When you’re sampling for aerosols with a sampler that contains a filter, particles do not always reach the filter or stick to it. For total dust samples, OSHA and NIOSH require all particles that pass through the opening of a sampler to be included in the assessment of worker exposure. Serious underestimation of exposure can ensue if non-filter “wall deposits” are excluded.  When filters first replaced impingers and other airborne dust sampling methods in the 1940s and 1950s, the filter was placed in an open holder that did not protect it from accidental or deliberate tampering. The closed-face cassette (CFC), first known as the “Millipore Monitor,” was designed in the late 1950s for cleanroom evaluation, and in 1960 it appeared in the first edition of the ACGIH book . In either configuration, two-piece (top and bottom) or three-piece (with middle ring), the CFC has been used ever since by the majority of industrial hygienists for “total” dust sampling. The cassette, with its small entry inlet, prevents disruption of the filter by typical environmental “hazards,” such as fingers or pencils, during sampling, and also protects it during transport to the laboratory. For respirable sampling, the entry inlet can also be connected to the “nylon” cyclone, or the middle ring can be used to make an attachment to the “aluminum” cyclone. The CFC is often used with PVC filters for gravimetric analyses (for example, in NIOSH methods 0500 and 0600). However, from their inception, OSHA and MSHA have used a different design of cassette housing a PVC filter. The “Filter Cassette with Removable Capsule” (once marketed as the “Woodchek”) was developed by the Mine Safety Appliances company. All particles entering the cassette are included in the analysis of the removable capsule, even if they deposit elsewhere than on the filter, or if they fall off the filter during transportation. OSHA and MSHA use capsules for all gravimetric analyses, including for respirable crystalline silica.  For metals sampling, OSHA and MSHA use the CFC without an internal capsule, a configuration that NIOSH also recommended in its past methods. Within the CFC, particles may deposit elsewhere than on the filter, and this is rarely obvious to the analyst or the hygienist. For very large particles, like wood dust, non-filter deposits may result from particles bouncing off the filter. For dense particles, wall deposits may result from gravitational settling before reaching the filter; for high-velocity particles, this might be due to impaction on the walls; and for smaller particles, wall deposition may be due to interception or electrostatic attraction to the walls. It seems likely that all these mechanisms may be in play, according to papers published in the journal Aerosol Science and Technology. In addition, of course, particles may fall off the filter during transportation, but that is unlikely where the collected mass is less than 4 mg. OSHA’s methods had indicated early on that if dust not on the filter was obvious to the analyst, then it needed to be included in the assessment of exposure, underscoring a policy that all aspirated dust in the cassette should be deemed part of the sample. NIOSH was also aware of the importance of wall deposits and had provided guidance in a background chapter (Chapter O in the 4th edition; Chapter AE in the 5th edition) in the preamble to the NIOSH Manual of Analytical Methods, or NMAM. During the 1980s the concept of “inhalability” was explored, leading to the development of the Institute of Occupational Medicine personal inhalable sampler. The IOM sampler includes a cassette containing the filter. Testing of this sampler revealed that a substantial mass of the aerosol entering the cassette deposited on the internal walls. This deposit was considered an integral part of the sample, and the cassette was designed to be weighed as a whole to ensure inclusion of all collected particles. The IOM sampler was tested against the CFC and results implying that the IOM would collect considerably more sample in most environments were summarized in a 1996 paper published in the journal . However, the data was from tests that had evaluated only the filter deposit for the CFC. Given the quantity of aerosol now known to deposit on the internal surfaces of the CFC, further “apples-to-apples” comparisons appear to be warranted (for an example of the type of study needed, see “Comparison of Lead and Tin Concentrations in Air at a Solder Manufacturer from the Closed-Face 37-Mm Cassette with and without a Custom Cellulose-Acetate Cassette Insert,” e, December 2014). In the 1990s, the French occupational health and safety institute (INRS) evaluated the deposits in its CFC field samples and realized that the quantities of particles not collected on the filter were a substantial portion of the total aspiration. INRS later made a similar discovery with its IOM samples. The agency relayed this information to OSHA, which quickly performed its own evaluation and arrived at the same conclusion. OSHA methods for metals sampling have now been updated to ensure inclusion of these deposits in every analysis.  In 2007, JOEH published a review of the state of the art at that time. By 2010, a substantial body of data on the under-reporting of exposures had accumulated, as summarized in Table 1.