Can you collect air samples using a method for the inhalable size fraction and use those results to demonstrate compliance with OSHA Permissible Exposure Limits for total size fractions?
Determining the appropriate aerosol particle size fraction(s) [1] to monitor is an important consideration in industrial hygiene exposure assessments. Often there may be different occupational exposure limits (OELs) for different particle sizes, with the inhalable fraction used for many OELs recommended by the ACGIH – American Conference of Governmental Industrial Hygienists, National Institute for Occupational Safety and Health (NIOSH), and in the literature, while the Occupational Safety and Health Administration (OSHA) Permissible Exposure Limits (PELs) generally rely on the respirable and/or total fractions. Sampling conventions for each aerosol size fraction specify the use of different sample collection methods. When developing air sampling plans, the ability to sample for all size fractions with an applicable OEL may be limited by practical considerations based on the number of personal sampling devices placed on a worker, or due to budget or other resource constraints.
In a Letter of Interpretation (LOI) from 2011, OSHA concurred that using an IOM Sampler, designed for sampling the inhalable fraction (particle aerodynamic diameters <100 µm), is more efficient in capturing particles compared to a 37-mm cassette, referenced by OSHA in total particulate sampling methods. OSHA stated that “a Total Dust measurement collected with an IOM Sampler may be used as an equivalent method when measuring an employee’s exposure to particulates not otherwise regulated (PNOR), and to determine whether exposure exceeds the PNOR permissible exposure limit (PEL) in 29 CFR 1910.1000.”
OSHA indicated in a 2019 LOI that results from inhalable fraction sampling methods (e.g., IOM, Button samplers) could be used to determine compliance with the OSHA PEL for total hexavalent chromium, as long as the monitoring and analysis method meets the specified accuracy and performance requirements.[2]
Thus, when sampling for airborne contaminants with OELs established for both inhalable and total size fractions, industrial hygienists can generally use an inhalable sampler to collect data for comparison with both OELs. Due to the increased efficiency of inhalable samplers compared to 37-mm cassettes for certain particle sizes, it may be necessary to apply a correction factor to establish the total fraction concentrations and demonstrate compliance with such OSHA PELs, particularly in environments where larger size particles (e.g., >20 µm) are predominant. Studies in the literature suggest correction factors for various industries and particle size distributions to convert inhalable sampler results to estimated total fraction concentrations.
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[1] International Standards Organization (ISO). ISO 7708: Air Quality: Particle Size Fraction Definitions for Health-Related Sampling. Geneva, Switzerland; 1995.
[2] The OSHA Hexavalent Chromium general industry standard [29 CFR 1910.1026] does not require a specific method and indicates that employers must “use a method of monitoring and analysis that can measure chromium (VI) to within an accuracy of plus or minus 25 percent (+/- 25%) and can produce accurate measurements to within a statistical confidence level of 95 percent for airborne concentrations at or above the action level (2.5 µg/m3)”.
