Last year, environmental geoscientist Mark Krekeler of Miami University in Ohio traveled to Gary, Ind., a former steel city of 77,000 people just outside of Chicago, Ill., to study the dust and debris that collect along the sides of roads.
Krekeler and his team chose Gary because it is analogous to many other urban areas throughout the world; it’s a medium-sized, formerly heavily industrial city now faced with a severely reduced industrial output. This makes Gary a good place to determine whether metal pollution is still pervasive in road sediment years after industrial activity waned, the team reported in Environment International.
They found high levels of some dangerous heavy metals, including lead and manganese, in road sediments. Other recent research has shown that road sediments can even contain highly dangerous pollutants like hexavalent chromium, perhaps released by aging yellow traffic paint. Street sediment is a way of exposing people to pollutants every day, researchers said, and it’s barely monitored, much less regulated.
Krekeler and his team scooped up samples from road sediment using plastic spoons as they walked transects down two busy streets south of the Gary Works plant operated by United States Steel Corporation. (The plant is still operational, though at a much diminished level from its heyday as the world’s largest steel mill in the 1970s.) They put the sediment samples into small plastic bags, recorded GPS coordinates, and headed back to the lab.
In the lab, researchers put the samples under a scanning electron microscope and a transmission electron microscope (TEM) to look at particle size and types and then used X-ray fluorescence (XRF) and inductively coupled plasma–optical emission spectroscopy to determine the chemical composition of samples in the sediments.
In all samples, scientists found significant levels of lead, as well as high levels of zinc, manganese, chromium, copper, and vanadium—heavy metals associated with steel production, Krekeler said. Spatial analysis of the metals showed concentrations of many of the metals decreasing the farther away from the steel plant that the sample was collected.
Krekeler said the lead has many different possible sources: atmospheric deposition from the steel plant, parts of cars like tire weights, road paint, and even leaded gas or lead paint from decades ago. Manganese and vanadium, however, have just one major source in Gary: the Gary Works steel mill. These metals showed the strongest spatial trends, Krekeler said. Of primary concern is that many of the heavy metal particles the team found were small enough to be ingestible and inhalable.
Chronic Health Impacts
Lead and the other metals found in Gary are well recognized to have human health implications, said Angela Arrington, a senior at Portland State University in Oregon who presented the team’s TEM and XRF results at the Geological Society of America’s (GSA) annual meeting in Phoenix, Ariz., in September as part of the Research Experiences for Undergraduates program.
Lead exposure can cause neurological and developmental issues; overexposure to manganese can lead to neurological toxicity, including Parkinson’s-like symptoms, and hexavalent chromium is a human carcinogen, exposure to which is associated with increased risk of asthma, renal damage, cancer, and negative effects on reproduction. In Gary, Arrington says, asthma rates, especially among children, are higher than elsewhere.
“It’s not that if you walk around the street for 1 day in Gary, Ind., you’re going to get asthma or lung cancer or have a change in your nervous system,” Krekeler said. “But if you’re around and directly exposed to those pollutants chronically, over years, then, yes, it’s more probable than not that you’d have a negative health impact.”
Indeed, chronic toxicity from heavy metal exposure can lead to asthma, allergies, chronic obstructive pulmonary disease, carcinomas, and even cardiovascular diseases, said Raihan Khan, a doctoral student in the School of Public Health at West Virginia University who wrote a 2018 review paper on road dust health implications. The human body is like a filter, he said. When we inhale small particles from road dust over time, we can’t get rid of them. And many of these particles are so fine that they aren’t filtered out by the lungs but can get into other parts of the body. Furthermore, Khan said, the larger particles that stay in the lung cells can cause chronic inflammation in the lungs themselves.
A Global Problem
Heavy metals have been found in road dust around the world, from Islamabad, Pakistan, and Sendai, Japan, to Hamilton, Ohio, and Philadelphia, Pa. Common sources include bits of car tires, brake materials, soils, churned-up asphalt, and residual metals from industrial activities. Those pollutants are found along rural roads too, along with sand and gravel dust, Khan said. None of it is healthy for people to breathe.
Another common source of road dust pollution is road paint.
Yellow road paint in Hamilton and Philadelphia—as well as in many other locations—can contain lead chromate. Over time, Krekeler said, lead chromate can become bioavailable as weathering, wear and tear from driving, and road treatments like road salts enhance dissolution and breakup. The composition of traffic paint varies widely, even within a city, said Michael O’Shea, an environmental geochemist at the University of Pennsylvania who presented research on yellow road paint at the GSA meeting. In Pennsylvania, more than 1.5 million gallons of paint are applied to roads each year, and information on the composition of the paints is not easily obtained, he said.
O’Shea and his colleagues took handheld vacuums out onto the streets of Philadelphia to collect road sediments from 30 sites across the city that had varied land uses (from heavily industrial to single-family housing) and varied amounts of traffic to get a representative sample of the city’s streets. They also collected samples of yellow paint. The team took the samples back to the lab, analyzed the elements in each, and then conducted a series of simulations on the yellow paint samples: They submerged the samples in simulated rainwater to test natural dissolution, simulated gut fluid to test ingestion effects, and simulated lung fluid to test inhalation effects.
The preliminary results, O’Shea said, showed dissolution of lead in gut fluid but not in lung fluid or rainwater, suggesting that the most dangerous pathway for interaction with lead chromate–bearing paint may be through ingestion. This makes children especially vulnerable to lead poisoning, he said: They may play outside, get dirt and sediment on their hands, eat a sandwich, and ingest this toxic substance.
O’Shea and his colleagues characterized the road sediment samples to learn their elemental composition, mineral phases present, organic content, and particle size distribution. “We wanted to see how mineralogic and chemical composition varied by site with road dust,” he said.
Other studies have shown that inhalation poses a larger risk, given the small size of many road dust particles, O’Shea noted. “The finer the particles, the more dangerous they can be,” he said. “And road dust can be a very mobile medium; it doesn’t just stick” in one place.
A complicating factor in these studies, O’Shea said, is that road dust itself is a very broad term. Nonetheless, “road dust can be a great resource to learn about our environment—it’s just one more tool in the toolbox.”
—Megan Sever (@MeganSever4), Science Writer