Aerosol Science Significance for COVID

Thomas Grillo, Contributor

This past October, The American Association for Aerosol Research conducted a virtual press conference to share insights on the airborne spread of the novel coronavirus. The conference was presented by a professional panel of aerosol researchers who shared convincing evidence that supports the fact that aerosols, not droplets, may be the leading cause of the pandemic.

Both the Center for Disease Control (CDC) and the World Health Organization (WHO) have held that large ballistic droplets and surface contact with the virus are the key modes of transmission.

Both organizations have been hesitant to accept aerosol transmission, partly due to their historical bias, even contrary to the evidence being presented. It is important to note that the CDC now has toned down their previous stance against aerosol transmission.

Jose Jimenez, Ph.D. of University of Colorado at Boulder provided multiple examples to support the premise that COVID 19 is primarily spread through aerosols expelled from infected persons.

The example of the super-spreading event at the Skagit Choir in Washington, U.S., demonstrated that during the 2.5-hour rehearsal, a single infected person caused 52 new infections, some of them more than 13 meters behind the spreader.

The super-spreading event evidenced low exposure to droplets (close proximity conversations) or fomites (touching common surfaces). The extremely high infection rate can be explained by the aerosol suspension of the virus, due to a low ventilation rate in the room that was well mixed in a space where all present were singing with no masks being worn.

It is important to understand that the size of the virus organism is typically between 0.06-0.13µm (microns) in diameter. Particles smaller than 5.0µm are easily suspended and remain airborne for many hours—even days—depending on their size and the conditions of the enclosed environment. These conditions can include convection, movement and activity from occupants, air handling systems, fans, etc.

Documented Study Confirms

During a study conducted in 2017 at the Healthy Building Summit, in Seven Springs, Pa., Particles Plus instruments were able to document the particle suspension times for 1.0µm particles which were ~ 8 to 10 hours, based on a sealed and activity-free control room. The room, when disturbed, created measurable and repeatable values that were monitored over multiple days and continued to demonstrate this approximate suspension time.

Smaller particles less than 1.0µm, down to 0.1µm, can stay airborne for days due to multiple conditions in the enclosed indoor environment, including Brownian Motion, with smaller air molecules impacting and creating chain-reaction movement of the larger airborne aerosols.

Practice the Three Cs

Linsey Marr, Ph.D., of Virginia Tech University-Blacksburg, stressed the importance of Japan’s “Three Cs” for infection control: avoid closed spaces; avoid crowded places; and avoid clos-contact settings.  Ventilation, social distancing and PPE are all key in helping to reduce the risk of transmission, but not completely eliminating it.

Professor Kelly Miller discussed the ventilation rates and air changes per hour, sharing results of a study that demonstrated that, in indoor spaces with very few air changes per hour, high transmission rates have been documented. However, spaces that have high air-change rates had much lower transmission cases.

Professor Miller also discussed a case of an outbreak of Tuberculosis at a university, where transmission flourished with air changes of 1.7 liters per second per person. Following a retrofit of the building’s ventilation system, it then increased the air changes to 24L/sec/person. In doing so, the university effectively stopped the outbreak in its tracks.

Air Purifying Systems

Air purification/air cleaners are other effective methods to clean the smaller particles from the air in an indoor environment. This is being highly recommended with many commercial facilities upgrading their filtration systems and improving the performance of cleaning out sub-micron particulates. Properly sizing an air purifier is important. You must know the clean-air delivery rate of the device, so you can determine how large of a room can be effectively used to properly clean that room.

Particle counters are being used by more contractors, facility engineers and property managers to understand their buildings’ indoor particle count levels, especially the respirable particles below 2.5µm, and then testing again following retrofits and improvements to their filtration and air purification systems or air handlers with filtration. Many filtration system resellers and installing companies have also been utilizing particle counters to demonstrate the effectiveness of their products, creating even more awareness of the amount of aerosols that remain airborne in indoor spaces that are not well-ventilated or -filtered.

It is important that our scientific and medical communities work together to better understand COVID 19, and to help develop and educate the public on sound, common-sense practices to help reduce the spread of this horrible virus.

Particles Plus is proud to support the many institutions, think-tanks, laboratories and government agencies who are involved in aerosol-dispersion testing and research. It is our goal to provide the tools needed to accurately and repeatedly quantify the submicron aerosol exposure in public and indoor settings. What is key is that as a society, we all employ common sense; accept that this is real and not something political; and be concerned for our well-being and the well-being of others through our actions and the use of faces masks and honoring the Three Cs. IHW

Thomas Grillo is Sales Manager at Particles Plus, Inc.

[Editor’s note: This article first appeared on Particles Plus’s “News & Education” blog: ]

For more information about the American Association of Aerosol Research visit them at

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