Coal Tar, Pavement Sealer, and PAHs: Facts and Potential Dangers
What Is Coal Tar?
Coal tar is a thick black liquid produced as a by-product when coal is processed. It's a mixture of thousands of different substances, which haven’t all been identified. Its composition varies and depends on the way in which the coal is processed. For many years, coal tar preparations have been used to treat skin and scalp problems such as psoriasis, seborrheic dermatitis, dandruff, and eczema. They’re also frequently used in pavement or driveway sealers.
For some time, scientists have strongly suspected that certain components of coal tar can cause cancer. The quantities of tar used in commercial products were thought to be too low to produce any problems, however. Recent research suggests that the amount of coal tar in pavement sealer could be a health risk. Pavement sealer is also known as sealant and sealcoat.
Coal is a natural substance that is processed into other materials. It's made from the bodies of ancient plants that have been altered by the heat of the Earth and the pressure created by overlying sediments.
Polycyclic Aromatic Hydrocarbons or PAHs
Coal tar contains chemicals called polycyclic aromatic hydrocarbons, or PAHs. The chemicals consist of fused rings that each resemble a benzene molecule. They are produced when substances that contain carbon undergo incomplete combustion. This process occurs when there isn't enough oxygen to turn all of the carbon into carbon dioxide. Some PAHs are thought to be carcinogens (chemicals that have the potential to cause cancer).
Most of us are exposed to PAHs when we eat food that has been cooked at high temperatures, such as by being grilled, roasted, or fried, or when the food is smoked. The chemicals also enter our bodies from air which is contaminated by emissions from factories, power plants, and the tailpipes of cars. Cigarette smoke and wood-burning stoves are other sources of PAHs.
Researchers say that PAHs are released in large quantities from pavement sealer containing coal tar and are transported in vapor and dust. The dust is especially dangerous for young children, who frequently put their hands into their mouths. They may ingest the dust after touching sealer or toys that have become contaminated. Children and adults may also inhale the dust or dangerous fumes released from the sealer.
PAHs are hydrocarbons, which means they contain only hydrogen and carbon atoms. Different PAHs contain a different number of rings. The black lines in the rings shown above represent chemical bonds.
An Oregon State University Toxicologist Describes PAHs
Pavement Sealer and the Environment
Pavement sealer containing coal tar is sprayed on parking lots and driveways to protect them and to give them a dark black color, which many people like. It's sometimes applied to playgrounds as well. According to the United States Geological Survey (USGS), a coal tar sealer is commonly used in the central, eastern, and southern parts of the United States, while an asphalt sealer tends to be used in the western United States. Sealers based on coal tar generally contain 20% to 30% coal tar pitch, which is the material that remains after coal tar is distilled. Pure coal tar pitch is considered to be a carcinogen (a substance that is capable of causing cancer).
A coating of coal tar sealer isn't permanent. In fact, the manufacturers recommend that it's reapplied regularly at one to five year intervals, depending on the product. Dust is released from the sealer due to abrasion by vehicle tires. Rain then washes it into soil or storm drains. The dust eventually reach ponds, rivers, or lakes. It's also blown by wind or enters homes on the soles of shoes. In addition, it's transferred to new areas on vehicle tires or by snow removal. Coal tar fumes are released from the sealed pavement in a process called volatilization.
Effect on Aquatic Life
Dust containing coal tar is toxic to aquatic life. It settles in the sediments at the bottom of lakes and rivers and can kill or injure amphibians and invertebrates, especially aquatic insects. It may also interfere with their reproduction. The dust is especially dangerous for creatures that live in the bottom sediments.
Coal tar dust is thought to be responsible for stunted growth in aquatic amphibians and for movement difficulties in the animals. Even fish are affected by an increase in PAH concentrations. PAHs cause cataracts, liver, and immune system problems in fish. In addition, they may be the cause of tumors that have formed in fish living in water contaminated by coal tar dust.
Asphalt is a mixture of stones, sand, and a substance known as asphalt cement, which is made from petroleum. Pavement sealer may be either coal tar based or asphalt based.
A USGS Scientist Describes PAHs
Polycyclic Aromatic Hydrocarbons in Pavement Sealer
There are different types of polycyclic aromatic hydrocarbons. At least some are mutagenic, which means that they cause mutations, or changes in genes. They are also teratogenic since they damage embyros and cause birth defects. Some PAHs have been identified as probable carcinogens.
