Benzene exposure to workers during tunnelling

The Challenge

As part of upgrades to the Belfast wastewater and sewers infrastructure, a main tunnel was under construction 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 and went well in excess of the UK workplace exposure limit. This corresponded with a rise in temperature (early May) - 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.

Benzene is a highly flammable liquid which also has some health effects. It occurs naturally in crude oil, natural gas and some ground waters and is also present in crude oil vapours. It can get into the body by inhalation, absorption or ingestion.

The effects of benzene exposure can include: headaches, tiredness, nausea, dizziness and unconsciousness if exposure is very high (thousands of ppm). Long-term exposure to lower concentrations of benzene can result in bone marrow suppression leading to serious blood disorders such as anaemia, forms of leukaemia and other cancers of the white blood cells.

Employers have requirements under The Control of Substances Hazardous to Health Regulations 2002 (COSHH) to assess and control worker exposure to benzene.

The Solution

HSE measured a specific metabolite of benzene called S-phenyl mercapturic acid (SPMA) in urine samples 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 loss of 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.

The Outcome

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 benzene contamination, the intermittent nature of these 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 any human factors issues that might influence exposure control to be identified.

The improvements to control measures and working practice, made in light of the elevated biological monitoring results, resulted in significant reductions in worker exposure to benzene. Biological monitoring enabled the job to be completed whilst giving continued assurance that the workforce was not being exposed to potentially hazardous levels of benzene.

With thanks to McCallum Safety & Health.

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