Tackling the risk of airborne infection

Andrew Carnegie discusses the importance of effective elimination of airborne pathogens in the healthcare setting and provides advice on key considerations for supplemental air cleaning.

The paper ‘Modelling aerosol transmission of SARS-COV-2 in a multi-room’ facility’1


out a clear case for using air filtration around SARS-COV-2 as an aerosol transmitted pathogen. However, there is a clear dichotomy between this well researched paper and current infection control practice. Scientifically, due to an experiment in 1956,2

only tuberculosis is currently deemed to be airborne. Richard Riley and William Wells took four years to prove their case to the scientific community and it is striking that, in 2020, we find ourselves no better informed while facing a pandemic with an airborne aspect to transmission. While coronavirus may well be the current popular topic, there are of course many other diseases such as aspergillus, which are well known to be transported via our air. Thus, the purpose of this article is not to focus purely on SARS-COV-2, but air in general, with a review of what approaches can be taken to reduce risk. This will also include clear explanations of the various filtration standards and approaches being seen in the current market.

In many clinical areas, approaches to ventilation fall outside of direct clinical input. There are well constructed guidance documents known as healthcare technical memoranda (HTM) which deal with how this should be approached; a list of these is included as an annex to this article. For the purposes of this article, the most important of these is HTM 03-01, which deals with ventilation.3


What should be noted is that all the documents are guidance and are not a legal requirement. This is important because they are not renewed as quickly as technology changes and HTM 03-01 is now 13 years old, having taken several years to draft. It is also an evolution of a previous standard, HTM 2025, which it replaced. It is often the case that these standards can perpetuate errors in earlier documents or indeed take so long to be updated that the update is already requiring refreshing when first published. I was involved in the ITU revision of HBN57 (2003) from the former HBN27 (1992). That document commenced review around 1996 and was verging being out of date by the time it was first published. The last amendment occurred in 2013. One glaring error with HTM 03-01 is its reliance on ceiling mounted points for incoming air and extracted air. This means that the main air flow is between these two points across the ceiling, and not across the full room as is often supposed. Research into this was conducted by BSRIA in 2015 in their project ‘Design of isolation rooms for infection control’.4

This showed that there is a clear benefit in creating a flow vector that crosses the room. In this manner, the required dilution being sought is effective, whereas in the majority of healthcare rooms this is not the case. Thus, it follows that over reliance on calculated air change rates, while well intentioned, actually gives very little information concerning the actual level of air dilution in a particular room. After over 100 years of ventilating healthcare buildings, what is certain is that using a dilution approach for air with a view to infection control works.5

For all pathogens

and the majority of contaminants, diluting the concentration significantly reduces the chances of infection or contamination of both patients and staff. This works equally well for chemicals, such as diathermy plumes, anaesthetic agents, even cigarette smoke, as it does for pathogens such as tuberculosis and SARS-COV-2. If something isn’t there or the amount is insignificant, then its propensity to cause harm is low. If we combine good air quality with sufficient PPE and infection control management then there should be very low


©Rainer Fuhrmann -

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