PCM NIOSH 7400 B Rules


PCM NIOSH 7400 B Rules


The PCM NIOSH 7400 B Rules represents a procedure employed in the testing of asbestos. This technique offers an alternative counting rule set to the NIOSH 7400 Method, potentially providing a more suitable approach for quantifying specific non-asbestos fiber types, including fibrous glass.

Overview of the Method

The PCM NIOSH 7400 B Rules method is based on the use of phase contrast microscopy (PCM) to count and size fibers present in air samples. This method is widely used due to its simplicity, speed, and relatively low cost.

Counting Rules

The counting rules limit the upper diameter of fibers to 3 microns. This restriction prevents the measurement of non-thoracic fibers, ensuring a more accurate count of asbestos fibers that can be inhaled into the lungs.

Sample Retention

After testing, the sample retention time is 60 days. This period allows for retesting or additional analysis if required.

Limitations of the Method

While the PCM NIOSH 7400 B Rules method is effective, it does have some limitations. For instance, it does not differentiate between asbestos and non-asbestos fibers. Additionally, it may underestimate the number of fibers present if they are clumped together or if they are smaller than the limit of resolution of the PCM.


In conclusion, the PCM NIOSH 7400 B Rules method is a valuable tool in asbestos testing. Despite its limitations, its simplicity, speed, and cost-effectiveness make it a popular choice in many situations.


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