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Sunbaked pavement sealcoat may release toxic compounds

Sunlight might convert carcinogenic PAH compounds in coal-tar-based sealants into more toxic derivatives

by Melissa Pandika
May 16, 2016

Credit: Pete Van Metre/U.S. Geological Survey
New research finds a broader array of highly toxic compounds in coal-tar-based sealcoats than earlier studies.
Photo of coal-tar-based sealcoat being applied to a test plot of pavement.
Credit: Pete Van Metre/U.S. Geological Survey
New research finds a broader array of highly toxic compounds in coal-tar-based sealcoats than earlier studies.

New research adds to a growing body of evidence that a pavement sealcoat made from coal tar emulsions—commonly used east of the Rocky Mountains to give parking lots and driveways a smooth, finished sheen—could harm the environment and human health (Environ. Sci. Technol. Lett. 2016, DOI: 10.1021/acs.estlett.6b00116).

Earlier studies have shown that compounds in the sealcoats known as polycyclic aromatic hydrocarbons (PAHs), many of which are known or suspected carcinogens, can volatilize in warm weather or enter the soil and waterways from rain runoff. As a result, some communities have banned these sealcoats. Staci L. M. Simonich of Oregon State University and colleagues set out to examine not only a wider array of PAHs than those previously investigated, but their derivatives, too. They found that despite their presence in small concentrations, these compounds pose major risks.

The researchers measured concentrations of 34 PAHs and 56 PAH derivatives in two coal-tar-based sealcoats and an asphalt-based sealcoat, a type more commonly used in the western U.S. They obtained dried sealcoat samples from the U.S. Geological Survey that were collected for a previous study. To measure changes in PAHs and PAH derivatives over time, they also painted each sealcoat product onto a University of Texas, Austin, parking lot and scraped off samples after 1.6 hours, one day, 45 days, and 149 days. They then extracted the PAHs and PAH derivatives with a series of increasingly polar solvent mixtures and analyzed the 11 resulting fractions using gas chromatography/mass spectrometry.

Using a standard method for estimating the toxicity of complex mixtures of PAHs by weighting their relative toxicities, the researchers found that the coal-tar-based products were up to three orders of magnitude more toxic than the asphalt-based product. The coal-tar-based products, but not the asphalt-based product, also contained high molecular weight PAHs, a highly toxic class of PAHs not examined in prior studies. Additionally, Simonich and her team saw a drop in the concentration of PAHs and a rise in the concentration of two classes of PAH derivatives—nitrated PAHs (NPAHs) and oxygenated PAHs (OPAHs)—over time. They suspected that sunlight exposure might convert PAHs to these even more toxic derivatives.

The researchers incubated each fraction with a culture of Salmonella bacteria to check for chemicals that can trigger mutations in DNA, a preliminary screen for cancer-causing potential. Fractions from the coal-tar-based products, but none from the asphalt-based products, caused mutations. Simonich’s group also suspended zebrafish embryos in the extracts and assessed them for malformations. (Zebrafish’s genetic similarity to humans makes them useful for assessing human health risks.) Fractions containing NPAHs and OPAHs had the most toxic effects.

Simonich says her findings suggest that asphalt-based sealcoats, though not completely safe, are significantly safer than coal-tar-based sealcoats.

Judy Crane of the Minnesota Pollution Control Agency would like to see more details on the quality assurance of their analysis but agrees that the findings only underscore the risks of these sealcoats. In another recent study, storm runoff from coal-tar sealcoat killed over half of juvenile coho salmon—more than seven months after applying the sealcoat (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.5b04928). Many of these waterways also supply drinking water. “We’re going to really need to rethink how we regulate this product,” says Dickinson College chemist Amy E. Witter.


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