks and social distancing should be combined indoors to reduce short range COVID-19 infection
  • Indoors is never totally safe from COVID-19 transmission.

Warwick Post asked Miller and Corsi two follow-up questions on indoor transmission of COVID-19:

Warwick Post: Is there a percent reduction in transmission risk (from air cleaners) available to give readers? 

MILLER: No you would have to do a risk calculation using our risk estimator tool: COVID infection risk calculator.

CORSI: Used in the right space and under the right conditions, portable HEPA filters with sufficient clean air delivery rate (CADR) can lower levels of virus-laden aerosol particles in air by as much as 60 to 90 percent as upper-bound values.  I recently delivered a national webinar on the use of these for school classrooms.  My slides are available at under the seminar pulldown and slides.  My webinar is also available with a link under seminars.  I showed how to estimate the percent reduction for a specific scenario.

Warwick Post:Does an air cleaner system make being indoors in a school or movie theater setting safe with COVID-19 circulating at a high rate (10 percent) within the community?

MILLER: Yes, air cleaners, filtration, ventilation can all help to make being indoor safer.  You must also wear masks and social distance.  You especially want to minimize your time indoors in crowded spaces or in large groups.

Community transmission rates are important but also contextual.  How many are outbreaks in a nursing home?  In a meat packing plant, linked to a bar?  How many are because of having birthday parties in your home?

CORSI: No.  No technology makes an indoor environment completely safe.  In the webinar that I refer to above I discussed layered risk reduction strategies.  If done right, it is possible to reduce risk by over 90 percent (up to 95 percent) with universal mask wearing, proper physical distancing, increased ventilation, improved filtration (from lower MERV-rated filters to MERV-13 if possible in a system), and possibly portable air cleaners or UVGI systems. [Editor’s note: HEPA filters are not MERV rated as they exceed the ASHRAE test protocol 52.2 used in determining the MERV ratings, according to industry sources.]

Portable air cleaners do not make sense in a movie theatre.  Significant increases in ventilation (outdoor air) with improved filtration, universal mask wearing (no eating), and de-densification (fractional capacity to allow distancing) might get you to 90% dose (and risk) reduction.  Upper room UVGI (ultraviolet germicidal irradiation) would also help.

Having said that, a 10 percent community infection rate is too high for me to personally recommend anyone going to a non-essential indoor environment.  I do consider schools as essential for the physical, social, mental and educational development of children.  For schools, we should do everything we can to achieve layered risk reduction so that kids can be back in school, but NOT without those risk reduction steps when community infection rates are 10 percent.

There remains an inferno of infection across much of the US and the only way we can deal with this until we have widespread vaccination (which is not likely to occur for at least another 6 to 9 months) is to starve this virus of its hosts as much as possible.  The more the public takes chances when they do not have to in non-essential indoor environments (gyms, restaurants, theaters, etc.), the more this virus will thrive, potentially mutate, and cause more suffering and deaths.  I understand the devastation that this advice has on businesses (I have good friends who are small business owners).  It’s a really difficult situation.  Had we done the right thing 10 months ago we would not be where we are now and we’d be living life more like those countries who did it right.   But we are where we are, and the best thing that we can do now is to take every possible precaution, avoid any non-essential indoor environments, and try to get things under control so that businesses can get back to some level of capacity with proper protections in place as opposed to opening and closing in fits and starts.

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