Frequently Asked Questions

With a growing site of information about the problem of coal tar sealant pollution, it seemed reasonable to put together an FAQ or “Frequently Asked Questions” section on the topic.  The answers have links to sites, posts, or videos that go into more detail.  Some references are quoted extensively because they are well-written and succinct.  The purpose here is to put the information in a single, digestible location.

The infographic, listed either at this link or in the right hand column, is designed to quickly answer many of the initial questions that might arise when learning about this subject.  So if you want text with references, then continue down this page.  But if you want a moving “Prezi” presentation, then go to the infographic.

We will add more as we are available.  If you have specific questions or think something is omitted here, feel free to contact us by email by clicking on the “Contact me” form in the right-hand column of this page. 

What is pavement sealant?

“Pavement sealcoat (also called sealant) is a black liquid that is sprayed or painted on some asphalt pavement. It is marketed as protecting and beautifying the underlying pavement, and is used commercially and by homeowners across the Nation. It is applied to parking lots associated with commercial businesses, apartment and condominium complexes, churches, schools, and business parks, to residential driveways, and even to some playgrounds. Most sealcoat products have a coal-tar-pitch or asphalt (oil) base. Coal-tar-based sealcoat is commonly used in the central, southern, and eastern United States, and asphalt-based sealcoat is commonly used in the western United States.” Source: USGS

What is coal tar? 

“Coal tar is a thick, black or brown liquid that is a byproduct of the carbonization of coal for the steel industry or the gasification of coal to make coal gas. Coal tar is a byproduct of the coking of coal for the steel industry and coal-tar pitch is the residue remaining after the distillation of coal tar. Coal-tar pitch is 50 percent or more PAHs by weight and is known to cause cancer in humans (International Agency for Research on Cancer, 1980).” Source: USGS

Where does coal tar for sealant come from?

Most of it is imported.  Yes, sorry to say it is true! Put in its simpliest terms: we import what should be hazardous waste to spread all over our nation to make our driving surfaces look prettier and last longer. What year is this?

Years ago, industry petitioned the EPA so coal tar could get an exemption from hazardous waste rules because if they didn’t, they said our landfills would fill up coal tar. Now some 20 years later, the basis for that argument, as well as the production of coke byproducts (coal tar) have moved offshore.

In 2003 about 2/3 of US supplies were imported and the foremost use is in the production of aluminum. With increased environmental scrutiny, the percentage imported is expected to increase. Details are available in this industry report:

This point was illustrated by the sealant industry’s coal tar shortage in 2006. The supply ran low here in the US of A because of factory problems outside our borders!  For more information see: Holy Tar Balls! Most Coal Tar for Sealants is Imported!

What is asphalt?

Asphalt primarily is a byproduct of the oil refining operations. Less frequently, there are asphalts that come from natural source like gilsonite, which is a mined, fossilized asphalt. Asphalt can be in both a liquid or solid form.

What’s the problem with coal tar sealants?

“Coal-tar-based sealcoat products typically are 20 to 35 percent coal-tar pitch. Product analyses indicate that coal-tar-based sealcoat products contain about 1,000 times more PAHs than sealcoat products with an asphalt base (City of Austin, 2005).” Source: USGS

What are PAH’s (polycyclic aromatic hydrocarbons)?

“PAHs are a group of chemical compounds that form whenever anything with a carbon base is burned, from wood and gasoline to cigarettes and meat. PAHs also are in objects and materials, such as automobile tires and coal tar, the production of which involves the heating of carbon-based materials. PAHs are of environmental concern because several are toxic, carcinogenic, mutagenic, and/or teratogenic (causing birth defects) to aquatic life, and seven are probable human carcinogens (U.S. Environmental Protection Agency, 2009).” Source: USGS

How toxic are coal tar sealants?

Potent. A few years ago, I came across this research that got little attention when presented back in 1997. It pre-dates any of the current understanding of the problem of coal tar sealants. The lead author is the retired head of the Mobil Corporation’s research laboratory. He developed an index to rate the mutagenicity of chemical solutions called the Ames Index. It has been used with other paving products as well.

