Airborne transmission of virus-laden respiratory droplets has drawn a great deal of attention since the outbreak of COVID-19. Ambient relative humidity is the key factor controlling the evaporation of water from airborne droplets. The evaporation processes of respiratory droplets due to decreases in relative humidity are not well understood because of the complexity of the chemical compositions of the droplets. This article reviews the current understanding of the effects of relative humidity on the physicochemical and aerodynamic properties of respiratory droplets and on the infectivity of enveloped viruses.

Severe acute respiratory syndrome coronavirus 2 (SARS‑CoV‑2) is the virus that causes the new coronavirus infection (COVID-19). The molecular structural basis of SARS-CoV-2 infection has been investigated by various structural biological studies including three-dimensional (3D) structural analysis of virus Spike protein, which directly mediates infection to human cells through the interaction with Angiotensin-converting enzyme 2 (ACE2), which locates on the cell surface of some of our tissues (e.g. lungs). Further, the 3D structures of main protease (3CLpro) and RNA-dependent RNA polymerase (RdRp) of SARS-CoV-2 provided useful information for understanding the mechanism of the amplification of viral genome RNA in our cells.

In this paper, I would like to give an easy-to-understand overview of the current state of structural biology studies related to COVID-19, and also discuss the possibility of rational design of drugs.

Worldwide pandemic by COVID-19 still continues. Ventilation is recommended as one of the important measures against virus infection. However, if ventilation measures are taken in the wrong way, not only the measures can’t reduce the risk of infection, but also they may make increase it. The effect of ventilation as an infection control, how to determine the volume flow rate requirement, and the ventilation method for effective air exchange has been studied. Based on these results, how we should consider effective ventilation to reduce the risk of viral infections were discussed in this paper.

There is considerable public interest in monitoring airborne viruses in ambient air. The typical method to monitor airborne viruses is to collect them into a liquid and analyze the liquid suspension to enumerate the viruses. As a result, an effective sampling device is a critical component of a system for monitoring airborne viruses. This study presents a novel electrostatic aerosol-to-hydrosol sampler that collects aerosol particles on the wet-inner surface of a rotating cylindrical chamber. The prototype's collection performance was evaluated in terms of physical and biological efficiencies, and the results were compared to those of BioSampler, a liquid impinger. The biological recovery efficiency (BRE) of our prototype for Influenza A virus subtype H1N1 was as high as the previously reported BRE of BioSampler. Furthermore, our prototype's aerosol-to-hydrosol enrichment capacity was approximately 50 times that of BioSampler. Additional tests were carried out to simulate the concentration of airborne viruses during flu season. The collected suspension was analyzed using real-time PCR after collecting the test aerosol for 10 minutes. The result shows that the measured Ct-values in the collected suspension are within the dynamic range of the real-time PCR indicating that our prototype can collect a large enough amount of viruses from ambient air during flu season.

A small air cleaner with the visible-light-responding photocatalyst was developed. This air cleaner is specialized in removal of the splashed droplets which flies between facing peoples to cope with various infectious diseases. Photocatalyst of a tungsten trioxide system which response efficiently to visible-light was sprayed on a conventional nonwoven fabric filter with a visible-light LED module and a silent type fan are applied. The photocatalyst was excited by visible-light to generate active oxygen that oxidizes and decompose various organic matters. In this study, the physical performance of the air purifier was estimated, 1) the removal action of splashed droplets from the oral cavity with special motion-picture system, 2) evaluation of the transmittance of droplets to the filter and 3) droplets capture performance in a clean bench. Furthermore, performance of the photocatalyst system was evaluated by formaldehyde gas decomposition rate in a small chamber. As a result, droplets of 5 micrometers or more can be removed effectively by the small air purifier, and it is considered that the virus contained in the droplets caught by the filter will be inactivated with the photocatalyst by degrees.

A particle collector that allows high time resolution monitoring of particulate matter was fabricated and coupled with an ion chromatograph for online analysis of particulate ions in the atmosphere. The system was applied to the atmospheric observation for over a month in Tokushima, Japan. The average particulate anion concentrations were 6.42 nmol m⁻³ for Cl⁻, 18.8 nmol m⁻³ for NO₃⁻, and 22.1 nmol m⁻³ for SO₄²⁻. The atmospheric particle collection efficiency was more than 98.4%, and the continuous observations were successfully achieved without problems.