In 2005, the city of Austin, Texas, made a controversial decision to ban coal tar-based pavement sealers within city limits, setting off a chain reaction that has influenced municipal policies across the country. The ban was based on studies linking polycyclic aromatic hydrocarbons (PAHs) in coal tar sealants to contamination of urban waterways. But as the debate unfolded, questions emerged about the quality of the science behind these regulations. For construction and building professionals, understanding both sides of this debate is essential when specifying pavement maintenance products. This examination of the controversy connects directly to broader discussions of how scientific evidence shapes building practice, as explored in Building Science in Action Key Takeaways From the symposium.
The Origins of the Coal Tar Ban in Austin
The Austin city ordinance banning coal tar-based sealers was passed rapidly in November 2005 and took effect on January 1, 2006. The decision was driven by concerns raised by the city’s Watershed Protection and Development Review Department (WPDRD) about PAH contamination in Austin’s streams and waterways. PAHs, or polycyclic aromatic hydrocarbons, are a group of more than 100 chemicals commonly found in the environment. They can be carcinogenic and toxic to aquatic life at certain concentrations.
How the Investigation Began
The supposed link between coal tar pavement sealers and PAH compounds in Austin’s stream sediments began with a hunch by a city environmental department worker collecting stream sediment samples from selected locations. City staff subsequently conducted a series of sediment sampling tests at these Austin stream locations. Local USGS researchers also performed a cooperative experimental study, collecting runoff water samples from simulated rain events across several paved areas, including parking lots sealed with coal tar products and those sealed with asphalt-based alternatives.
On the basis of this parking lot runoff data and stream sediment information, the WPDRD concluded that the use of coal tar pavement sealants is a primary cause of PAH contamination in Austin’s waterways. Their next step was to determine that eliminating coal tar sealer would significantly improve aquatic habitat for sediment-dwelling organisms.
The Rapid Regulatory Response
Following Austin’s decision, other municipalities began taking notice. Madison, Wisconsin, started investigating its own ban on coal tar sealer. The New York Academy of Sciences, under the direction of the New York Harbor Consortium, began conducting studies to assess the impact of various pollutants entering the harbor, including PAHs. Senators Jim Jeffords and John Warner cited the Austin USGS study in a letter asking the EPA to revisit a 1992 ruling that had exempted recycling of coke oven wastes, claiming the study disproved earlier EPA assertions that coal tar-based products do not pose risks to human health and the environment.
The pavement coatings industry responded forcefully. By mid-2006, a group of sealer industry companies initiated a lawsuit against the City of Austin, seeking to have the State of Texas overturn the ban ordinance. They also challenged the objectivity and data quality of the USGS study used to justify the council’s decision.
Understanding PAHs and Their Environmental Sources
To evaluate the scientific claims on both sides, it is necessary to understand what PAHs are and where they come from. PAHs are organic compounds composed of multiple aromatic rings. They are formed through incomplete combustion of organic materials and are found virtually everywhere in the modern environment.
Natural and Anthropogenic Sources of PAHs
Research has found that increased urbanization results in higher PAH levels in the environment. Major sources of PAH compounds include:
- Combustion of fossil fuels in vehicle use (gasoline and diesel engines)
- Industrial processes and manufacturing emissions
- Wood burning and residential heating
- Charcoal grilling and cooking operations
- Municipal waste discharge
- Forest and prairie fires (natural sources)
- Volcanic activity
- Biosynthesis by microbes and plants
Given these diverse sources of PAHs, it is not surprising that urban areas show increasing accumulations of PAHs in the environment. The critical question in the Austin debate is not whether PAHs are present, but what proportion comes from coal tar sealers compared to other sources.
The Problem of Source Attribution
Industry representatives argue that concentration of a contaminant in a particular substance has no direct relationship to the amount of that contaminant actually entering the environment or its bioavailability. They claim the Austin studies failed to consider the broader picture of PAH contributions from all sources. A more scientifically rigorous approach would involve evaluating the potential quantity of PAHs released by each source, determined by analyzing the yield in weight of PAHs released per unit source area over time, compared across all relevant sources.
| PAH Source | Relative Contribution Factor | Scale of Impact |
|---|---|---|
| Vehicle exhaust emissions | 34x that of sealed pavement runoff | City-wide, continuous |
| Industrial discharges | Variable by locality | Localized, often high |
| Coal tar sealed pavement runoff | 1x (baseline reference) | Site-specific, episodic |
| Wood burning and fires | Comparable to industrial | Seasonal, widespread |
| Atmospheric deposition | Continuous, low level | Regional, diffuse |
This table draws on data from the USGS report’s own tables and published EPA emissions factors. When vehicle exhaust contributions are calculated using Texas Transportation Institute data on vehicle miles traveled in the Austin metropolitan area, the results suggest that PAHs from vehicle exhaust are approximately 34 times greater than the amount projected from sealed pavement runoff.
Flaws in the Austin Studies: The Industry Perspective
The pavement coatings industry contends that the Austin city council’s actions ignored the substantial body of scientific knowledge compiled by researchers in environmental toxicology over decades. According to industry representatives, several fundamental flaws undermine the analyses presented by the Austin WPDRD and USGS studies.
