COVID-19 is known to be transmitted mainly in aerosol droplets exhaled from the lungs. Data shows that when people speak, sing, shout, cough, or sneeze they emit aerosol droplets of diameter ranging from less than 1 micron to greater than 100 microns. A 1995 study showed that 80 to 90% of such droplets are of diameter 1 micron or smaller. An earlier study showed the droplet size range to be 1 to 100 microns and roughly estimated the size and quantity occurring from different types of activity, as shown below.
| Activity | Qty of 1-100 micron droplets | 1 to 2 micron droplets |
| Normal breathing, 5 minutes | from none to a few | some |
| Single strong nasal expiration | few to a few hundred | some |
| Talking (counting) loudly | few dozen to a few hundred | mostly |
| A single cough (mouth open) | none to a few hundred | some |
| A single cough (mouth closed initially) | few hundred to many thousand | mostly |
| A single sneeze | few hundred thousand to few million | mostly |
Droplets of 1-micron diameter (a human hair is about 75 microns thick) have a long settling time in still air, and potentially much longer in moving or turbulent air. According to CDC data, a 1-micron particle takes 12 hours to settle from a height of 5 feet in still air, while a 3-micron particle takes 3 hours; a 10-micron particle about 8 minutes; and 100-micron, only 6 seconds. The virus within an aerosol droplet may remain viable for about 3 hours.
Within a building, a long settling time means that the droplet will travel with the HVAC (heating, ventilating, and air-conditioning) airflow throughout an HVAC zone until it is exfiltrated in the normal exchange of indoor and outdoor air or is trapped on the media of an HVAC filter, or becomes loosely attached to a surface by electrostatic forces. For energy efficiency and cost reasons, normal HVAC filters are multiple-pass filters for small aerosols; that is, their removal efficiency is such that very small particles are not likely to be captured in a single encounter with filter. Also, these filters are not perfectly sealed to their frames, so there is usually substantial bypass that effectively lowers the removal efficiency another 10 percent or so.
If people infected with COVID-19 enter an office building, they can produce an invisible cloud of mostly 1 to 10 micron droplets of variable concentration that will remain airborne in the HVAC zone and be recirculated until removed by HVAC filtration or indoor-outdoor air exchange. The HVAC zone of the highest vulnerability is one that includes the building lobby, open to walk-in customers, and is part of the same HVAC zone as employee offices. The building maintenance supervisor should check the mechanical drawings of the building to determine if this configuration exists and seek to isolate the lobby from the rest of the zone, either by expedient or permanent measures.
For an office, the normal rate at which fresh air is introduced, per ASHRAE Standard 62, is 20 cubic feet per minute (cfm) per person. An office space for one person is thus likely to have only 20 cfm of outdoor air (fresh air, assumed clean air) in its air supply diluting any contaminants. This amount of fresh air entering an HVAC system can be varied, but in summer months, bringing in too much fresh air defeats the benefits of air conditioning and drives up energy costs.
A practical, low-cost, near-term solution for increasing the clean air delivery to occupied spaces is to employ portable indoor air cleaners having High Efficiency Particulate Air (HEPA) filtration media. These units, which are mainly used to remove mold aerosols, particulates of tobacco smoke, and pollen, are rated by clean-air-delivery rate (CADR) for a specific aerosol size. The CADR is not usually advertised, although it can be calculated by multiplying the fractional efficiency of the filter against a certain aerosol size by the flowrate through the filter. Minimum sizes of the air cleaners are shown in the table below, from the EPA’s Guide to Air Cleaners in the Home (2018).
With a true HEPA filter, the removal efficiency for aerosols is 99.97 percent of 0.3 micron particles. The 0.3-micron particle is the most penetrating size. Aerosols larger than 0.3 are filtered at higher efficiency by impaction and those smaller than 0.3 are filtered at higher efficiency by diffusion. For the 1-micron size, the efficiency is about 99.99% or better, so the CADR is essentially the same as the flowrate. That is, against a challenge of variable aerosol size, a 100-cfm filter unit with true HEPA and well-sealed filter media delivers 100 cfm of clean air.
A compact, quiet commercial unit of this type can boost this clean air delivery rate by 100 to 300 cfm when operated continuously in a one-person office. Multiple units can be employed in common areas, such as the lobby. With a 100-cfm unit in an office, the clear-air delivery rate would be boosted from 20 cfm to 120 cfm.
Because the COVID-19 virus is an internal threat transmitted by infected people, these indoor units cannot, within a range of minimum CADR recommended by the EPA, provide a high level of protection as would a positive-pressure filter system employed against an external airborne threat. The indoor air cleaners can, however, substantially reduce the dose to which employees could be exposed from infected visitors’ entering the building for a short time or an infected (asymptomatic) employee in the building for a full workday. The exposure dose – concentration over time — is a significant variable affecting the rate of transmission as indicated by the recent experience of hours-long exposures in bars, meat processing plants, and hospitals.
A Honeywell portable indoor clean air unit was tested and used for enhanced sheltering in place in communities near U.S. Army chemical stockpile sites under the Chemical Stockpile Emergency Preparedness Program about 15 years ago. Those filter units contained activated carbon filters because the threat was an accidental release of low levels of chemical warfare agents.
Some of the indoor air cleaners on the market contain very thin activated carbon pre-filters to remove volatile organic compounds (VOCs) such as formaldehyde or nicotine in addition to HEPA media; such carbon pre-filters have low efficiency and capacity for VOCs and are not needed.
In selecting a portable indoor air cleaner, it is also unnecessary to have one with an ultraviolet light source for the purpose of deactivating microbes that pass through or are retained by the media.Shadowing and short residence times in passing through render the UV light ineffective for this.
There are many portable indoor air cleaners on the market. Those with true HEPA with well-sealed periphery, an airflow rate of greater than 100 cfm, and quiet operation can be selected based on best price without compromising on aerosol filtration performance.
Good candidates are the Honeywell True HEPA HPA100, HPA200, and HPA300, which have 100, 200, and 300 cfm airflow respectively. The flow rate or CADR are not stated in the advertising, but the flow rates can be calculated by multiplying the stated room square footage by 8-foot room height by stated air changes per hour and dividing by 60. The flow rate would yield a CADR of 100 cfm with true HEPA. The three Honeywell units are priced at $140, $178, and $250 respectively.
–William (Bill) Blewett July 13, 2020
About the author: Bill has nearly 50 years of experience in research, development and testing of systems for protection in airborne toxic agents and chemical, biological, and radiological defense. He holds a Master’s in Industrial Engineering from Texas A&M University and a Bachelor of Science in Mechanical Engineering from the University of Oklahoma. Bill is a freelance writer with numerous publications on chemical-biological defense and sports.