HVAC's role in swine-flu transmission

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HVAC systems may contribute far more to disease transmission than thought previously

The latest research data suggests that influenza transmission occurs as much through the airborne route as it does through direct contact or large droplets. This also means HVAC systems have a greater potential for reduction of the transmission risk, said Gordon Holness, president of the American Society of Heating, Refrigerating and Air-Conditioning Engineering (ASHRAE)

As health and school officials worldwide deal with a second wave of the H1N1 virus, commonly referred to as swine flu, ASHRAE has developed new guidance on the health consequences of exposure to such airborne infectious diseases and the implications on the design, installation and operation of HVAC systems. ASHRAE’s Airborne Infectious Diseases Position Document addresses the impact of ventilation on disease transmission, the diseases for which ventilation is important for transmission or control, and control strategies for buildings.

First case

Since the first reported case in the spring of 2009, the H1N1 virus has spread to nearly 170 countries, resulting in 1,154 deaths and some 160,000 illnesses. With a better understanding of ventilation’s effect on the transmission of disease, future incidents of the H1N1 virus may now be easier to prevent, according to Holness.

Several technical solutions are available to assist in avoiding transmission.Ventilation represents a primary infectious disease control strategy through dilution of room air around a source. Directed supply and/or exhaust ventilation, such as laminar flow and displacement, is important in operating rooms, for example.

Room pressure differentials are important for controlling air flow between areas in a building. For example, TB isolation rooms are kept at negative pressure with respect to the surrounding areas to keep potential infectious agents within the rooms; hospital rooms with immuno-compromised individuals are kept at positive pressure to keep potential infectious agents out of the rooms.

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Another strategy from an exposure control perspective could be the use of personalised ventilation systems that supply 100% outdoor air, highly-filtered, or UV disinfected air (that is, the ventilation provision per person) directly to the occupant-breathing zone.

Additionally, providing supplemental (either general dilution or exhaust/capture in a specific location) ventilation in locations in which infectious sources are located will reduce exposure potential, such as what is done in TB isolation rooms.

Particle filtration

The addition of highly-efficient particle filtration to central ventilating systems is likely to reduce the airborne load of infectious particles. This control strategy may prevent the transport of infectious agents from one area, such as patient rooms in hospitals or lobbies in public access buildings, to other occupied spaces, when these areas share the same central ventilation system. Such systems are common in buildings in the US, for example.

Additionally, local efficient filtration units (either ceiling-mounted or portable) reduce local airborne loads and may be very beneficial in specific areas such as healthcare facilities or high-traffic public areas.

There are three general ultraviolet germicidal irradiation (UVGI) strategies: installation into ventilating ducts, irradiation of the upper zones of occupied spaces, and in-room irradiation after one occupant and before the next. All depend upon inactivation of viable agents carried in droplet nuclei.

ASHRAE cautions that additional research is needed showing clinical efficacy, specifically in occupancies with high-risk sources (such as jails, homeless shelters and healthcare facilities) and facilities where high-risk susceptible individuals congregate, such as nursing homes. Such research may lead to other recommended changes in HVAC system design. More research is also needed to document intrinsic (specific to micro-organism) airborne virus and bacteria inactivation rates.

Other relevant ASHRAE guidelines in this regard include Standard 170, Ventilation of Healthcare Facilities, and Standard 611, Air Quality Within Commercial Aircraft.

The full position paper can be read at: www.ashrae.org/positiondocuments

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