Coal-tar-based sealcoat, a potent source of polycyclic aromatic hydrocarbons (PAHs), is used commonly on residential driveways in the central, southeastern, and northeastern U.S. In Lake in the Hills, Ill., about 90% of residential driveways are sealcoated, as shown here.

Coal-tar-based sealcoat—a product marketed to protect and beautify asphalt pavement—is a potent source of polycyclic aromatic hydrocarbons (PAHs) to air, soils, streams and lakes, and homes. Does its use present a risk to human health?

Results from a new study by researchers from Baylor University and the USGS indicate that living adjacent to a coal-tar-sealed pavement is associated with significant increases in estimated excess lifetime cancer risk, and that much of the increased risk occurs during early childhood.

What are sealcoat, coal tar, and PAHs?

Pavement sealcoat—also called sealant or driveway sealer—is a black liquid sprayed onto the asphalt pavement of many residential driveways, parking lots, and even some playgrounds. Sealcoat used in the central, southern, and eastern U.S. commonly contains coal-tar pitch, and sealcoat used in the western U.S. commonly contains asphalt.  Coal-tar pitch, the residue remaining after distillation of coal tar (a byproduct of the coking of coal), is a known human carcinogen and contains about 200 different PAHs.  Coal-tar-based sealcoat typically is 20 to 35 percent coal-tar pitch and contains from 50,000 to 100,000 milligrams per kilogram (or parts per million) PAHs, about 1,000 times higher than PAH concentrations in asphalt-based sealcoat products, and hundreds of times higher than PAH concentrations in tire particles, used motor oil, or other urban sources. At least seven PAHs, including benzo[a]pyrene, are probable human carcinogens.

The pavement of many parking lots is coated with coal-tar-based sealcoat, which is marketed to increase the longevity of the underlying asphalt and improve appearance. Car tires grind the dried sealcoat to a fine powder that is a potent source of polycyclic hydrocarbons (PAHs). The sealcoat particles wash down storm drains, are blown onto adjacent soil and pavement, and are tracked into homes on the soles of shoes.

PAHs from coal-tar sealcoat find their way into soil and house dust

Friction from vehicle tires grinds pavement sealcoat into small particles, which are incorporated into the dust on the pavement surface.  Dust on coal-tar-sealed pavement contains PAHs at concentrations that are hundreds of times higher than those in dust on concrete or unsealed asphalt pavement.  Some of that contaminated dust is transported by wind, rain, and snowplows to nearby soil, and some is tracked into homes, where it becomes part of the house dust.

People, especially children, are exposed to PAHs in soil and housedust

We all consume soil and dust through what is called incidental (non-dietary) ingestion, which occurs when we put our hands or objects into our mouths.  Incidental ingestion is a pathway to exposure to many chemicals, especially for children.  The study focuses on incidental ingestion of the seven cancer-causing PAHs.  A single number known as the benzo[a]pyrene equivalent (BaPEQ) was determined that represents the cancer-causing concentration of the seven PAHs combined.   Using existing data, a representative BaPEQ was determined for soil adjacent to coal-tar-sealcoated pavement and for house dust in nearby residences, and also for soil adjacent to unsealed asphalt pavement and house dust in nearby residences.  The average estimated lifetime BaPEQ dose for someone living adjacent to coal-tar-sealcoated pavement was 38 times greater than for someone living adjacent to unsealed asphalt pavement.  About one-half of that dose occurs during childhood, that is, 0 to 6 years of age.

Estimated lifetime cancer risk is substantially higher for people that live near coal-tar-sealcoated pavement

USGS researchers collect a sample of dust from a sealed parking lot for analysis of PAHs.

Excess lifetime cancer risk—the probability that an individual will develop cancer by age 70 because of exposure to a contaminant—was determined for individuals who spend their lifetime living adjacent to pavement with coal-tar-sealcoat (for example, a sealcoated driveway or a sealcoated parking lot), those who spend just the first 6 years of their life living adjacent to pavement with coal-tar-sealcoat, and those who spend their lifetime (70 years) living adjacent to unsealed asphalt pavement (urban background exposure).

For someone who spends their entire lifetime living adjacent to coal-tar-sealcoated pavement, the average excess lifetime cancer risk is estimated to be 38 times higher than the urban background exposure.  More than one-half of the risk occurs during the first 18 years of life, and most of it (84 percent) is from ingestion of soil.  The estimated lifetime cancer risk also is elevated for someone who spends just the first 6 years of their life living adjacent to coal-tar-sealed pavement—about 25 times higher than urban background exposure.

A USGS researcher vacuums up a sample of house dust for analysis of PAHs. Concentrations of PAHs in house dust from residences adjacent to parking lots with coal-tar-based sealcoat were 25 times higher than in housedust from residences adjacent to parking lots with other types of pavement surfaces.

For the average individual who lives adjacent to coal-tar-sealed pavement for either their entire life or just the first 6 years, the excess lifetime cancer risk is estimated to be greater than 1 in 10,000.  Estimated cancer risk associated with coal-tar-sealcoat is even higher for children that consume larger-than-average amounts of soil and dust.  In general, the U.S. Environmental Protection Agency considers excess cancer risks greater than 1 in 10,000 to be sufficiently large that some sort of remediation is desirable.

Young boys working in a newly cropped field in Africa.

In parts of eastern Africa, drought is of increasing concern, as poor families suffer from food shortages and the inability to grow crops and sustain livestock. Stunted growth in children due to malnutrition has also been linked to climate trends in Africa.

Drought conditions are expected to continue as global temperatures continue to rise and rainfall declines across parts of eastern Africa.

This poses increased risk to millions of people in Africa who currently face potential food shortages.

What’s being done to help?

The USGS is involved in a variety of research efforts to help understand current and future conditions in Africa, helping to inform plans to provide aid.

