Guest blogger Dr Alexander Edwards (bio below) has written a post in preparation for IPC Journal Club on Wednesday (details and registration for that here)…

Why I chose this article?

The COVID-19 pandemic highlighted the importance of ventilation in mitigating against the transmission of airborne respiratory pathogens and improving air quality in indoor environments. However, as is the case in many UK hospitals, reliance on natural ventilation as the main source of airflow in patient ward means that attainment of the recommended ventilation rates is difficult to manage. Airflow in indoor spaces is affected by external weather conditions and can lead to uncertain airflow patterns and consequently, the unexpected transport of infectious material to connecting zones. However, the effects of weather conditions on airborne transmission are often overlooked.

Guidance in the Health Technical Memorandum 03-01 (HTM03-01) recommends that patient wards have fresh air ventilation rates of 6 air changes per hour (ACH), with preference for natural ventilation where possible. Although natural ventilation can have cost and energy benefits, the internal design of hospitals has changed with the addition of doors for increased patient privacy and changes to windows or other external openings for safety, security or efforts to improve energy efficiency. This puts emphasis on considering internal airflow and ventilation for future building design and retrofit.

Key findings:

• Natural ventilation and consequent indoor airflows can lead to unexpected transport of infectious respiratory pathogens to connected indoor spaces. This leads to uncertainty in infection risk predictions, meaning particular time periods or zones may experience a higher risk than others.

• The predicted ventilation rates achieved through natural ventilation fall far below the recommended HTM03-01 guidance of 6 ACH. Even with higher ventilation rates being attainable with natural ventilation, it is possible that it is not as efficient at removing infectious concentration as the alternative of mechanical ventilation.

• Implementing mechanical ventilation can lead to a large reduction in infection risk and provide a more reliable airflow, dominating uncertainties caused by time-varying weather conditions.

• Despite the addition of 3 ACH mechanical ventilation, the overall average ventilation rate still fell below the recommended 6 ACH. However, a large reduction in infection risk was predicted, illustrating that “something is better than nothing”.

Limitations of the study

One of the main challenges with a study such as this is validation. Characterising a disease outbreak to allow for accurate modelling and quantification can be complex. It is often difficult to know the exact origin and original transmission route of the outbreak, and to quantify occupant factors such as infectiousness, immune response, symptoms, and behaviour.

However, it is not the aim of this study to predict exact outbreak patterns; rather by using modelling approaches, it is possible to develop a better understanding of the effects of natural ventilation on infection risk.

What this means for IPC?

Modelling studies can help in developing a better understanding of factors which affect airborne disease transmission and through predictions, quantify the likely infection risk and potential for outbreaks, whilst assessing the effectiveness of mitigation strategies.

In this case, the study highlights that many wards may be suffering from limited or insufficient ventilation, increasing the risk to occupants not only from a disease transmission perspective, but also the indoor air quality of hospitals more broadly. The work emphasises the need for careful consideration of ventilation and airflow in retrofits and future building design, ensuring that future guidance on ventilation is consistent with and not inhibited by other recommendations e.g., for privacy, safety and security.

Bio: Dr Alexander Edwards

Dr Alexander Edwards is a Lecturer in Aerosol Science at the University of Bristol. He completed his undergraduate degree in mathematics at The University of Manchester and his MSc. and PhD in Fluid Dynamics at the University of Leeds. His research focuses on modelling airborne transmission and associated infection risk using a combination of airflow modelling and mathematical risk, with specific focus on UK healthcare settings.