Using a breathing manikin (one of a kind in Australia), together with an inert, traceable gas that simulated SARS-CoV-2 aerosols, tests were conducted by researchers from University of Sydney's Indoor Environmental Quality (IEQ) Laboratory, School of Architecture, Design and Planning and ARBS Education & Research Foundation.
Recognition that COVID-19 infections are largely due to airborne transmission raises several questions as we learn to live with the virus.
Key findings:
· The 'virus' concentrations did not vary significantly over distances ranging up to 6m from the index patient within indoor environments. While the '1.5 m social distancing' rule indoors was originally conceived to minimse infection risks posed by larger SARS-CoV-2 droplets (sneezing and coughing), it may be less effective in limiting airborne infectious aerosols emitted from an index patient simply breathing or talking.
· Indoor dining settings with 100 percent fresh air ventilation showed significant reductions in the 'virus' concentration levels, offering increased safety. Outdoor dining offers even more safety.
· Indoor venues with large air volumes and high ceilings, (e.g. entry foyers, places of worship, shopping malls, social halls) potentially have lower 'virus' concentrations, provided there are no interferences to indoor airflows from associated mechanical ventilation systems.
· Wearing masks is an additional safety measure in indoor settings, where 100 percent fresh air ventilation cannot be guaranteed, however, a loose-fitting normal mask showed noticeable leakage around the nose. A properly fitted N95 surgical mask leaked the least.
This is the first study in Australia in a realistic indoor scenario, using a breathing thermal manikin, with the experiment conducted at the Australian National Maritime Museum's theatre, cafeteria, and the entry foyer.
The primary objective of the tests was to evaluate the impact of ventilation on the 'virus' concentrations in a 'real world' setting.
The experiment simulated airborne infectious aerosol (SARS-CoV-2) dispersion from an infected person. The breathing thermal manikin, "Laura", provided by IEQ Lab, played the role of the infectious person by exhaling the tracer gas: nitrous oxide. The researchers measured concentrations of the gas at distances from the manikin, under different ventilation modes. This represented different concentrations of viral loading of SARS-CoV-2 in the air.
The tracer gas was collected from multiple sampling points via a network of thin flexible tubes, and then photo-acoustically assayed in units of parts-per-million accuracy, in real-time.
The overwhelming majority of studies of this type utilise computational fluid dynamics (CFD) to numerically simulate how infectious aerosols are transported within indoor air. While CFD is a very powerful tool, it struggles to realistically represent the myriad subtle details in the model. Moreover, CFD simulations are exceedingly hard to validate.
The researchers, led by Professor Richard de Dear from the IEQ Lab in collaboration with Ashak Nathwani from the ARBS Education and Research Foundation, also used theatre smoke in place of tracer gas in the manikin's lungs to visualise the exit of "infected breath" around the manikin's face mask.
Professor de Dear said: "In July last year, a group of 239 scientists (including me) from around the world published an open letter to the WHO, imploring it to acknowledge that COVID-19 was an airborne disease, and that aerosols emitted by an infected person could stay suspended longer and travel much further than the 1.5-2m recommended in social distancing guidelines.
"Eighteen months later these points are accepted wisdom in both scientific and policy arenas, so now attention must focus on how air conditioning and mechanical ventilation systems move infectious aerosols between and through the breathing zones of multiple people inside buildings.
"Numerical simulation of indoor air currents (CFD) is one solution to this challenge, but it is incapable of capturing the myriad subtle influences on the way air moves inside a quarantine hotel, hospital ward, office, theatre, foyer, etc. 'Laura' on the other hand, with her "infected lungs", sitting inside actual rooms in real buildings, affords a reality check on the CFD approach. As is often the case, measurements from idealised simulations (CFD) and reality (manikin) diverge considerably."
Mr Nathwani said: "These unique tests have enabled us to quantify the impact of fresh air when introduced into an indoor space, either through natural ventilation or an air conditioning system. The outcomes provide valuable insights into airborne transmission of COVID-19 for air conditioning and ventilation professionals, who need to work closely with health authorities to make indoor spaces safer, as per the latest guidelines by US Centers for Disease Control and Prevention (CDC)."
Declaration: This research was funded by the ARBS Education & Research Foundation