Until recently, although some scientists were concerned about our exposure to PAHs, it was generally thought that their concentration in the environment was too low for them to be a major health threat to humans. In recent years, however, researchers in the United States have noticed that the level of PAHs in urban lakes has increased, sometimes by a large amount, which has stimulated their interest in the chemicals.
Here are some recently discovered facts about PAHs in coal tar pavement sealer, according to the United States Geological Service.
- Two hours after a coal tar sealer has been applied to pavement, the release of PAHs into the environment from the pavement is 30,000 times higher than the release from unsealed pavement.
- Release of PAHs from sealed pavement that is three to eight years old is 60 times higher than the release from unsealed pavement.
- The concentration of PAHs in house dust is 14 to 25 times higher in houses next to a parking lot covered with coal tar sealers, or in houses with a driveway coated with a sealer containing coal tar, compared to the dust in houses near unsealed parking lots and driveways. The increased concentration of PAHs is also more than twice the usual dose obtained from the diet.
- Coal tar sealers contain about 1000 times more PAHs than asphalt based sealers do.
Coal Tar Sealer Dust and Contamination
Possible Dangers of PAHs
Is the higher concentration of PAHs in the environment near pavement sealer causing an increased incidence of human cancer? Will it do so in the future, since cancer often takes time to develop after exposure to a carcinogen? If the PAHs are carcinogenic, what dose and exposure time are needed to produce cancer? Scientists don't have the data needed to answer these questions definitively, but they strongly suspect that the PAHs are dangerous.
The USGS says that the estimated lifetime cancer risk for people living near pavement sealer is increased. Someone who spends their whole life near the sealer is estimated to have a 38% increase in cancer risk. Someone who spends only the first six years near the sealer is estimated to have a 25% increase in cancer risk. Some communities are sufficiently concerned about the safety of coal tar that they have banned the use of pavement sealers containing the substance.
Coal Tar in Skin Creams and Shampoos
Coal tar has been a popular treatment for skin conditions such as psoriasis for more than a hundred years, yet it's still not clear how it works. Psoriasis is a disorder characterized by red, itchy, thickened, and scaly patches on the skin. Coal tar reduces inflammation and itching and is absorbed into skin cells. Here it's thought to interfere with DNA replication and slow down cell division, reducing skin thickening.
Coal tar skin cream and shampoo are also used to treat seborrheic dermatitis. This condition is known as dandruff when it occurs on the scalp and cradle cap when it occurs on the scalp of young children. Seborrheic dermatitis is an inflammatory condition in which white or yellow flakes are shed from oily areas of the skin. The skin may also be red and itchy. Coal tar creams are sometimes used to treat eczema as well.
Wright's coal tar soap is still sold and still has its original name, but it no longer contains coal tar. Some other products with "coal tar" in their name contain a synthetic chemical instead of a coal tar derivative.
Safety of Coal Tar Creams and Liquids
It’s known that in industries or jobs which involve frequent exposure to high levels of coal tar there is an increased incidence of several types of cancer. Coal tar medications, which contain much lower concentrations of the tar, generally aren't considered to be dangerous, however. They are somewhat controversial, though, since we don't know the exact composition of the tar or exactly how the medications work. Scientists haven’t noticed any increase in the incidence of cancer in people who use coal tar skin creams.
Coal tar skin preparations may stain clothing and temporarily stain skin and hair. They may also make the skin more sensitive to ultraviolet radiation, so people who use coal tar shampoos or skin creams should be very careful about sun exposure. The creams can be helpful, but some people find that they irritate the skin.
The Problem of Coal Tar in the Environment
We know that frequent exposure to a high concentration of coal tar is associated with an increased risk of cancer in humans. What we don't know is the maximum concentration and exposure frequency that is harmless. Apparently coal tar skin creams and shampoos are safe. What about PAHs from coal tar in the environment? These are likely dangerous. It seems like a good idea to reduce our exposure to coal tar and polycyclic aromatic hydrocarbons as much as possible
Coal tar pitch dangers from the New Jersey Department of Health
Ban on pavement sealant in Austin, Texas from the ACS (American Chemical Society)
Facts about polycyclic aromatic hydrocarbons from Tox Town (an NIH or National Institutes of Health site)
Information about PAHs and coal tar in pavement sealant from the USGS (United States Geological Service)
Coal tar ointment information from the Mayo Clinic
© 2012 Linda Crampton