PCM versus TEM Testing
District Health and Safety officers often request information with respect to the most appropriate clearance testing following a type 3 asbestos abatement. The goal is to ensure that testing accurately reflects the number of airborne asbestos fibres following a type 3 abatement. The following paper summarizes 2 common clearance tests and provides advice for Joint Health and safety Committee members regarding the most appropriate type of test to request.
PCM : The PCM method uses a typical laboratory optical microscope to count the number of fibres greater than 5 microns (μm) in length. This method was established to measure occupational exposures in jobs where asbestos was being handled, used or processed. The method is not able to accurately detect low level concentrations (i.e. less than 0.1 fibres/mL). Another major difficulty with this method is that it cannot distinguish between asbestos fibres and non-asbestos fibres. In the occupational environment these are not serious drawbacks since exposures generally exceed the 0.01 fibres/mL level and if asbestos is known to be present then it is reasonable to assume that most of the fibres collected are asbestos. These assumptions are not reasonable for building environments since in typical buildings asbestos makes up only a minority of the total fibres present in the air. For this reason the PCM method is not recommended for use to assess building occupant exposures. It is, however, a valid method for evaluating the personal exposures of maintenance staff performing tasks where asbestos containing products are being disturbed.
TEM : The TEM method use a very complex electron microscope which is able to "see" much smaller and thinner fibres than the optical microscope used in the PCM method. The transmission electron microscope (TEM) uses at least a 20,000X magnification as opposed to about 500X for PCM. PCM can only identify a fibre if it is at least 0.2 microns (μm) thick, whereas, TEM can detect much thinner fibres. These thinner fibres are much more prevalent in building environments than in mining or industrial environments. Converting TEM derived data to PCM data is very complex but usually the TEM method can detect more fibres in the same sample than the PCM method can. The transmission electron microscope is usually fitted with additional systems for analysis. One of these methods uses a system called energy dispersive X-ray analysis (EDXA) and the other is called selected area electron diffraction (SAED). These additional options allow the TEM method to distinguish between asbestos and non-asbestos fibres by analyzing the chemical components and crystal structure of the fibres. For this reason TEM is the more appropriate method for asbestos measurements in building environments.