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Key Summary: Occupational Exposure Limits (OELs) define the maximum allowable concentration of hazardous substances in workplace air, expressed in mg/m³ or ppm. Three airborne types exist: the 8-hour Time-Weighted Average (TWA), the Short-Term Exposure Limit (STEL, typically 15 minutes) and the Ceiling value (never to be exceeded). A fourth type — the Biological Limit Value (BLV, also BEI, BGW or BAT depending on jurisdiction) — measures the internal dose in blood or urine. OELs are set by national governments and scientific bodies, with typically 500–800 substances covered per country. DOHSBase aggregates over 15,000 OELs from national and international sources, plus 5,300+ REACH DNELs and over 100,000 kick-off values for substances that lack a formal limit.
If your risk assessment flags an airborne concentration, you need a limit value to compare against. Occupational exposure limits (OELs) provide that benchmark: the maximum concentration of a substance in a worker’s breathing zone — gases, vapours, particles, aerosols or fibres — at which the exposure is considered acceptable over a defined period. Put more simply: the maximum permitted concentration of a hazardous substance in workplace air. The breathing zone is where it is measured, because that is the air the worker actually inhales. Without an OEL, “safe” becomes a subjective judgment. With one, the compliance question is quantitative.
In EU regulatory texts and in EN 689 the same benchmark is called an occupational exposure limit value (OELV) — indicative values are IOELVs, binding values BOELVs. OEL and OELV refer to the same quantity; which term you meet depends on the document, not on the value.
The complication is that OELs come in several forms, from multiple sources, with different units and different legal weight. This article covers the four types you will encounter in practice and how DOHSBase structures them.
The four types of limit value
At a glance — three airborne limits plus the biological limit:
| Limit type | Averaging period | What it measures | Protects against |
|---|---|---|---|
| 8-hour TWA (TGG-8u) | Full 8-hour working day | Substance concentration in workplace air | Chronic effects from full-shift exposure |
| STEL (short-term) | ~15 min, max 4×/day, ≥60 min apart | Substance concentration in workplace air | Acute effects from short peaks — irritation, narcosis, sensitisation |
| Ceiling (C) | Instantaneous — never exceeded | Substance concentration in workplace air | Rapid-onset toxicity from highly irritant or acutely toxic substances |
| Biological limit (BLV / BEI / BGW / BAT) | Internal dose at a defined sampling time | Substance or metabolite in blood, urine or breath | Total uptake across all routes, including skin absorption |
8-hour Time-Weighted Average (TWA)
The 8-hour TWA — also written as TWA-8h or TGG-8u in Dutch — represents the average concentration over a standard working day. Brief peaks above the TWA are acceptable provided the daily average stays within the limit and no short-term or ceiling value is exceeded. The 8-hour TWA is the primary reference value in most risk assessments and compliance checks.
Short-Term Exposure Limit (STEL)
A STEL is the maximum concentration to which workers may be exposed over a short period, typically 15 minutes, usually with a maximum of four such exposures per day and at least 60 minutes between them. STELs protect against acute effects — irritation, narcosis, sensitisation — that brief peaks can cause even when the 8-hour average is within bounds. Not every substance has a STEL; they are assigned where short-term peaks are known to be particularly hazardous.
Ceiling value
A ceiling value is an instantaneous maximum that must never be exceeded, not even momentarily. Ceiling values apply to substances with very rapid-onset toxicity — highly irritant gases, acutely toxic vapours — where even a few breaths at high concentration can cause serious harm.
Biological Limit Value (BLV / BEI / BGW / BAT)
Biological limit values work differently from the three airborne types above. Instead of measuring the substance in workplace air, they set a maximum concentration of the substance (or a metabolite) in biological samples from the worker — blood, urine, or occasionally exhaled breath. Biological monitoring captures the internal dose regardless of how the substance entered the body: inhalation, skin absorption, or ingestion. That makes it especially useful for substances with significant dermal uptake, where an airborne limit alone under-estimates the actual exposure.
Jurisdictional names vary:
- BLV — the generic EU term, used in the Chemical Agents Directive
- BEI — Biological Exposure Index, the ACGIH term (United States)
- BGW — Biologischer Grenzwert, the regulatory value in Germany
- BAT — Biologischer Arbeitsstoff-Toleranzwert, the health-based equivalent from the German MAK Commission
DOHSBase aggregates biological limit values alongside airborne OELs so you can check both types in a single lookup.
Statutory or private: who sets the value?
The four types above differ in averaging time. A second distinction runs alongside it and often matters more in practice: who set the value, and whether compliance is legally enforceable.
- A statutory (public) limit value is set by government or the European Union and is binding. In the Netherlands it appears in annex XIII to the Working Conditions Regulation; the Labour Inspectorate assesses exposure against it.
- A company limit value — also called a private limit value or private standard — is set by the employer for a substance that has no statutory value. The general duty of care still applies in full: the chosen value must be low enough that no health damage occurs, and the reasoning behind it must be reproducible.
The two axes are independent. A statutory and a private limit value can each be a TWA, a STEL or a ceiling value. Where no value exists at all, a REACH DNEL or a kick-off value provides a defensible starting point.
Which value takes precedence when several exist side by side is covered in the limit-values hierarchy.
