How far away is “safe”?
Perhaps that’s not the right question. Maybe the question should be, How far away from the danger does the risk level become acceptable? But what constitutes an acceptable risk is also hard to define. And when we’re talking about asbestos and people’s lives, it gets even more complicated because the effects of asbestos exposure are cumulative and can take decades to show up in the body.
Asbestos has been found naturally occurring in the rocks and dust just south of Las Vegas in Clark County, Nev. Clark County is one of the fastest-growing counties in the United States, and building and development are booming in this dry, dusty place. Every time the dust is disturbed, asbestos can be kicked up into the air, where it can then be inhaled, not only by nearby construction workers but also by children playing at a park, teens operating all-terrain vehicles (ATVs), outdoor enthusiasts riding mountain bikes or horses in the hills, and people leaving their windows open in their homes.
In 2011, University of Nevada, Las Vegas (UNLV) medical geologist Brenda Buck found an asbestos-like mineral known as palygorskite at Nellis Dunes Off-Highway Vehicle Recreation Area, about 15 minutes northeast of Las Vegas.
It turns out, Buck said, “there was evidence in the Nevada Cancer Registry data that people in southern Nevada were being exposed to asbestos minerals through an environmental mechanism, not an occupational one.”
Buck then partnered with UNLV geologist Rodney Metcalf, who had found asbestos-like minerals in the desert around Lake Mead just across the Nevada border in Arizona, and Brett McLaurin of Bloomsburg University of Pennsylvania. Together, they found that more than 1 million acres—20% of the surface area of Clark County—likely contain asbestos-like minerals. And that doesn’t even include eolian deposits, Metcalf said.
At the annual meeting of the Geological Society of America earlier this year, Buck, Metcalf, and McLaurin spoke about naturally occurring asbestos and increased urbanization in Clark County.
A Rocky Problem
“Asbestos” is not a mineralogical term but an industrial term used to describe several naturally occurring hydrated silicate minerals made up of long, thin fibers typically a micrometer or less in diameter.
The two primary families of asbestos minerals are the serpentines and the amphiboles. Serpentines are characterized by curly fibers and include chrysotile, the asbestos type most frequently used commercially in the United States. “Chrysotile is fibrous because it’s rolled up like a sheet of paper or a rug,” Metcalf said.
Asbestiform amphiboles are “more like string cheese, with strands pulling off.” Their fibers are long and thin, are easily respirable, and persist in the body for longer periods, making amphibole asbestos much more dangerous, he said.
Amphiboles are the types of fibrous minerals found in Nevada as well as at a Libby, Mont., Superfund site.
Libby is the site of high incidences of asbestos-related mortality and respiratory disease in vermiculite miners and nearby residents. This disease cluster has been attributed to fibrous amphibole particles within the vermiculite ore body that was mined and milled near the town from 1923 to 1990. Some of the amphiboles in Libby are similar or nearly identical to those found in Arizona and Nevada.
Prior to Buck and Metcalf’s work, the type of plutonic granites found in the Clark County region had not been known to contain asbestiform minerals, said Bradley Van Gosen of the U.S. Geological Survey, who has mapped naturally occurring asbestos locations throughout the lower 48 states.
Geologists had previously identified eight magnesium-rich rock types in specific, special geologic conditions that form asbestiform minerals, but “[Buck and Metcalf’s] model opened up other areas with potential,” Van Gosen said.
Prior to Buck and Metcalf’s research, most of what scientists knew about these types of rocks and fibers came from mining, and many geologists thought the hydrothermal alteration required to turn magnesium-bearing silicates into fibrous rocks occurred at the large “mine” scale. Now geologists know that the alteration occurs not at the mine scale but at the grain scale.
“It turns out,” Metcalf said, “any amphibole can be changed into asbestos” with hydrothermal alteration. Replacement of nonfibrous magnesium hornblende by fibrous winchite requires mineral dissolution-reprecipitation and exchange of ions between amphibole and fluid. Hydrothermal alteration of common hornblende-bearing plutonic rocks should be considered a potential source of nonoccupational exposure to amphibole asbestos, said the researchers.
