Occupational exposure limit values

Introduction

Occupational exposure limit values (OELs) are set to prevent occupational diseases or other adverse effects in workers exposed to hazardous chemicals in the workplace. OELs assume that exposed persons are healthy adult workers, although in some cases the OELs should also protect vulnerable groups – e.g. pregnant women or other more susceptible people. They are tools to help employers protect the health of workers who may be exposed to chemicals in the working environment. OELs are usually set for single substances, but sometimes they are also produced for common mixtures in the workplace, for example solvent mixes, oil mists, fumes from welding or diesel exhaust fume.

Basic definitions

Council Directive 80/1107/EEC, as amended by Council Directive 88/642/EEC, on the protection of workers from the risks related to exposure to chemical, physical and biological agents at work, introduced into EU legislation of the objective of establishing occupational exposure limits (OELs)^[1]. Directive 98/24/EC on the protection of the health and safety of workers from the risks related to chemical agents at work replaced 80/1107/EEC and defined OEL as the limit of the time-weighted average of the concentration of a chemical agent in the air within the breathing zone of a worker in relation to a specified reference period^[2].

This Directive sets out general principles for assessing and preventing risks at work from the use of chemical agents, and includes the legal framework for indicative occupational exposure limit values (IOELVs), binding occupational exposure limit values (BOELVs) and binding biological limit values:

Table 1: Acronyms

• Indicative OELs are health-based limits conventionally established only for substances for which it is possible to establish a threshold or a no effect level considered to be protective of health. To establish OEL, a thorough assessment of the available scientific information is essential as a first step (see below). These limit values should be established or revised taking into account the availability of measurement techniques. Member States should keep workers' and employers' organisations informed of IOELVs set at a Community level. For any chemical for which an IOELV is established at a Community level, Member States should establish a national occupational exposure limit value, taking into account the Community limit value, determining its nature in accordance with national legislation and practice.
• Binding OELs (BOELVs) may be drawn up at a Community level and, in addition to the factors considered when establishing IOELVs, socio-economic and technical feasibility factors should be taken into account and intend to provide a level of minimum protection for all workers in the Community. For any chemical agent for which a BOELV is established, Member States should establish a corresponding national binding occupational exposure limit value based on, but not exceeding the Community limit value.
• Biological Limit Values (BLVs) are reference values for the evaluation of potential health risks in the practice of occupational health. The BLVs define maximum levels of substances in humans, their metabolite, or indicator of effect e.g. in blood, urine or breath. For many substances, the data are too limited to support a biological monitoring method, or a metabolite or indicator cannot be defined.
• Binding biological limit values (BBLVs) may be drawn up at a Community level on the basis of a thorough assessment process and on the availability of measurement techniques, and should reflect feasibility factors while maintaining the aim of ensuring the health of workers at work. For any chemical agent for which a binding biological limit value is established, Member States should establish a corresponding national binding biological limit value based on, but not exceeding, the Community limit value. A BBLV is established for lead and its ionic compounds and integrated in Directive 2004/37/EC on the protection of workers from the risks related to exposure to carcinogens, mutagens or reprotoxic substances at work^[3].
• Skin notation is used to warn about skin contact where it can add significantly to the body burden, in addition to that caused by inhalation. Skin notation setting is not standardised across countries and agencies. A skin notation assigned to an OEL identifies the possibility of significant uptake through the skin. 

Setting OELs

When selecting candidate priority substances for setting OELs in the EU, the following criteria are taken into account:

• Epidemiological evidence including reported cases of ill-health in the workplace
• Availability of toxicological data
• Severity of effects
• Number of persons exposed
• Availability of data on exposure
• Availability of measurement methods

Scientific evaluation: SCOEL and RAC

Before 2019 the process of harmonising exposure limits in EU countries began with the establishment of IOELVs by the Scientific Committee on Occupational Exposure Limits to Chemical Agents (SCOEL), an advisory body of the European Commission. The first stage in the OEL setting process was to assemble all the information available on the hazards of the substance and decide whether this provides an adequate database on which to proceed. The SCOEL prepared a short summary document (SCOEL/SUM) on each compound, and, if the SUM document was agreed upon by the SCOEL members, it was circulated to interested parties for comments. After a comment period of about 6 months, the SCOEL re-discussed the document in light of the received comments. After clarification of the raised questions, the final version was adopted and submitted to the Commission for publication. When the European Commission disposed of a sufficient number of approved SCOEL recommendations, it prepared a draft Commission Directive setting out proposed new IOELVs.

