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 â€œParticulates Not Otherwise Regulatedâ€ (PNOR) (Table Z-1) or Inert or Nuisance Dust (Table Z-3). OSHAâ€™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 â€œtotal dustâ€ is 15 mg/m3. If only particles less than 10 micrometers (microns) in diameter are considered, then the PEL for â€œrespirable fractionâ€ (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 TLVsÂ® provide an updated view of particulates. The 2003 TLVÂ® for â€œParticles (Insoluble or Poorly Soluble) Not Otherwise Specifiedâ€ (PNOS) includes three â€œParticle Size-Selective TLVsÂ®â€:
ACGIH is recommending (guideline not TLVÂ®) 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 TLVÂ® apply only to particles that:
- Do not have an applicable TLVÂ®;
- Are insoluble or poorly soluble in water; and
- Have low toxicity.
The 2003 PNOS TLVÂ® 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 TLVÂ® for â€œwelding fumes â€“ total particulates (not otherwise specified) at 5 mg/m3 because â€œwelding fumes cannot be classified simply.â€ 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 TLVÂ®. 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 challengesNot 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.
Thereâ€™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 TLVÂ®. 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 TLVsÂ®. The ACGIH advises: â€œThese 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.â€ If you must address particulate exposures, make sure your IH skills are up-to-date.