Where the values come from
DOHSBase draws from a wide range of national and international sources:
National legal limits from the Netherlands (Arbeidsomstandighedenregeling, SER), France (Valeurs Limites d’Exposition Professionnelle), Germany (TRGS, MAK list), the United Kingdom (WEL) and others.
European Union values — the binding and indicative OELs set under the Chemical Agents Directive (CAD) and the Carcinogens, Mutagens and Reprotoxins Directive (CMRD).
Health-based recommendations from scientific committees: DECOS (Netherlands), SCOEL / RAC (European Union), the MAK Commission (Germany), and ACGIH — whose recommendations are published as Threshold Limit Values (TLVs), the US equivalent term for OEL.
REACH DNELs — Derived No-Effect Levels from ECHA registration dossiers (5,300+ currently).
DOHSBase kick-off values — 100,000+ conservative exposure benchmarks for substances that lack a formal OEL. See the background article on kick-off values for the methodology.
Each source uses different methodologies, levels of conservatism and units. DOHSBase normalises them, cross-references them against the primary source documents, and ranks them according to the DOHSBase hierarchy — see Limit Values - Hierarchy.
Where to look a limit value up yourself
The section above describes which sources DOHSBase brings together. To verify a value at source, these are the places to go — each with its own reach and its own limitation:
- SER — Databank Grenswaarden Stoffen op de Werkplek (Dutch) — the Dutch public limit values, and the starting point for “is there a statutory value here?”. The database presents annex XIII; the binding form is the Staatscourant publication and the consolidated annex itself.
- GESTIS Limit Values (IFA/DGUV) — limit values from dozens of countries side by side. Indispensable once the question turns international, but without the national legal context.
- ECHA registered substances — REACH DNELs for substances with no limit value. Full per-dossier justification, but laborious: several registrants may report diverging values.
- DOHSBase — those sources side by side per substance, each with its provenance and ranked by hierarchy.
No single source answers the practical question. That question is rarely “what is the limit value?” but “which of the available values applies here, and why that one?” — which requires setting a statutory value, a health-based recommendation and possibly a DNEL or kick-off value alongside each other.
How DOHSBase validates the data
Compiling limit values is not a matter of copying numbers into a database. Different sources express values in different units (mg/m³ vs ppm), reference different temperature and pressure conditions, or apply to different forms of a substance (total dust vs respirable fraction). Some values are superseded. Some apply only under specific working conditions.
The DOHSBase team reviews every data point before it enters the database. Values are normalised, cross-referenced against original source documents, and placed within the hierarchy. Discrepancies between sources are flagged and resolved. Over 5,500 internationally accepted measurement methods are linked to the substance records so you can go from limit value to compliant sampling in a single step.
DOHSBase in numbers
- 325,000+ chemical substances
- 15,000+ substances with at least one occupational exposure limit
- 5,300+ REACH DNELs
- 100,000+ kick-off values
- 5,500+ measurement methods
- Three regional databases: Netherlands, France, Europe
Every record is searchable by name, synonym, CAS number or EC number, with real-time suggestions as you type.
Frequently asked questions
What does OEL mean?
OEL stands for Occupational Exposure Limit — the maximum airborne concentration of a hazardous substance at which a worker may be exposed without experiencing adverse health effects, over a defined averaging period.
What is the difference between TWA and STEL?
The 8-hour TWA is the daily-average limit: exposure averaged across a full working shift must stay below it. The STEL is a short-term peak limit, typically over 15 minutes, that protects against acute effects from brief high-concentration exposures even when the daily average is acceptable.
What does TLV mean?
TLV stands for Threshold Limit Value — the ACGIH-specific term for an occupational exposure recommendation in the United States. ACGIH publishes TLVs as TLV-TWA, TLV-STEL and TLV-C (ceiling), corresponding to the same three airborne types used elsewhere.
How are OELs set?
An OEL is derived by a scientific committee from toxicological studies, animal data and epidemiological evidence from exposed worker populations. The committee identifies the critical health effect, the dose-response relationship, and applies uncertainty factors to arrive at a concentration considered protective. National authorities then decide whether to adopt the value as a legally binding limit.
What if my substance has no formal OEL?
Most substances in commercial use do not have a formal OEL — national committees typically only cover 500–800 substances per country. For the remainder, DOHSBase provides kick-off values: conservative exposure benchmarks derived from the statistical distribution of existing OELs within each GHS/CLP hazard class. The methodology has been recognised by the Dutch Labour Inspectorate since 2012.
What is a biological limit value used for?
A biological limit value is the benchmark for biological monitoring — measuring the substance or its metabolites in a worker’s blood or urine. Use it when skin absorption is a significant route, when respiratory protection is used and airborne monitoring under-reports real exposure, or when you need confirmation of actual internal dose rather than estimated external dose.
How do you statistically test TWA measurements against an OEL?
EN 689 requires compliance verdicts to incorporate workplace variability and measurement uncertainty explicitly — not a naïve “average versus OEL” test. The standard tool is the Upper Tolerance Limit (UTL): a statistical upper bound below which, with specified confidence, a specified percentage of exposures fall. DOHSBase Online computes UTL via Monte Carlo simulation and ties the verdict directly to the OEL from the DOHSBase database. Read more about EN 689 compliance assessment with UTL and Monte Carlo simulation.