The Real Issue with Wall Deposits in Closed Filter Cassettes—What's the Sample?

Abstract

The measurement of aerosol dusts has long been utilized to assess the exposure of workers to metals. Tools used to sample and measure aerosol dusts have gone through many transitions over the past century. In particular, there have been several different techniques used to sample for beryllium, not all of which might be expected to produce the same result. Today, beryllium samples are generally collected using filters housed in holders of several different designs, some of which are expected to produce a sample that mimics the human capacity for dust inhalation. The presence of dust on the interior walls of cassettes used to hold filters during metals sampling has been discussed in the literature for a number of metals, including beryllium, with widely varying data. It appears that even in the best designs, particulates can enter the sampling cassette and deposit on the interior walls rather than on the sampling medium. The causes are not well understood but are believed to include particle bounce, electrostatic forces, particle size, particle density, and airflow turbulence. Historically, the filter catch has been considered to be the sample, but the presence of wall deposits, and the potential that the filter catch is not representative of the exposure to the worker, puts that historical position into question. This leads to a fundamental question: What is the sample? This article reviews the background behind the issue, poses the above-mentioned question, and discusses options and a possible path forward for addressing that question.

Keywords:

• exposure assessment
• workplace monitoring
• air sampling

ACKNOWLEDGMENTS

The Beryllium Health and Safety Committee (www.sandia.gov/BHSC), which organized the Third International Symposium on Beryllium Particulates and Their Detection, is acknowledged for its interest and contribution, though the views of the authors do not represent any officially endorsed position of the BHSC.

The assistance of Kevin Ashley, CDC/NIOSH, as well as the reviewers of the manuscript, is gratefully acknowledged.

This article was prepared as an account of work sponsored by an agency of the United States government. The views and opinions expressed herein do not necessarily state or reflect those of the United States Government, any agency thereof, or any of their contractors.