The qualitative evaluation provides information for estimating the fibre release potential of the asbestos and the potential exposure of building occupants to asbestos containing materials. In order to set priorities, algorithms have been developed with combine relevant factors to generate a numerical index for the potential asbestos hazard within a building or complex of buildings. Several types of risk assessment algorithms have been utilised to date. The Ferris Index and modified Ferris Index have been used respectively by the Massachusetts Asbestos Commission and the Colorado Department of Education. The EPA has used the Sawyer Index entitled “EPA’s Asbestos Hazard Evaluation Algorithm”, and the United States Navy developed a risk evaluation logarithm entitled “US Navy Asbestos Hazard Index.”
All of the above referenced algorithms compile relevant factors relating to the amount of asbestos present, such as the condition of the asbestos at the time of the inspection, a potential for physical change, and the proximity to air currents and air plenums, and the degree of potential for occupant exposure to asbestos fibres. Subjective factors such as friability, accessibility, and percentage of asbestos in a bulk sample of suspect material are combined with the relevant variables and assigned a numerical score. The factors compiled into overall indices which provide the risk assessment analyst with relative rather than absolute rankings of hazard potential.
Note: We are aware that a correlation study funded by the EPA compared airborne asbestos levels with their algorithm score for 48 sampling sites in 25 schools. This study concluded that the existing algorithm was not a valid indicator of exposure to airborne asbestos. In addition, wide variances existed between different inspectors both trained and untrained. As a result, the EPA withdrew its Asbestos Hazard Evaluation Algorithm as an assessment tool in 1983.
However, most assessment/management experts agree that despite this lack of official government endorsement, these algorithms are good for general purposes and they are currently the most practical options available in evaluating asbestos hazards.
Recognising that these studies are very subjective, care needs to be taken in ensuring that the inspectors are well trained and have extensive experience with these procedures. In each survey we undertake, we complete, as a policy, both the Ferris and Sawyer indices and compare these results with a data base of several hundred prior surveys conducted by Healthy Buildings International Inc. since 1981.
Conclusions and recommendations are then based on the data generated. However, any conclusion based on these studies should be recognised as potential risk evaluations based on qualitative data only. In all cases, when asbestos is identified, those areas adjacent to the ACM materials should then be checked for airborne asbestos.