Twenty personal samples and one bul

Twenty personal samples and one bulk samples have been collected
and tested. It was found that the bulk sample contained silica
and 93% of silica existed as quartz. Therefore, only the quartz
was tested during the X-ray analysis.
Twenty respirable samples have been collected and eighteen of
them were valid. They show in Table 2, the results indicated that
none of the measurements exceeded the exposure limits for the
respirable dust concentration (5 mg/m3) and seven samples
exceeded the respirable quartz limit (0.05 mg/m3). Since there
are not more than half of respirable quartz concentration values
(samples) that were below LOD, the non-detectable values were
replaced as follows: LOD=ð
ffiffiffi 2
p
 volumeÞ (since the GSD were less
than 3).
The hypothesis of normal distribution could not be rejected for
logarithmically transformed respirable dust (p = 0.745 > 0.05) and
respirable quartz (p = 0.236 > 0.05) concentration values (Kolmogorov–
Smirnov test), so the GM is a better way to describe
the central tendency of lognormal distributions.
A non-parametric statistical comparison (Wilcoxon Test) was
conducted to investigate whether there were significant differences
between the respirable dust and respirable quartz dust concentrations
of operators with DustBubble and without DustBubble.
The results showed that there were significant differences between
the exposure levels to total respirable dust (p = 0.025 < 0.05). The
workers using DustBubble exposed to low level of respirable dust,
which meant the use of DustBubble could reduce the respirable
concentrations by 63% (GM: from 0.27 mg/m3 to 0.10 mg/m3; estimated
reduction = (1  0.09/0.29)  100% = 63%).
For respirable quartz dust, the estimated reduction of respirable
quartz by DustBubble is 43% ((1  0.04/0.07)  100%). However, the
results of statistical comparison indicated that there were no significant
differences between the exposure levels to total respirable
quartz dust (p = 0.144 > 0.05), which suggested that the use of Dust-
Bubble could not significantly reduce the quartz exposure level of
workers when drilling concrete. Moreover, both the GM of quartz
concentration values of two groups (0.07 mg/m3 and 0.04 mg/m3
respectively) were over or close to the exposure limit, which suggested
the concrete drilling process can generate high level of
quartz dust and DustBubble is inadequate to protect workers. Other
dust control measures (e.g. wet methods, and LEV) should be taken
to reduce the quartz exposure level to below the limit. The workers
should also wear appropriate respirators.The workers using DustBubble were also interviewed to collect
their feedbacks on the use of DustBubbles. They suggested that
the use of DustBubble could reduce the dust generated for there
was less settled dust or visible dust during the drilling with
DustBubble (as shown in Fig. 6). However, it was commented that
it is not so convenient to use DustBubble for there are several steps
to prepare before the drilling. It was also claimed by the workers
that it would take far much more time (normally two or three times
longer than the normal duration) to finish drilling a hole, which may
stop the contractors from using DustBubble. Sometimes, the
DustBubble could not stick to the slab firmly during the drilling
and dust was leaked out in that case.