Phase contrast microscopy (PCM) analysis (NIOSH method 7400) is a technique used for determinations of airborne particulate aerosols like dust characterizations, enumeration of airborne asbestos fibres or fungal spores and airborne mould fragments. An air sample for PCM analysis is collected on a filter medium which is dissolved during sample preparation so that the collected particulate can than be viewed under the microscope.

Phase contrast microscopy is often used for analysis of samples for airborne asbestos fibres. The sample is collected on a mixed cellulose ester membrane (MCEM) filter with a 0.8 mm (micron) pore size. MCEM filter is then cleared with a chemical solution and the collected fibres are counted under the 400X magnification. All fibres longer than 5 mm, wider than 0.25 mm and of aspect ratio of 3:1 or greater are enumerated. The airborne fibre concentration is obtained from the microscopic count and air sampling data.

The PCM analysis is fast and inexpensive and can be performed on site during asbestos removal operations for fast turnaround. Inability to distinguish between the asbestos and non-asbestos fibres is the main disadvantage of PCM. Therefore all fibres that meet the counting criteria are counted as asbestos fibres. Where precise determining of airborne asbestos fibres is needed transmission electron microscopy (TEM) should be used. Air sampling for the determination of airborne asbestos and phase contrast microscopy analysis is fully described in the US NIOSH analytical method 7400.

Depending on needs, up to five types of air samples are collected on asbestos abatement projects:

•  Background or prevalent air samples - collected prior to
   asbestos abatement activities to determine pre-existing or
   reference fibre concentrations,

•  Ambient area air samples - in vicinity to asbestos removal
   work areas,

•  Work area air samples - inside asbestos removal work areas,

•  Occupational air samples - to control worker’s exposure,

•  Clearance air samples - inside asbestos removal work areas
   prior to re-occupancy.

The NIOSH analytical method 7400 is specified in Ontario Regulation 278/05 as analytical method for analysis of clearance air samples after Type 3 asbestos removal operations.

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Phase Contrast Microscopy (PCM)

Analysis of Air Samples for Airborne Asbestos Fibres (NIOSH method 7400)