Exposure Assessment of Polycyclic Aromatic Hydrocarbons in Refined Coal Tar Sealant Applications

Coal tar sealants are applied as a protective coating for paved surfaces. Many polycyclic aromatic hydrocarbons (PAHs) are found in these sealants. Several of these PAHs are known or suspected to cause, or increase the risk of developing cancer, but to date there has been no published research on workplace exposures to coal-tar-based sealant. Researchers from the National Institute for Occupational Safety and Health (NIOSH) undertook the first occupational exposure assessment study for PAHs among coal-tar-based sealant workers. The results were recently published in the International Journal of Hygiene and Environmental Health and suggest that coal tar sealant workers are exposed to PAHs on the job at levels that may increase their risk of developing cancer.

Workplace Exposures to Coal Tar Sealant and PAHs

Coal tar pitch is a by-product of coke plants and contains several PAHs that can cause cancer in humans (IARC, 2012). It is the residue that remains after distilling crude coal tar. This residue contains several PAHs that are known or possible carcinogens, meaning they are known or suspected to cause, or increase the risk of developing cancer. Coal tar pitch is often separated, or refined, into a product known as refined coal tar pitch (RT-12). When the coal tar pitch is refined into 12 different viscosities, or thicknesses, RT-12 is the most viscous. RT-12 is the product used in refined coal tar sealants.

Refined coal tar sealant is prepared immediately before application to paved surfaces. The sealant is a mixture of RT-12, clay, water, and sand. Small differences in PAH content between batches of sealant were observed in this study, but overall, they were very similar. The final sealant product applied by workers contains up to 35% RT-12 (McClintock et al., 2005).

Exposures to PAHs occur through ingestion, skin absorption, and inhalation. However, exposure limits or guidelines currently focus on airborne PAH concentrations. Seven of the PAHs identified in coal tar pitch in this study have airborne occupational exposure limits or guidelines. Four of the coal tar pitch PAHs do not have an acceptable airborne exposure level because they are thought to be cancer causing. It is recommended that all exposures to these compounds be reduced to the lowest possible levels (ACGIH, 2019).

The Study

Three construction contracting companies with expertise in pavement sealing participated in the study. Samples were taken over 1-4 days from the 21 workers who volunteered to participate. Personal breathing zone filter samples mounted on the upper chest area of the workers were collected using sampling pumps for each full work shift. Skin wipe samples and urine samples were also collected from each worker before and after each work shift. Urine samples were taken at the beginning and end of each work shift to ensure the results were from work-related duties. For example, higher levels of PAHs in urine after a shift shows that exposures happened during the workday.

What We Found

Refined coal tar sealant worker exposure to PAHs is demonstrated by the results of personal breathing zone, skin wipe, and urinary samples. Lighter, more volatile PAHs were more likely to be found in airborne exposure samples, while the heavier, less volatile PAHs were more likely to be found on skin wipe samples. PAH profiles on skin wipes were consistent with refined coal tar sealant products. Concentrations of 1-hydroxypyrene exceeded the American Conference of Governmental Industrial Hygienists (ACGIH) Biological Exposure Indices (BEI) benchmark of 2.5 μg/L for all participants, which indicates occupational exposure and risk of genotoxicity (cell damage which may lead to cancer).

Researchers observed that workers did not wear personal protective equipment (PPE) consistently. Many workers wore long pants and work boots, while others wore shorts and shoes. There was intermittent use of gloves, booties, dust masks, and splash-protective suits. Applicators worked more directly with coal tar sealant than non-applicators. No consistent differences in PPE usage were observed between these two subgroups, except that applicators wore gloves when conducting certain tasks, such as mixing. One applicator was observed in a full Tyvek suit and face covering when operating the boom sprayer/squeegee apparatus on the back of a truck. Some workers were observed wearing the same clothes every day, which likely contributed to chronic and take-home exposures.