Dr. Mackerer decided to do this study after seeing some college kids sealing his neighborhood driveways. He wondered just how toxic the sealants are. So he went to a hardware store and bought 12 separate products. As the above graph shows, anything above 1.0 is considered a mutagen. The coal tar sealants are an average of about 450! Dr. Mackerer said that while the absolute number can go higher, after a few hundred the real toxicity is maxed out. For more see this blog post.

What effect does coal tar sealant have on the environment?

“Some PAHs are toxic to mammals (including humans), birds, fish, amphibians (such as frogs and salamanders), and plants. The aquatic invertebrates—insects and other small creatures that live in streams and lakes—are particularly susceptible to PAH contamination, especially those that live in the mud where PAHs tend to accumulate. These invertebrates are an important part of the food chain and are often monitored as indicators of stream quality (analogous to the “canary in the coal mine” concept). Possible adverse effects of PAHs on aquatic invertebrates include inhibited reproduction, delayed emergence, sediment avoidance, and mortality. Possible adverse effects on fish include fin erosion, liver abnormalities, cataracts, and immune system impairments.”   Source: USGS

What effect does coal tar sealant have on people?

“PAHs are an alarming group of substances for humans and environmental organisms. Many PAHs are carcinogenic, mutagenic, and/or toxic for reproduction (Crone and Tolstoy, 2010). Some PAHs are at the same time persistent, bioaccumulative, and toxic for humans and other organisms. Persistent means that the substances remain in the environment for a long time and are hardly decomposed there. Bioaccumulative chemicals accumulate in organisms – including the human body. Substances that combine these three characteristics represent a particular level of concern under an environmental aspect. Experts speak of PBT substances in this context (Persistent, Bioaccumulative, and Toxic substances). If such chemicals are released, they can no longer be removed from the environment due to their characteristics. On the contrary: They accumulate and can harm plants, animals, and ultimately humans.”   Source: German Federal Environment Agency

How does coal tar sealant enter the environment?

“Friction from vehicle tires abrades pavement sealcoat into small particles. These particles are washed off pavement by rain and carried down storm drains and into streams. Other sealcoat particles adhere to vehicle tires and are transported to other surfaces, blown offsite by wind, or tracked indoors on the soles of shoes. Some of the PAHs in sealcoat volatilize (evaporate), which is why sealed parking lots and driveways frequently give off a “mothball” smell. Sealcoat wear is visible in high traffic areas within a few months after application, and sealcoat manufacturers recommend reapplication every 2 to 4 years.” Source: USGS

Does product type really matter?

“PAH concentrations in the coal-tar-based sealcoat product are about 1,000 times higher than in the asphalt-based product (more than 50,000 milligrams per kilogram [mg/kg] in coal-tar-based products and 50 mg/kg in asphalt-based products [City of Austin, 2005]).”  Source: USGS

Is this an isolated problem or a nationwide one?

“PAHs are increasing in urban lakes across the United States.  To better understand why this might be happening, USGS scientists collected sediment cores from 40 lakes in cities from Anchorage, Alaska, to Orlando, Florida, analyzed the cores for PAHs, and determined the contribution of PAHs from many different sources by using a chemical mass-balance model. The model is based on differences in the chemical “fingerprint” of PAHs from each source. Coal-tar-based sealcoat accounted for one-half of all PAHs in the lakes, on average, while vehicle-related sources accounted for about one-fourth.”   Source: USGS

Is this only a problem outside the home?

 “House dust is an important source for human exposure to many contaminants, including PAHs. This is particularly true for small children, who spend time on the floor and put their hands and objects into their mouths. In 2008, the USGS measured PAHs in house dust from 23 ground-floor apartments and in dust from the apartment parking lots. Apartments with parking lots with coal-tar-based sealcoat had PAH concentrations in house dust that were 25 times higher, on average, than concentrations in house dust from apartments with parking lots with other surface types (concrete, unsealed asphalt, and asphalt-based sealcoat). PAH concentrations in the dust from the parking lots with coal-tar-based sealcoat were 530 times higher, on average, than concentrations on the parking lots with other surface types.”  Source: USGS