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Negative pressure of the contaminated room with respect to adjacent spaces is essential to control contamination.

Simply balancing a system is not effective unless the air movement is away from the patient. The addition of an exhaust fan for example in an exterior window/ wall is an acceptable method to create negative pressure in a room provided it does not discharge near an adjacent intake for another system or an open window. For areas where exterior facades are not available, exhaust ducting should be considered.

Increasing the quantity of supply air to the contaminated space will dilute the contaminants. As with the exhaust air the direction of the supply air away from the patient is important. Too much air however, tends to create drafts or turbulence and will recirculate the contaminants within the space.

Other conditions, which create turbulence, are open doors, poor distribution patterns, etc. Any negative pressure effect will be negated by allowing doors to remain open and by unnecessary movement of staff and equipment.

As a guide a minimum supply air rate of six changes per hour should be used with all air exhausted to the outside. The exact quantity will depend on the type of space.

If air changes and exhaust requirements cannot be met, germicidal ultraviolet (UV) lights are a possible alternative control method. These lights can be located in the occupied areas and adjacent space. They can also be located in common ducts to control the distribution of contaminants to other systems.

Where large quantities of supply air are impractical, recirculating room air through HEPA filters within the space is an alternative method to reduce contaminants. HEPA filters, which are very efficient even at low particle sizes, will collect over 99% of air borne particles.

When HEPA filters are used experienced personnel must replace the media to avoid contamination of the space or maintenance personnel. Systems such as bag-in/bag-out are relatively inexpensive and reduce the chance of accidental spillage of dirty media which could contaminate the ductwork, etc.

Periodic maintenance is critical to assure negative pressures are maintained and dirty filters replaced. UV lights also need to be regularly maintained.