NIOSH Pocket Guide Exposure Limits
How do I use the NIOSH Pocket Guide to check chemical exposure limits?
To check a chemical in the NIOSH Pocket Guide (NPG), find its entry, then read the "Exposure Limits" field. The NIOSH Recommended Exposure Limit (REL) comes first and the OSHA Permissible Exposure Limit (PEL) second, followed by a separate IDLH value. The guide covers 677 chemicals or substance groupings, and NIOSH's main page says it was last updated February 18, 2020 (NPG home; NPG Introduction).
Approach: The NPG itself isn't in the datasets I can search. The guidance below comes from NIOSH's own pages on cdc.gov.
1. Find the entry (NPG home)
- Chemical name index (A–Z): search by name, including group entries such as cyanides, fluorides or manganese compounds.
- CAS number index: this is the most reliable route, because each CAS number belongs to only one chemical.
- Other indexes: RTECS numbers, and synonyms and trade names.
- Formats: the guide is available online, as a PDF download, or as the mobile app for iPhone or Android.
2. Read the Exposure Limits field (Introduction)
- NIOSH REL (listed first):
- TWA: time-weighted average for up to a 10-hour workday in a 40-hour week.
- ST: short-term exposure limit, a 15-minute TWA unless stated otherwise.
- C: ceiling, not to be exceeded at any time.
- Ca: NIOSH considers the substance a potential occupational carcinogen.
- OSHA PEL (listed second):
- The PELs shown are those in effect as of July 1, 1993.
- TWAs are for an 8-hour shift, so the averaging time differs from the REL's 10 hours.
- A † marks a case where the 1989 limits were vacated and the PEL went back to the 1971 value. The 1989 values are in Appendix G.
- Other notations:
- [skin]: the substance can be absorbed through the skin.
- (total) or (resp): the limit applies to total or respirable particulate.
- Units: ppm, mg/m³, mppcf or fibres/cm³.
- Footnotes: check Appendices C, D, F and G for substance-specific notes.
3. Use the other fields with the limits
- IDLH: listed for more than 380 substances. Its purpose is to make sure a worker can escape if respiratory protection fails. "10%LEL" means the value is based on the lower explosive limit, "N.D." means not determined, and bracketed values are revised values.
- Conversion factor: converts ppm to mg/m³ at 25 °C and 1 atm, so you can compare air-sampling results with the limit in either unit.
- Measurement method: NIOSH methods from the NIOSH Manual of Analytical Methods (4th edition) and OSHA methods. "None available" means there is no standard method.
- Respirator recommendations: these are based on whichever is more protective, the REL or the PEL. Each entry lists the maximum use concentration and respirator class with its assigned protection factor. NIOSH states that air-purifying respirators do not protect against oxygen-deficient atmospheres and "are not to be used in IDLH conditions."
4. Things to keep in mind
- RELs are recommendations from NIOSH. PELs are OSHA's enforceable limits, but the NPG shows them as of 1993.
- For chemicals with no exposure limit, NIOSH points users to its Occupational Exposure Banding page (NPG home).
I can look up a specific chemical by CAS number in OSHA's annotated PEL tables (29 CFR 1910.1000, Tables Z-1 to Z-3) or the OSHA Occupational Chemical Database. That would let you compare the current OSHA figures with the values printed in the NPG.
Sources:
How is it different from OSHA’s chemical database?
The NIOSH Pocket Guide is a NIOSH reference handbook. It gives NIOSH's recommended limits (RELs) next to OSHA PELs as they stood in 1993, with IDLH values, NIOSH sampling methods and respirator advice. OSHA's Occupational Chemical Database is OSHA's own compilation. It puts the current OSHA PEL beside the NIOSH REL, the ACGIH TLV and the Cal/OSHA PEL in one record, and adds OSHA's own sampling details. OSHA calls it "OSHA's premier one-stop shop for occupational chemical information," compiled from "several government agencies and organizations" (OSHA Occupational Chemical Database).
Approach: I read OSHA's Occupational Chemical Database, using the toluene record as a worked example, and OSHA's overview page. The Pocket Guide details come from the NIOSH pages I opened in the previous answer.
| NIOSH Pocket Guide | OSHA Occupational Chemical Database | |
|---|---|---|
| Publisher and purpose | NIOSH reference guide for 677 chemicals or groupings, last updated February 18, 2020 (NPG home) | OSHA compilation of identity, physical properties, exposure limits, sampling information and further resources (OSHA) |
| Limits shown | NIOSH REL first, then OSHA PEL as of July 1, 1993 (NPG Introduction) | OSHA PEL, NIOSH REL, ACGIH TLV (with the year) and Cal/OSHA PEL side by side, plus a skin notation for each [1] |
| Sector PELs | General-industry PEL only | Points to the relevant 29 CFR tables and gives construction and maritime PELs where they differ. For toluene: "See 29 CFR 1910.1000 Table Z-2", with a construction and maritime PEL of 200 ppm (750 mg/m³) under 29 CFR 1926.55 Table 1 and 1915.1000 Table Z-Shipyards [1] |
| Emergency and hazard values | IDLH, with notations such as 10%LEL and N.D. | IDLH, AIHA ERPG-1/2/3 values and carcinogen classifications (IARC, ACGIH, EPA). For toluene: IDLH 500 ppm, ERPGs 50/300/1000 ppm, IARC-3, TLV-A4, EPA-II [1] |
| Sampling | NIOSH (NMAM) and OSHA method numbers | OSHA's working sampling details: IMIS analyte code, sampling media, time, volume and flow rate, instrument, validated OSHA methods (toluene: OSHA 5000 and OSHA 111), and detector-tube or chip screening devices [1] |
| Respirator guidance | Maximum use concentrations and respirator classes, based on the more protective of the REL and PEL | None in the record. It links out to the Pocket Guide entry, NOAA CAMEO Chemicals, EPA and literature references instead [1] |
| Physical data | Physical properties and conversion factor | Physical properties plus NFPA health, fire and reactivity ratings [1] |
| Currency | One update date for the whole guide | Each record has its own "Last Updated Date" (toluene: 06/03/2022) [1] |
What this means in practice (toluene example)
- The OSHA record shows how much the limits can differ for the same chemical [1]:
- OSHA PEL-TWA: 200 ppm
- NIOSH REL-TWA: 100 ppm
- ACGIH TLV-TWA: 20 ppm [2006]
- Cal/OSHA PEL-TWA: 10 ppm
- For toluene, OSHA's ceiling is 300 ppm, with a 500 ppm peak ("10 min maximum in an 8 hr shift"). Cal/OSHA marks a skin notation (Y); OSHA, NIOSH and ACGIH do not (N).
- For compliance: OSHA's enforceable PELs are in 29 CFR 1910.1000, 1926 and 1915, as the OSHA page states (OSHA). The OSHA database reflects these better than the 1993 PEL snapshot in the Pocket Guide.
- Use the Pocket Guide for: NIOSH recommendations, IDLH-based planning and respirator selection.
- Use the OSHA database for: the current OSHA PEL next to the other US limits, sector-specific PELs, and planning sampling the way OSHA does it.
I can pull the OSHA database record and the annotated PEL table entry for a particular chemical or CAS number so you can compare them.
Sources: