Types of OELs
In principle, OELs are based on the measurement of chemical agent concentrations in working air, but in a few cases it is possible to define biological exposure indices (BEIs), which are based on measurements on certain biological media. The biological limit value is the limit of the concentration in the appropriate biological medium of the relevant agent or its metabolite, or an indicator of effect. However, considering that BEIs are used as a complement to environmental OELs, and considering also that they do not have much application to industrial ventilation, we will not devote attention to them.
With this in mind, it is possible to define two types of OELs:
• Time-weighted average exposure limit with a specific reference period
• Instantaneous exposure limit
The time-weighted average exposure limit might refer to long (TWA = time — weighted average) or short (STEL = short-term exposure limit) periods of time. These OELs always refer to the concentration of a chemical agent in gaseous, vaporous, or suspended form in the air within the breathing zone of a worker.
It is normal to establish OELs for long periods (TWAs) in relation to a reference period of 8 hours, which is a typical working-day schedule, Values above the OEL are allowed provided these are compensated by equivalent excursions below the TWA during the working day. OELs are also normally set on the basis of a nominal 40-hour work week, with a maximum of 240 working days in a year and for a working lifetime that might reach 45 years. The long reference period could be a different one, such as a month, a year, and so on. In general, an 8-hour OEL is recommended as the most satisfactory and practical way of monitoring airborne agents for the purpose of preventing adverse health effects, although, in any case, OELs do not constitute an absolute dividing line between harmless and harmful concentrations.
In addition, it may be necessary to limit permissible upward excursions from the TWA. In practice, concentrations of chemical agents in workplace air fluctuate frequently and to a considerable extent. The amount by which the OEL-TWA may be exceeded for short periods without impairment of health depends upon several factors, such as the nature of the substance, the frequency with which high concentrations occur, and the duration of such periods.
According to the ACGIH (American Conference of Governmental Industrial Hygienists, USA), variations in worker exposure levels may exceed three times the OEL-TWA for no more than a total of 30 minutes during a workday, and under no circumstances should they exceed five times the TWA.
However, there are substances for which an 8-hour OEL-TWA alone provides insufficient protection. In such cases the OEL-STEL is used in relation to a 15-minute period, unless otherwise specified, in order to prevent adverse health effects, immediate or delayed, due to peaks in exposure that cannot be controlled by the application of an 8-hour OEL-TWA. The OEL-STEL indicates a limit value above which exposure should not occur, and it is needed when there are recognized acute effects from a substance whose toxic effects are primarily of a chronic nature.
For ACGIH, exposures above the OEL-TWA up to the OEL-STEL should not be longer than 15 minutes and should not occur more than four times per day. Also, there should be at least 60 minutes between successive exposures in this range.
In any case, the OEL-STEL is not a separate, independent exposure limit; rather, it supplements the OEL-TWA and is intended to be used in normal work situations, not for emergencies. However, the OEL-STEL needs to be complemented by other precautions for substances that may be lethal at very high concentrations and for substances whose toxic or irritant effects are pronounced upon exposure to high concentrations for very short periods.
Another type of time-weighted average exposure limit is the decision level (DL), which is expressed as a fraction of the OEL. In general, it is based on judgment, and it is greater than a dose of 50% and usually corresponds to one-fourth of the dose. For special substances, such as carcinogens, it should be one-tenth. Unlike the OSHA (Occupational Safety and Health Administration, USA) action level, the European DL is primarily used for monitoring decisions implying the need to adopt certain types of preventive measures.
Instantaneous OELs represent concentrations that should not be exceeded during any part of the working exposure; that is, they represent the maximum permissible concentration of a chemical compound (or element) present in the air within a working area that, according to current knowledge, generally does not impair the health of the employee or cause undue annoyance. These OELs are also called ceiling values and are used for agents that present acute effects such as irritants or for other type of substances that present chronic and acute effects simultaneously and therefore may be used with appropriate medical surveillance. This OEL uses no specific time reference period, but for some situations instantaneous monitoring is not feasible, requiring the use of a 15-minute sampling period.
For some substances, in order to consider all absorption pathways—including dermal, which may lead to skin penetration and consequently increase the total body burden—a skin notation must be assigned to an OEL. Although it is recognized that for many substances with a skin notation, quantitative data on dermal penetration may not be available, for other chemical agents uptake through the skin may be on the order of 10% or more of the uptake from respiratory exposure at the 8-hour OEL, especially in operations involving high airborne concentrations where significant areas of the skin are exposed for a long period of time. In general, a substance with a skin notation requires additional risk management measures to ensure the best possible level of protection and to control total systemic exposure at the workplace.
The OELs are expressed in terms of concentration, such as ppm (volume/ volume) or mg/m3. For gases and vapors it is stated in terms independent of temperature and air pressure variables, in tnL/m3, or parts per million by volume in air, and in terms dependent on those variables in milligrams of substance per cubic meter of air at a temperature of 20 °C and a pressure of 101.3 kPa,
The limit value for suspended matter is given in mg/m3 or multiples thereof for actual environmental conditions (temperature and pressure) at the workplace. The limit values for fibers are given in fibers/m3 or fibers/cm3 for actual operating conditions of temperature and pressure. It is possible to con vert one expression to the other using the formula
TWA (mg/m!) = (TWA (ppm) x MW)/24.04,
Where MW is the molecular weight of the substance and 24.04 is the molar volume in liters under these conditions.
Correct and appropriate use of OELs in practice requires considerable knowledge and experience, particularly in cases where there is exposure to mixtures rather than to one substance in isolation, where it is not practical to evaluate the effects of all possible conditions of exposure, or where the work ing patterns are nonstandard.
Established OELs such as OSHA PELs and ACGIH TLVs, cover only a small fraction of the substances that are found in the workplace, and even for those chemicals discrepancies are commonly encountered among the different lists. For this reason, the European Union (EU) is trying an approach in which
The OELs represent levels of exposure that are perceived to be realistic and attainable at the time they are established. Political and socioeconomic aspects are combined with scientific considerations in order to avoid the imposition of administrative, financial, and legal constraints that would hold back the development of enterprises while maintaining the aim. of ensuring the health of workers.
Toward this goal the EU defines two types of OELs:
• Binding limit value
• Indicative limit value
Binding occupational exposure limit values reflect feasibility factors related to social acceptability. When the results of environmental monitoring have to be compared with OELs, factors that influence exposure and that entail preventive policies are considered. For these values member states shall establish a corresponding national binding occupational exposure limit value based on but not exceeding the community limit value. That is, a binding limit means a minimum requirement.
An indicative limit value is a more common type of limit that reflects expert evaluation based on scientific data where it is possible to identify the highest level of exposure along with the corresponding reference time period for which one can have confidence that there will be no adverse effects on health. For any chemical agent for which an indicative occupational exposure limit value is established at the community level, member states shall establish a national occupational exposure limit value, taking into account the community limit value and determining its nature in accordance with national legislation and practice.
This approach may provide an opportunity for consolidating and simplifying old legislation on chemicals and for bringing OEL setting up to date with respect to social aspects, balancing the participation of all interested parties (the scientific community, industry, unions, and governments) and therefore advancing the establishment process much more rapidly.
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