Assessment of Exposure to Chemical Agents
It is appropriate here to give a few ideas on the assessment of workplace exposure to chemical agents, although specialized literature in this field should be consulted for details.
Determination and assessment of the risks of hazardous chemical agents requires knowledge of, among other things, the nature of the agents, the type and duration of the exposure, the gravity of risk, and the criteria chosen for the OEL. The purpose is to make quantitative measurements of the work environment in order to compare the exposure with the limit value by means of an independent scientific assessment using the best available scientific methodology. ‘
However, assessing occupational exposure to airborne contaminants in a representative way is not an easy task and requires a good deal of professional judgment and reliance on a good methodology. There are many issues to be resolved, such as
• Patterns of contaminant generation that change with time
• Interaction among several release points
• Worker variability in relation to location and time
This implies, for example, that measurements taken over a given period of time may give insufficient data to integrate workplace factors in the series of activities in the period under consideration in such a way as to permit assessment of the likely maximum risk to which the individual workers are exposed.
The occupational exposure assessment (OEA) should take into account the future working conditions for the employee, which implies some degree of uncertainty, especially if the exposure is close to the limit value. Therefore, the initial assessment of exposure may require periodic measurements to check for any change in exposure conditions that may increase the risk to workers—for example, if there has been a significant change in working conditions caused by the introduction of a new industrial process or technology or new chemical agents, which could make the risk assessment out of date. In the case of a new risk-involving activity, work could commence only after an assessment of the risk of such activity has been made and any identified preventive measures have been implemented.
T he standardized procedure for evaluation of exposure to chemical agents is done in three steps:
• Identification of potential exposure
• Determination of workplace factors
• Assessment of exposure
The first step is to identify the substances present at the workplace. As a starting point, knowledge of the process is needed in order to formulate a list of all chemical agents used in the establishment. The list should include not only primary products but also intermediate and final products, as well as reaction products and by-products. For the chemical agents in the list, it is necessary to know their chemical properties, especially hazardous ones; their OEL values, including biological limit values; and, where these are not available, other technical criteria that can be used to evaluate the risk. It is also helpful to include any information on the safety and health risks of those substances provided by the supplier or other readily available sources. This information on dangerous substances and preparations, in the form of safety data sheets, is intended primarily for industrial users, to enable them to take the measures necessary to ensure the safety and health of workers.
The next step is the determination of workplace factors that have to be considered in estimating the potential for exposure to chemical agents. This includes all points related to work processes and procedures which may be relevant to exposure, such as
• Operations, technical installations, and processes
• Emission sources
• Job functions and activities of each worker
• Work patterns and techniques
• Process layout and workplace configuration
• Engineering prevention measures taken or to be taken
• Workloads and exposure times
The identification of chemical agents and the data on workplace factors leads to the assessment of exposure, which can be done in three different stages that depend on the risk level for the worker and the type and a mount of data required:
• An initial appraisal
• A basic survey
• A detailed survey
The assessment of exposure brings together all the data and compares the results of this integration with the chosen OEL. It begins with an initial appraisal and continues with a basic survey and then a detailed one until it is confirmed either that exposure exceeds the limit value or that it is sufficiently below that limit.
The initial appraisal allows consideration of the likelihood of exposure due to the presence of chemical agents. When the conclusions of the initial appraisal show a possible risk exposure for the worker, more data are gathered taking special account of tasks with high exposures. If necessary, workplace measurements are taken if the input data are not sufficient to enable valid comparison with the limit values.
Finally, for situations in which it is not clear that exposure is and will continue to be below the limit values, a detailed survey with a measurement strategy is required. The methods used are based mostly on instruments for sampling the environment in order to obtain reliable measurements of workplace air concentrations. In general, the measurement strategy must consider the most efficient use of resources for the purpose of obtaining quantitative data on exposures by having a clear idea of what the data will be used for or whether it meets the specified need. For example, if the data pertain to peak exposures, these peaks have to be assessed according to the STEL requirements, if any.
For this reason, in order to save time and money, it may be convenient to subdivide the exposed workers into more or less homogeneous groups with respect to exposure—that it is to say, based on similar work patterns—-and take direct measurements of air samples within the breathing zone of a worker in the area or areas in which the work activities are carried out during a reference period. However, the use of homogeneous groups should be reconsidered if any individual exposure differs greatly from the average value of the group, which may occur if workers do not perform repetitive tasks.
