This section covers the steps that should be taken to evaluate the effectiveness of an existing negative pressure isolation room. Failed engineering controls in negative pressure isolation rooms have been identified as factors in documented hospital TB outbreaks. Regularly scheduled assessment of engineering controls will identify and may help prevent such failures. Items that should be checked include the exhaust and supply airflow rate, negative pressure, and exhaust duct termination location.

To determine the ACH of a space, you will need to measure the airflow and calculate the room volume. See Appendix A. The airflow measurements and calculations should be performed by a certified testing and balancing agency or by in-house engineering staff.

The airflow of a room is usually measured at the individual registers and diffusers using a balometer. This is a device that consists of a hood, a velocity sensor, and a microprocessor.

The hood is placed over a register or diffuser and should completely cover the air outlet. The top of the hood should have a foam gasket that establishes a good seal between the hood and the ceiling or wall around the outlet.

The hood directs all air entering or leaving the outlet past a velocity-sensing grid. The area of the grid is fixed. Therefore, the microprocessor can calculate and display the quantity of air being exhausted or supplied by the air outlet. Balometers usually provide an airflow reading in cubic feet of air per minute (CFM).

The standard size of a balometer hood outlet is 24" X 24", although adapters are provided to adjust the hood size. This size hood can be used to measure the airflow of any outlet equal to or smaller than this (e.g., 12" X 24" or 18" X 18" diffuser). For other size outlets, such as a 36" X 6" slot diffuser, the hood size on the balometer may need to be changed.

There may not be sufficient space in front of some outlets to place the balometer. In this case, the airflow should be measured by a pitot traverse in the duct that serves the outlet. A pitot traverse is a specialized measurement that requires access above the ceiling. Air velocity is measured at a number of sample locations inside the duct. Airflow is calculated based on these velocity readings and the area of the duct cross-section.

If a dedicated exhaust fan serves the isolation room suite, it may be possible to estimate the airflow at the room by measuring the airflow at this fan. Because of duct leakage, this measurement will not be as accurate as one taken at or near the outlet. Inadequately sealed duct joints can result in extra air being sucked into the duct between the isolation room exhaust grille and the fan, which would result in an overestimate of airflow at the room. To compensate for this, an allowance of at least 10% should be made. This allowance should be increased in the case of a long duct run. If room airflow is found to be inadequate, i.e., less than 12 ACH, it should be increased. See Section 6, Upgrading or Converting an Existing Room.

After establishing the airflow, the next step is to evaluate how effectively this air is used in the isolation room. This assessment is not as straightforward as calculating the airflow rate because there is no clearly defined numerical standard to meet.

Smoke testing can be used to visualize the direction of room air and to estimate how well air is mixing. Consequently, ventilation problems can be identified, such as undesirable directional airflow patterns and poor mixing. Ideally, the clean supply air will be introduced near a health-care worker, while exhaust air will be removed near the patient. Good air mixing is confirmed by

rapid dissipation of the test smoke in all parts of the room, which demonstrates that particles generated in the room are being diluted and removed.

The engineering department staff at the facility should trace the path taken by the exhaust air duct after it leaves the isolation room. If applicable, they should also check the exhaust duct serving the bathroom and anteroom. For the record, a set of drawings should be generated (or an existing design set marked) to show the ductwork and fan.

The exhaust ductwork and fan should also be checked for optimum performance. Conditions that should be corrected include: excess air leakage at duct joints, damaged ductwork, incorrectly adjusted dampers, and fans in need of servicing.

If air from an isolation room is returned to a recirculating ventilation system that does not include HEPA filtration, this room should no longer be used for isolation. Staff and patients in rooms served by this system may be exposed to M. tb from patients in isolation.

The risk of exposure from a recirculating mechanical system is affected by dilution of the return air with outside air and by the filter in the mechanical system. The risk is reduced as the percentage of outside air is increased and the efficiency of the filter is increased. Filtration in hospital ventilation systems is usually better than in clinics because hospitals are typically covered by stricter building codes and have larger facilities and maintenance budgets.