From 2019, the scientific evaluation of the relationship between the health effects of hazardous chemical agents and the level of occupational exposure is conducted by the Risk Assessment Committee (RAC) of the European Chemicals Agency (ECHA)^[4]. RAC has taken over the role of SCOEL in giving scientific opinions on OELs for the European Commission. Members of RAC are appointed by ECHA's Management Board based on candidates nominated by the Members States^[5]. The initial report of RAC is subject to open consultation and is complemented by ECHA’s scientific report on the subject. This is transmitted to the tripartite Advisory Committee on Safety and Health at Work with an opinion from the Working Party on Chemicals in the Workplace (a subgroup of the Advisory Committee for Safety and Health at Work). The Advisory Committee gives its opinion on the proposal, if necessary by voting. If the Advisory Committee has an agreed position in favour of the proposal, binding OELs are processed via the ordinary legislative procedure for adoption by the Council and the European Parliament whereas indicative OELs are published in Commission directives. The Directives are published in the Official Journal of the European Union. Member States then have a fixed timescale (typically 18 months) to implement the Directive in their national legislation^[6].

Basic principles

Indicative or binding OELs are established on the basis of sufficient evidence in relation to a reference period of a typical 8-hour working day, i.e. as 8-hour time-weighted average (TWA) exposure limits. They are also generally set on the basis of a nominal 40-hour working week and a working lifetime of 40 years (48 weeks/year; 5 days/week; i.e. 9600 days or 76,800 hours) ^[7].

OELs are usually expressed as milligram per cubic meter (mg/m³) of air. They may be expressed in parts per million by volume in air (ppm) for gases and vapours, if the substance exists as a gas or vapour at normal room temperature and pressure. OELs for metals and other non-vapour-forming compounds at ambient temperature and pressure are expressed in mg/m³ only. Some OELs may be expressed in units such as fibres per cm³ (e.g. for asbestos)^{[8][6] [9]}.

OELs are usually established for single substances and do not take into account combined effects^[10].  When substances have similar toxicological effects (i.e. similar toxicological effects on the same target organ/same mechanism of action), the combined effect should be considered as additive (the sum of the individual effects)^[11] [10]. Based on this principle, OELs for mixtures can be calculated. However, this principle does not apply to more complex mixtures, to mixtures suspected of having a synergistic effect or to carcinogenic substances^[10].

Health effects of chemicals and OELs

Chemical substances occur in the form of gases, vapours, liquids, dusts or fumes in the working environment. They are absorbed into the body mostly through the respiratory tract and skin or from the gastrointestinal tract. Gases and vapours are absorbed directly by the respiratory tract, depending on the physical activity. Fumes in the liquid phase can be absorbed directly by the lung alveoli (terminal dilations of the air passageways). Biopersistent aerosols (dusts and fumes) are not totally absorbed – some dusts can be eliminated with mucus, coughed up with sputum or swallowed.

The body’s response to chemicals depends on the dose/concentration, the chemicals’ physicochemical properties and absorption route, the health, sex and age of the exposed person and the condition of both the endocrine (hormonal) and immune systems, in addition to external factors such as temperature, exposure period and humidity.

If the harmful effects of a substance occur in a relatively short period of time (within 24 hours), this indicates an acute type of exposure. This is most often the case with a sudden event, referred to as an ‘accident’ at work. If the harmful effects occur after prolonged exposure to low doses or concentrations of substances present in the workplace, it is a chronic type of exposure. Occupational exposures are usually chronic.

Harmful (hazardous, dangerous) substances are the substances that can cause adverse effects, including (sensory) irritation from the airways and the eyes, headache, as well as sedation and narcotic effect depending on the dose (the amount of substance that is ingested, inhaled, or absorbed through the skin). Research methods on the toxicity of chemicals and principles of classification are globally harmonised. The ever growing European list of dangerous substances can be found in the CLP Regulation. The list is available in the Classification & Labelling Inventory^[12]. Chemical substances fall into at least one of the following categories, according to health effect: highly toxic, toxic, harmful, skin corrosive/irritant, eye damage/irritant, sensitising, carcinogenic, mutagenic, toxic for reproduction, endocrine disruptive^[13].

