In the ongoing global pandemic it has become paramount to find novel solutions to help reduce the risk of viral exposure. Photo Electrochemical Oxidation (PECO) technology has been under development for 25 years and was commercialized by Molekule, Inc. in the form of portable air purification devices in 2017. PECO is designed to clean the air by both capturing particles and oxidizing pollutants in a powerful and targeted way. Filters coated with the PECO nanocatalyst readily inactivate microscopic organic matter such as viruses, bacteria, and mold spores. Given that many viruses require intact outer envelopes to successfully infect, a technology like PECO designed to quickly degrade the surface of viral particles is well-suited to their removal.
University of Minnesota College of Science and Engineering aerosol science and engineering experts and a College of Veterinary Medicine disease transmission expert undertook a project to test PECO on viral inactivation. They sought to understand the difference in survivability of a virus caught on a PECO filter. To this end they designed a series of experiments to show both the efficient capture rate of the filters in a Molekule unit and the impact on the virus viability after capture.
They found that Molekule Mini inactivated the viruses at a rate as efficient as 99.99% on a single pass through the unit.
Selection of virus species
In seeking viruses to test for this project, the team at University of Minnesota sought species that would have similar characteristics to SARS-CoV-2, the virus that is currently responsible for the global pandemic. SARS-CoV-2 is an airborne coronavirus, so the team picked three virus strains for the experiments, bovine coronavirus (BCoV), porcine respiratory coronavirus (PRCV), and H1N1 influenza A virus.
Coronaviruses were first classified in a 1968 publication and are so named for the club-shaped protrusions that stick out of their viral envelopes. These shapes look like the jewels of a crown, which in Latin is corona. This corona of viral proteins is vital to the function of the virus particle because it uses them to trick its host into gaining entry. Coronaviruses exclusively infect birds and mammals, including humans, and can cause a number of different ailments.
Bovine coronavirus (BCoV) is similar to SARS-CoV-2 as it is in the same subgroup, betacoronavirus. This group also contains the strain of SARS that spread in 2003, MERS, and many other coronaviruses that infect humans. BCoV infects both cows and calves and can cause respiratory and intestinal disease. There is currently an effective vaccine available.
Porcine respiratory coronavirus (PRCV) is a more mild coronavirus in the subgroup alphacoronavirus. It shares this group with a few coronaviruses that infect humans, but this particular species only infects pigs and causes mild or no symptoms.
A recent article published in the Journal of Clinical Microbiology looks at BCoV and PRCV specifically as viable surrogate species to study in lieu of SARS-CoV-2 which would present a risk to the researchers. It points out the most bare similarity, that BCoV and PRCV are airborne coronaviruses that infect mammals. For all three virus species the authors also note that “other shared aspects include an array of clinical symptoms and syndromes in the host (depending on age, coinfections, and stress), the patterns of respiratory disease, lung lesions, and a potential for recurrent nasal shedding.”
The H1N1 subtype of influenza A is very well-known and responsible for the Spanish flu in 1918, the 2009 flu pandemic, and countless other outbreaks. The choice to include influenza in addition to coronaviruses also helps to generalize the findings to more real-world conditions. While it lacks the same corona as coronaviruses, it is still an enveloped virus that uses proteins on its surface to access its host. The different H and N subtypes of influenza A represent different envelope proteins that vary infectivity between host cell types and even host species.
Duct testing
This testing was designed very specifically to test the survivability of viruses filtered from the air by Molekule Air Mini after a single pass through the unit using an air duct.
The team was already familiar with the scientific consensus-based industry standards around this type of duct testing. They drew from the ASHRAE standards 52.2 and 145.2 which both offer concrete and reproducible methods for this type of work. A duct was adapted from these specifications that fit around Air Mini and only allowed air to flow into the inlet at the bottom of the device.
The team used three different techniques to look for the presence of the virus, viral titer, polymerase chain reaction (PCR), and fluorimetry. Titer is the most interesting factor as it is determined by measuring the viability of the virus to infect a test line of host cells. PCR amplifies and quantifies any viral genetic material remaining, while fluorimetry measures the raw physical volume of viral material present.
For all three viruses, Molekule devices achieved at least a log 2 reduction on any of the measures, which means at least 99 out of 100 viruses were removed or inactivated with each pass through the device.
This graph shows titer and fluorimetry. Looking just at the black bars that represent virus titer, or how many viruses were inactivated from in the airstream after passing through the device, BCoV came in at log 3 reduction after a single pass. This means at least 99.9% of these viruses were inactivated by passing through Air Mini. The results with influenza reached log 4 or 99.99% inactivation. When compared with the fluorimetry results, the oxidative effects of PECO in a single pass are well represented by the additional inactivation.
Inoculation and sampling
In addition to testing the airstream, it was also important to be sure the filters were inactivating the viruses, not just capturing them. The team first tried extracting viruses from the device’s PECO-filter, but could not find any. In order to get concrete data for virus survivability on the filter, they applied the viruses directly to the filters.
The results, while not surprising, were nonetheless dramatic. Viruses exposed to the PECO filter began to be inactivated immediately.
This graph shows the effect of PECO on the virus particles over time on a logarithmic scale. After one hour the viruses on the PECO filter were more than 90% inactivated. After four hours, less than 1% of the viruses remained viable on the PECO filter.
While no air purifier can completely reduce exposure to viruses, this experiment tested some very sophisticated techniques for introducing viruses into an airstream and sampling their presence as they travel through it. Molekule encourages the use of PPE and medical countermeasures suggested by government authorities.
You can read the whole published article about this research, Wind tunnel-based testing of a photoelectrochemical oxidative filter-based air purification unit in coronavirus and influenza aerosol removal and inactivation, in Indoor Air.