Failure to Account for Other PAH Sources
The most significant criticism is that none of the information presented or cited in the USGS 2005 report or any of Austin’s WPDRD work estimated or even considered PAH contributions from other sources. If PAHs introduced into the sediment load from fuel combustion, vehicle oil spills, or leakage are not evaluated or quantified for comparison, singling out coal tar sealant as the major contributor lacks scientific foundation.
An earlier 2000 study by USGS researchers themselves had noted the effect of increased traffic on PAH levels in the Austin area: “The large increases in traffic offer an explanation for recent PAH increases in Austin’s waterways. This suggests that urban sprawl in outlying areas may affect traffic patterns and water quality in the inner city.” This earlier study attributed recent PAH increases to burgeoning traffic, not parking lot sealers, contradicting the later conclusions.
Methodological Concerns
Industry members point to several methodological issues:
- Sample size limitations – The simulated rain event tests were conducted on a limited number of parking lots, which may not represent the full range of conditions across the city.
- Lack of baseline data – The studies did not establish pre-existing PAH levels before the introduction of coal tar sealers in the area.
- No bioavailability assessment – The presence of PAHs in sediment does not automatically mean they are biologically available to harm aquatic organisms.
- Selective data reporting – Industry analysts claim that Table S2 in the USGS report shows higher PAH yields from coal tar sealed plots, but Table S3, when analyzed with EPA emissions data, tells a different story.
Implications for the Building and Pavement Maintenance Industry
The coal tar sealant debate has significant practical implications for contractors, specifiers, and property owners. Coal tar-based sealers have been widely used because of their durability, resistance to petroleum products, and cost-effectiveness. As more municipalities consider restrictions, the industry faces pressure to develop alternatives and to defend the scientific basis for existing products.
Comparing Sealant Options
Professionals evaluating pavement sealants should understand the differences between the main product categories. The choice between coal tar-based and asphalt-based sealers involves trade-offs in performance, longevity, and environmental considerations. For a deeper understanding of how material chemistry affects performance, see Art Science Paints, which explores the intersection of material science and application techniques.
- Coal tar sealers – Made from refined coal tar, these offer excellent resistance to gasoline, oil, and deicing salts. They provide a deep black appearance and typically last 3 to 5 years with proper application.
- Asphalt-based sealers – Made from asphalt emulsion, these are less resistant to petroleum spills but are generally considered to have lower PAH content. They have a shorter lifespan of 1 to 3 years.
- Acrylic sealers – These polymer-based options offer good UV resistance and flexibility but are typically more expensive than both coal tar and asphalt alternatives.
The Role of Proper Pavement System Design
Effective pavement maintenance involves more than just selecting the right sealer. The entire pavement assembly, including drainage, base preparation, and proper ventilation of adjacent structures, contributes to long-term performance. Understanding Roof Ventilation Science When and How to Vent insulated assemblies offers parallels for pavement systems, where moisture management and vapor control are equally critical for durability.
Regulatory Landscape and Industry Response
Industry support for coal tar is centered at the Pavement Coatings Technology Center, a research organization made up of more than 25 sealer producers and crude tar distillers. The PCTC is joined by the American Coke and Coal Chemicals Institute, which represents producers of refined coal tar, the base material in coal tar sealer emulsions. These organizations argue that regulations should be based on comprehensive science rather than studies that ignore the majority of PAH sources.
For building professionals, the takeaway is that the science behind coal tar regulations remains contested. Until more comprehensive studies are completed that account for all PAH sources and their relative contributions, the debate will continue. Meanwhile, contractors should stay informed about local regulations and be prepared to discuss the trade-offs between different sealant options with clients.
As the industry awaits the ongoing study by the New York Academy of Sciences, which may provide more comprehensive data, property owners and contractors must navigate an uncertain regulatory landscape. Specifications written today should account for the possibility that coal tar sealers may face additional restrictions in the future. This kind of forward-looking material selection parallels other areas of building science where understanding underlying chemistry matters for long-term performance, such as in the analysis of Electric Radiant Floor Heating and Electromagnetic Fields Separating science from concern in building systems.
Key Considerations for Pavement Maintenance Contractors
- Monitor local municipal ordinances regarding coal tar sealer use in your service area.
- Document the scientific basis for product recommendations to clients who may have concerns about environmental impact.
- Evaluate alternative sealant products and understand their performance characteristics before switching from coal tar formulations.
- Consider that the ongoing scientific debate may lead to broader regulatory changes, and plan your product inventory accordingly.
- Engage with industry organizations like the Pavement Coatings Technology Center to stay current on research and advocacy efforts.
The Austin coal tar ban may have been based on studies that, upon closer examination, raise significant scientific questions. For building and pavement maintenance professionals, the lesson is not to dismiss environmental concerns but to demand rigorous science from all sides. Understanding the full picture of PAH sources in the urban environment from vehicle emissions to industrial discharge to pavement sealers allows for more informed decision-making about materials and methods that affect both performance and environmental outcomes.