Specific findings include:

• PAH air and skin concentration levels were substantially higher for coal tar sealant workers than what was found in previous studies of asphalt workers.
• Of the 22 PAHs evaluated in this study, nine ranged from possible (Group 2B) to known human carcinogens (Group 1). All nine carcinogenic PAHs were detected in personal breathing zone air samples at all companies. One exception was dibenzo[a,h]anthracene, which was not found in personal breathing zone air samples at one site visit.
• Applicators worked more directly with coal tar sealant than non-applicators and had higher total PAH concentrations in personal breathing zone air samples than non-applicators. Applicators were also more likely to have post-shift hand wipe concentrations significantly higher than pre-shift concentrations.
• Urine results indicated naphthalene, fluorene, phenanthrene, and pyrene were present in all workers and were higher for post-shift samples than those collected pre-shift.
• Urinary concentrations of the pyrene metabolite, 1-hydroxypyrene (a common indicator for PAHs), were greater than the ACGIH BEI in 89 percent of post-shift samples collected on the final day of the work week or field survey. The BEI for PAHs is a method used to determine the likelihood that exposures are from someone’s job and the associated risk of genotoxicity from long-term exposures to PAHs.

The results of this research suggest that refined coal tar sealant workers were occupationally exposed to PAHs at levels that may increase their risk of developing cancer. These results indicate the need to increase safety controls and reduce occupational exposures to PAHs in coal tar sealant.

Safety and Health Considerations

Although there is currently no research specific to coal tar sealant safety controls, the Hierarchy of Controls provides general guidance. The Hierarchy of Controls shows that the most effective safety practices are to eliminate refined coal tar sealant or substitute it with a less toxic sealant product.

Concentrations of 1-hydroxypyrene exceeded the adjusted BEIs for air, hand wipes, and neck wipes in most cases. Therefore, long-sleeved shirts, neck covers, and long pants may be considered in conjunction with gloves and shoe covers to minimize PAH skin exposures.

Recognizing that adding skin protection may increase heat stress to workers who may already dehydrated at the end of the workday, it is important that employers also protect workers from heat-related hazards. NIOSH provides Heat Stress Recommendations that can be applied to many working environments. Shade, rest, and water must be provided, and workers should be encouraged to use these to prevent heat stress as needed. It is recommended that the OSHA-NIOSH Heat Safety Tool application be used to aid in heat stress evaluation.

One final consideration is that of take-home exposures. Take-home exposures occur when chemicals from work are brought home via skin, hair, clothes, or shoes. More information regarding take-home exposures, associated risks, and prevention measures can be found here.

Next Steps

Additional research is needed in this area. More detailed data on PPE, demographics, and post-shift cleaning practices (i.e., hand washing methods) could provide additional insight.

Research on how to best control refined coal tar sealant exposure is also needed. Additionally, occupational exposure research for other types of sealants for paved surfaces is lacking. If you are interested in participating in future exposure assessment research for sealant workers, please contact Seth McCormick (SMcCormick@cdc.gov).

Seth McCormick, MPH is a Research Industrial Hygienist in the NIOSH Division of Field Studies and Engineering (DFSE)

John Snawder, PhD, DABT, was a Research Toxicologist in the NIOSH Health Effects Laboratory Division (retired)

I-Chen Chen, PhD is a Biostatistician in the NIOSH Division of Field Studies and Engineering (DFSE)

Marissa Alexander-Scott, DVM, MS, MPH is a Senior Service Fellow Researcher in the NIOSH Health Effects Laboratory Division (HELD)

Michael Breitenstein, B.S., is a Biologist in the NIOSH Health Effects Laboratory Division (HELD)

Yuesong Wang, Ph.D., is a Research Chemist in the NCEH Division of Laboratory Sciences (DLS)

Lei Meng, MS is a Chemist in NCEH Division of Laboratory Science (DLS)

Juliana Meadows, Ph.D., is a Research Biologist in NIOSH’s Health Effects Laboratory Division.

Cheryl Fairfield Estill, PhD is the Deputy Associate Director for Science in the NIOSH Division of Field Studies and Engineering (DFSE)