The CDC Guidelines do not address the issue of dedicated exhaust air systems serving isolation rooms. However, in some jurisdictions this is mandated by the building code for new or renovated rooms. Because most building codes are not retroactive, it is usually acceptable for an existing isolation room to combine the exhaust air with other exhaust systems, such as those serving toilet rooms.

If the existing exhaust system is dedicated, make sure that the ductwork is labeled as recommended for a new isolation room ("Caution Ð Negative Pressure Isolation Room Exhaust"). For a shared system, only the ductwork between the isolation room and the main exhaust trunk needs to be labeled. The exhaust fan, whether dedicated or shared, should have a warning label as recommended for a new system ("Negative Pressure Isolation Room Exhaust Fan Ð Contact Infection Control Coordinator Before Turning Off Fan"). See Section 4.F, Isolation Room Exhaust, for additional information on labeling of exhaust ductwork and fans.

Negative pressure is the easiest characteristic of an isolation room to check. Several methods are available to qualitatively assess negative air pressure, including smoke trail testing and tissue testing. If the isolation room is operating as intended, there will be an air current moving into the room under the door. The existence and direction of this current should be verified.

Smoke trail testing helps visualize the current near a room door. In this simple procedure, smoke is released near the air gap under an isolation room door. See Appendix E for more detailed instructions on smoke trail testing. Commercially available smoke-generating kits produce a visible cloud, which usually consists of water and acid. The quantity of smoke typically issued from the tube is minimal and is undetectable at short distances from the tube.

Because inhalation of this smoke in concentrated form can cause irritation, care should be taken not to expose workers or patients until the smoke has been diluted. The amount of smoke used should not be excessive. There are many different types of easy-to-use smoke-generating kits available from safety supply companies. A typical design is the disposable self-contained puff bottle. Another common design is the disposable smoke tube, which attaches to a rubber bulb that acts like a bellows.

1. If commercial smoke-generating devices are not available, incense sticks can be used. ICS recommends that two sticks be used side-by-side to generate the smoke trail. However, incense smoke does have a strong odor, and is not as visible or controllable as commercial smoke.

If smoke-generating devices are not available, or if the room is occupied by a patient who may be vulnerable to the irritant properties of smoke, a thin strip of tissue can be used to determine whether a room is at negative, neutral, or positive pressure. A thin strip of tissue should be held parallel to the door with one end of the tissue in front of the gap. The direction of the tissue’s movement will indicate the direction of air movement.

Relative room pressurization can also be verified using a handheld pressure gauge or manometer, which is similar to a direct room pressure monitor, except it is portable. A length of rubber tubing is attached to each of the two ports on the manometer. The manometer displays (in " W.C.) the pressure difference between the two spaces at the termination of the tubes. If one of the tubes is threaded under the door into the isolation room and the other is in the hallway, the manometer will indicate the pressure difference between the two paces. A negative symbol verifies that the room is at negative pressure.

Air speed is measured by a velometer, usually in units of feet per minute (FPM). These devices can be placed near the gap under the isolation room door to measure the speed of the airstream. Velometers are available in a number of different configurations. Many only indicate air speed regardless of air direction. For instance, some velometers indicate how fast the air is moving, but not whether the air is entering or leaving the room. However, there are models available that can also be used to determine airflow direction.

All of these tests to verify negative pressure should be conducted at least three times until the results are consistent.

If the existing room is equipped with a permanent room pressure monitor, one of the above tests should be performed to confirm negative pressure and to validate the monitor. Also, the Isolation Room Pressure Monitor Checklist (Appendix D) should be completed for the monitor.

After negative pressure has been verified, it should be measured. The table below summarizes three ways to quantify negative pressure. The corresponding units of measurement, the measuring device for each method, and the approximate costs are also shown.

If the existing room is equipped with a permanent room pressure monitor, verify that it has been calibrated within the last 12 months.

Clinic Case Study: Episode 1

Background

The room was square-shaped (15 feet each side), with a ceiling height of 8.5 feet. The exhaust air change rate was calculated as follows: