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
Air Sampling Instruments
. 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,”
Journal of Occupational and Environmental Hygien
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. 
Air Sampling Instruments
, 1st edition (1960).
Aerosol Science and Technology
: “Size Distributions of 0.5 to 20 µm Aerodynamic Diameter Lead- Containing Particles from Aerosol Sampler Walls and Filters” (October 2009).
Aerosol Science and Technology
: “Comparison of Filter and Wall Deposits from Samplers Used to Collect Airborne Lead-Containing Dusts at Field Sites” (April 2012).
American Industrial Hygiene Association Quarterly
: “Use of Membrane Filters in Air Sampling” (September 1957).
: “Investigation into the Impact of Introducing Workplace Aerosol Standards Based on the Inhalable Fraction” (September 1996).
Annals of Occupational Hygiene
: “Aerosol Evaluation Difficulties Due to Particle Deposition on Filter Holder Inner Walls” (August 1990).
Annals of Occupational Hygiene
: “A New Personal Sampler for Airborne Total Dust in Workplaces” (January 1986).
Applied Occupational and Environmental Hygiene
: “Field Comparison of 37-Mm Closed-Face Filter Cassettes and IOM Samplers” (March 2002). Brown, C.E.: Filter-Paper Method for Obtaining Dust-Concentration Results Comparable to Impinger Results (1944). International Organization for Standardization: ISO/IEC 17025:2017,
General Requirements for the Competence of Testing and Calibration Laboratories
Journal of ASTM International
: “Evaluation of Quartz Residue on Cassette Interiors of AIHA Proficiency Samples” (April 2005).
Journal of Occupational and Environmental Hygiene
: “On Wiping the Interior Walls of 37-mm Closed-Face Cassettes: An OSHA Perspective” (December 2009).
Journal of Occupational and Environmental Hygiene
: “Acid-Soluble Internal Capsules for Closed-Face Cassette Elemental Sampling and Analysis of Workplace Air” (June 2013).
Journal of Occupational and Environmental Hygiene
: “Closed-Face Filter Cassette (CFC) Sampling—Guidance on Procedures for Inclusion of Material Adhering to Internal Sampler Surfaces” (March 2013).
Journal of Occupational and Environmental Hygiene
: “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” (December 2014).
Journal of Occupational and Environmental Hygiene
: “Comparison of a Wipe Method with and without a Rinse to Recover Wall Losses in Closed Face 37-Mm Cassettes Used for Sampling Lead Dust Particulates” (October 2015).
Journal of Occupational and Environmental Hygiene
: “Concerning Sampler Wall Losses in the Chemical Analysis of Airborne Metals” (September 2007).
Journal of Occupational and Environmental Hygiene
: “Interlaboratory Evaluation of Cellulosic Acid-Soluble Internal Air Sampling Capsules for Multi-Element Analysis” (January 2016).
Journal of Occupational and Environmental Hygiene
: “Preliminary Studies on the Use of Acid-Soluble Cellulose Acetate Internal Capsules for Workplace Metals Sampling and Analysis” (July 2012).
Journal of Occupational and Environmental Hygiene
: “Quartz in Coal Dust Deposited on Internal Surface of Respirable Size Selective Samplers” (December 2014). NIOSH Manual of Analytical Methods:
Consideration of Sampler Wall Deposits: Inclusion of Material Adhering to Internal Cassette Surfaces during Sampling and Analysis of Airborne Particles
. NIOSH Manual of Analytical Methods: Factors Affecting Aerosol Sampling, Chapter O (4th edition) and Chapter AE (5th edition) (
, 2016). U.S. Patent 36868354:
Filter Cassette with Removable Capsule
(August 1972).

It was clear that the issue was indeed serious, with the potential for some samples to have more metal-containing particles on the walls of the cassette than on the filter. The median data in the table could potentially be combined to provide a “correction factor,” which could be applied statistically to a large dataset, but the variance between individual results does not allow a correction on a single sample. Applying this correction factor to the results provided by the OSHA States program laboratory indicated that not considering wall deposits resulted in a likely 10 percent over-estimation of compliance with applicable permissible exposure limits. Nor is this situation confined to “total” or “inhalable” samples. A project was initiated in the early 2000s to investigate reasons for the large data spread in samples from AIHA’s respirable crystalline silica Proficiency Analytical Testing program. The samples used cyclone-cassettes from laboratory-generated aerosols. As described in the April 2005 issue of the
Journal of ASTM International
, wall deposits in the cassettes were found to be present in half the samples at up to 26 percent of the total sample. The same laboratory then analyzed its field samples and found up to 32 percent on the walls of two-piece cassettes and up to 55 percent on three-piece cassettes. Recent work by NIOSH has confirmed this observation, especially for nylon and aluminum cyclones (see “Quartz in Coal Dust Deposited on Internal Surface of Respirable Size Selective Samplers,” JOEH, December 2014).
It is the policy of both OSHA and NIOSH to ensure that all particles entering the cassette are included as part of the sample. A NIOSH review of the issue published in the March 2013 JOEH represents agency policy, and the NMAM website now links to a
statement reiterating the policy
.  So, how can inclusion of non-filter (“wall”) deposits be assured? OSHA, as noted above, has its special cassette with capsule for gravimetric analysis. OSHA also has evaluated rinsing the interior of cassettes with dilute acid for metals and found recovery to be lower than the alternative procedure of using a single manual wipe with a wetted absorbent material. The wipe is added to the filter for analysis; although this increases the limit of quantitation and uncertainty of the procedure, the increase is not likely significant. (The AIHA Laboratory Accreditation Programs, LLC considers any contribution that is greater than or equal to one-third of the largest measurement uncertainty contributor for a test method to be a significant contributor to measurement uncertainty.) A paper from NIOSH published in the October 2015 JOEH confirms that use of a single wipe is effective. However, wiping is a manual procedure carrying potential risks of inconsistency and contamination.  As a less risky option to wiping, internal capsules have been designed: PVC capsules bonded to PVC filters for weighing (or hexavalent chromium analysis) and cellulose capsules bonded to MCE filters for metals. The former have been validated and are now incorporated in the new NIOSH methods 0501 and 5100. The latter have been evaluated in ASTM International interlaboratory precision and accuracy studies, NIOSH interlaboratory user checks, and field studies, and are now incorporated in the new NIOSH method 7306. For respirable dust samples, wiping can be used, but because the dominant mechanism leading to wall deposition for fine particles is electrostatic attraction, conductive (“static-dissipative”) plastic cassettes do not exhibit large wall deposit losses and are an acceptable alternative to wiping, as shown in the December 2014 JOEH paper on quartz in coal dust.
It is very important to be aware of the obligation placed on laboratories by ISO 17025,
General Requirements for the Competence of Testing and Calibration Laboratories
, section 7.1.2, which reads, “The laboratory shall inform the customer when the method requested by the customer is considered to be inappropriate or out of date.” Following ISO 17025 is necessary for laboratories if they are to be accredited against its requirements. For those who wish to maintain a database of consistent sample results over time, an option is to use the wiping procedure and to analyze the wipe and filter separately. Once sufficient samples have been analyzed this way, it may be possible to develop and apply a correction factor to the older sample results. Separate analyses can then be discontinued, and users can choose between wiping, with the wipe and filter combined into a single analysis, or using the internal capsules for a single analysis without risk of increasing uncertainty from the manual wiping procedure. It is up to everyone in the sampling and analytical community to understand this issue and act accordingly, and to remember that if you are not accounting for wall deposits, you’re likely underestimating exposures; you’re not following current NIOSH guidance; and you won’t obtain results equivalent to OSHA compliance samples. Also, don’t forget that we no longer use “sugar tubes” and “impingers” for dust sampling. Science leads to change, and we do change with it.  
is director of Scientific Research, Zefon International Inc., in Ocala, Fla., and courtesy professor, Department of Environmental Engineering Sciences, University of Florida, Gainesville, Fla. 
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If you are not accounting for wall deposits, you’re likely underestimating exposures; you’re not following current NIOSH guidance; and you won’t obtain results equivalent to OSHA compliance samples.
Table 1. Median Wall Deposits for Closed-Face Cassette (CFC) and Institute of Occupational Medicine (IOM) Samplers* 
Tap on the table to open a larger version in your browser.
What Wall Deposits Mean for Your Samples—And What You Should Do about It
Part of the Sample