The Famine Early Warning Systems Network, or FEWS NET, is one endeavor that has already made great strides in helping to address this issue. FEWS NET helps target more than $1.5 billion of assistance to more than 40 countries each year.

FEWS NET examines the populations of the developing world with the most food insecurity, identifying critical situations in which food aid will be needed. These are populations whose livelihoods are typically tied to subsistence rain-fed agriculture and pastoralism.

FEWS NET is sponsored by the U.S. Agency for International Development (USAID) Office of Food for Peace and the USGS is actively involved.

FEWS NET at the United Nations Climate Convention

A USGS presentation on FEWS NET will be a featured side event on November 30, 2011, at the United Nations 17^th annual Conference of the Parties (COP-17) in Durban, South Africa. The convention’s purpose is to develop international agreements and a declaration of policies and practices for combating climate change and its impacts around the world.

A herder moves cattle through a barren landscape in eastern Africa.

Climate forecasts and remote sensing help spot future trouble

FEWS NET has developed its own climate services to provide decision makers with early identification of agricultural drought that might trigger food insecurity. Scientists use climate forecasts to develop forward-looking food security assessments that are based on expected agricultural outcomes for the season ahead.

Since networks of ground observation stations are often sparse or reported late in FEWS NET countries, satellite remote sensing of vegetation and rainfall fills in the gaps. Remote sensing from space allows for rapid, accurate assessments of a broad range of environmental and agricultural conditions. USGS scientists provide the technologies and expertise to support remote sensing for FEWS NET activities.

Early warning of famine in Somalia helps pre-position food supplies

On July 20, 2011, the United Nations declared parts of Somalia as a region of famine. The decision was supported by FEWS NET and USGS observational evidence of conditions in the area.

The declaration was the culmination of early warning communications encouraging — months before the crisis — that government and other agencies pre-position food and supplies in the region.

“None of the many uses of Earth-observing satellites is more vital — or has as much potential for prompting timely humanitarian intervention — as famine early warning,” said USGS Director Marcia McNutt. “Remote sensing from space allows USGS scientists to provide rapid, accurate assessments of a broad range of environmental and agricultural conditions.”

The eastern Horn of Africa, the continental region that encompasses Somalia, has experienced two consecutive seasons of very poor rainfall resulting in the worst drought in 60 years. Crops have failed, livestock deaths are widespread, and food prices are very high. While the rains this winter have been good, food prices remain high, and the food security situation remains insecure.

Stunted growth linked to malnutrition and climate change

Other USGS research is helping to identify the impacts of a changing climate on Africa’s people. Scientists recently discovered that malnutrition and dry hot living conditions are linked to stunted growth in Mali, West Africa.

USGS research found that Mali was becoming substantially warmer and a little bit drier. Scientists also knew that farmers and those who make a living raising sheep, cattle, goats, or camels were poor, and that stunted growth was occurring throughout Mali.

Scientists wondered if there could be a link between human health and increasingly warm and dry conditions.

To investigate, the USGS worked with the University of California, Santa Barbara, to study climate observations and demographic and health data. The Demographic and Health Survey program routinely compiles data from surveys in 90 countries to study trends in health and population. Scientists analyzed statistics on specific villages in Mali and found that there was a link between a warmer climate and increased stunting.

Population growth combined with the impacts of warming will further increase these health impacts.

Stunting was also linked to other factors, such as mother’s education and the water supply system. Women’s education, improved water supplies, and agricultural development could help to address malnutrition and stunting in Mali.

An article on this research was published in in the journal, Applied Geography, by San Diego State University, the University of California, Santa Barbara, and the USGS.

A Food Security Assessment in Somalia found severe impacts on livestock due to drought conditions.

Other studies underway

Other new research includes the discovery that the warming of the Indian and western Pacific oceans (which is linked to global warming) affects rainfall over large areas of the Horn of Africa. As the globe has warmed over the last century, the Indian Ocean and western Pacific have warmed especially fast.

The resulting warmer air and increased humidity over the Indian and western Pacific oceans produce more frequent rainfall in that region. The air loses its moisture during rainfall, and then flows westward and descends over Africa, leading to decreased rain in parts of eastern Africa. Trends toward increased frequency of drought that we are seeing now are likely to continue into the future as warming continues.

A few recent articles on this research were published in the journal, Climate Dynamics, by scientists with the USGS, the University of California, Santa Barbara, and Los Alamos National Laboratory. The most recent article concludes that global warming will lead to a decrease in rainfall during the summer monsoon season, from June to September, across southern Sudan, southern Ethiopia, and northern Uganda.  Another article concluded that eastern Africa, particularly Kenya and southern Ethiopia, will also have a significant decrease in rainfall during the long-rains season from March to June.

USGS scientists are working hard to translate these technical studies into reports for decision makers. To date, they have completed summary fact sheets focused on Sudan and Kenya.

Scientists also found that some regions, like northern Ethiopia, are not getting drier due to current warming temperatures. Rainfall varies dramatically across all of eastern Africa, with high mountainous areas typically receiving many times the rainfall received in low-lying areas. Therefore, agricultural growth in these climatically safe regions could help offset rainfall declines in other locations.

Start with science

Scientists are looking at clues and changes in nature to understand the impacts of global warming. In Africa, impacts are seen across the landscape — on farms and even in humans.

By starting with science, well-informed decisions can be made to help Africa as it faces drought, famine, and health concerns.

FEWS NET partners include the USAID, Chemonics International, the USGS, NASA, NOAA, and the USDA. The Geography Department at the University of California, Santa Barbara, is a partner to the USGS in this effort.

Want more information?

Listen to a podcast interview with USGS scientists as they discuss ongoing efforts to understand conditions in Africa.