Under the Lens: Sealing the Science on PAHs

utl-pahsNew evidence shows how tar-based pavement sealants affect public health

By Paul Mathewson, Staff Scientist

PAHs, or polycyclic aromatic hydrocarbons, are a class of chemical pollutants that can persist for a long time in the environment and bind to solid materials, leading to accumulation in soils and waterbody sediment. When aquatic invertebrates, fish or amphibians are exposed to them, they can cause reproductive problems, organ defects, tumor growth, immune system impairment and increase death rates. They can also cause cancer in humans and are linked to decreased cognitive development in children if exposure occurs while in the womb.

More specifically, PAHs are chemical compounds made of fused rings of carbon and hydrogen molecules. There are many different forms of PAHs, some of which are more or less toxic than others, but they generally appear together and are found in especially high levels in fossil fuels like oil and coal. While they are also formed when organic material is burned, such as during forest fires, human sources are the biggest causes of PAH pollution in most places.

Unfortunately, there has been a continued rise in environmental concentrations of PAHs over the past 40 years in the United States. This is partly because we have been operating under the assumption that since there is a wide range of potential sources, the PAH problem cannot be easily addressed with one or two targeted practices. However, there is new hope to reverse the trend as recent research and experimental evidence have begun to identify a single major and primary cause of PAH pollution in aquatic sediments: tar-based pavement sealants.

Pavement sealants, also called sealcoats, are applied to driveways, parking lots and playgrounds to give them a dark black appearance, often with the idea that they protect the asphalt surfaces. They are generally made from either asphalt or coal tar, but it is the latter type that is the biggest concern: tar-based sealants contain 20% to 35% coal tar, a byproduct of refining coal, which is a known human carcinogen and is up to half PAHs by weight. Because of the high levels of PAHs in coal tar, tar-based sealants can contain 1,000 times more PAHs than the alternative asphalt-based sealants.

The problem is that sealants don’t last forever and weather and wear off the surfaces where they are applied; water runoff then washes loose particles into nearby soils and waterbodies. As a result, due to the high amounts of PAHs in those loose particles, research has found that tar-based sealants are the largest source of PAHs to urban waterbody sediment where they are used. Experiments have also shown that runoff from pavement with tar-based sealants contains PAH concentrations orders of magnitude higher than runoff from asphalt-based-sealed or unsealed pavement. Of local interest, the U.S. Geological Survey has recently completed a study examining PAH sources to waterbodies in Milwaukee, and we look forward to seeing the results when they are published.

In addition to contaminating water sediments, studies have also documented how tar-based sealants can be a major contributor of PAHs to the air, with one study calculating that annual PAH emissions from new applications of tar-based sealants can exceed those from annual vehicle emissions. Finally, emerging research is showing how tar-based sealant use may be increasing human PAH exposure through inhalation and ingestion of contaminated dust and soil. This happens when dust containing PAHs from the pavement is transported into homes by wind and on shoes and clothing, and is a particular threat to young children who are both more sensitive and more highly exposed because they spend time on the floor where they incidentally ingest soil and dust. In fact, concentrations of PAHs in dust in homes adjacent to parking lots with tar-based sealants were found to be 25 times higher than in homes adjacent to unsealed asphalt parking lots. This increased exposure was estimated to raise lifetime excess cancer risk by a factor of 38, with much of the increased risk occurring during early childhood.

There is an economic cost of using tar-based sealants as well. Many municipalities use detention ponds to control stormwater pollution, and accumulated sediment must be periodically removed from these ponds. However, sediment containing high levels PAHs may need to be landfilled at significant expense to the municipality. In the Minneapolis-St. Paul metropolitan area, for example, total cost estimates for disposing of PAH-contaminated sediment are in the millions of dollars.

In response to the emerging science documenting how tar-based sealants are contributing PAHs to the environment, Washington, Minnesota, Massachusetts, the District of Columbia, and numerous counties, including Dane County in Wisconsin, and other municipalities have taken action to ban or restrict the use of tar-based sealants. Some schools, school districts and universities have policies to not use coal tar-based sealants, and many home improvement stores have chosen to stop selling tar-based sealants, including The Home Depot, Lowe’s, True Value, Ace Hardware and Menards.

If sealing your driveway is on your 2016 to-do list, check the ingredients list to make sure you are not using a tar-based sealant if you are doing it yourself or find an applicator that will use an alternative to tar-based sealants. Numerous applicators in Wisconsin have pledged to stop using tar-based sealants; a list of these companies can be found here.