We need to talk about ventilation

While the public debate about COVID has largely been dominated by a discussion regarding vaccination for good reason, as States currently locked down emerge from lockdown and schools, restaurants, cafes, retail shops, pubs and bars and offices open up, the conversation needs to shift to the next layer in the hierarchy of controls – engineering controls and in particular, ventilation.

Businesses in those States should use the time available before re-opening in a few weeks to prepare their workplaces to welcome back workers, contractors and the public. Recognising that a proportion of the population such as those under 12 still cannot be vaccinated and others such as the under 16 group will not have had a sufficient opportunity to be vaccinated, controllers of premises used as a workplace have a duty of care to ensure that their premises are safe and free from risks so far as reasonably practicable. That includes considering the design of the premises and specifically the question of the adequacy of airflow and ventilation in enclosed premises such as shopping centres, offices, restaurants, cafes, bars and pubs and schools.

Professor Mary-Louise McLaws and I considered this issue in our Managing COVID-19 Risks in the Workplace: A Practical Guide, LexisNexis, 2021 book drawing on the vast expertise of my co-author. The SARS-CoV-2 is thought to mainly spread by larger droplets and aerosols. When free air exchange of the indoor built environment is low, aerosol size SARS-CoV-2 particles may build up because these size particles are light enough to remain suspended in the air longer than larger droplet size viral particles. To prevent transmission in offices, cafes, restaurants, bars and pubs the clean air exchange must be more than the pre-COVID-19 adequate ventilation requirements (Tooma & McLaws (2021)). That is, the approach to ventilation and airflow must be revisited.

The science

A combination of a low temperature and higher relative humidity favours the spread of droplet particles and transmission when physical distancing cannot be maintained. A slightly higher temperature but lower humidity accelerates indoor transmission of aerosol transmission. Therefore, the survival rate of the virus indoors can be reduced with high rates of clean air exchange. The phase of disease that changes the mucous composition of the airways may increase the number of particles and the size of the particles. So when airway mucous interacts with a cough, the number of aerosol particles  increases. The combined factors of the stage of the disease state, and the indoor environment (temperature, humidity and ventilation) may increase the risk of transmission (Tooma & McLaws (2021)).

Improving airflow will reduce the risk of transmission of both aerosol particles and droplet particles in crowded conditions in shopping malls, restaurants, bars and offices. Setting the air-conditioning to lower humidity may be a key method of preventing transmission of  viral aerosol particles. Refreshing the indoor air may increase the dispersion of both droplet and aerosol sized particles from building up and reduce the risk that just one COVID-positive person (who might not yet know they are infectious) could infect customers and staff (Tooma & McLaws (2021)).

A COVID-19 hospital ward of average size requires 60–80 litres per second per patient of full air change: that is 10–12 full changes of clean air per hour per patient. That gives us a useful baseline for what we should be striving to achieve so far as reasonably practicable.

The US Centre for Diseases Control and Prevention (CDC) has released guidelines to assist businesses to improve ventilation and airflow in indoor environments to address the risk of COVID transmission. These Guidelines are a useful reference for Australian businesses as they prepare for re-opening and in addressing their duty of care as controllers of premises used as a workplace. Safe Work Australia has also released guidelines on Improving ventilation in Indoor workplaces: COVID-19.

The CDC Guidelines recommend amongst other things:

• Greater use of outdoor space – this will be particularly relevant for schools, cafes, restaurants and bars. It may require the cooperation Local Council Authorities to provide permits to allow greater use of walkways, laneways and open spaces but Councils will have their own duty of care and a duty to consult, cooperate and coordinate with overlapping duty holders.
• Opening windows and use of fans to augment airflow – this may be relevant for office and school environments. Care should be taken to assess the risks of the combination of fans with windows to ensure the directional flow of air reduces the risk of transmission
• Assess and Maintain existing ventilation and air-conditioning systems – the adequacy of controls should be constantly under review. While traditionally air conditioning systems have been seen as a control of climate temperature their dual use in the context of COVID risk requires specific assessment and review.

The CDC guidelines also recommend consideration of ultraviolet germicidal irradiation and High-efficiency particulate air filters (HEPA). HEPA filters are used in a hospital in combination with a negative air pressure room (with the main aim of preventing airborne particles from escaping into the corridors where masks are not worn) and the healthcare worker in the negative air pressure room continuously wearing a mask. The scientific position in relation to the use of HEPA filters outside healthcare settings is not well settled and businesses should seek expert advice on this issue.

Acknowledgement: The author would like to acknowledge the contribution of Professor Mary-Louise McLaws in reviewing and commenting on this article.