trols, fire and smoke alarms, and special alarms and monitors for gases, should thus be inspected and maintained on a regular basis.

Each laboratory should be evaluated periodically for the quality and quantity of its general ventilation and any time a change is made, either to the general ventilation system for the building or to some aspect of local ventilation within the laboratory. The size of a room and its geometry, coupled with the velocity and volume of supply air, determine its air patterns. Airflow paths into and within a room can be determined by observing smoke patterns. Convenient sources of smoke for this purpose are the commercial smoke tubes available from local safety and laboratory supply companies. If the general laboratory ventilation is satisfactory, the movement of supply air from corridors and other diffusers into the laboratory and out through hoods and/or other exhaust sources should be relatively uniform. There should be no areas where air remains static or areas that have unusually high airflow velocities. If stagnant areas are found, a ventilation engineer should be consulted, and appropriate changes should be made to supply or exhaust sources to correct the deficiencies.

The number of air changes per hour within a laboratory can be estimated by dividing the total volume of the laboratory (in cubic feet) by the rate at which exhaust air is removed (in cubic feet per minute). For each exhaust port (e.g., hoods), the product of the face area (in square feet) and the average face velocity (in linear feet per minute) will give the exhaust rate for that source (in cubic feet per minute). The sum of these rates for all exhaust sources in the laboratory yields the total rate at which air is being exhausted from the laboratory. The rate at which air is exhausted from the laboratory facility should equal the rate at which supply air is introduced to the building. Thus, decreasing the flow rate of supply air (perhaps to conserve energy) decreases the number of air changes per hour in the laboratory, the face velocities of the hoods, and the capture velocities of all other local ventilation systems.

Airflows are usually measured with thermal anemometers or velometers. These instruments are available from safety supply companies or laboratory supply houses. The proper calibration and use of these instruments and the evaluation of the data are a separate discipline. An industrial hygienist or a ventilation engineer should be consulted whenever serious ventilation problems are suspected or when decisions on appropriate changes to a ventilation system are needed to achieve a proper balance of supply and exhaust air.

All ventilation systems should have a device that readily permits the user to monitor whether the total system and its essential components are functioning properly. Manometer, pressure gauges, and other devices that measure the static pressure in the air ducts are sometimes used to reduce the need to manually measure airflow. "Telltales" and other similar simple devices can also serve as indicators of airflow. The need for and the type of monitoring device should be determined on a case-by-case basis. If the substance of interest has excellent warning properties and the consequence of overexposure is minimal, the system will need less stringent control than if the substance is highly toxic or has poor warning properties.

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