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The NIOSH Pocket Guide to Chemical Hazards (NPG) is a CDC/NIOSH reference that summarises workplace exposure limits, health effects, protective measures and properties for hundreds of chemicals and chemical groups. Look a chemical up by name or CAS number, read the exposure limits and incompatibilities, then go to the safety data sheet and the legal OSHA tables for anything that decides compliance.
Short answer
The NPG is a quick lookup, not a complete record. NIOSH describes it as offering “key facts, but does not give all relevant data”. Each entry places NIOSH recommended exposure limits (RELs), OSHA permissible exposure limits (PELs) and immediately dangerous to life or health (IDLH) values side by side. Those columns use different averaging times and different legal weight, so reading them correctly matters more than finding them.
What the NIOSH Pocket Guide covers
NIOSH publishes the guide as general industrial hygiene information for workers, employers and occupational health professionals, to help them recognise and control workplace chemical hazards. It covers hundreds of chemicals and chemical classes, including groupings such as cyanides, fluorides and manganese compounds, so a search for one compound may land on a group entry.
It is available as an online search, an index of names, synonyms and trade names, an index of CAS numbers, a PDF and a mobile web app. NIOSH no longer distributes print copies; the printed edition is sold through the U.S. Government Publishing Office and the PDF through NTIS. The CDC page was last reviewed on February 18, 2020 and shows no edition number, although the PDF file path refers to publication 2005-149.
How an NPG entry is organised
Every entry follows the same layout, which is what makes the guide quick to scan. According to the CDC description, an entry includes:
- Names, synonyms, trade names, CAS and RTECS numbers, and DOT ID and guide numbers
- Structure or formula and conversion factors
- NIOSH RELs, OSHA PELs and IDLH values
- Physical and chemical properties
- Measurement methods
- Personal protection, sanitation and respirator selection
- Incompatibilities and reactivities
- Exposure routes, symptoms, target organs and first aid
Reading REL, PEL, STEL, ceiling and IDLH
The guide’s introduction defines the limit types, and the definitions differ in ways that are easy to miss:
| Entry field | What the NPG says it means | Lab question it answers |
|---|---|---|
| REL | NIOSH recommended exposure limit, a time-weighted average (TWA) for up to a 10-hour workday within a 40-hour week | What does NIOSH recommend as an airborne limit? |
| PEL | OSHA enforceable limit under 29 CFR 1910.1000, a TWA over an 8-hour shift in a 40-hour week | What is the legal limit that applies? |
| STEL (ST) | Unless noted otherwise, a 15-minute TWA not to be exceeded at any time during a workday | How high can a short exposure go? |
| Ceiling (C) | A concentration not to be exceeded at any time | Is there an instantaneous cap? |
| IDLH | A concentration from which a worker could escape without injury or irreversible health effects if respiratory protection failed; based on a 30-minute exposure, but workers should leave immediately | When does a respirator failure become an emergency? |
| Ca | Potential occupational carcinogen | Does NIOSH treat this as a possible carcinogen? |
| [skin] | Skin absorption is a significant exposure route | Does air monitoring alone miss part of the exposure? |
| N.D. | No IDLH determined | Is there an IDLH value to use? |
The 10-hour REL basis versus the 8-hour PEL basis means two numbers in the same entry are not directly comparable without noting the shift length. For the wider comparison of PELs, RELs and ACGIH TLVs, see permissible exposure limits (PELs, TLVs and RELs) in the laboratory.
Worked example: reading the acetaldehyde entry
The acetaldehyde entry (CAS 75-07-0) shows how the fields fit together. Its synonyms are acetic aldehyde, ethanal and ethyl aldehyde. The REL reads “Ca” with pointers to Appendix A and Appendix C, so NIOSH flags it as a potential occupational carcinogen and points to appendix notes rather than giving a single number. The PEL is a TWA of 200 ppm (360 mg/m3) with a pointer to Appendix G, the appendix where the guide lists OSHA PELs that were vacated. The IDLH is shown as “Ca [2000 ppm]”.
