Michael Riediker

This article was first published in Nanotoxicology. It is reprinted here under the terms of the Creative Commons Attribution-NonCommercial-NoDerivatives License (http://creativecommons.org/licenses/by-nc-nd/4.0/), which permits non-commercial re-use, distribution, and reproduction in any medium, provided the original work is properly cited, and is not altered, transformed, or built upon in any way.

Citation: Michael Riediker, ‘Nano-Safety Research Lessons for Dealing with Aerosol Transmissions of COVID-19’, Nanotoxicology, 2020, 1–3 <https://doi.org/10.1080/17435390.2020.1786185>.

The coronavirus disease 2019 (COVID-19), which emerged in Hubei Province, China is caused by the severe acute respiratory syndrome virus (SARS-CoV-2) (Gorbalenya et al. 2020). A particularity of the novel Coronavirus disease 2019 (COVID-19) is the occasionally very high viral load in respiratory lining liquid, which can render the liquid highly contagious. Viruses have many properties that are well known to nanomaterial researchers. They are bilayer lipid membrane nanovesicles in the nanoscale range (roughly 200 nm diameter in the case of SARS-CoV-2) coated with proteins and sugars that keep it well dispersed and that allow cell targeting. To infect a human, these vesicles must reach the target cells to unload the contents of the vesicles, similar to what a nanomedical membrane vesicle does (Schütz et al. 2013). In other words, after the virus is released from the host cell into the surrounding bodily liquid, it corresponds to a large part to a nanovesicle dispersed in an aqueous salt and protein solution. Many of the strategies developed for safe work with high-activity nanomaterials dispersed in aquatic solutions can thus be expected to work also for the virus.

When working with hazardous chemicals and biological agents, touching and ingestion must be prevented. This is well covered by COVID-19 recommendations to wash hands. In addition, processes that can – even accidentally – spray and aerosolize the nanodispersion need to be checked in detail. An efficient risk management addresses the risk at the source, during transmission in the air, and at the target (WHO 2012). What do we know about these processes and what are effective strategies? To understand this question, we need to look at the processes governing release, transfer and exposure.

In the case of nanodispersions, we need to understand the processes leading to aerosolization and how big the resulting droplets will be, as this is decisive for the fate of the droplets. Humans that have respiratory symptoms typically “spray” (a directional high-speed release process) large droplets of “nanodispersions” (the liquid present in the nose and upper airways) also known as sneezing and coughing. In addition, humans always, whether sick or not, release a cloud of much smaller aerosols during breathing, speaking, talking, singing, chanting loudly or coughing (in increasing order) (Johnson and Morawska 2009; Asadi et al. 2019), which corresponds to a fogging process. This “personal cloud” consist of micro-droplets of lung lining liquid that becomes airborne when the terminal bronchioles re-open during inhalation (Johnson and Morawska 2009). In both processes, the nanoparticles dispersed in the lining fluid are included in the droplets, which is known from studies of nanoparticle exposed individuals (Sauvain et al. 2014). The nanoparticle source strength (how much is released per time unit) corresponds to the cumulative volume of liquid that got airborne.

To understand the fate and transfer of aerosolized nanodispersions, it is important to look at the different aerosol sizes and their physical behavior (Willeke and Baron 1993): large “sprayed” droplets, larger than about 10 to 20 µm emitted during sneezing and coughing will sediment rapidly, while smaller aerosols can remain airborne much longer. Once a microdroplet has become airborne, it will disperse in the air. How many of the droplets remain airborne depends not just on their size but also on the turbulence and speed of air. In calm air, the larger droplets – those carrying the biggest load of viruses – will sediment more rapidly than in fast turbulent air. Also, if the aerosol is released into dry air, the droplets will shrink in size and be more likely to remain airborne. Initially, a shrinking size will only result in an increase of the virus concentration in the droplet, though complete drying may passivate the virus (Vejerano and Marr 2018).

Once we understand the release and transfer of large and small droplets, we can estimate the resulting exposure in different situations. For large droplets released during spray processes, distance and orientation to the spray path are essential, while for the smaller airborne droplets it is a question of how much time a person spends in a shared airspace and the virus concentration in the air. The safety strategies can be defined along these lines. They include technical and organizational measures and appropriate use of personal protective equipment and aim to keep the cumulative exposure (the dose) as low as possible.

Protection against spray exposure by keeping distance or by using face shields seems effective for situations where the time spent in the joint airspace is short and where the room is large and well ventilated. This may explain why there are no reports about excessive infection numbers of supermarket cashiers. The airborne route becomes important when being for a prolonged time in the same room as a virus carrier, especially if that person is singing, shouting or physically active, as seen during superspreading events in choirs (Hamner et al. 2020) and fitness centers (Jang, Han, and Rhee 2020) and having a high viral load in the lining liquid (Riediker and Tsai 2020). In contrast, passing on the street through the “personal cloud” of an individual virus carrier will be a very low risk, as evidenced also by the landfall of the Princess Diamond in Taiwan where 627,386 persons had been in proximity of any of the 3000 embarked passengers, but nobody got sick as a result of the landfall (Chen et al. 2020). As soon as people have to spend prolonged time together, wearing face masks becomes important. Masks can be used as emission control devices (surgical masks) and to protect the carrier (respirators). Wide spread use of simple face coverings and surgical masks aimed to control emissions were found to be very effective at reducing community transmissions, while respirators along with splash guards were effective to protect medical staff in high risk situations (Chu et al. 2020).

With the end of the lockdown and the reopening of the borders in many parts of the world, it is important to keep the number of infections low. Namely super-spreading events need to be prevented, for which the airborne route seem very important. Wearing masks should be enforced in situations where many people spend more than a few minutes in the same room, such as in trains, buses, or theaters, and also at work. There will be situations where masks cannot be worn. They may interfere with the intended activity, such as eating and drinking in a restaurant, or they can cause a serious burden such as difficulty breathing and thermal stress in situations of heavy physical work, especially when it is combined with high temperature and humidity. However, especially in situations where people exhale and inhale heavily during physical work, protection is needed, and it will be important to assess the possibilities to reduce the airborne route. In professional settings, cooling wests, positive pressure masks and more aggressive ventilation may help, while non-essential activities such as discos may have to be limited to open-air situations where the sky is the limit for ventilation.


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