01 September 2015

Hospital Air Filtration: Practical Considerations for Critical Care Environments

RCH operating theatre

There are few environments within a hospital where air quality has a more direct impact on patients and hospital staff than in operating theatres and isolation rooms.

Thankfully there are well established and very practical steps that guide the design and implementation of these important environments. However, the reliable achievement of these goals is complicated by variations in the state-by-state standards and sometimes by the interpretation of these guidelines to save short-term costs.

It should be noted that the behaviour of hospital staff and other environmental factors have massive impacts on the control and transmission of various infections. The professional behaviour of staff and cleaning protocols has proven to be reliable in Australia, so the ventilation system can perform its part of the overall disease control landscape.

General Design Parameters

Design parameters that have measureable impact on the hospitals ventilation system include, relative room pressurisation, grade and location of filtration, air change rates, the source of air (external / recycled), temperature and humidity. Information contained in Standards Australia HB 260-2003 is particularly informative as a general reference. AS/NZS 1668.2 is also an invaluable and prescriptive reference.

A number of clear and rational criteria are outlined in these references – such as the requirement for a “respiratory isolation” room having a dedicated air conditioning, single pass and dedicated exhaust ventilation system that are then isolated as much as possible from air inlet systems. In some quarantine rooms – a case can be made for bag-in-bag-out systems to further protect the public from exhausted pathogens.

Airepure Gel Sealed HEPAAny discussion of ventilation for medical environments must include HEPA filtration. Most state and Australian standards are very prescriptive about the nature and efficiency of these important items. Rigorous testing of filters by NATA accredited testing services adds a further level of protection for the patients and hospital staff.

Of course the supply air to HEPA filters should be pre-filtered to prolong the life of the HEPA filter, and normal guidance is to comply with AS 1324.1. Suitable pre filtration is obtained by using a combination of filters with a rating of G4 followed by higher efficient filters with ratings of either F7 or F8. However, this standard requires filters to be tested and validated every 5 years. The author is not aware of any Australian independent NATA certified testing authority that can test and certify filters to AS1324 so manufacturers and specifiers need to rely on overseas facilities that can provide acceptable data to ASHRAE 52.1 or 52.2 or EN779.2012.

In many cases the HEPA filter that supplies air to a given room will be housed in a simple HEPA module, and these units will be sized to achieve an effective airflow to the room at approved face velocity rates. Generally these plenum boxes will have side or top air inlet spigots, and sometimes have options of inspection plates and damper adjustment units for air flow balancing. The fascia sections of these units are precision devices to afford a flat, square mounting for the HEPA filter.   Materials of construction are typically 304 grade stainless steel with protective mesh covers which serve to diffuse air flow from the HEPA and protect the delicate HEPA filter from in house cleaning processes.Airepure Laminar Airflow System for Operating Theatres

In more specific circumstances such as operating theatres, HEPA filters are often arranged in purpose built, multi-filter housings to promote superior laminar air flow control and simplified installation. Refinements such as accommodation of various pendants and lighting systems are further enhancements.

State by State and National Standards

In any given state, both 1668.2 and state guidelines may apply. AS 1668.2 is documented in legislation via BCA, and its guidance should be adhered to in all cases.

As an example – for a protective isolation room, the standard indicates that designers “shall”, have at least 15 air changes per hour, negative pressurization to half that of surrounding rooms, HEPA filtration to 99.99%, and specified outdoor air rates of the greater of 10L/s per person or 2L/s.m2.

There are some excellent state level guidelines such as the Queensland Health –Infrastructure Design Guidelines that deliver very specific information on fine details of design.

The guideline values for an airborne infection isolation room augment those above for 1668.2.

Negative pressure, air changes per hour: as per 1668.2, total air changes per hour 12, full outdoor exhaust, Humidity a max of 60% and design temp 21 to 24C.

Final Thoughts

Some state guideline may be less stringent, but it is worth noting that these are critical facilities, and erring on the side of “best possible practice” may be a wise direction, even if it results in modest short –term cost increases.

The designers, specifiers and suppliers should pause to ask themselves the question “when I am lying on this operating table” do I want best practice or lowest price above me?

Written by Dr. Allan Heckenberg (PhD), BDM for Airepure Australia for The Australian Hospital Engineer Journal, Vol 38, No 3, September 2015.

About the Author

Allan Heckenberg Airepure

Written for AHE Journal by
Dr. Allan Heckenberg (PhD)
Airepure Australia
1300 886 353

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