Contaminant Clouds Part 2 – Asphalt Fume
Asphalt is comprised of aggregate material mixed with bitumen. Bitumen is the product of crude oil distillation in petroleum refining. Asphalt applications include roofing, road paving and repair works. Fumes generated from hot asphalt consist of particles containing adsorbed polycyclic aromatic hydrocarbons (PAHs). Asphalt can also contain coal tar where PAH exposure is more likely although coal tar use has largely been discontinued. Still, some PAH exposure via bitumen fume may occur.
As asphalt is a complex mixture of hydrocarbons and other compounds such as sulphur, nitrogen and oxygen, other airborne hazardous substances may be generated as thermal decomposition by-products during heating. These airborne hazardous substances may include hydrogen sulphide (a natural gas found in crude oils) and volatile organic compounds as well as the bitumen fume itself and PAHs such as napththalene, acenaphthene, and phenanthrene. But the exact composition of the contaminant cloud will depend on the specifics of that asphalt material being heated.
These hazardous substances can be absorbed into the body by fume inhalation or skin absorption. Exposure to asphalt fume is linked with respiratory tract irritation and the development of lung diseases potentially including lung cancer. The ill-health effects may be serious. And so a high level of control would be needed i.e. control measures must be proportionate to the health risk.
The risk assessment will help to identify what controls are needed. Should exposure monitoring being needed in order to aid this risk assessment, then this brings up the question of an exposure limit to compare against. The HSE publication EH40/2005 gives a workplace exposure limit (WEL) of 5 mg.m-3 (8 hour TWA) for asphalt, petroleum fumes. On the other hand, the American Conference of Governmental Industrial Hygienists (ACGIH) threshold limit value (TLV) gives a significantly lower limit of 0.5 mg.m-3 (8 hour TWA) for asphalt (bitumen) for the solvent extractable material i.e. the solvent soluble aerosol. The ACGIH TLV is more relevant to a solvent (e.g. cyclohexane) extractable material (CEM) analysis whereas the UK WEL is understood to apply to gravimetric analysis (total particulates). One may choose to sample both routes i.e. the samples to undergo both gravimetric analysis and CEM analysis.
One of our site visits took us to a road where workers were laying asphalt to fill potholes. It was observed that the by-standing workers tried to keep out of the visible fume as much as possible, although the main machine operative was required to stand relatively close to the burner (which heats the bitumen/aggregate mix) during operation. Keeping in mind that the ‘visible’ fume was avoided where possible, the monitoring results of the by-standers show notable levels of napththalene and bitumen fume (as the solvent extractable material), indicating exposure occurred although the workers believed they were not being excessively exposed i.e. standing away from the cloud of fume they could see. The burner operative’s results were slightly more elevated.
These results show that bitumen fume exposure can be inadvertently elevated when applying hot asphalt. With the CEM results up to 2.1 mg.m-3 (8 hour TWA) and the gravimetric results up to 2.9 mg.m-3 (8 hour TWA) in this situation. These results are elevated and show that the process was not being adequately controlled. A simple Tyndall beam analysis with a dust lamp was also undertaken during this process and showed high levels of fume being created, much greater than what was visible under normal lighting, confirming that improvements to the exposure controls were needed.