The pavement maintenance industry has faced increasing regulatory pressure in recent years over the use of refined tar-based sealers, with some jurisdictions implementing outright bans based on concerns about polycyclic aromatic hydrocarbons (PAHs). However, new studies sponsored by the Pavement Coatings Technology Council (PCTC) present compelling evidence that challenges the prevailing assumptions about these sealers. Understanding the science behind these findings is essential for contractors, specifiers, and pavement professionals who want to make informed decisions about materials and methods. As with many innovations in the field, Understanding 6 Types of Construction Technology You Will use in modern pavement work requires separating verified data from unsubstantiated claims.
Understanding PAHs in the Pavement Maintenance Industry
What Are Polycyclic Aromatic Hydrocarbons?
Polycyclic aromatic hydrocarbons, commonly referred to as PAHs, are a class of organic compounds consisting of multiple fused aromatic rings. These compounds are formed naturally through the incomplete combustion of organic matter and are present throughout the environment. PAHs are produced by a wide range of sources including forest fires, volcanic eruptions, internal combustion engines, fossil fuel power plants, and even the grilling of meats and vegetables. They are also found in products derived from coal and petroleum, including used motor oil.
Because PAHs are so widespread, virtually every urban and suburban environment contains measurable levels of these compounds in soil, water, and sediment. The key question for the pavement industry is not whether refined tar-based sealers contain PAHs (they do, as do many other materials), but whether they are a significant contributor of PAHs to the broader environment relative to other sources.
The Ubiquity of PAH Sources
To put the issue in perspective, it is helpful to consider the many pathways through which PAHs enter the environment:
- Atmospheric deposition of combustion byproducts from vehicles, power plants, and industrial facilities
- Used motor oil and automotive fluids that wash off road surfaces during rain events
- Tire wear particles that accumulate along roadways and in drainage systems
- Natural sources such as forest fires and coal seam erosion
- Residential sources including wood-burning stoves and fireplaces
- Asphalt pavement itself, which is petroleum-derived and contains PAH compounds
When all of these sources are considered together, it becomes clear that attributing PAH contamination in urban sediments primarily to pavement sealers requires careful analysis that accounts for the full range of contributing factors. The recent PCTC studies were designed specifically to conduct this level of thorough investigation.
The 2005 Austin Studies and Their Regulatory Fallout
How a Single Study Sparked Widespread Bans
In 2005, a study conducted in Austin, Texas, became the catalyst for regulatory action against refined tar-based sealers across the United States. That study showed that refined tar-based sealant contains PAHs and that soil and sediment samples taken adjacent to sealcoated parking lots sometimes contained high levels of PAHs. The study proposed a direct link between PAHs in refined tar-based sealant and PAHs found in rivers, streams, and lakes.
The city of Austin acted swiftly, banning the use of refined tar-based sealant within city limits effective January 1, 2006. Other municipalities followed suit, and the pavement maintenance industry suddenly faced a regulatory challenge that threatened the continued use of a widely accepted product. However, a critical piece of information was overlooked: data from sediment samples collected from Austin’s streams in the years before the 2005 study had never been considered in the analysis.
The Before-and-After Evidence That Changed the Picture
A local environmental company had collected sediment samples from Austin’s streams before the ban, in October 2005, and again after the ban took effect, in April 2008. This dataset provided a natural experiment: if refined tar-based sealers were the principal source of PAHs in sediments, then PAH concentrations should have declined measurably after the ban. The results were unequivocal: total concentrations of PAHs in sediments before and after the ban did not change significantly.
This finding alone casts serious doubt on the theory that pavement sealers are the dominant source of PAHs in aquatic sediments. If sealers were the primary contributor, removing them from use should have produced a detectable reduction in PAH levels. The fact that PAH concentrations remained stable after the ban points to other sources, particularly atmospheric deposition of combustion byproducts, as the primary contributors.
The PCTC-Sponsored Research: Environmental Forensics in Action
Applying Chemical Fingerprinting to the PAH Question
In response to the regulatory challenges, the Pavement Coatings Technology Council commissioned two rigorous scientific studies to examine whether refined tar-based sealants are a significant source of PAHs in the environment. PCTC engaged Dr. Paul Boehm, a leading expert in environmental forensics widely regarded as the foremost authority on PAH fingerprinting, to lead the investigation. Boehm’s previous work includes high-profile cases such as the Exxon Valdez oil spill, where his forensic methods were used in court to distinguish PAHs from the spilled oil versus natural coal seam sources.
Study One: The Austin Sediment Analysis
The first study, published in the journal Environmental Forensics under the title “Polycyclic Aromatic Hydrocarbons (PAHs) in Austin Sediments after a Ban on Pavement Sealers,” analyzed sediment samples collected before and after the Austin ban. Using forensic techniques, researchers examined the PAH profiles in these samples to determine whether the fingerprint of refined tar-based sealant was present in the broader environment.
