NIOSH Pocket Guide Chemical Hazards and OSHA Limits Compared
The NIOSH Pocket Guide explains chemical hazards and recommended limits. Compare RELs with OSHA PELs, read IDLH values, and check the applicable standard.
The NIOSH Pocket Guide to Chemical Hazards is an occupational-hazard reference covering 677 chemicals or substance groupings, including exposure limits and IDLH values. Use it for NIOSH recommendations and hazard information, compare limits in OSHA's Occupational Chemical Database, then check the applicable OSHA standard.
A NIOSH recommended exposure limit (REL) is not itself a federal OSHA limit. An applicable federal OSHA permissible exposure limit (PEL) is mandatory. OSHA's annotated exposure-limit tables show that distinction and warn that many PELs are outdated and inadequate for ensuring protection of worker health. Compliance with a PEL does not mean the exposure is harmless.
What the NIOSH Pocket Guide contains
An entry lists identity, synonyms, exposure limits, physical properties, symptoms, target organs, measurement methods and protection information. It also lists exposure routes, incompatibilities and first aid. The introduction calls this key information, not all relevant data.
Start with identity. The CDC landing page offers text search, an alphabetical index of names and synonyms, and a CAS-number index. Match the entry to the substance. A similar name is not enough. The count of 677 includes substance groupings, so it is not 677 distinct chemicals.
For the NIOSH guidebook, use that same CDC page. It lists online, PDF and mobile web app versions. The mobile web app requires an Apple iPhone or Android phone. Read the introduction with the chemical entry, or the notation will not make sense.
OSHA vs NIOSH roles and limits
NIOSH is the federal institute that researches work-related injury and illness and makes prevention recommendations. It sits inside CDC, in the Department of Health and Human Services. OSHA is part of the Department of Labor and sets and enforces standards. They do not answer the same question.
| Question | NIOSH Pocket Guide | OSHA Occupational Chemical Database |
|---|---|---|
| What does it provide? | Concise hazard, symptom and protection information | Chemical properties, exposure limits, sampling information and other resources |
| How are limits presented? | NIOSH RELs alongside OSHA PEL information | Comparison columns, with contents varying by record |
| What sampling help is available? | Measurement-method references | Monitoring methods and sampling details in records such as benzene |
| Does a listed number establish a federal legal duty? | A REL does not. A PEL printed in the guide is not a substitute for the current standard | Only if the applicable OSHA standard and jurisdiction impose it |
The NIOSH introduction, OSHA database description and benzene record explain these contents. A comparison column does not make every number federal law. Cal/OSHA PELs apply within their jurisdiction, not automatically elsewhere, as OSHA explains in its annotated tables.
There is a historical limit on the PELs printed in the guide. The introduction identifies the general-industry Table Z-1, Z-2 and Z-3 values as those effective on July 1, 1993. That baseline does not mean every guide field dates from 1993, or that every listed PEL has since changed. It does mean you should check the current OSHA standard before using a guide value for compliance. The chemical database can point you to a record. It is not the standard.
Read the averaging period with the number
A limit without its time basis is incomplete. The Pocket Guide introduction explains the notation:
- A NIOSH REL time-weighted average, or TWA, normally covers up to a 10-hour workday during a 40-hour workweek.
- An OSHA PEL TWA generally must not be exceeded during any eight-hour workshift of a 40-hour workweek. Check the substance and sector requirements.
- ST or STEL normally means a 15-minute time-weighted average, unless otherwise noted. A NIOSH STEL should not be exceeded during a workday unless the entry says otherwise.
- C means ceiling. Read the specific provision instead of treating it as another ordinary TWA.
Keep the units attached too. A comparison should preserve the value, the units, the averaging period and any notation or footnote.
For a NIOSH ceiling REL, the introduction says the value should not be exceeded at any time unless otherwise noted. An OSHA ceiling must not be exceeded during any part of the workday. If instantaneous monitoring is not feasible, it is assessed as a 15-minute TWA. Some Table Z-2 substances allow specified excursions. Don't turn the letter C into one universal peak-exposure rule. Read the specific explanation.
Benzene shows why the distinction matters
The NIOSH benzene entry marks benzene "Ca," a potential occupational carcinogen. Its RELs sit next to the OSHA benzene-standard limits:
| Source and limit | Value | Averaging period |
|---|---|---|
| NIOSH REL TWA | 0.1 ppm | Up to a 10-hour day, 40-hour week |
| NIOSH short-term limit | 1 ppm | 15 minutes |
| OSHA benzene-standard PEL | 1 ppm | Eight-hour TWA |
| OSHA benzene-standard STEL | 5 ppm | Any 15-minute period |
The REL values and periods come from the benzene entry and NIOSH introduction. The OSHA values come from §1910.1028.
Do not apply the 1 ppm OSHA value to every benzene operation. The benzene standard has scope exceptions. The OSHA database record points to Table Z-2 where the benzene-standard limits are stayed or otherwise not in effect.
That record has federal OSHA, NIOSH, ACGIH and Cal/OSHA columns. Some cells are blank, and the ACGIH column directs readers elsewhere. But a blank cell does not mean the hazard or the legal requirement is absent. The table is not the whole record.
The same record shows monitoring methods, sampling media and flow rates. Sampling duration is not necessarily the exposure limit's averaging period. OSHA describes the sampling instructions as recommended guidelines for its Compliance Safety and Health Officers, not as a complete plan for every workplace.
IDLH is not a routine working limit
IDLH means immediately dangerous to life or health. OSHA's respiratory-protection standard defines an IDLH atmosphere as one that poses an immediate threat to life, would cause irreversible adverse health effects, or would impair escape from a dangerous atmosphere. The Pocket Guide introduction uses IDLH values for escape and respirator-selection decisions.
The 30-minute figure is a safety margin, not permission to remain. The introduction says to exit immediately. An IDLH value also does not replace a daily or short-term exposure limit.
Under §1910.134(d)(1)(iii), an employer must treat the atmosphere as IDLH when employee exposure cannot be identified or reasonably estimated. IDLH respiratory protection requires specified atmosphere-supplying equipment, not an ordinary air-purifying respirator. Use the standard for that decision. A Pocket Guide line is not enough.
That edition did not revise the respirator recommendations for OSHA's end-of-service-life-indicator or change-schedule requirements. Required respirator use still needs a worksite-specific program, including applicable evaluation, fit-testing and training. Read the guide's warning with the OSHA standard.
Use both resources before a compliance decision
Confirm the identity, read the full entry and its notation, and compare the OSHA record without dropping the units or averaging periods. Check scope and jurisdiction before treating a number as a legal duty. The OSHA database also points to hazard communication and protective equipment. An exposure-limit lookup is not a complete compliance assessment.
RegAffairs AI explains how the Pocket Guide differs from OSHA's chemical database.
Check the dates, and don't treat a page date as a full revision history. The Pocket Guide landing page was last reviewed on February 18, 2020. The introduction shows a March 7, 2016 review date. The OSHA benzene record shows a last updated date of October 1, 2026. None of those dates proves that every field changed then, or that either resource is current through that date.