Biomedical Engineering Reference
In-Depth Information
standard deviation of the emission concentrations are calculated. If personal moni-
tors are available, workers may be equipped with these instruments during the same
measurement times, and the arithmetic mean and standard deviation of the individual
exposure concentration in the breathing zone are determined as previously described..
The net emission or exposure concentrations are then determined by subtract-
ing the mean background concentration from the mean emission or exposure
concentration:
C
)
=−
C
C
(11.1)
(
netE
−
E
B
C
)
=−
C
C
(11.2)
(
netBR
−
BR
B
Here,
C
stands for the particle concentration and the indices E for emission, BR
for breathing zone, and B
for background.
To judge whether a potentially increased particle concentration in the workplace
is significantly higher than in the background, the standard deviations of the back-
ground and emission or breathing zone measurements, respectively, are taken into
account. The workplace concentration is considered to be significantly increased
over the background if the net exposure or release concentration (
C
(
net-BR
)
C
(
net-E
)
)
is
larger than 3 times the standard deviation
s
D,B
of the background concentration:
C
)
>⋅
3
s
(11.3)
(
netE
−
DB
,
C
)
>⋅
3
s
(11.4)
(
netBR
−
DB
,
It should be noted that this procedure is only applicable if the background
concentration does not fluctuate strongly; that is, if the standard deviations of the
background and the near-field measurements are of the same order of magnitude.
If a screening measurement according to tier 2a.1 reveals a significantly increased
workplace concentration, this may be double-checked by a long-term monitoring
(tier 2a.2). In tier 2a.2, small, easy-to-use devices are installed in the facility under
surveillance, near the detected potential particle source or near the ventilation inlet
(see Figure 11.3), if available. Such measurements can be conducted over several
days to weeks. Since particle concentrations may vary significantly over such long
time, the background particle concentration should frequently be measured, best by
installing an additional long-term monitor in a representative location. Only instru-
ments that do not require frequent attention or maintenance are suitable for the moni-
toring task; that is, diffusion charger based instruments are usable, whereas handheld
CPCs are not an option, because they need a frequent refill of their alcohol reservoir.
If aerosol monitors are permanently installed in the facility, they are used to con-
tinuously measure the workplace concentration. As long as no legally binding limits
exist, certain threshold concentration can be defined as a trigger for further investi-
gations on “suspicious concentrations.” Monitoring data are examined as described
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