Study of urban roadside dirt reveals potentially toxic mixture of metals
 truck idles at a stoplight; when the light turns
green, it roars away in a cloud of dirt and exhaust. For many years,
researchers have studied tailpipe emissions as they correlate to human
exposure and adverse health effects. Recently, however, scientists at
UW-Madison learned that there’s more to that resuspended roadway
dirt than meets the eye: What looks like ordinary dirt actually is a
potentially toxic mixture of non-tailpipe vehicle emissions, including
microscopic metal particles from brake and tire wear.
Epidemiologists link roadway emissions to a higher
incidence of chronic conditions like asthma or lung disease, particularly
in people living near roadways. “We know that metals can be important
in toxicity and adverse health,” says Associate Professor James
Schauer.
Schauer, former doctoral student Glynis Lough and
other researchers studied the composition of resuspended road dust.
Their results showed that, in addition to platinum-group metals from
catylytic converters, high levels of metals in resuspended road dust
come from brake and tire wear, and fragments of the lead weights used
to balance tires.
Published in a report available through the Health
Effects Institute, the group’s findings might help epidemiologists
correlate adverse health effects with exposure to specific pollutants—and
specific levels of pollutants.
Traditionally, researchers and epidemiologists have
focused on tailpipe emissions as key to health concerns, says Schauer.
“The broader picture of this is that these metals need to be thought
of in the context of adverse health effects from roadways,” he
says. “It doesn’t necessarily mean the metals are a part
of that, but it gives us a basis that would need to be investigated.”
Schauer’s group conducted much of its study
in the Milwaukee area. “To get representative samples was not
a trivial thing,” he says.
To measure pollutants at the entrances and exits
of Milwaukee’s Howell Street and Kilborn tunnels, the researchers
set up air samplers and collected hundreds of filters, which they analyzed
using unique techniques developed at UW-Madison for measuring very low
metal levels. They counted the number of cars that passed through each
tunnel and tracked their speed. They calculated the air flow through
each tunnel, which helped them determine the difference in emissions
at the entrances versus the exits.
The researchers “vacuumed” the roadways,
resuspended the dirt samples in the lab, collected them on filters and
analyzed them. Every sixth day for a year, they collected ambient air
samples in Milwaukee and Waukesha, Wisconsin, and in Denver, Colorado,
to contrast and compare metal concentrations they observed in the air
with the metals they found on the roadway.
In addition, to identify non-tailpipe auto emissions,
they ran vehicle tests in a sealed room on a California Air Resources
Board running-loss-shed dynamometer. “Historically, this has been
used to look at gasoline vapors leaking out of a car, but we actually
looked at how the brake dust and tire wear came out and built up in
this room,” says Schauer. “So with all of these profiles,
we get ‘fingerprints’ for each source, and then we can reconstruct
where the emissions come from.”
His group also developed a synthetic fluid similar
to human lung fluid, then leached metals off samples of particulate
matter to see what fraction of metals was soluable in the fluid. “A
high percentage of the metals that are in these particles actually are
liquid-soluable or soluable in this surrogate lung fluid, which again
could be important to the health community to begin to understand what’s
important,” says Schauer.
Resuspended road dust generally falls into what researchers
classify as the “coarse” particle category—meaning
that it is larger than 2.5 microns, or 40 times smaller than the diameter
of a human hair. Recently, the U.S. Environmental Protection Agency
(EPA) proposed new regulatory standards for urban particles that fall
into this category, says Schauer. “Our data would suggest that
the reason we see adverse health effects is because of all of these
metals that are present in these materials—which makes it different
from rural dirt,” he says.
Auto-industry decision-makers could use his group’s
findings to devise ways to reduce the environmental impacts of their
products, while EPA officials can draw on the knowledge to assess current
or establish new emissions regulations. “It’s very difficult
to say what are healthy levels,” says Schauer.
He and other researchers recently began similar studies
in large cities, including Los Angeles. They are collaborating with
epidemiologists to try to correlate metal
concentrations with observed health effects.
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