The Science Behind Pavement Sealers: Environmental Impact, Regulatory Challenges, and Best Practices

Pavement sealers are a fundamental component of asphalt maintenance programs across North America, protecting parking lots, driveways, and roadways from weathering, oxidation, and water damage. Yet in recent years, these essential products have faced intense regulatory scrutiny based on concerns about polycyclic aromatic hydrocarbons (PAHs) and their potential environmental impact. Understanding the science behind these concerns, evaluating the actual evidence, and applying proven sealcoating strategies for high-traffic pavement maintenance can help contractors and property owners navigate this complex landscape with confidence.

The Environmental Debate Around Pavement Sealers

The controversy surrounding refined tar-based pavement sealers centers on a single question: do these products contribute significantly to PAH contamination in urban waterways and sediments? PAHs are a class of organic compounds that occur naturally in coal, crude oil, and gasoline, and they are also produced when organic matter burns. They are found in sources as varied as vehicle exhaust, grilled food, forest fires, and used motor oil.

In 2005, a study conducted in Austin, Texas, suggested that refined tar-based sealants were the primary source of PAHs in urban sediments. This study led to regulatory actions, including a ban on the use of these sealers within Austin city limits effective January 2006, and similar measures in other jurisdictions. However, subsequent research funded by the Pavement Coatings Technology Council (PCTC) has raised serious questions about the methodology and conclusions of the original Austin study.

The Ubiquity of PAHs in the Environment

One of the fundamental flaws in the original Austin study was its failure to account for pre-existing PAH levels in the environment. PAHs are present everywhere; they are formed by natural processes such as forest fires and volcanic activity, as well as by human activities including:

• Internal combustion engine exhaust from cars and trucks
• Emissions from fossil fuel power plants
• Wood burning and residential heating
• Industrial processes and manufacturing
• Natural coal seam erosion in certain geological areas

The presence of PAHs in soil or sediment samples taken near a sealed parking lot does not automatically mean the sealer is the source. Comprehensive sampling before and after sealer application is required to establish causation, something the original study did not adequately perform.

Comparing Tar-Based and Asphalt-Based Sealers

There are two primary types of pavement sealers on the market today: refined tar-based sealers and asphalt-based sealers. Understanding their differences is essential for making informed product choices:

Each type has performance advantages, and the choice should be based on project requirements, local regulations, and client preferences. Understanding different sealcoating mix designs for lasting pavement protection can help contractors select the right product for each application.

Understanding PAH Fingerprinting and Source Attribution

Environmental forensics, and specifically PAH fingerprinting, has emerged as a critical tool for determining the actual sources of PAHs found in sediment samples. This scientific approach uses analytical chemistry to distinguish between different sources of contamination based on the unique chemical profiles (fingerprints) of PAH mixtures from various origins.

How PAH Fingerprinting Works

Different sources of PAHs produce distinct ratios of individual PAH compounds. By analyzing the relative proportions of these compounds in an environmental sample, scientists can identify which sources are contributing to the contamination. For example:

• Combustion-derived PAHs are dominated by higher-molecular-weight compounds such as pyrene and benzo(a)pyrene, produced when fossil fuels, wood, or other organic materials are burned.
• Petrogenic PAHs (from crude oil and its products) typically contain higher proportions of alkylated PAHs and lower-molecular-weight compounds.
• Coal tar-derived PAHs have a distinctive profile with specific ratios that can be differentiated from combustion and petroleum sources.

Key Findings from Forensic Studies

Dr. Paul Boehm, a leading expert in PAH forensics, applied these fingerprinting techniques to sediment samples collected nationwide. His research, published in Stormwater magazine in 2010 and subsequently expanded, reached several significant conclusions:

1. Refined tar-based sealant has a distinct PAH fingerprint that is not uniquely reflected in environmental samples except in sediments taken immediately adjacent to sealcoated lots.
2. PAH profiles in urban sediments are more similar to atmospheric deposition patterns than to sealant profiles.
3. Combustion sources, particularly vehicle emissions and power generation, represent the vast majority of PAH inputs to urban sediments.
4. Localized contributions from sealcoating are measurable but do not dominate regional or watershed-scale PAH concentrations.

These findings align with the understanding that PAHs are ubiquitous pollutants with multiple sources. Asphalt pavement itself, vehicle tire wear, and atmospheric deposition all contribute to the background levels measured in urban environments. Understanding how asphalt pavement deteriorates and requires maintenance provides important context for why protective sealcoating remains essential despite regulatory concerns.

