Benzene exposure to workers during tunnelling
A main tunnel was under construction, as part of upgrades to the
Belfast wastewater and sewers infrastructure, on the site of a
former gas works.
Soil samples had indicated the presence of various volatile
organic chemicals, of which benzene was the major component.
Initial monitoring (using both air and biological sampling) showed
that benzene exposure was being well controlled, however, benzene
levels began to rise rapidly; well in excess of the UK workplace
This corresponded with a rise in temperature (early May) and
workers were complaining about the heat and were sweating profusely
inside their suits. A small number exhibited signs of what was
believed to be heat stress/dehydration.
What We Did
We measured a specific metabolite of benzene called S-phenyl
mercapturic acid (SPMA) in a urine sample from exposed workers. A
guidance value (equivalent to an 8-hour exposure at the workplace
exposure limit) allowed us to interpret the results.
In response to the high temperatures, workers had been provided
with additional bottled water. However, the subsequent series of
biological monitoring results showed significant benzene exposure
with 20% of samples exceeding the guidance value for urinary SPMA;
in the worst case by over 10 fold.
On investigation, it was found that due to the heat and the need
to drink water more frequently, workers had been removing their PPE
in the tunnel during work, when leaving the tunnel at break times,
and at the end of their shifts. Some also reported taking off their
respirators to answer their mobile phones.
A decision was taken to stop working and during this time a
chiller was installed to improve working comfort. PPE was upgraded
and changes were made to working practice.
Work resumed with at least weekly biological monitoring. Prompt
analysis and reporting of the results allowed site management to
quickly intervene if biological monitoring indicated a reduction in
exposure control. Following these improvements, a dramatic
reduction in SPMA levels was seen, with only three results out of
432 (0.7%) exceeding the guidance value.
This case study illustrates the value of biological monitoring
in situations where control of exposure primarily relies on
RPE and other PPE. Although air monitoring had identified 'hot
spots' of contamination, the intermittent nature of these pockets
of benzene contamination and the extensive use of PPE meant that it
was not sufficient to assess the risk of exposure.
Biological monitoring was able to give an integrated measurement
of actual systemic exposure (despite the PPE) and highlight issues
with both the PPE and its use. Furthermore, since biological
samples are specific to an individual, it enabled the
identification of any human factors that might influence exposure
The improvements to control measures and working practice, in
light of the elevated biological monitoring results, resulted in
significant reductions in worker exposure to benzene. Thus
biological monitoring enabled the job to be completed whilst giving
assurance that the workforce was not being exposed to potentially
hazardous levels of benzene.
With thanks to McCallum
Safety & Health
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