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How to Choose a HEPA Filter Air Purifier to Clean a Classroom or Dentist Office

In March of 2020, the American Dental Association asked dentists to only perform urgent procedures during COVID-19 outbreak. Schools went remote and parents began counting down the days for a return to normalcy.

Lots of guidance has been issued around the use of personal protective equipment (PPE) as well as routine cleaning and disinfection procedures that include the use of “high-efficiency particulate air (HEPA) filters or filters of equivalent or superior efficiency.”

For important spaces like schools and dental offices, proper air ventilation and purification is an important element in staying open and minimizing airborne infections. This guide is written for both schools and medical offices to assist in how to choose the right HEPA filtration system.

It's based on questions we've received from teachers, dentists, and other medical professionals. Our focus will be on how to clean rooms in a classroom, dental operatory and other shared areas like waiting rooms and school libraries. These spaces have the most traffic, so they're also where air purifiers can have the most benefit.
First, we'll share a brief overview of HEPA filters and how to compare air purifier performance.

HEPA Filter Efficiency

Our smaller models (OV200, Finn, and Max) remove more than 99% of airborne particles down to 0.3 microns in size.

In comparison, NIOSH N95 masks are rated to remove 95% of particles down to 0.3 microns.

What Size Are Viruses And Bacteria?

Virus particles range in size from 0.02 to 0.4 microns. Most bacterial cells range in size from 0.2 to 10 microns. Common E.coli and rod-shaped bacteria are generally 1 micron by 2 microns.

For viruses how well do your air filters work from 0.02 to 0.4 microns?

Testing standards are set at the 0.3 micron size so testing data is not readily available for the smaller sizes. To address this, we sent our Max and mod HEPA filters to one of the few labs that will test the performance against nano-sized particles.

Here is a summary of the results.

mod Air Purifier Removal efficiency (each air pass):

0.02 microns - 99.4%

0.04 microns - 98.7%

0.10 microns - 99.4%

0.12 microns - 99.5%

Max Air Purifier Removal efficiency (each air pass):

0.02 microns - 96.6%

0.04 microns - 95.7%

0.10 microns - 98.2%

0.12 microns - 98.67%

0.04 microns is the point with the lowest efficiency for our filter. The filter efficiency is higher for particles smaller and larger than 0.04 microns. As described below the effect of diffusion will result in higher efficiencies at particles sized smaller than the 0.04 microns (40 nanometers).

There are two forces at work in a HEPA filter. Interception is the effect of the fibers catching particles that are larger than the opening in the filter fibers. Diffusion is the effect of the smallest sized particles that collide with one another and therefore are able to be collected in the HEPA filter fibers. The efficiency related to diffusion increases significantly as the particle size approaches zero.

According to Science Direct, “High Efficiency Particulate Air (HEPA) filters are available that have efficiency greater than or equal to 99.97% even at the MPPS (most penetrating particle size). For particles with diameters between 2 nanometers (0.02 microns) and the MPPS, HEPA filter users should be confident that the efficiency will be at least 99.97% when the filter is new.”

In short, for particle removal of nanometer sizes the higher the HEPA filter efficiency, the better.

How long does it take to clean the air in a room after aerosol producing activity?

This is a function of the amount of air flow. Air purifiers are tested to a standard called the CADR (Clean Air Delivery Rate). This represents the volume of clean air produced at various particle sizes in CFM (cubic feet per minute). It is based on a ceiling height of 8 feet.

You will want to use the Smoke CADR since that is based on the smallest particle sizes (0.3 microns). The other two measurements are called Dust and Pollen and should be ignored in this case.

Here is a chart that shows the smoke CADR for our air cleaners and the number of air changes per hour for various sized rooms.
How to know room sq ft coverageFor example, the EJ120 can clean a room of 2,483 square feet once per hour and a room of 497 square feet five times per hour (so every 12 minutes).


What would the CADR be to change air in a room 150 square feet with 9 foot ceilings in 5 minutes?

CADR represents the amount of filtered air in cubic feet per minute (CFM). To clean the air once every five minutes would require a CADR of 1,350/5 = 270. The 150 square foot space with 9 foot ceilings = 1,350 cubic feet.

The EJ120 with a 331 CADR (CFM) cleans 41 square feet per minute. The 169 square feet (150 square feet with 9 foot ceilings) room can be cleaned in 4.1 minutes on the highest fan speed. In ten minutes it will filter the air in a 169 square foot room about 2.5 times.

Speed 4 is 200 CADR which is 25 square feet per minute. It will take 6 min 45 sec to clean your space on speed 4.

Tip: Take the cubic foot size of your room and divide by the number of minutes you want it to be cleaned. The result is the minimum CADR of the air purifier for that space.

Can they be wall mounted?

Our air purifiers are meant to be placed on flat, secure surfaces. They can be placed on a shelf provided there is at least one foot of clearance to the front, top and left/right sides.

Wall mounting your air purifier can be a great space saver. Keep in mind that you also want maximum air flow to it for maximum performance. Any air purifier that's mounted to a wall will have decreased air flow to it and therefore decreased performance. 


Which air purifier is suitable for a classroom or medical office?

Any of our models could work depending upon the room size. Our mod air purifier models have the highest efficient filters so they will do better with catching the smallest sized airborne particles. The mod air purifier is the best combination of high efficiency, high air flow plus a lower price point.

When considering different options, you'll also want make sure the air purifier has sufficient air flow for the space size. This is because, no matter how powerful the filter, the air needs to reach the filter first in order to be passed through it. 

This is why the process of choosing the right air purifier takes different considerations and calculations.

You'll want a motor that's powerful enough to pull all the air in your space to the filter. You'll want a filter that can truly clean the air of the particle sizes that you're concerned about. And you'll want all of this in a package that's energy efficient, attractive, priced for your budget, designed for many years of use, and easy to use.

How many air changes per hour for a waiting area?

We suggest four to five air changes per hour for a waiting area. In general, the Max could clean 400 square feet, the mod 500-600 square feet. Multiple units will achieve a cumulative effect. So, two mod units will cover 1,000-1,200 square feet. We also offer an automatic multi-unit discount if this is the right solution for your needs.


How long does it take for a delivery?

Our warehouse usually ship orders placed Monday through Friday by 2 pm EST. If we're out of stock, we'll share the expected ship date on the product page.


Any other questions?

Our Customer Service team is happy to help with any questions you have about Oransi products or air purifiers in general. Just email us at


*The Erik air purifiers are retired as of October 2021. The EJ air purifiers are retired as of January 2022. Oransi still offers filters for the Erik and EJ air purifiers and will still honor the 10 year warranty for anyone who has purchased an Erik or EJ model.