What is an OEL?
An Occupational Exposure Limit (OEL) is the maximum permissible concentration of a given gas, vapour, fibre or dust in the air in the workplace. It is intended to be the level at or below which a given substance can be present in the air in the workplace without harming the health of employees and their offspring, based on current knowledge. This should be the case even if exposure to the substance at that level occurs repeatedly or over a long period of time, even an employee’s entire working life.
Why are OELs important?
Clear-cut, quantified standards governing working conditions are required to ensure the health and safety of the working population. Such standards let employers know what they need to provide and let employees what they can expect. They also give the labour inspectors an unambiguous basis for enforcement of such standards. The presence of such standards is particularly important when hazardous substances or irritants are involved. OELs specify the permissible degree of exposure to such substances. The greater the risk involved in exposure, the stricter the OEL must be.
What are OELs used for?
OELs are used by the authorities and by industry to check exposure levels and to assess working conditions. They are also used to monitor the effectiveness of measures (e.g., occupational hygiene measures) taken to minimise exposure levels. Furthermore, OELs are used as a guideline for the minimum level of protection that needs to be provided when designing new installations or monitoring emission sources.
Types of OEL
There are two types of OEL in the Netherlands (see Procedure for setting OELs):
- Public (i.e. statutory) OELs, set by the government
- Private OELS, set by companies.
Both types of OEL are health-based OELs, which means that exposure to these concentrations of substances in the workplace may not harm health.
OELs are not absolute limits, but rather time-weighted averages measured over an 8-hour period (expressed in the Netherlands as TGG-8u). During this period, exposure may at times exceed the OEL, providing that such higher levels of exposure are balanced by lower levels, so that the average level for the 8-hour period does not exceed the OEL.
OELs may also be defined by a ceiling value (expressed as -C). This is an absolute OEL that must not be exceeded at any time. To apply the concept of the ceiling limit in the workplace, a practical solution must be chosen for the way in which a ceiling limit should be measured in order to ensure compliance with and enforcement of the limit. According to the OEL Subcommittee, a substance-specific measurement method should be used to determine that substance’s level of concentration in the workplace in cases where the ceiling limit has been reached. The method should take as little time as the current state of technology and science allow. That means that scientific and technological advances in measuring methods will lead to shorter measuring times for compliance with ceiling limits. In addition, in some cases, to prevent high exposure levels over a short period of time (peak exposures), an OEL may be defined as a 15-minute time-weighted average (expressed as TGG-15min).
As far as possible, OELs are also set for carcinogens. If a safe threshold has already been determined for a particular carcinogen, the OEL is set at the level of that safe threshold.
- In the case of carcinogens for which no safe threshold can be determined (i.e., genotoxic carcinogens – substances that cause an immediate change in DNA that may lead to cancer), it is impossible, given current scientific knowledge, to indicate a safe OEL that would prevent cancer. The only way to eliminate any risk would be to impose a complete ban. However, as long as such substances are indispensable (or at least as long as society requires the use of such substances, e.g., cytostatics), the risk of exposure, and thus the risk of cancer, cannot be avoided. In setting OELs for these substances, use is made of the system of risk levels defined by the former Dutch Working Environment Council (now succeeded by the Council’s Working Conditions Committee) in its advisory report on the setting of standards for genotoxic carcinogens (1992). In that report, the Council recommended using a ‘prohibitive risk level’ (prohibiting an additional risk of cancer higher than 10-4 per substance per year) and a ‘target risk level’ (10-6 per substance per year).
- In the case of non-genotoxic carcinogens, a safe OEL can basically be defined. These OELs have the same status as OELs for ‘normal’ health-damaging substances.
Exposure to inhalant allergens in the workplace can lead to work-related respiratory allergies. This generally begins with sensitisation (to become sensitive to the relevant substance). Eventually, however, an allergy develops that is harmful to health, even when exposure is minimal.
It is often impossible to define a safe OEL for this group of substances. That is why we have chosen to apply the same approach as in the case of carcinogens for which no safe exposure limits can be set. Instead of an OEL, then, a target risk level will be identified, feasibility testing will be carried out, and a repeat test will be conducted every four years where necessary.
The target risk level states the extent to which exposure must be minimised in order to ensure that the extra risk of harm is negligible or will be reduced to a natural background risk. The feasibility test should not focus on technical feasibility alone, but also on operational and economic feasibility. The target risk level is a 1 per cent extra risk of sensitisation owing to exposure to an inhalant allergen (beyond any inherent sensitisation to a substance). The level of corresponding exposure may differ from one inhalant allergen to the next.
Caveats and qualifications regarding OELs
A number of caveats and qualifications regarding OELs should be noted. They specifically relate to:
a. The health basis of OELs
b. The level of protection provided by OELs
c. The practical implementation of OELs.
a. The health basis of OELs
Most OELs are defined on the basis of health considerations. Some, however, are not. There are various reasons for this, including the following:
- There may be insufficient toxicological information about the substance to draw sound conclusions about its effects on human health.
- People’s sensitivity to the harmful effects of a substance may vary too much to establish a realistic and generally applicable limit on health grounds.
- OELs for carcinogens for which there is no safe threshold may not be based on health grounds.
b. The level of protection provided by OELs
OELs may provide insufficient protection under certain special circumstances:
- OELs are designed to provide protection in everyday work situations and processes. They have no application in the case of emergencies or abnormal incidents.
- OELs are geared to a standard workload for healthy employees. They may not provide protection in the event of heavy physical labour, or to employees who are already in poor health. In these cases, the company’s own OEL will need to take the workload and employees’ health issues into account.
- Some employees may experience some discomfort or damage to their health even when exposed to levels below the OEL, due to large variations in people’s sensitivity to substances (see The health basis of OELs).
- OELs do not take into account the possibility that people may be exposed to the substance or substances in question outside the work environment (e.g., during leisure activities, odd jobs or voluntary work).
- OELs are based on an 8-hour working day and a 40-hour working week. Employees who work longer hours may be exposed for longer periods, which may increase the risk to their health.
- In principle, OELs only apply to exposure to the pure substance. They do not necessarily apply to a mixture containing the substance. Although, in some cases, the harmful effect of a mixture may be the sum total of the effect of its component substances, in other cases exposure to a combination of substances may have an effect that is either more or less harmful than the effect of exposure to the substances separately. When there is simultaneous exposure to several substances, experts (e.g., from the Occupational Health & Safety Services) must take this into account.
c. The practical implementation of OELs
In some cases, there may be no available adequate methods of measurement to determine the concentration of a given substance in the air in the workplace.