The first stage in the OEL setting process is to assemble all the information available on the hazards of the substance, as well as physicochemical properties. High quality human data (individual case reports, studies in human volunteers, or cohort and case-control studies) are preferred to animal data, but frequently may not be available, and clear dose-response relationships (the change in effect on an organism caused by different doses or concentrations after exposure time) are rarely demonstrated. The evaluation if the data includes an assessment of the adequacy, relevance and reliability for human health hazard assessment in the occupational context^[7].

The second stage is to identify the adverse effects that may arise from exposure to the substance and then establish which adverse effect is crucial in deriving the level of OEL^[14]. From the key study (or studies) describing the critical effect(s) of the chemical substance, result in the identification of hazardous properties relevant to the workplace and, if possible, to conclude on points of departure (PoD) relevant for deriving limit values. The most relevant adverse effect(s) are taken as a basis for the PoD(s)^[7]. A PoD is for example the NOAEL (No Observed Adverse Effect Level), the highest level of a test substance to which organisms can be exposed without causing any observed and statistically significant adverse effects on the organism compared with the controls. 

The process of selecting the final recommended limit values may be iterative. It takes into account all available evidence, as opposed to basing a limit value on a single study result. The lowest limit(s) will normally be recommended if the evidence does not allow one limit to be chosen over another^[7].

In cases where a substance is known to cause reproductive harm, the OEL should protect workers of both sexes from these effects. Pregnant and lactating women are particularly vulnerable because developing individuals (like a foetus or newborn) can be more sensitive to specific toxins. Employers must assess risks to pregnant or breastfeeding workers and take necessary measures to avoid exposure that could harm the mother or child (Directive 92/85/EC^[15]). A key challenge is that workers may not always be aware of their pregnancy in its early stages, potentially leading to unintended exposure to harmful substances. Therefore, OELs should also account for the protection of the unborn child from developmental harm.

When recommending an OEL, available data on reproductive and developmental toxicity must be considered in choosing the point of departure (PoD) for deriving the limit. If information is lacking, this uncertainty should be recognised and factored into the OEL, with clear identification of the risks where possible^[7].

Occupational exposure limits for carcinogenic compounds

Exposure to some chemicals can cause uncontrolled growth of cells leading to cancer. For most carcinogenic substances, an effect threshold cannot be determined. It is extremely difficult to derive a safe level of exposure to a genotoxic carcinogen (genotoxic substances – substances that damage genetic material). Genotoxic chemicals are not considered to have a safe threshold or dose.

For carcinogenic substances in many countries, exposure limits are not established because it is not possible to determine safe exposure levels. Instead of proposing an exposure limit, a quantitative risk assessment may be carried out. Different government agencies and national or international organisations active in establishing or proposing admissible exposure levels for carcinogenic substances use the concept of so-called acceptable risk. The level of acceptable risk depends on commonly accepted social and economic criteria. In this respect, the decision is usually taken based on a tripartite process (government, employer and employee representatives) . A few Member States (e.g. Germany, the Netherlands, Poland) are reported to apply criteria on acceptability of risk^[16].

The approach developed by the RAC, based on the work of SCOEL^[7], distinguishes between carcinogens for which a threshold for carcinogenic effect can be identified and those for which no threshold can be identified^{[7] [17]}. 

For non-genotoxic carcinogens, it is generally accepted that a threshold concentration exists and that it is theoretically possible to establish a concentration below which the chemical agent in question will not be carcinogenic. Recommendations for health-based OELs can then be derived. 

For most genotoxic carcinogens, the available data are often insufficient to identify a safe threshold. The default assumption for these substances is that no safe threshold exists, meaning any exposure carries some risk of cancer. However, in some cases where more detailed information is available, it may be possible to identify a threshold-based mode of action (MoA) for certain genotoxic carcinogens, allowing for the determination of a safe exposure level.

The RAC distinguishes two types of genotoxic carcinogens:

- Direct-acting genotoxic carcinogens (DNA-reactive carcinogens): These substances (or their metabolites) directly interact with DNA, leading to mutations. For these substances, the risk is generally assessed using a linear dose-response model, assuming no threshold (i.e. any exposure carries some risk). However, in cases where sufficient substance-specific data exist, it may be possible to deviate from the linear model and establish a threshold-based OEL. For example, if there is evidence that DNA repair mechanisms effectively prevent mutations at low exposure levels, or if the substance is naturally present in the body, a threshold can be identified below which there is no significant increase in cancer risk.