The incompatibilities line lists strong oxidizers, acids, bases, alcohols, ammonia and amines, phenols, ketones, HCN and H2S, which is the part most useful when planning chemical storage. The measurement methods line cites NIOSH methods 2018, 2538 and 3507 and OSHA method 68, which tells an industrial hygienist where to find the sampling procedure. Taken together, one entry yields a limit, a flag, a legal footnote, a storage warning and a sampling reference in a single page.
Where the guide falls short
NIOSH states that the guide supplements, and does not replace, general industrial hygiene knowledge. The introduction lists specific cautions that apply to lab use:
- Some PELs were vacated by the courts (enforcement stopped in 1993) and are noted in Appendix G.
- No respirator recommendations are given for 186 chemicals that lack an IDLH value.
- Air-purifying respirators do not protect in oxygen-deficient atmospheres or at IDLH concentrations.
- Respirator recommendations have not been updated for the end-of-service-life indicator rules.
- Measurement methods may not suit every sampling situation.
- The introduction page was last reviewed March 7, 2016, so values may have changed since.
OSHA’s position on limits adds a second layer. OSHA says its mandatory PELs in the Z-tables remain in effect, but that many are outdated and inadequate, most having been set shortly after 1970. It recommends that employers consider alternative occupational exposure limits, and notes that TLVs are not intended as legal standards. For a chemical that is not in the guide or has no occupational exposure limit, NIOSH directs readers to its occupational exposure banding topic page.
Where it fits in lab practice
In a lab, the NPG works best as a first lookup that is then cross-checked. The safety data sheet remains the document that travels with the product (see how to read a safety data sheet), and the employer duties behind it are covered in the Hazard Communication Standard in laboratories. The guide’s respirator and IDLH columns connect to respiratory protection programs in laboratories, and its incompatibility lines pair with the common lab chemical hazard classes primer.
Common misunderstandings
The NPG is not a legal standard
It is a NIOSH publication. Only the OSHA PEL column reflects an enforceable limit, and OSHA’s own Z-tables are the legal reference.
A blank or N.D. field does not mean a chemical is safe
N.D. means no IDLH was determined, and the guide gives no respirator recommendation for the 186 chemicals without one. The absence of a value is missing information, not a clean bill of health.
REL and PEL numbers cannot be compared at face value
One is a TWA for up to 10 hours and the other a TWA over 8 hours, and the REL is a recommendation while the PEL is enforceable.
FAQ
What is the NIOSH Pocket Guide to Chemical Hazards?
A CDC/NIOSH reference giving key workplace exposure facts for hundreds of chemicals and chemical classes, including exposure limits, properties, protective measures and incompatibilities.
Is the NIOSH Pocket Guide the law?
No. The RELs are NIOSH recommendations. The PELs it prints are OSHA limits under 29 CFR 1910.1000, and some were vacated by the courts, with enforcement stopping in 1993, and are flagged in Appendix G.
How do I search it by CAS number?
The online guide provides an index of CAS numbers and one of RTECS numbers, along with an index of chemical names, synonyms and trade names, and the same data is in the PDF and the mobile web app.
Is it current?
The CDC landing page was last reviewed on February 18, 2020, and the introduction page on March 7, 2016. The page shows no edition number, so treat values as a starting point and confirm limits against OSHA’s tables and the current safety data sheet.
Sources
- CDC/NIOSH, NIOSH Pocket Guide to Chemical Hazards: https://www.cdc.gov/niosh/npg/default.html (checked 2026-10-29)
- CDC/NIOSH, NPG Introduction: https://www.cdc.gov/niosh/npg/pgintrod.html (checked 2026-10-29)
- CDC/NIOSH, NPG acetaldehyde entry: https://www.cdc.gov/niosh/npg/npgd0001.html (checked 2026-10-29)
- OSHA, Annotated PELs: https://www.osha.gov/annotated-pels (checked 2026-10-29)
