Using BIOLOGICAL Particles as TEST AEROSOLS

Research Project Aeropore Provides Innovative Test Methods

By Bernadette Führer

Many biological hazards such as viruses, bacteria, fungi and allergens being present as airborne infectious or allergenic particles had a low profile in aerosol research until the COVID-19 pandemic. At the beginning of the pandemic, the main control strategies focused on the disinfection of surfaces and hand hygiene rather than virus transmission via air, which turned out to play the most important role regarding the source of hazardous infections. Measures, such as ventilation or air purification and face masks, that lead to a reduction in the viral load in the air we breathe, are therefore considered as the most effective protective strategy against infectious airborne pathogens (e.g., Ebola, influenza, endemic coronaviruses or molds) and against airborne allergens such as pollen spread via air.

Status Quo: Air Filter Evaluation with Non-biological Aerosols

The greatly increased transparency on the topic of bioaerosols in the scientific community and social media caused a significant increase in research activities as well as the need for solution concepts on efficient reduction options of these airborne pathogenic aerosols in indoor air, and vice versa. Thus, many innovative concepts are developed to enable lownoise and energy-efficient reduction of pathogens in indoor air. However, the efficiency of air purification to remove airborne pathogens in a real indoor environment can still only be estimated to a very limited extent at present. While established test standards with precisely defined test germs and conditions have been developed for decades in the field of surface hygiene, there is currently a strong need in the field of air hygiene for test methods that can reliably measure and quantify the efficiency of measures to reduce infectious pathogens such as viruses in indoor air.

In current routine standard operations, non-biological aerosols such as salt particles, oil-like droplets or fine dust are applied for air filter evaluation (ISO 16890, DIN EN 71460-1, ÖNORM/EN 149). Regarding the investigation of air cleaners, there are promising studies aiming at more realistic indoor test scenario using e.g. NaCl aerosols (Küpper et al., 2019). While these particles represent the size of viruses, nanometerscale particle deposition is also largely related to the physicochemical properties of, for example viruses, which cannot be easily simulated by a saline aerosol. It becomes particularly difficult when it is not a matter of pure filtration, but when the viruses are to be inactivated (e.g., by UV light ionization or electrochemical processes). Here, realistic experiments with infectious pathogens cannot be substituted, since the determination of the infectivity of the particles plays an essential role.

Testing with Real Airborne Microorganisms

Testing with real biologically active viruses, bacteria or allergenic particles, especially at large scales can be challenging and complex. However, they could support better understanding of infection control and pathogen transmission, especially for risk minimization instruments such as filtration systems in health care settings. As working with very hazardous and harmful pathogens is often not possible, the application of less harmful microbial model strains is recommended in several aerosol studies.

One possibility to mimic pathogenic viruses are bacteriophages, which are bacterial viruses infecting specific bacterial host cells, which means, they are nonpathogenic to humans. Although these bacterial viruses reflect the properties of critical pathogens such as Ebola, influenza or various endemic or pandemic coronavirus strains quite well, they can-