Coal-tar sealcoats pollute nearby soil and water.
Van Metre, PC, BJ Mahler and JT Wilson. PAHs underfoot: Contaminated dust from coal-tar sealcoated pavement is widespread in the United States. Environmental Science and Technology doi: 10.1021/es802119h.
|US Geological Survey.|
Parking lots treated with coal-tar-based sealcoats are a major source of cancer-causing contaminants that can pollute air, soil, water and wildlife, posing a significant health risk to humans who may breathe, drink or eat them in fish and other food.
Parking lots and other paved areas are sometimes sealed to protect them from the elements. Two main sealants are used in the US: asphalt (crude oil base) and coal-tar base. Asphalt is more common in western states, while coal-tar base is the dominant form used in eastern and central states.
Coal-tar is formed when coal is coked, a process to prepare coal for use as a fuel. Coal-tar base sealants contain high levels of a class of chemicals known as polycyclic aromatic hydrocarbons (PAHs). A mix of different types of PAHs are found in the sealant.
The long-lived PAHs widely pollute the environment, wildlife and people. These highly dangerous chemicals are expected to cause cancer in people.
PAHs form if oil, coal, wood and petroleum do not burn completely. Sources include vehicles, factories, power plants and pavement sealcoats. Of the many sources of PAHs in the environment, it is not known how much of it comes from sealcoated pavements.
As the black, shiny sealcoats wear away over time, small, dusty particles form. The specks can contain PAHs and other chemicals found in the sealants.
Wind and water disperse the particles into the surrounding environment. Wind carries the contaminated dust almost everywhere -- into the water, onto other pavements or onto land used for gardens or crops. Potentially, the dust, if very fine, could be breathed in by animals and people.
The dust is also washed off the surfaces into local lakes, streams and other waterways by rain and snow melt. The stormwater runoff can contain high levels of PAHs. Some researchers suggest that coal-tar based sealcoats are a major source of PAH pollution in streams. Often, high levels of PAHs are found in the sediments of the lakes and streams accepting the stormwater runoff.
This pollution poses an environmental health risk for the organisms that live in the waterways, including fish that may be eaten by humans. A study of sealcoats in runoff in Austin, Texas, has linked adverse effects in local amphibians to sealcoat dust runoff (Bryer et al. 2006).
The authors studied paving in nine US cities, choosing the cities because of their proximity to lakes and their regional location in the western (Seattle, WA; Portland, OR; Salt Lake City, UT), eastern (New Haven, CT; Washington, DC) and central (Minneapolis, MN; Chicago, IL; Detroit, MI; Austin, TX) parts of the country. The pave study sites within each city were not near industry and represented different uses, such as home driveways, schools, office parks and retail businesses. Dust was also gathered from adjacent roads and soils.
The samples were collected, sieved and analyzed for total PAHs. Depending on the comparisons made, data were either combined -- to examine PAHs in dust from surfaces with or surfaces without sealants -- or analyzed individually -- to determine site-to-site variability. The researchers compared PAH levels in the dust samples: 1) among different US cities, 2) between the nonsealcoated and sealcoated pavements, 3) between the two major types of sealcoatings and 4) with levels in nearby lake sediments as measured by other studies.
The amount of PAHs in the dust samples reflected whether the pavement was unsealed or sealed. Very low levels of PAHs were found in all unsealed pavements.
However, the level of PAHs in dust on sealed pavements depended on which sealant was used. Surfaces sealed with an asphalt-based product contained low PAH levels, similar to those of unsealed pavements.
In sharp contrast, samples from coated pavements in the east and central US had mean total PAH levels up to 80 times higher than those from unsealed lots. In the six central and eastern cities, where coal-tar based products dominate, the average levels of total PAHs were 2,200 milligrams per kilogram (mg/kg). Samples from uncoated pavements from the same cities were just 27 mg/kg. The sealcoated samples also had large variablity -- an order of magnitude -- among sites, with levels ranging from 345 to 3,400 mg/kg.
Samples from both coated and uncoated sites in cities in western states had low levels of PAHs. All but one were less than 13 mg/kg, which is 1,000 times lower than levels found in the eastern and central sites. One of the nine coated lots was sealed with a coal-tar base product and had PAH levels of 850 mg/kg. These data highlight the predominant use of asphalt base rather than coal-tar base sealants.
Samples of soil and dust collected from areas close to sealcoated lots had elevated levels of PAHs, from 2 to 39 times higher than samples obtained near unsealcoated lots. A similar trend was seen between PAH levels in local lake sediments near sealed lots.
Two residential homes in suburban Chicago had the highest levels of PAHs at 5,800 and 9,600 mg/kg.
In general, the amount of PAHs in dust samples clearly differed among regions. Eastern and central cities had higher levels of PAHs than the study sites in western states. This held for PAH levels in the dust from the paved areas, in the samples from nearby streets/soil and from records of lake sediments.
These differences reflect the type of sealant predominantly used in different areas of the country. Some of the contamination found in eastern and central cities was chemically traced to coal-tar based sealcoats. Also, the difference between eastern and western samples in PAH levels -- reported as a ratio of 1,000 to 1 -- mimics the total PAH concentrations found in the coal-tar base and the asphalt base products, respectively.
This study adds more informtion about regional and national PAH pollution from coal-tar sealants. It also highlights the potential human health risk from coal-tar based products and shows a need to better understand and reduce use and impacts.
Coal-tar based sealcoats contain very high levels of PAHs -- up to 30 percent by weight. PAHs are known to cause an array of health effects and pose a significant threat to wildlife and humans.
Sealcoats are not stable. The coatings break down over time, forming dust that moves the PAHs from the paved surfaces to surrounding areas, including soil, water and air.
Of particular concern are the levels found in residential driveways. PAH levels in the collected dust were above toxic guideline limits for indoor dust and soils. Potentially, the outside dust could pose a human health risk either directly, by someone touching the surface, or indirectly through objects that contact the surface, such as shoes or basketballs.
Hand to mouth contact is a major route of exposure for infants, children and adults. People can ingest contaminant-laden house dust by touching it then eating without washing their hands. Mouthing toys and other objects is another way babies are exposed (Stapleton et al. 2008).
Outdoor, PAH-laden dust could also be ingested by hand to mouth contact. The dust could be picked up on hands or carried indoors on the bottom of shoes. It could also be breathed in while working or playing outside (driveways, playgrounds, etc.).
PAHs in the lake sediments and surrounding soils is also a concern for wildlife and people. Humans could eat contaminated fish or agriculural crops grown on soils polluted with PAHs.
Clearly, the high levels of PAHs observed in these urban and suburban areas warrant further research as to the health risk of these common and widely used sealing products.
Coal-tar sealants are banned from use in some places. One simple solution is use asphalt containing sealcoats, particularly for residential use and playgrounds.
Stapleton, HM, SM Kelley, JG Allen, MD McCleanw and TF Webster. 2008. Measurement of polybrominated diphenyl ethers on hand wipes: Estimating exposure from hand-to-mouth contact. Environmental Science and Technology. 42(9); 3329-3334.
Van Metre, P, BJ Mahler, M Scoggins and PA Hamilton. 2006. Parking lot sealcoat: A major source of polycyclic aromatic hydrocarbons (PAHs) in rrban and suburban environments. US Geological Survey and the City of Austin, Fact Sheet 2005–3147.
Polycyclic aromatic hydrocarbons (PAHs)