Strategies designed to limit contact with the pathogen, such as physical distancing or the use of protective equipment, are the first line of defense against viral outbreaks.
When the route of entry of the virus is our airways, knowledge of how the virus is transmitted in the air is key to applying effective strategies. Viruses can only survive in an aqueous environment, so they are in fact always transmitted through the aqueous particles we naturally excrete, and not as single particles.
Here, epidemiology teaches us that the minimum size of the aqueous particles in which the virus retains its infective potential makes a huge difference to the rate of transmission . Droplets is the term used to describe respiratory particles larger than 5 µm. They are mostly produced when we talk, scream, sing or sneeze, do not project very far, and tend to fall to the ground quickly. Particles below 5 µm in diameter are termed aerosols because they have the ability to remain airborne for much longer periods than the droplets and can travel much longer distances. Thus, respiratory viruses that are able to retain infectious potential in aerosols generally have a much higher transmission rate than those that can only retain infectivity in droplets.
In this regard, the SARS-CoV-2 pandemic has proved a textbook case. At the beginning of the pandemic, as virus transmission was thought to be through droplets alone, mitigating measures such as hand disinfection and surgical masks were regarded as sufficient to prevent the spread of the disease. As it became increasingly clear that transmission was also airborne, recommendations became more stringent, with countries such as Austria making the wearing of respirator masks (N95 or FFP2) compulsory in enclosed public spaces .
Respirator masks are tested for filtering efficacy using sodium chloride particulate aerosols and are qualified to filter ≥95 % of particles over 300 nm . While they offer significant protection to the wearer, they do not constitute an absolute barrier against a virus contained in a nanometer-range aerosol. This is why the SARS-CoV-2 pandemic has also seen the rapid development of portable HEPA filters, which are qualified to retain ≥99.97 % of particles below 300 nm, for room air purification .
Water-borne viruses, which generally use our digestive system as a portal of entry, constitute a different challenge. Poliovirus, hepatitis A, and noroviruses are amongst the smallest known human viruses, averaging about 30 nm in diameter (Figure 1). As they do not use carrier particles, but are directly infectious, a water filtration system must ensure that particles in the size range of the virus itself are blocked.
Here, progress is being made with the development of innovative nanofilters (e.g., nanofibrillated cellulose filters), which have added specific functionalization to increase virus retention. As viruses tend to be negatively charged, functionalization aimed at increasing the net charge of the filter membrane can increase their filtering capacity. In this regard, Anton Paar’s SurPASS 3 electrokinetic analyzer is used to characterize the zeta potential of antiviral water filters  .