Membrane Filter Method

The membrane filter method measures airborne asbestos by pumping air through a thin filter, making the filter transparent, and counting the fibers on it under a phase contrast microscope. It counts fibers longer than 5 micrometers with a length at least three times their width, and reports results in fibers per cubic centimeter of air (f/cc).

Origin

The method was developed in Britain and the United States in the 1960s to replace particle counting with instruments such as the impinger and thermal precipitator. It was the basis of the British Occupational Hygiene Society's chrysotile standard in 1968 and of the U.S. Public Health Service and NIOSH methods of the early 1970s, which led to today's NIOSH Method 7400.

Significance in Asbestos Context

The switch to membrane filters changed what was measured: from all dust particles (in mppcf) to asbestos-sized fibers (in f/cc). Every OSHA asbestos limit since 1971 is expressed in fibers per cubic centimeter, measured this way. For a claim, this matters in two ways. First, old particle counts cannot be translated exactly into fiber counts, so experts reconstructing exposure before the 1970s must explain their conversions. Second, the light microscope cannot see the thinnest fibers and cannot tell asbestos from other fibers, so a membrane filter count can undercount asbestos or count non-asbestos fibers. When identification matters, samples may be analyzed by electron microscope instead. The method's adoption also marks a point in history when companies and regulators had a practical way to measure fibers.

Example Sentence

The plant switched from impinger surveys to the membrane filter method in 1971, and the first fiber counts at the bagging station were well above the new federal limit.

Related Terms

Phase Contrast Microscopy (PCM), NIOSH Method 7400, Fibers per Cubic Centimeter (f/cc), Countable Fiber, Mixed Cellulose Ester Filter, Midget Impinger, Thermal Precipitator, Permissible Exposure Limit (PEL)

Notes

• A membrane filter count shows how many fibers were in the air, not which kind; ask whether any samples were confirmed by TEM.
• If your work history spans the 1960s and 1970s, expect both particle counts and fiber counts in the records; they are not interchangeable.