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MANAGING BEST PRACTICES: Getting particular about particulates

January 5, 2004
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The air we breathe is filled with a variety of particles that vary in shape, size, structure, composition, solubility and other parameters. Particles are formed from solids and may be generically called dust, mist, smoke or fume. Illness or discomfort may arise if we inhale too many particles.

Some particles may be distinguished as a specific substance with known toxic effects and have their own exposure limits. Silica is an example. OSHA regulates particles that cannot be classified as a specific substance as “Particulates Not Otherwise Regulated” (PNOR) (Table Z-1) or Inert or Nuisance Dust (Table Z-3). OSHA’s permissible exposure limits for PNOR and inert or nuisance dust are based upon particle size. If particles of all sizes are included, then the PEL for “total dust” is 15 mg/m3. If only particles less than 10 micrometers (microns) in diameter are considered, then the PEL for “respirable fraction” (particles small enough to reach the lungs) is 5 mg/m3.

A micron (one millionth of a meter) is very small. The diameter of a human hair ranges from 50-75 microns. A red blood corpuscle is about 8 microns. The naked eye has a hard time seeing particles less than 50 microns in diameter.

The ACGIH TLVs® provide an updated view of particulates. The 2003 TLV® for “Particles (Insoluble or Poorly Soluble) Not Otherwise Specified” (PNOS) includes three “Particle Size-Selective TLVs®”:

  • Inhalable Particle Mass “for those materials that are hazardous when deposited anywhere in the respiratory tract” (medium cut-point at 100 microns);

  • Thoracic Particulate Mass “for those materials that are hazardous when deposited anywhere within the lung airways and the gas-exchange region” (medium cut-point at 10 microns); and

  • Respirable Particulate Mass “for those materials that are hazardous when deposited in the gas-exchange region” (medium cut-point at 4 microns).

    ACGIH is recommending (guideline not TLV®) that inhalable particles be kept below 10 mg/m3 and respirable particles be kept below 3 mg/m3. The ACGIH further advises (because of misuse in the past) that the PNOS TLV® apply only to particles that:

    • Do not have an applicable TLV®;
    • Are insoluble or poorly soluble in water; and
    • Have low toxicity.

    The 2003 PNOS TLV® does not address a limit for thoracic particulate. OSHA, however, addressed this concept in 1999 when the OSHA Metalworking Fluids Standards Advisory Committee recommended a PEL of 0.4 mg/m3 thoracic particulate mass for metalworking fluid exposure. The UAW sued OSHA in November 2003 to force the agency to establish the limit in a standard.

    Not enough info?

    Lumping particulates into a single group and then setting an exposure limit for the group is acceptable when it is difficult to qualify or quantify the individual constituents in the group. The ACGIH established a TLV® for “welding fumes – total particulates (not otherwise specified) at 5 mg/m3 because “welding fumes cannot be classified simply.” The ACGIH did not intend that this was the only measurement necessary for welding fume exposure, but this is how some EHS pros applied the TLV®. Substances in some welding fumes with known toxic effects such as chromium, nickel, and manganese still must be measured.

    Knowing the individual constituents in welding fume was underscored in November 2003 when an Illinois jury awarded a former welder $1 million in damages when he claimed exposure to manganese during welding caused him to develop a central nervous system disorder.

    New challenges

    Not that long ago it was easy to sample for PNOR/PNOS. Total particulate was captured with a filter cassette. A cyclone attachment to the filter cassette was used to capture the respirable fraction. Results in both cases were obtained by simple gravimetric analysis.

    There’s a greater challenge today in sampling for PNOR/PNOS. First, we must make sure that no constituents of the particulates have a PEL or TLV®. Next, we must know about the solubility and toxic properties of the particulates. Then we need to decide other things such as what size fraction we are looking for. Will we also look for bioaerosols (viable or non-viable) during the analysis? Accuracy and precision during sampling and analysis are more important today, too.

    The best advice in dealing with particulate exposure comes from the ACGIH in its policy statement on uses of TLVs®. The ACGIH advises: “These recommendations or guidelines are intended for use in the practice of industrial hygiene, to be interpreted and applied only by a person trained in this discipline.” If you must address particulate exposures, make sure your IH skills are up-to-date.

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