Midget Impinger

The midget impinger was the standard instrument for measuring workplace dust from the 1930s into the 1960s. It drew air through a glass tube into liquid, where the dust was trapped; a technician then counted all particles under a light microscope and reported the result in millions of particles per cubic foot of air (mppcf).

Origin

The impinger was developed by Greenburg and Smith in the 1920s, and the smaller "midget" version by the U.S. Bureau of Mines in the 1930s. It was used in the U.S. Public Health Service's 1938 study of asbestos textile plants, which led to the recommendation of 5 mppcf as a dust limit.

Significance in Asbestos Context

The impinger counted every particle, not asbestos fibers, and could not see the thinnest fibers, so its results cannot be converted precisely into today's fiber counts. Defendants have relied on companies' compliance with the 5 mppcf guideline to argue that workplaces were considered safe; plaintiffs point out that the limit was known to be uncertain and that disease kept appearing in plants that met it. Old impinger surveys remain important historical evidence of how dusty a workplace was and of what a company measured and knew.

Example Sentence

The 1952 survey used a midget impinger and reported 12 mppcf at the carding machines, more than twice the accepted limit.

Related Terms

Air Sampling, Threshold Limit Value (TLV), Industrial Hygiene, Quebec Standard Test, Exposure Reconstruction, Personal Breathing Zone Sample

Notes

• Historical dust surveys are often in company files and Bureau of Mines or state records; ask your lawyers whether any exist for your workplace.
• Impinger counts are not fiber counts. Be wary of any expert who converts them without explaining the uncertainty.