The importance of an effective ventilation decontamination programme in hospitals and care environments

During such critical times, when the Covid-19 pandemic has impacted just about every country on the planet, there has been a sharp rise in the interest and discussion surrounding infection control, from both the general public as well as public and private authorities. More column inches in newspapers have been dedicated to decontamination methods, infection control and disease prevention than ever before.

Whilst this is great to see – we should all be aware of the risks of the spread of infection, whether it is in the office, at University, schools, or simply within hospitals – there is one area that is overlooked and that we need to consider: the potential for ventilation systems within hospitals to represent a source of infection.

The possible problems posed by ventilation systems in the hospital and care environment isn’t exactly a new threat; it has long been considered that air distribution systems including both extraction and ventilation systems could harbour microbial pathogens.

This week, the University of Cambridge published research from the Journal of Fluid Mechanics that suggested ventilation systems, particularly within modern office buildings, may increase the risk of COVID-19 exposure.

The study found that ventilation systems “disperse airborne contaminants evenly throughout the space”, which could include viruses:

“Small respiratory aerosols containing the virus are transported along with the carbon dioxide produced by breathing, and are carried around a room by ventilation flows. Insufficient ventilation can lead to high carbon dioxide concentration, which in turn could increase the risk of exposure to the virus.”

Whilst it is encouraging to see the team at Cambridge publish such a study, when it comes to other literature reviews and research into the area, there is little to be found. A detailed search on PubMed and Web of Science has produced just 41 papers on the topic, a number that is dwarfed by the vast numbers of published material relating to more common or popular processes of decontamination, for example PPE.

This isn’t a critique of what’s been published, nor is it an attack on why we have failed to produce hard-hitting research to date; rather it is simply a way to kickstart a conversation on the topic and argue for a more robust and dedicated decontamination programme for ventilation systems.

So, what do we know from the reviews that have been published?

Firstly, there is evidence to suggest that the efficacy of 3 of the most common types of air ventilation cleaning processes is poor. A 2001 study titled “Effectiveness of HVAC duct cleaning procedures in improving indoor air quality“, published in Environmental Monitoring and Assessment, found that vacuum cleaning, compressed air sweeps and rotary brush cleaning methods can contribute to the spread of airborne microorganisms due to disturbance caused during cleaning. Whilst further examination showed the cleaning methods to be successful to some extent two days after cleaning, it appears that only air sweeping can be considered a viable method of cleaning.

Another study, “Environmental contamination and hospital-acquired infection: factors that are easily overlooked.”, published in PubMed in 2015, set out to assess the impact of hospital ventilation systems on Hospital Acquired Infections (HAIs).

As the study confirmed, there is an ongoing debate about the factors that contribute to HAIs, which in part has extended to the culpability of ventilation systems.

The study suggested that, whilst hand hygiene is the main focus to prevent the spread of infection within hospitals, “environmental contamination” is an overlooked area:

“it is clear that many unknowns persist regarding aerial dissemination of bacteria, and its control via cleaning and disinfection of the clinical environment.”

The study went on to call for a “carefully designed study to determine the impact of environmental contamination on the spread of HAI”.

Moving away from healthcare environments and looking at commercial ventilation systems, a 1996 study published in Environment International, titled “Assessment of growth on three HVAC duct materials”, sought to better understand the emission of spores and volatile organic compounds from heating, ventilating and air-conditioning ducts. They study placed a particular focus on the spread of infection via ventilation ducts.

The study tested the susceptibility of three types of ventilation duct, fibrous glass duct board, galvanized steel, and insulated flexible ducts. The results suggested that, of the three, only flexible ducts supported moderate fungal growth. In fact, the levels of spores on the steel duct decreased during the test period. The study concluded that dust accumulation and/or high humidity should be properly controlled in any HVAC duct to prevent fungal growth.

Another recent study set out to monitor the prevalence of fungi in ventilation ducts during major construction and refurbishment works within hospital environments. The study collected dust samples in the hospital evert six days and analysed over 70 duct dust samples. The study found that concentrations increased following three major renovation incidents.

Looking at the reviews, you can see that there are various factors that come into play when assessing ventilation systems, and there are still many unknowns. Little is known about how best to assess duct build-up in ventilation systems, whilst opinion differs in terms of the viable duct materials and the impact of environmental contamination when it comes to HAIs.

Whilst there isn’t a catch-all solution to ventilation system cleaning, we know that if we can control and guarantee ventilation decontamination, we can reduce the exposure of ‘outdoor’ or external particles, and therefore reduce the spread of infection.

As to how best we deploy a decontamination approach, the review papers suggested various methods, most of which posed certain difficulties. Two such methods which are perhaps overlooked but are proven to be effective are UV-C light and hydrogen peroxide vapour decontamination systems.

Reviewing the 40 aforementioned articles that have been published, one does in fact focus on the efficacy of UV irradiation. The study, titled “Ultraviolet germicidal irradiation inactivation of airborne fungal spores and bacteria in upper-room air and HVAC in-duct configurations”, reviewed UV efficacy within ventilation ducts, concluding that UV lamps did indeed have a significant impact on inactivating fungal spores:

“Ultraviolet lamps enclosed in ventilation system ductwork inactivated fungal spores and vegetative bacterial cells at single-pass efficiencies of 75% and 87%, respectively, at an air stream velocity of 2.2 m s(-1)”

This implies that UV-light decontamination is a viable solution for ventilation decontamination, however more research is clearly needed to arrive at a certain conclusion.

Both UV-light and hydrogen peroxide vapour play an important role in the efficacy of cleaning practices and controlling the spread of infection across various areas of hospital and care environments. UV-C light and HPV systems are residue-free and safe to use.

As to whether both systems can play an important role in ventilation system decontamination, more reviews and studies need to be undertaken, but it should be considered a viable method.

In theory, UV-C light could be deployed to decontaminate ventilation ducts and the various materials found in such systems, be it steel or fibrous glass, whilst HPV could have a significant impact on decontaminating the airflow of both ventilation and extraction systems, therefore reducing the risk of indoor microbes from downstream airflow.

Encouragingly, the team at the University of Cambridge are currently working with the Department of Transport and Department of Education to look at the impacts of ventilation on aerosol transport on trains and in classrooms. It’ll be interesting to see the progress that is made on this front in the coming weeks.

The reviews that have been undertaken to date are important and a great platform from which to further improve our knowledge of ventilation decontamination. What we need to focus on now is developing an effective programme that can confidently and safely decontaminate such systems and reduce the spread of infection.