The measurement procedure must have been tested previously under realistic conditions and should give representative results of worker exposure. Thus, it should not be influenced by unusual work schedules, where tasks may not be well defined or planned. The procedure should include
• The chemical agents
• The sampling procedure
• The analytical procedure
• The sampling location
• The duration of sampling
• The timing and the interval between measurements
• The calculations that yield the occupational exposure concentration from the individual analytical values
• The jobs to be monitored
The measuring procedure used must be appropriate to the chemical agent to be measured, its limit value, and the workplace atmosphere so that the results show the concentration of the agent in the same terms as the limit value. Also, the limits of detection, sensitivity, and precision of the measuring procedure must be appropriate to the limit value.
The sampling period and interval of sampling are related to concentration patterns and to the method of analysis used.
For the sampling location, fixed-point measurement may be used if the re sults make it possible to assess exposure of the worker at the workplace. In general, however, personal sampling devices are preferred, which give more representative results of worker exposure.
Recently in the surveillance of exposed workers, it has proved useful to carry out analysis of the harmful compound or its metabolites using biological material or a change of biochemical parameters caused by the harmful chemical or its metabolites.
The coordination of ambient chemical agents and biochemical analyses of exposed workers and medical data is intended to confirm the validity of risk assessment.
Calculation of the occupational exposure concentration (OEC) depends on the type of OEL. For example, when the limit value has been set as an eight-hour time-weighted average, the cumulative exposure for an eight-hour work shift should be computed as follows:
DE = Citj/’y tt = (cjij + c2t2+ ■■■ + c„t„)/S,
DE is the equivalent exposure for the working shift, or daily exposure
C, is the concentration during any period of time t where the concentration remains constant
Tj is the duration in hours of the exposure at the concentration c,-
When the limit value is a STEL type, usually based on a 15-minute period, the exposure concentration is
Where ct is the measured concentration from direct-reading instruments or from air samples during a 15-minute period.
Finally, for some substances it may be convenient to define a weekly exposure WE:
WE = y (DE),- /5,
Where (DE), represents the daily exposure corresponding to successive days of the working week.
The daily or weekly exposure should not exceed the eight-hour time — weighted average OEL for the chemical agent involved, and exposure peaks that may occur during the shift must fulfill the STEL; that is, any measured short exposure, SE, during the whole working shift must be lower than the STEL for that agent, if any.
If workers are exposed simultaneously or successively to more than one chemical agent, the risk shall be assessed on the basis of the risk presented by ail such chemical agents in combination. Usually, additive effects are assumed for the mixture of chemical agents, so the cumulative exposure is calculated as follows:
E(m) = ^ (DE/TWA),
= (DE/TWA), + (DE/TWA)2 + ■■• + (DE/TWA)„,
E(m) is the equivalent exposure index for the mixture
DE is the daily exposure for a particular substance
‘TWA is the OEL for that chemical agent
To comply with the OEL, the value of the cumulative exposure index shall not exceed unity.
Comparison of the results of the occupational exposure concentration with the OEL leads to three different possibilities.
When the exposure is well below the limit values and it is possible to be confident that on a long-term basis the probability of exceeding the limit value is very low, the risk assessment may conclude that the nature and extent of the risks related to those chemical agents make a further detailed risk assessment unnecessary unless work conditions are modified in a significant way. In these cases, however, it must be regularly checked whether the assessment leading to that conclusion is still applicable.
If the assessment shows that the exposure approaches the limit values, so that the OEL is met but there is a probability of exceeding the limit values, subsequent measurements at appropriate intervals must be taken to ensure that the assessment situation continues to prevail. The frequency of these measurements will depend on the previous results, so that the nearer the concentration recorded comes to the limit value, the more frequently measurements must be taken under normal working conditions. However, if the values are borderline, the decision of whether exposures are below the limit values within the OEA is not clear, and a more comprehensive sampling exercise may be required using, for example, worst-case measurements. This becomes more important the fewer the measurements that have been taken, so in case of doubt the evaluation results should be verified through additional selective measurements.
When the results of the assessment reveal a risk to the safety and health of workers, with exposure values clearly above the OEL for the chemical agent, or if the probability of exceeding the limit value is high, specific protection, preventive, and monitoring measures must be applied as soon as possible to remedy the situation. The reasons for the limit value being exceeded must be
Identified to allow elimination of the hazardous agents through process design, engineering control, or other administrative practices to keep exposures within acceptable limits. A new assessment should be made to check the new situation, and periodic measurements should be done to check that control measures remain effective.
In any case, the risk assessment must be documented in a suitable report, including the existing working conditions and the reasons for the chosen pro cedures. It should contain, at least, the following data:
• The names of the chemical agents considered
• The description of the workplace factors
• The description of the working conditions during the measurements
• The measuring procedure
• The time schedule
• The results of occupational exposure concentrations
• All events or factors liable to influence the results
® Results of the comparison with the OEL
Retention of this data is essential for future work in order to compare and analyze successive sets of assessment exposures.
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