Thermal Precipitator

A thermal precipitator is a dust-sampling instrument that draws air slowly past a heated wire. The heat pushes dust particles out of the air and onto glass slides or cover slips, where they are counted under a microscope. It was the standard dust sampler in Britain for decades before membrane filters replaced it.

Origin

The thermal precipitator was developed in Britain in the 1930s, notably by H.L. Green and H.H. Watson for the Medical Research Council. It became the usual instrument for British dust surveys in mines, mills and factories, much as the midget impinger was in the United States.

Significance in Asbestos Context

Many historical British and Commonwealth dust surveys of asbestos textile mills, factories and mines were made with thermal precipitators, and some of that research is cited in American cases on what was known about asbestos dust and when. Like the impinger, it sampled very little air and counted particles rather than asbestos fibers, so its results cannot be converted precisely into modern fiber counts. Its samples did preserve particle shape better than the impinger, which allowed some fiber counting. When an expert relies on old thermal-precipitator data to estimate exposure, the conversion method and its uncertainty should be explained. The instrument also matters as history: it shows that industry and government in Britain were measuring dust in asbestos workplaces long before modern limits existed.

Example Sentence

The 1950s survey of the weaving shed used a thermal precipitator, and the expert explained why its particle counts could not simply be converted into fibers per cubic centimeter.

Related Terms

Midget Impinger, Membrane Filter Method, Million Particles per Cubic Foot (mppcf), Air Sampling, Exposure Reconstruction, Industrial Hygiene, Asbestos Textile Industry

Notes

• If an expert converts old dust counts into fiber counts, ask what conversion was used and how uncertain it is.
• Historical British dust research is part of the record of what was known about asbestos; your lawyers may use it even in a U.S. case.