- Indirect-acting genotoxic carcinogens: These substances cause DNA or chromosomal damage indirectly, often through interactions with proteins. In cases where there is sufficient evidence to establish a threshold for this indirect genotoxicity, a MoA-based OEL can be derived. These carcinogens are usually weakly genotoxic, and the evidence suggests that their cancer-causing effects are driven more by other mechanisms rather than direct DNA damage. In such cases, the relevant genotoxic effects typically occur only at exposure levels above the identified threshold^{[7] [17]}.

From 2017 a series of binding limit values were drawn for many carcinogen substances based on the relevant scientific expertise from SCOEL and the RAC, and the opinions of the Advisory Committee on Safety and Health at Work (ACSH) and the monographs of the IARC.

Where possible, a health risk assessment for regulating carcinogens should be based on epidemiological studies. However, this approach is only possible for a limited number of compounds, and risk assessment has to be based on animal studies in most instances. In this case, several points of departure are used in risk assessments in combination with various extrapolation models^[6]. The ECHA website provides an overview of the evaluations of OEL substances carried out by the RAC^[18].

Short term exposure limits (STEL) and ceiling (STEL-C)

Some substances have set short term exposure limits (STELs). STEL is the concentration that workers can be exposed to continuously for a short period of time without risking acute effects, such as throat irritation, that will not be controlled by the application of an 8-hour OEL. In these cases, one substance has two limit values. In addition to the ‘normal’ 8-hour OEL, there is a STEL. These exposure limits are often set for 15 minutes and referred to as Short-Term Exposure Limits – 15 minutes. Some countries have also limited the frequency of peaks to a maximum of 4 peaks/day with a minimal interval of 1 hour. Short term exposure limits are intended for use in normal work situations. They must not be used to protect against emergency situations. 

For other substances, peak concentrations are determined, where this level should not be exceeded during any time of the workday. A ceiling limit may be set without setting an OEL. Ceiling exposure limits are used for substances, for which short-term peaks of exposure could result in serious health effects – for example, respiratory irritants such as chlorine. For those substances, continuous, direct-reading or short-term instantaneous measurements should be available. If such monitoring is not feasible, sampling should be conducted for the minimum period of time sufficient to detect exposure at or above ceiling value^[1].

Legislative aspects

The legal basis for setting Indicative Occupational Exposure Limit Values (IOELV) is included in the Chemical agents Directive 98/24/EC^[2]. These IOELVs were established by the SCOEL (Scientific Committee on Occupational Exposure Limits), and later also by the RAC. IOELVs are health-based, non-binding values established on the basis of the latest data and with the use of available measuring techniques. They determine threshold exposure levels below which exposure is not expected to lead to adverse effects. Employers can use IOELVs in the risk assessment process as required by the Chemical agents Directive^[2]. IOELVs have to be considered by the Member States when establishing their own national OELs.

The indicative occupational exposure limit values are published in six directives:

• Commission Directive 91/322/EEC of 29 May 1991 on establishing indicative limit values by implementing Council Directive 80/1107/EEC on the protection of workers from the risks related to exposure to chemical, physical and biological agents at work^[19]
• Commission Directive 2000/39/EC of 8 June 2000 establishing a first list of indicative occupational exposure limit values in implementation of Council Directive 98/24/EC on the protection of the health and safety of workers from the risks related to chemical agents at work^[20]
• Commission Directive 2006/15/EC of 7 February 2006 establishing a second list of indicative occupational exposure limit values in implementation of Council Directive 98/24/EC and amending Directives 91/322/EEC and 2000/39/EC^[21]
• Commission Directive 2009/161/EU of 17 December 2009 establishing a third list of indicative occupational exposure limit values in implementation of Council Directive 98/24/EC and amending Commission Directive 2000/39/EC^[22]
• Commission Directive (EU) 2017/164 of 31 January 2017 establishing a fourth list of indicative occupational exposure limit values pursuant to Council Directive 98/24/EC, and amending Commission Directives 91/322/EEC, 2000/39/EC and 2009/161/EU^[23]
• Commission Directive (EU) 2019/1831 of 24 October 2019 establishing a fifth list of indicative occupational exposure limit values pursuant to Council Directive 98/24/EC and amending Commission Directive 2000/39/EC^[24]

