PAH and Seal Coats: A No-Brainer?

by Bill Chameides | July 20th, 2011
posted by Erica Rowell (Editor)

Permalink | 2 comments

More than 80,000 chemicals are produced, used, and present in the United States. This is one of their stories.

Want a spiffy-looking, seal-coated driveway? You may get more than you bargained for.

The Culprit

PAHs — short for polycyclic aromatic hydrocarbons — are a group of some 100 semi-volatile compounds that are found in products made from petroleum. They are also produced from burning stuff that contains carbon. What does that include? Cooking meat on the grill, lighting up a cigarette, burning gasoline or coal, you name it.

The Chemical Marketplace
A series that looks at chemicals in everyday consumer products
     Alkylphenols and laundry and such »
     Aluminum and antiperspirants »
     BPAF »
     Dioxin and eggs »
     Flame retardants and pets »
     Fluoride and water »
     Formaldehyde and no-iron shirts
     Insect repellents »
     Nanoparticles and food »
     PAHs and seal coats
     PBDE and fire retardants »
     PFOA and popcorn »
     Piperonyl butoxide, a pesticde »
     Propoxur and bedbugs »
     Rotenone, a pesticide »
     Spray foams, sealants, diisocyanates »
     TDCPP and the air »
     Triclosan and toothpaste »
     Trihalomethanes (THM) and
     showering »

The problem with PAHs once they get into the environment is two-fold. For one, PAHs can be nasty. The Environmental Protection Agency classifies a number of PAHs as probable carcinogens in people, and they are also linked to numerous other adverse health effects both in people and the environment. Definitely a baddie.

Second, PAHs persist. They break down slowly, and as a result, they remain in the environment and accumulate. Even small sources of PAH can eventually lead to significant effects. Currently, the U. S. Geological Survey reports that the concentration at which PAHs “are expected to have harmful effects on bottom-dwelling biota — is exceeded in [the bottom sediments of] one-third of the central and eastern U.S. urban lakes where PAH sources were studied.”

The Seal Coat Connection

For the uninitiated, seal coats are black coatings that are painted or sprayed on asphalt to improve longevity and give them that “new” look. Thinking of seal-coating your driveway to improve your home’s curb appeal? One group highly recommends it, claiming that doing so can “save the owner of a property $100,000 or more.” Who is this organization? None other than the National Pavement Contractor’s Association (NPCA), a trade group. Hmm, why am I not convinced? Well, let’s suppose you are. What kind of seal coat should you get?

Seal coats come in two flavors: the traditional (derived from coal tar) and the asphalt-based. What’s the difference? While coal tar has been in use since the 1950s, asphalt-based seal coats have only come onto the scene in recent years. The asphalt-based products appear to have a reputation among some for being not quite up to snuff. The NCPA, for example, says they have “deficiencies”; at the same time other industry groups say coal-based seal coats are considered to be “most effective,” more “durable,” and the “most popular.”

Sounds like a no-brainer, right? Let’s all get out there and spruce up our driveways with coal tar, right? Maybe not. There is another major difference between the two types of seal coats: The stuff made with coal tar carries up to 1,000 times the PAHs found in asphalt-based seal coat.

A 2010 USGS study that analyzed and fingerprinted the source of PAH contamination in eight urban lakes found coal tar-based seal coats have been the largest source of PAHs since the 1960s in seven of the lakes.

Home Invasion of the PAHers?

PAHs from seal coats aren’t just showing up in urban lakes either. They’re showing up in our homes. A separate 2010 USGS study found that apartments with parking lots sealed with coal tar-based seal coats had PAH concentrations in house dust that were 25 times higher, on average, than house dust in apartments next to parking lots surfaced with concrete, unsealed asphalt, and asphalt-based seal coat. They also found that the absence or presence of a tar coal-based seal coat explained almost 50 percent of the PAH concentration in the house dust.

Point – Counterpoint

The conclusions of these and other studies highlighting the negative aspects of coal tar-based sealants are challenged on a website “dedicated to show[ing] that … the USGS studies have overstated the link between Refined Tar-Based Pavement Sealers and environmental PAHs as a health or environmental threat.”  It is not clear who is sponsoring this site; it is stated that all rights and copyrights for material on it are reserved to Wonder who they are?

But you know, even if is correct and these other studies have overstated the link be
tween coal tar seal coats and urban PAH pollution, it still doesn’t change the fact that both coal tar seal coats and the dust from such coats have substantially higher PAH contents than the alternatives. Austin, Texas found that the median asphalt-based seal coat had a content of 50 parts per million for 16 PAH compounds whereas the median concentration for coal tar seal coats was larger than 50,000 parts per million PAH compounds.

A Turning Tide?

Indeed, there are folks out there who are rethinking the whole seal coat issue. Citing research and advances in asphalt-based seal coats, some in the industry now tout the superior benefits of these products over coal tar seal coats. Also certain suppliers (like Home Depot) no longer carry coal tar seal coats and some cities, like Austin and Washington D.C., have banned the use of coal tar seal coats. In Washington state a ban on the sale and use of coal tar seal coats is set to go into effect next year.

So, are you considering sprucing up your driveway with a seal coat? You just might read the label before plunking down your cash. Turns out that choosing between coal tar and asphalt-based seal coats is not exactly a no-brainer. Then again, maybe it is.

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  1. Deborah
    Jul 23, 2011

    According to [] is owned by Xycom Technology Group, which is a technology consulting company based in Durham, NC. So, I’m guessing that Xycom Technology Group was hired to consult for a client with a vested interest in seal coatings. Maybe their client is none other than the NPCA?

  2. Tom Ennis
    Jul 20, 2011

    Bill: I couldn’t agree more with you article. Thanks for publishing it. Through your influence, would Duke University be the first in the US to commit to ending the practice of coal tar sealing at all of its campus facilities? As you know, our universities often lead the nation in efforts such as this. Let me know if you need more info to bring this about. Tom Ennis Coal Tar Free America

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