Introduction:- Class II safety cabinets are designed to control airborne contamination of the product and reduce the exposure of the operator to any airborne contamination generated within the cabinet from work procedures. The work surface of the safety cabinet is flushed with a unidirectional filtered downward airflow and is constructed to minimise, by means of an inward airflow through the working aperture, the escape of airborne contamination generated within the cabinet. Ideally there should be no escape of hazardous aerosol from a safety cabinet and the protection factor provided to the operator should be infinite. In reality safety cabinets do not give complete protection as, in use, operators disturb airflows and hence the protection provided by the cabinet. The test to determine the protection factor involves generating an aerosol of particles e.g. potassium iodide generated by a spinning disc, in the cabinet and the effect of an operators arms on the airflows and hence the effect on the protection factor. The following is a brief overview of the KI discus test used to determine the operator protection factor (OPF) described in BS EN 12469: 2000, Annex C. It is described here in order to allow an interpretation of test results. It is likely that this test will not be carried out in-house and will usually be carried out by an external service contractor.
Equipment:- The equipment required includes an aerosol generator assembly for producing the potassium iodide aerosol (concentration specified) and the detection system (air samplers) where filter papers are used to detect KI aerosol that has escaped from the cabinet. Also included are metal cylinders, which simulate the presence of operator’s arms during the test and the likely disturbance that they may cause to the airflows. Equipment used should be calibrated and traceable to national standards. Where the test is performed by an external service contractor a valid certificate of calibration for the test equipment used should be supplied with the test report.
Sample locations:- For safety cabinets up to 1m wide, 5 replicate protection factor tests are taken in the centre of the working aperture according to the test procedure. Cabinets larger than this will require additional sample positions.
Frequency of sampling:- The test need only be carried out annually as the test is usually performed by an external contractor with the necessary equipment and expertise.
Method of sampling:- The method of sampling is defined in the standard. With the cabinet running a volume of KI solution is generated into an aerosol and the air sampler detection system is operated. Aerosol escape will be picked up by the filter membranes and show up as brown spots. The number of brown spots counted can be used in a formula to determine the operator protection factor. For example a protection factor of 1 x 105 would correspond to 62 spots on the filter membrane. The OPF is taken to be the worst figure achieved by the cabinet in the 5 tests.
Results and interpretation of results:- The cabinet is considered to provide sufficient operator protection where all values of the OPF are greater than 1 x 105, when tested in accordance with the standard. Values less than this mean that the escape of contamination from the cabinet may be greater than is acceptable.
Action:- Where a safety cabinet fails to meet requirements for the OPF an investigation into the problem should be carried out. The OPF can be influenced by poor siting of the cabinet, air flows/currents causing turbulence near the aperture, cabinet performance (fan speed, filter blockage etc) and work activities in/around the cabinet. Steps should be taken to eliminate each of these potential causes from the situation. Any corrective action taken as a result of the investigation should be recorded. The test may then be repeated.
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