Binding Occupational Exposure Limit Values (BOELV) and Binding Biological Limit Values (BBLV) may also be adopted based on the Chemical agents Directive 98/24/EC2 and included in annex I (BOELV) and II (BBLV) of the Directive. They are established on the basis of currently available scientific data, socio-economic criteria and technical possibilities to achieve these values in the industry. Member States are obliged to establish a corresponding national binding occupational exposure limit. Initially the Chemical Agents Directive included one BOELV and BBLV for lead and its compounds. Amending Directive 2024/869/EU^[25] changed the annexes of the Chemical Agents Directive. A new binding limit value was introduced for diisocyanates in Annex I. At the same time, the binding occupational and biological limit values and the accompanying health surveillance measures for lead were removed from Annex I and Annex II. Lead is a reprotoxic substance and, since 2022, reprotoxic substances fall within the scope of CMR Directive 2004/37/EC^[3]. Therefore, the amending Directive (2024/869/EU) has removed the limit values for lead from the Chemical Agents Directive and placed them (while making them more stringent) in the CMR Directive.

A BOELV for asbestos (actinolite, anthophyllite, chrysotile, grunerite, crocidolite, tremolite) is included in Directive on the protection of workers from the risks related to exposure to asbestos at work (Directive 2009/148/EC)^[26].

BOELVs for other carcinogens, and also for mutagens or reprotoxic substances are listed in annex III of the CMR Directive^[3]. Between 2017 and 2024, five amending Directives were published introducing additional and/or more stringent limit values. Annex III of the CMR Directive lists a total of 41 BOELVs, some of which are for a single chemical (e.g. benzene), while others relate to process-generated agents such as diesel engine exhaust emissions or respirable crystalline silica dust.

Relations between occupational exposure limits and derived no effect levels

Within the framework of the REACH Regulation (2006/1907/EC)^[27], DNELs (Derived No Effect Levels) have been introduced in Europe. These represent levels of exposure above which humans (inclusive of consumers, workers, etc.) should not be exposed. Manufacturers and importers are required to calculate DNELs as part of their chemical safety assessment (CSA) for any chemicals used in quantities of 10 tonnes or more per year^[27].

The DNEL is used in the risk characterisation part of the chemical safety assessment as a benchmark to determine adequate control for specified exposure scenarios. DNELs reflect the likely routes and duration and frequency of exposure. If more than one route of exposure is likely to occur (oral, dermal or inhalation), then a DNEL must be established for acute and repeated exposure, for each route of exposure and for the exposure from all routes combined. It may also be necessary to identify different DNELs for each relevant human population (e.g. workers, consumers or humans subject to exposure indirectly via the ambient environment), and possibly for certain vulnerable sub-populations (e.g. children, pregnant women).

The starting point in establishing DNELs is a ‘No Observed Adverse Effect Level’ (NOAEL) or a ‘Lowest Observed Adverse Effect Level’ (LOAEL) from human data or animal studies^[28]. The next step in the calculation of a DNEL is to address assessment factors extrapolating from experimental data to a real human exposure situation. This may result in a very conservative figure, perhaps two or three orders of magnitude lower than that from the traditional OEL setting process^[29]. When an EU IOELV has been set, this may be applied as a DNEL for workers. Where an EU Binding Exposure Limit Value (BOELV) has been set taking into account socio-economic factors and technical feasibility, this cannot be used as a DNEL. Where a health-based national OEL has been set, the toxicological information used must be evaluated, and any differences to the REACH calculation must be taken into account^[30].

A study comparing OELs and DNELs found that many more chemical substances are covered by DNELs than by OELs but on the other hand, no DNEL is available for many occupationally relevant pollutants because they are not covered in REACH. If a DNEL and an OEL is available for the same substance, they are often quite similar but sometimes there are large differences. Among the 418 substances that were studied, 27 (6.5%) had a DNEL that was more than 10 times higher than one or several OELs^[30]

Conclusions

Recent developments in European limit values can help preventing many deaths and ill-health among workers. However, values should be considered as starting points (and not final goal): everyone should constantly strive for minimising workplace exposure^[31].