Page 415 - Industrial Ventilation Design Guidebook

3 70 CHAPTER 6 TARGET LEVELS

• 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 amount ot
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 ap-
praisal 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 ap-
praisal 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 con-
tinue to be below the limit values, a detailed survey with a measurement strat-
egy is required. The methods used are based mostly on instruments for sampling
the environment in order to obtain reliable measurements of workplace air con-
centrations. In general, the measurement strategy must consider the most effi-
cient 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 re-
spect 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 refer-
ence 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 real-
istic 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