Lessons from the Austin, Texas Sealant Ban

The Austin ban serves as a natural experiment in environmental policy. By comparing sediment PAH levels before the ban (October 2005) and after the ban took effect (April 2008), researchers could determine whether eliminating refined tar-based sealers actually reduced PAH concentrations in local waterways.

Before-and-After Analysis

The results were striking. Total PAH concentrations in Austin stream sediments showed no significant change after the ban was implemented. If refined tar-based sealers had been the principal source of PAHs in sediments, concentrations should have declined once the product was no longer used within city limits. The fact that they did not decline provides strong evidence that sealants were not the dominant source.

Methodological Considerations

Critics of the original 2005 study point out several methodological issues that undermined its conclusions:

• Incomplete baseline data: Pre-existing sediment PAH data from Austin streams, collected years before the study, were not incorporated into the analysis.
• Confirmation bias: The study assumed sealers were the source without testing alternative explanations or conducting forensic fingerprinting.
• Scale mismatch: Localized PAH elevations near sealed lots were extrapolated to watershed-scale impacts without supporting data.
• Mass balance errors: Subsequent models assigning large percentages of sediment PAHs to sealers relied on assumptions and input values that were not empirically validated.

As the old modeling adage goes: garbage in, garbage out. When mass balance models are fed assumptions unsupported by field data, the output percentages are meaningless regardless of how sophisticated the mathematics may be.

Regulatory Implications for the Industry

Despite the growing body of evidence supporting the safety of refined tar-based sealers, several municipalities have followed Austin’s lead and implemented restrictions or outright bans. This regulatory patchwork creates challenges for contractors who work across multiple jurisdictions. Key considerations include:

• Verifying local regulations before bidding on sealcoating projects
• Maintaining certification records for applicators in regulated areas
• Using appropriate product substitutions where bans are in effect
• Documenting environmental compliance measures on job sites
• Staying informed about emerging regulations through industry associations like PCTC

The performance characteristics of modern pavement materials continue to evolve, and the sealcoating industry must adapt its practices to meet both performance expectations and regulatory requirements.

Best Practices for Pavement Sealcoating in a Regulated Environment

Regardless of the ongoing scientific debate, contractors must continue delivering quality sealcoating services while navigating an increasingly complex regulatory landscape. The following best practices can help ensure successful outcomes:

Surface Preparation and Application

Proper surface preparation is the single most important factor in sealcoating longevity and performance. Best practices include:

1. Clean the surface thoroughly using power washing or mechanical sweeping to remove dirt, oil stains, and loose debris.
2. Repair cracks and potholes before applying sealer to prevent water infiltration and ensure a uniform finish.
3. Apply in proper weather conditions with ambient temperatures above 50F and no rain forecast for at least 24 hours.
4. Use the correct application rate as specified by the manufacturer, typically 0.10 to 0.15 gallons per square yard per coat.
5. Apply two coats for optimal protection and appearance, allowing sufficient drying time between coats.

Environmental Stewardship Measures

Contractors can demonstrate environmental responsibility through proactive measures that address regulator and community concerns:

• Use containment booms and absorbent materials to prevent runoff into storm drains
• Schedule work during dry weather periods to minimize wash-off risk
• Properly dispose of washout water and empty containers according to local regulations
• Maintain detailed records of products used, quantities applied, and weather conditions
• Consider offering asphalt-based sealer options in jurisdictions where tar-based products face restrictions

Communicating with Clients

Property owners may have questions about sealcoating safety and environmental impact. Contractors should be prepared to explain the difference between product types, the regulatory status in their area, and the protective benefits of sealcoating for extending pavement life. A well-informed customer is more likely to approve necessary maintenance and understand the value of professional application.

The Economic Case for Regular Sealcoating

Sealcoating represents one of the most cost-effective pavement maintenance investments available. Regular application every three to five years can extend asphalt pavement life by 50 to 100 percent, delaying the need for expensive reconstruction. When property owners understand that sealcoating protects their investment, the decision to proceed becomes much easier.

The pavement maintenance industry continues to advance, with ongoing research into lower-impact formulations, improved application techniques, and better environmental stewardship practices. By staying informed about both the science and the regulations affecting their work, sealcoating professionals can continue to provide essential services that protect and preserve asphalt infrastructure across the country.

As the evidence continues to mount that refined tar-based sealers are not the primary source of PAHs in the environment, the industry has a strong scientific foundation on which to base its advocacy and operations. Contractors who equip themselves with this knowledge will be better positioned to navigate regulatory challenges and continue delivering quality pavement protection services for years to come.