The key findings of the Austin sediment study included:
- PAH concentrations in Austin stream sediments did not decline after the 2006 ban on refined tar-based sealers
- PAH fingerprinting of sediments collected before and after the ban showed no marked changes in PAH source profiles
- The findings indicate that sealants were not the principal source of PAHs even before the ban took effect
Study Two: Nationwide PAH Forensics
The second study, published in Stormwater magazine in September 2010, expanded the analysis to environmental samples collected nationwide. Boehm and his team evaluated PAH fingerprints across a diverse range of environmental samples to determine whether refined tar-based sealant signatures could be identified in sediments beyond the immediate vicinity of sealcoated lots.
The results showed that refined tar-based sealant has a distinct PAH fingerprint that is not uniquely reflected in the analysis of a broad range of environmental samples. The one exception was sediments collected in the immediate vicinity of sealcoated lots, where localized contributions could be detected. However, when looking at urban sediments as a whole, the PAH profiles were far more similar to those associated with atmospheric deposition of combustion byproducts than to those of refined tar-based sealants.
Comparative Source Analysis
The following table summarizes the PAH source comparison from the Boehm research findings:
| PAH Source | Contribution to Urban Sediments | Geographic Scope | Forensic Fingerprint Detectability |
|---|---|---|---|
| Atmospheric deposition (combustion) | Primary (majority of inputs) | Regional to national | Widespread and consistent |
| Refined tar-based sealant | Localized, minor | Immediate vicinity of sealcoated lots only | Distinct but not reflected in broader samples |
| Used motor oil | Moderate in urban areas | Localized to roadways | Variable |
| Tire wear particles | Minor to moderate | Roadside environments | Detectable with specific markers |
| Natural sources (forest fires, coal) | Variable by region | Regional | Distinct natural signatures |
Preliminary results presented at the annual meeting of the Society for Environmental Toxicology and Chemistry further reinforced the conclusion that sealants are not a primary source of PAHs in urban sediments. The analysis found that PAHs in sediment samples collected nationwide were more similar to PAHs typically found in atmospheric particles than to PAHs in samples directly associated with refined tar-based sealant.
What the Findings Mean for Contractors and Industry Professionals
The Case for Science-Based Regulation
The PCTC studies raise an important question: if the weight of scientific evidence shows that refined tar-based sealants are not the principal source of PAHs in the environment, why do some researchers and regulators continue to assert otherwise? According to PCTC, the answer lies in the flawed foundation on which later calculations were built. By basing all subsequent analysis on the incomplete 2005 Austin study, subsequent researchers have constructed a theory on shaky ground.
In more recent publications, some researchers attribute volumes of PAHs to refined tar-based sealant that are not supported by actual data. These unsupported figures are then fed into mass balance models previously developed for atmospheric studies to produce equally unsupported percentages of sealer-derived PAHs in sediments. As the old saying goes, the quality of any model output depends entirely on the quality of its input.
Practical Implications for Pavement Maintenance Work
For contractors and pavement maintenance professionals, these findings have several practical implications. The continued availability of refined tar-based sealers as a legitimate product option depends on the industry’s ability to advocate for science-based regulation. Understanding the research allows contractors to respond knowledgeably when customers or regulators raise concerns about PAHs. Furthermore, the PCTC studies help clarify that the most significant source of PAHs in urban environments is not pavement sealer application but rather the combustion byproducts that are already present in the air we breathe.
This does not mean that responsible application practices are unimportant. Proper application techniques, including careful management of runoff and cleanup of overspray, remain essential elements of professional pavement maintenance work. However, the regulatory narrative that has emerged around refined tar-based sealers is not supported by the best available science.
Industry Resources and Continuing Education
PCTC has actively worked to disseminate the results of its research to the pavement maintenance industry. The council sponsored a free seminar at the National Pavement Expo in Nashville titled “Sealcoating: Regulatory Challenges and Industry Initiatives,” where contractors could hear directly from the scientists who conducted both studies. The seminar covered the two major research initiatives: the analysis of PAHs in Austin streams before and after the ban, and the nationwide forensics study of PAHs in environmental samples.
Contractors seeking to deepen their understanding of pavement materials and technologies can explore related topics such as Asphalt Pavement Engineering Mix Design Construction Methods Rehabilitation to see how sealcoating fits within the broader context of pavement management. For those working in building construction, understanding material performance characteristics is critical, much like understanding a New Spin On Toilets How American Standards flush technology demonstrates the importance of evaluating product claims against performance data. Similarly, the principles of evaluating material properties apply across construction disciplines, from Residential Glass Technology Low E Coatings Gas Fills to pavement coatings.
The Path Forward
The PCTC studies represent a significant contribution to the scientific literature on PAHs in the urban environment. By applying rigorous forensic methods that have been validated in court cases and peer-reviewed research, these studies provide a foundation for evidence-based discussion of pavement sealer regulation. The findings are clear: while refined tar-based sealants do contain PAHs and can produce localized effects immediately adjacent to treated surfaces, they are not the primary source of PAHs in urban sediments. Combustion-related sources, particularly atmospheric deposition from vehicles and industrial processes, play a far larger role.
For the pavement maintenance industry, the implications extend beyond any single product category. The broader lesson is about the importance of questioning assumptions and demanding rigorous science before accepting regulatory restrictions that affect business operations. Contractors who stay informed about the research underlying product regulations are better equipped to advocate for their industry and make sound decisions about the materials they use.