Page 365 - Industrial Ventilation Design Guidebook

5,3 TOXICITY AND RISKS INDUCED BY OCCUPATIONAL EXPOSURE TO CHEMICAL COMPOUNDS 32 !

may be lower (1.1-1.5), whereas s g may exceed 2.5 in some manual occu-
pations. The lognormal distribution becomes a straight line on logarithmic
probability paper. The concentration corresponding to the probability of
50% is m g (also the median) and s ? is obtained from the ratio c 5Q/c l5 q or
C //C as 192
( 84.i 5o) shown in Fig. 5.52.
A European standard (EN 689/95) has been set for occupational ex-
posure assessment. However, this is primarily intended to be used to
guarantee that the concentrations of air impurities are in compliance
with OELs. According to the standard, exposures exceeding 10% of OEL
level should be followed with repeated measurements, the interval of
which depends on the concentration observed. The interval decreases as
193
the concentration approaches the OEL. The standard also includes the
concept that workers should be divided into homogeneous exposure
groups (HEGs). These consist of workers who have similar jobs and are
exposed to the same agents. This is practical because it would be unnec-
essarily laborious to investigate every worker. On the other hand, the
prerequisite of the standard that the exposure levels of the members of a
HEG remain within the range 0.5-2 times the mean exposure level is im-
practically tight. In addition, airborne concentrations usually fluctuate
greatly with time. The within- and between-worker components of expo-
sure variability can be calculated by using the random-effects analysis of
194
variance. However, this would require extensive sampling. Even
though repeated random personal sampling is, in principle, the most ac-
curate method for exposure assessment, it has the serious limitation that
it does not provide information on the reasons for the exposure. Without
this basic knowledge, it may be difficult to institute effective remedial
measures. 52
It is appropriate to consider the differences between manual tasks and
process industries (see Section 5.3.2.1) while assessing the exposure, and
to perform air sampling so that it also can support planning of engineering
control. Because of steep concentration gradients, breathing zone sampling
must be performed when investigating manual tasks. A worker often per-
forms several tasks, and the exposure may be very different during differ-
ent tasks. Therefore, all major tasks done by the worker should be studied
under various conditions. If the position of the local exhaust is not fixed,
its influence should also be examined. The time-weighted average (TWA)
concentration is obtained using the lengths of various tasks as the weights.
It is common practice to determine the TWA of a working day (shift). Since
the health effects usually depend on long-term average exposure level, this
should also be estimated. Past exposures are often very difficult to assess
because working conditions and methods may have been changed. How-
ever, the present (e.g., annual) average exposure level can be estimated by
asking the worker how much time he/she spends on average (e.g., during
the past year) for various tasks and use these as weights. For example, if
we want to assess a construction painter's exposure to organic solvents, we
must first list all tasks in which solvent-based paints are used. The expo-
sure during painting depends mainly on the size of the surface painted (or
on paint consumption rate), the room volume, and the ventilation. Since