Section 8 of a safety data sheet carries three families of number that look similar and are not interchangeable: occupational exposure limits, derived no-effect levels and predicted no-effect concentrations. One is law, one is a registrant’s calculation, and one is about the environment rather than about people.
Occupational exposure limits
An OEL is a concentration in workplace air that must not be exceeded, set by an authority. It is a legal value, and it is jurisdiction-specific: the same substance carries different limits in different countries, and a sheet that quotes one country’s value without saying which country has not communicated anything useful.
Two forms appear:
- Eight-hour TWA — a time-weighted average over a working shift.
- STEL — a short-term exposure limit, typically over fifteen minutes, for substances whose acute effects matter.
Some substances also carry a skin notation, indicating that absorption through skin contributes materially to the total dose. Where that notation exists, controlling air concentration alone does not control exposure.
At EU level, indicative and binding limit values are set in directives; Member States implement them and may set stricter national values. Outside the EU, the United States has OSHA permissible exposure limits and NIOSH recommended limits, which are set by different bodies to different criteria and frequently differ from each other.
DNEL — derived no-effect level
A DNEL is calculated by the registrant during the chemical safety assessment: the level of exposure above which humans should not be exposed. It is derived from experimental data by applying assessment factors for interspecies differences, intraspecies variation, exposure duration and the quality of the underlying study.
DNELs are specific in four ways at once, and a value stripped of any of them is not usable:
- Population — worker or general population; workers are assumed to be healthy adults under supervision.
- Route — inhalation, dermal, oral.
- Duration — acute or long-term.
- Effect — systemic or local.
So a substance can carry six or eight DNELs, and “the DNEL” is rarely a meaningful phrase.
Where a threshold cannot be established — typically for non-threshold carcinogens and mutagens — a DMEL, a derived minimal effect level, is used instead. A DMEL is not a safe level; it is a level of residual risk considered tolerable, which is a different statement.
PNEC — predicted no-effect concentration
A PNEC is the environmental counterpart: the concentration below which adverse effects on an ecosystem compartment are not expected. Values are given per compartment — freshwater, marine water, sediment, soil, and sewage treatment plant — and derived from ecotoxicity data by applying assessment factors.
PNECs say nothing about human health, and DNELs say nothing about the environment. They appear in different parts of a risk assessment and are not comparable.
How they differ in nature
| OEL | DNEL | PNEC | |
|---|---|---|---|
| Set by | an authority | the registrant | the registrant |
| Status | legally binding where adopted | a calculated value | a calculated value |
| Protects | workers | people | ecosystems |
| Varies by | jurisdiction | population, route, duration, effect | environmental compartment |
Why the distinction matters in practice
Compliance with a DNEL is not compliance with an OEL. The DNEL is the registrant’s own derivation; the OEL is the value an inspector will measure against. Where both exist and they differ, the legal value governs — and a workplace assessment built on the DNEL alone can be short of what the law requires.
Conversely, the absence of an OEL is not permission. Many substances have no limit value anywhere, which reflects the fact that nobody has set one rather than a finding that exposure is unproblematic.
On a MolGod card
Where exposure limit values are available, the substance page shows them with the body that set them and the form they take. Where none is recorded, the page states that rather than leaving the reader to infer that none applies. The same rule governs toxicity values — see LD50, LC50 and EC50.
Legal basis: DNEL and PNEC derivation is set out in Regulation (EC) No 1907/2006 (REACH), Annex I. Occupational exposure limits are set in national and EU legislation and vary by jurisdiction. This page explains the concepts and is not a substitute for the applicable limit values in your country.