Nanofiltration must be combined with laminar vertical flow to minimize virus infection risk

Abstract

A SARS CoV-2 infected person may emit up to 10 million viruses with each m3 of exhaled air, and on the other hand, the infection dose may be as low as 500-1000 viruses. Given these numbers, the infection risk in a room cannot be sufficiently reduced by periodic window ventilation or by partly cleaning with mobile air cleaners but requires a continuous perfect virus elimination. Furthermore, lateral flow from the infected persons to their direct or distant neighbors must be avoided. This sounds utopic but can indeed be solved by body heat induced vertical aminar flow and ventilation from floor to the ceiling, extracting the contaminated air at the ceiling and recycling it via a filter back to the floor. Filtration must be very close to 100%. Since the « naket » Corona virus has a size of 60-150 nm, which is the typical size of particles emitted by combustion engines. The diesel particle filter DPF, a honeycomb type ceramic wall-flow filter, was selected for this nanofiltration task. These filters reach > 99% efficiency for soot particles of 10-500 nm and have attractive properties since their filtration surface is > 1 m2 per liter bulk volume, they can easily be cleaned in situ, if needed thermically disinfected or catalytically coated, and have the life, the quality, low bulk and low cost of an automotive product. To test the bioaerosol filtration properties, bacteriophages MS2, which are non-pathogenic to human, animals and plants and have a viral particle size of 30-40nm were used for the test as a proxy for SARS-CoV-2 virus. The wall flow filters reached a filtration rate of >99 % for these bacteriophages and the survival test resulted in 1% active viruses after 24 hours, zero after 48 hours. To test the whole system, a classroom was selected as a pilot case. The ventilation was designed to ex-change the room’s air volume five times per hour. Contaminated air was extracted at the ceiling and recycled to the floor corners after filtration. Fresh air from outside supplied for CO2-control was also filtered to clean it from ultrafine traffic related carcinogenic particles. The dynamic cleaning process and room distribution was tested with salt nanoparticles simulating the virus source by a cloud concentration of 80'000 particles per cubic centimeter. The lateral flow reaching the neighbor desk of the infected schoolboy contained only 2-300 P/cc, demonstrating that the contaminated air escapes vertically, showing a cross-contamination risk reduction of 2-3 orders of magnitude. This virus protection system is universally applicable, not only for applications in classrooms, but can also be scaled and adapted to industry, hospitals and public transport environments including aircraft cabins.