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Biological/Chemical - In our world today, biological probably means anthrax, which is a bacteria, or smallpox, which is a virus. Bacteria are usually large molecules and viruses are usually small. Obviously, no one can be sure whether these will remain as the only materials used to disrupt our society. When discussing the size of particles, it is important to understand that the best particulate filters available are designed for particle sizes from 0.1 to 0.3 microns, depending on filter type and testing procedures. While this type of filter will collect the majority of contaminants, it will not stop all. Further, chemical attacks can be in the form of particulates or gas, the latter requiring special filter media or scrubbers. At the moment a number of firms are involved in developing special biological filters, some of these have the ability to partially neutralize gaseous material. The type of filters and their media available, which also include spray type scrubbers, is very large and it is not the purpose of this bulletin to go into detail of all the various types and their uses.

The Building - Most office type buildings are not airtight. This means that the building must be kept at positive pressure at all times which is difficult to achieve during the winter months, e.g. stack effect which draws outside air into the building at the lobby level. It is essential that this effect be eliminated by over-pressurizing the lobby and also increasing the positive pressure on the office floors. The overpressure will also reduce window infiltration, particularly in older buildings where often the windows are operable. Finally, the building HVAC systems cannot be shut down or outside air will enter the building through natural infiltration.

HVAC Systems - Many older buildings use air side economizers to reduce energy whether the air systems are central or local. This system varies the outside air quantity depending on season. They also vary the relief air quantity to the outside depending on the amount of outside air being introduced to the building. Since it is impractical to provide filters for all operating conditions, buildings would have to operate at minimum outside air regardless of outdoor conditions, i.e. the filters would be provided for minimum outside air only. The maximum outside air damper section and the relief dampers would have to be sealed or gasket dampers provided to keep leakage to a minimum. It is totally impractical to expect an installation to be leak-proof. Some buildings utilize water side economizers; these buildings operate with minimum outside air throughout the year making outdoor air filtering simpler. Some also have ventilation units supplying tempered, filtered air to each floor which could more easily and effectively be fitted with special filters and scrubbers. The amount of outside air to the building should be increased depending on the condition of the window wall, to offset the effect of wind infiltration and general changes in the building's operation. VAV systems need to be controlled to ensure that the amount of outdoor air supplied is maintained regardless of the supply quantity delivered to the space.

Filters - Whatever filter type is decided upon, they must be installed so that leakage does not occur which in an office building is a very difficult concept. If HEPA and ULPA filters are used, the fan power required is probably more than the existing system will be able to accommodate requiring the fan to be changed, or as noted above, a ventilation system added. This is probably the only practical solution for most buildings but will require that a new system be installed. Where an air side economizer is used, this operation may have to be abandoned which will increase the energy consumption and the operating cost of the building. The increase in existing fan HP to accommodate new filters is also an addition to overall operating costs.

Observations - This represents a very brief overview of the problems and solutions. A study needs to be made based on the building, HVAC systems, and equally important, the practical needs of the building owner and occupants. It is worthwhile referring to ASHRAE Standard 62-1999 which deals with indoor air quality. This standard and its proposed revisions pay considerable attention to the quality of air inside a building and the quality of air introduced to a building. Many consultants are considering the idea of a separate filtered outdoor air system as a standard. Airports, for example, often require that outdoor air be chemically filtered. Protection against a terrorist attack is a further development. It must be noted that besides the practical issues and cost to retrofit an existing building, the degree of protection is limited.