Air Filtration
If the climate does not allow for sufficient outside air dilution or the building systems do not provide the recommended air exchanges per hour, localized filtration is a fast and economical solution to providing a sufficient number of air changes per hour (ACH), which is defined as the number of times the equivalent volume of air in a room is turned over, either by exchange with outside air or passing through a filtration device.
The design standard for minimum ventilation in classrooms is approximately 3 ACH (ASHRAE 62.1 (2019)). However, the recommended ACH for reducing airborne contaminants is 5 or more. This would mean that the air in a classroom would be completely exchanged at least 5 times an hour, or once every twelve minutes. Greater than 5 complete air changes per hour offers more dilution but overall diminishing returns make any additional performance over 6 ACH less economical and not as impactful.
The figure below models the probability of infection from being in a room for 5 hours with between 1 and 4 infected persons and no masks, with varying levels of clean air changes per hour. Whether there were 1 or 4 infectors exhaling an average of 31 infectious particles (quanta) per hour, the increase is ACH from 2 ACH (which is slightly better than the 1.5 ACH average air exchange rate in schools based on studies) to 5 ACH reduced the probability of infection by ~45%. In a 200 square foot room with 8 foot ceilings, Jaspr Pro provides over 16 ACH without considering contributions from the HVAC system.
As a consideration, the airflow rate from existing building systems (dilution/filtration) and portable air purifiers equipped with HEPA filters are additive, so the ACH from each can be combined to provide improved air flow and circulation for occupants.
Building systems should have HVAC systems with MERV 13 filtration or better. While portable air purifiers should have HEPA filters which provide a minimum of 99.97% capture efficiency for all particle sizes with even higher performance for particles of diameters below 0.2 microns and greater than 0.5 microns.
The reality of getting people back to work and students back into classrooms for learning and socialization is impacted by the logistical considerations of the cost of improving or replacing existing HVAC systems and the time it would take to undertake such capital projects, especially as there is currently a dearth of supply.
Again, portable air purifiers with HEPA filters offer a quickly deployable, economical, and highly effective solution for reducing the risk of airborne virus transmission in classrooms and workspaces.
Use our guide to selecting air purifiers to understand what performance metrics and features are most important to helping you achieve safer indoor air quality.
Key Considerations
Purification Technology
▢ Does the unit have a certified HEPA filter?
EVERY single certified HEPA filter is tested 2-3 times each, depending on the certification standard, to ensure it meets the stringent performance required of HEPA certification : 99.97% capture efficiency or better.
▢ Avoid features that add cost without really being additive to performance.
The residence time of air through a purification unit is not long enough for UV technologies to have an effect on most passing microbes. AND if you have a HEPA filter, that filter is already capturing 99.97 - 100% of all particle sizes.
▢ Definitely avoid features that are ineffective and harmful
Ozone generators and electrostatic cleaners will not prevent re-distribution of harmful particles into the air and ozone in the breathing zone has been shown to be harmful at higher concentrations - so you don’t want to be generating this molecule that is the major component of smog in the closed spaces of your practice. Ozone belongs in the upper atmosphere, not in our classrooms and offices.
Cleaning Capacity and Placement
▢ Airflow: What are the CFM levels at different speeds?
Be sure that the units you are studying can move high volumes of air with their filters in place like they would be in use. The higher the cubic feet per minute (CFM) of air it can handle the more frequently you’ll get the room air changed over.
▢ What ACH will you get?
The CFM tells you what volume of air will be handled per minute, but what matters to your staff and students is the resultant air changes per hour (ACH) your room will experience. This will be based on room size - and configuration matters to get it exactly right, but for a good starting estimate a calculator for ACH can be found here.
▢ Placement and Flow Patterns
Consider how you will fit a unit into your space so that it will be able to function properly.
Noise Level
▢ What is the noise level of the unit at its high and medium settings?
▢ How many CFM do you get at an acceptable noise level for all day operation?
Noise is an important factor to consider. If the unit is too loud and gets turned off, the cleaning efficiency is ZERO. Look for a unit with a noise level below 70 decibels at its fastest setting.
▢ Does the unit automatically detect air quality and adjust its speed to match?
