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hospital codes and Standards. While there are guidelines for movement to specific areas within medical

facilities most of these do not have a pressure differential value as part of the Standard. It is recognized that

most public buildings should be positively pressured to the outside in most cases but by what amount?

Israel has been working with the threat of Bio/Chem terrorism for over 20 years and their research leaders

have found that for protection of intrusion into a space the differential pressure must be 0.20 inches water

gage minimum. When we review the system pressure guideline found in most of our documents, pressure

differential regularly is design with thousandths of an inch water gage. The potential for disruption is significant with these low values when someone opens doors or changes thermostat settings.

When we look at an entire building rather than an Isolation room it becomes a more daunting task to ensure that the ingress and egress from the building is controlled effectively at nominal pressure differentials. While air will always move to the negative side of an air conveyance system, particulates, due to the physics of their particle sizes do not necessarily follow readily at low airflows.

Originally the reaction from our engineering community in the case of a WMD event was to shut down all air-handling units to prevent contaminants from entering the building. Unfortunately the ability to shut down exhaust systems is not as easily connected as the HVAC systems to central controls, meaning that if HVAC were shutdown the exhaust system would draw air into the building. If the HVAC is turned off this eliminates the only method of ensuring that positive pressure to the outside is maintained. It also eliminates the possibility of reducing the concentration of contaminants through filtration within the building if WMD intrusion to occupied space occurs.

Remember that man made organisms will be dispersed in similar sizes to the naturally occurring organisms, thus any HVAC system with filter efficiency of MERV 10 or higher will reduce concentrations of particulates in the target size. When HVAC systems are allowed to operate, the building pressurization is maintained as well as the relationships from clean to less clean spaces within the facility. If any part of the system were to be shut down it would be those exhaust systems that are not directly removing toxic substances from the facility to improve the

positive pressure relationship to the outside.

SYSTEM INTEGRITY

One extremely important part to the system performance is the “filter system” integrity. All to often we see standards for specific filter efficiencies only to find when the end user receives his final product the framing systems do not allow proper seal integrity to be achieved. Often there are clips required for the filters to seal, and installing contractors commonly leave these out. It is common to find that the filter section is designed to fit the ‘Air Handler’ rather than a filter section designed to fit filters of common, normal configurations.

When specifying filters it becomes just as important to specify the framing system that retains the filters. Gasket, pile seal, spring clips, swing bolts, doors, all are integral parts of a properly designed filter system and when any one of these components is compromised the entire system is compromised as well. Research done in existing HVAC systems to verify filter performance indicates that none are working effectively as found. Most have intrusion of contaminants associated with improper fitting of filters into the framing systems. It is not only the fault of the installers of filters as many times it is the selection of framing hardware itself.

In contrast to improving the filter efficiency by increasing the MERV value in a system, if a system leaks - a higher pressure drop means it will leak more. Air follows the path of least resistance, and dirt follows the path of most velocity. Meaning simply that increasing the filter efficiency of product selected does not necessarily improve the end result. Years ago one of the major filter manufacturers reported that an eighth of an inch gap on a 24 inch filter is equal to 18% of the dirt in the air stream bypassing the higher efficiency filters. The following graphs present a picture of what potentially happens with a minimal bypass. It is possible to drop two or three MERV numbers due to improperly sealing filters.

Insert Power point MERV Graph followed by MERV 13 Graph

PERFORMANCE VERIFICATION

When the system has been specified and installed to meet the current standards there is little or no effort made to document that it actually performs as intended. In fact there is not even a protocol for verification that exists for systems commissioning persons unless we get to HEPA filtration. Even with the specification of HEPA filter systems there are seldom access ports for the injection of test aerosol or access to the downstream side to measure effectiveness.

With the tools that we currently enjoy with ASHRAE 52.2 and the ability to know what particle size that we should design to remove it is now equally important to verify that we in fact accomplish the design. While it is difficult sometimes to obtain sufficient ambient particle challenges to have good representative comparison values, modern equipment allows real time particle counting in multiple size ranges simultaneously. The proper training and experience using these devices will allow end users the ability to review performance and system integrity within their own facilities.

If we use “minimum” Standards in the specifications for a filter system whether it is for WMD protection,

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