Photo: Ashak Nathwani

An experiment with the now well-known breathing thermal manikin, Laura, using an inert tracer gas presents some interesting results.

As we learn to live with the SARS-CoV-2 virus, the belated recognition that the main infections pathway is through airborne microdroplets raises several building-related questions around fresh air ventilation and also the effectiveness of the 1.5 metre social distancing rule, to name a few.

Answers to these questions are in the outcomes of an Australian-first test using a breathing manikin (one of a kind in Australia), named Laura, together with an inert tracer gas that simulated microscopic SARS-CoV-2 aerosols. 

The experiment was conducted at the Australian National Maritime Museum, in the theatre, cafeteria and entry foyer.

The key outcomes of these tests include:

  • The “virus” concentrations did not vary significantly within a six metre radius of the infected index patient, raises questions about the effectiveness of the 1.5 metre social distancing rule when indoors
  • Face masks need to be considered as additional safety measures in indoor settings where 100 per cent fresh air ventilation cannot be guaranteed
  • Indoor dining settings with 100 per cent fresh air ventilation showed significant reductions in the “virus” concentration levels, reducing risks of infection
  • Outdoor dining affords the highest level of safety
  • Indoor venues with large volumes and high ceilings, (for example, entry foyers, places of worship, shopping malls, social halls) could have potentially lower virus concentrations, provided there are no conflicting interferences to indoor airflows from associated mechanical ventilation systems
  • A loose fitting normal mask showed noticeable leakages around the nose. A properly fitted N95 surgical mask showed the least amount of leakage

The research was funded by ARBS Education and Research Foundation.

The first set of tests involved use of theatre “smoke” to visualize dispersion patterns in the index patient i.e. manikin’s breath, with air conditioning system turned off, on, and with minimum and maximum quantities of outdoor or fresh air, and with the infected person wearing different types of masks.

The results showed that the infectious breath was entrained in the convective plume of warm air rising above the index patient and then mixing into the room air.

With regards to different types of masks, a properly fitted N95 surgical mask had the least amount of leakage.

The tests involved assessment of virus concentrations at various distances around the infected person sitting in a theatre with raked seating.

No ventilation

With no outdoor ventilation at all, after 15 minutes there was already an increase of simulated virus concentration of 37 per cent inside the theatre. And the concentration at 4.5 metres distance from the infected person who was seated at the back of the theatre was virtually identical to that within their immediate vicinity.

With full ventilation

With full maximum fresh air ventilation of 100 per cent, the concentration (viral loading) fell by 55 per cent in the first 15 minutes.

Similarly, some of the observations from the tests in the cafeteria are:

With the bi-folding doors to the outside fully opened along length of one wall, the concentration levels at the dining tables fell by 77 per cent within 15 minutes.

The results validate the previous recommendations included in articles posted through The Fifth Estate last year regarding the importance of not recirculating air in indoor spaces and the use of fresh air economiser cycles.

In search of the missing piece of the Coronavirus puzzle

Guidelines from US Centre for Disease Control and Prevention (CDC) recommend consulting experienced heating, ventilation, and air conditioning (HVAC) professionals with regards to implementation of strategies and improvements that can increase the delivery of clean air and dilute potential contaminants.

Building systems that provided healthy, code-compliant indoor air quality prior to the pandemic can be improved for pandemic occupancy often using less costly interventions.

The experiment was conducted at the Australian National Maritime Museum in Sydney: Ashak Nathwani AM – director, ARBS Education & Research Foundation; Professor Richard de Dear AM and Dr Jing Xiong – IEQ Laboratory, School of Architecture, Design and Planning at The University of Sydney.

The research was funded by ARBS Education and Research Foundation.

Ashak Nathwani, AM, is director of the ARBS Education & Research Foundation

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  1. CDC recently carried out a similar analysis, albeit using recirculating HEPA units rather than outdoor air to consider the longer distance airborne transmission vector and the benefits of the HEPA units. This vector shouldn’t be confused with the shorter distance (within about 2m of the infectious individual) large droplet airborne vector which I believe is considered the main transmission path for Covid-19 and the argument for maintaining a separation distance. Their findings indicated that mask wearing alone provided a greater percentage reduction in viral concentration exposure than ventilation alone 72% reduction vs 55 to 65% dependent on relative positions within the room. The benefit of both measures used together was in the order of a 90% reduction. Its a little difficult to draw conclusions from the above testing as as the “smoke” used appears from the results description to have been buoyant and may or may not have been particularly representative of naturally exhaled droplets/aerosols. It would also be useful to know what the room ventilation air change rate was as this would have a significant impact on smoke/aerosol clearance from the space.