Creative natural ventilation solutions may hold the key to infection control

Posted: 19 September 2012 by Zulfikar Adamu

The need for energy efficiency, combined with the menace of airborne pandemics and threats from drug-resistant strains of, for example tuberculosis, are forcing a rethink about mechanical ventilation in hospitals.

At the same time, researchers are taking another look at natural ventilation. It has considerable potential to deliver high air change rates, so reducing the risk of airborne infections in hospitals, particularly in ward spaces that are in constant use.

Natural ventilation has been under-utilised for many reasons, probably because hospital designers tend to think mainly in terms of windows. New facilities using natural ventilation frequently utilise windows for ventilation, thereby failing to take account of progress in contemporary research.

Same-side dual openings and advanced natural ventilation

Alternative strategies include same-side dual openings where the air enters the room via one low-level opening and leaves by another (high-level) opening, both on the same wall. Another more sophisticated alternative is advanced natural ventilation (ANV) where fresh air comes from a low-level inlet while stale air escapes via an outlet in a chimney-like stack. With this arrangement, windows are fixed, serving only daylight and visual purposes. With both concepts, ‘openings’ (inlets and outlets) should not be construed to mean ‘windows’.

Both alternatives offer a feature that is essential for controlling infectious bio-aerosols in wards, namely segregation of air inlets from air outlets. This separation is a prerequisite for acceptable direction and pattern of airflow. Windows can only provide this capability when used in cross ventilation, which is usually impractical in modern hospitals. Additionally, the alternative strategies permit buoyancy-driven natural ventilation which does not rely on wind but requires low and high-level openings.

A great deal of attention has been paid to rates of airflow (e.g. six air changes per hour or 20 litres per second). However, for hospitals, this often leads to ventilation that may be efficient but which fails to be effective in enabling proper dilution of the air, which is required for infection control.

Both ANV and dual opening are advantageous because they can provide adequate dilution. Also they require only one facade of a ward and can be retrofitted into existing buildings. Comparing the two options, dual-opening is easier and cheaper than ANV. However, its use for hospital ward ventilation is restricted to single floor buildings. This restriction is due to the possibility of contaminated air from high-level outlets (on lower floors) entering the low-level inlets of spaces on upper floors.

For buoyancy-driven ventilation in such alternative strategies, heat generated by people, lighting and equipment will warm indoor air which rises and escapes, prompting the flow of cooler outdoor air in the process. For this to work well, careful sizing and location of the air inlets and outlets is paramount.

ANV has the edge

At urban locations, the ANV strategy has the edge over same-side dual-openings since it has extra benefits like security and safety as well as curtailing environmental pollution (e.g. pollen, fumes, smoke, dust and noise). These benefits come about because fresh air is supplied not from street-facing inlets, but from dedicated narrow shafts which could be linked to underground labyrinths. From the labyrinths and shafts, cooler air specifically protected from outdoor corruption, is fed into each space from low-level inlets. When used this way, buoyancy-driven ANV technically eliminates having openings that connect a space to the street, which can occur with traditional windows or with same-side dual openings.

Natural personalised ventilation

A new buoyancy-driven strategy with potential to protect patients from airborne pathogens in wards is the natural personalised ventilation (NPV) system. Traditional personalised ventilation (PV) is a mechanical system that protects sensitive patients with a direct supply of conditioned air over their bed space, but it consumes energy, is expensive and cannot be used on a wide scale. In the natural version, a ceiling-level duct connected to the external wall collects (and protects) fresh air, which then drops from an outlet above a patient’s bed. This happens due to gravity as well as the density of cooler outdoor air being greater than that of warmer indoor air. The warm/stale indoor air escaping via a stack induces the continuous ingress of fresh air from the duct. Whereas buoyancy-driven natural ventilation used with other strategies work with displacement, i.e. cool air floods the space at low levels, rising gradually to ‘displace’ warm air, the NPV is technically different and has new valuable features.

Firstly, the cool air falling over the patient guarantees very fresh air around the bed, protecting the patient from surrounding contaminated air. Secondly, on its descent and subsequent warming up, the escaping air creates miniature turbulence or mixing ventilation, which has hitherto been the privilege of mechanical systems. Mixing has been shown to be good for dilution of airborne contaminants. So both the personalised delivery and the mixing qualities of NPV offer a two-prong defence against airborne pathogens. Thirdly, the NPV system saves energy because it relies on buoyancy and not fan power to drive airflow into the duct and towards the patient.

Continuing research intends to capitalise on the NPV system for protecting individual patients in multi-bed wards by providing each bed area with dedicated supply of fresh air. This can help reduce the strain of airborne hospital-acquired infections in existing facilities where multi-bed wards are plentiful.

Florence Nightingale got it right

In her ‘Notes on Nursing’, published in 1860, Florence Nightingale got it right when she said that the ‘first rule of nursing is to keep the air within as fresh as the air without’. With modern tools like computational fluid dynamics, we can achieve this goal thanks to reliable predictions of airflow movement in our hospital spaces. What is left is the willingness to be creative. As shown by both ANV and NPV systems, creative natural ventilation solutions, which decouple the process of airflow from the visual and daylight functions of windows, probably hold the key.

Zulfikar Adamu is a HaCIRIC researcher at the School of Civil and Building Engineering, Loughborough University