A Possible Public Health Problem
Asbestiform minerals, wherever they are, become a health problem once they are disturbed. When contained in rock or in the vesicular soil horizon—the centimeters-deep layer of wind-deposited dust called desert “pavement” or “skin”—asbestiform minerals are not dangerous to anyone. But if they’re disturbed via construction, recreational activities, or even natural dust storms, they become dangerous. Breathing in the fibers is the primary way that people are exposed to asbestos, according to the Centers for Disease Control and Prevention (CDC).
Mesothelioma, a lung cancer, is the best known “indicator” disease for asbestos inhalation, said Jean Pfau, an immunologist at Montana State University. Most people who die from mesothelioma are elderly men who were exposed to asbestos from working with or near asbestiform material decades earlier.
According to a report by the Environmental Working Group (EWG) Action Fund, 2,000–3,000 Americans are diagnosed with mesothelioma each year, and 12,000–15,000 Americans die from asbestos-related diseases each year.
For the 2006–2010 period, Nevada had a global age-standardized incidence rate of 10 cases of malignant mesothelioma per million inhabitants each year. (In Clark County, about 55 people die each year from asbestos-related diseases, according to EWG.) This rate is similar to the mean U.S. rate. An analysis of all malignant mesothelioma deaths recorded in Nevada for the 1999–2010 period shows a constant mortality rate.
However, Nevada is not listed among the states having produced asbestos or showing a high occupational exposure. For this reason, the standardized incidence of 10 per million is unexpectedly high, compared with the background incidence of 5 per million in other states without known occupational exposure. In addition, Pfau said, many of the cases are in women and younger people.
Though mesothelioma is the proverbial canary in the coal mine to indicate asbestos exposure, Pfau said, it isn’t the only disease caused by asbestos fibers. Chrysotile exposure can lead to mesothelioma by suppressing the immune system, allowing cancer to grow. Amphibole asbestos, on the other hand, causes an elevated immune response, leading to systemic autoimmune diseases like lupus, rheumatoid arthritis, and scleroderma, as well as mesothelioma.
A Logical Plan
Some places with known asbestiform minerals have taken logical steps to mitigate risk.
The first step is knowing where the risk is. Van Gosen has added the southern Nevada locations to the national map of naturally occurring asbestos sites and is hoping someday to map terrains that may have naturally occurring asbestos as well. Using Buck and Metcalf’s research outlining the grain-scale pathways to forming asbestiform minerals, Van Gosen has identified possible sites of naturally occurring asbestos based on the presence of amphibole minerals and potential hydrothermal alteration.
The “ultimate goal,” Van Gosen said, is to map those areas.
In areas where asbestos fibers are found, officials can mitigate risks, Van Gosen said. In Fairfax County, Va., for example, a health department official recognized a “log of asbestos” unearthed during a building project. Scientists, engineers, and health officials outlined the rock formation and executed a plan to monitor the air during construction; to restrict dust, such as by keeping the site moist so the dust could not become airborne; and to provide protection for workers, such as coveralls and even respirators.
“That model was the best I’ve seen from a public planning perspective,” Van Gosen said.
Naturally occurring asbestos can be managed. Land managers can help reduce dust and exposure by, for example, not developing areas where the fibrous dust is rampant so they don’t disturb the fragile desert crust, Buck said, or by mitigating dust during construction.
An interesting case study in the management of naturally occurring asbestos, Metcalf added, is the one posed by the construction of huge solar arrays for power plants in the Mojave Desert.
Construction efforts are “tearing the desert to pieces,” Metcalf said, by kicking up tons of dust as they build the arrays and then as workers drive around between the arrays to maintain them. (Environmental impact statements compiled before the solar arrays’ construction predicted this disturbance to the desert environment.)
“Look, we all want those solar farms, if we’re concerned about climate change. Do you say ‘don’t make solar farms’ because you might kick up asbestos? No,” Metcalf said. “Just do it smarter” by, for example, paving the roads workers drive on and revegetating the land after the arrays are built.
And people can be smarter about their own risks to some degree as well, Buck said. Don’t stir up dust by riding horses or ATVs in areas that contain asbestos; clean your house with water as opposed to a dry dust rag; choose to do outdoor activities on less windy days, for example.
The Clark County Department of Air Quality (DAQ) monitors weather conditions for dust and issues dust warnings on days when high winds and dry conditions are expected, but the department does not monitor for asbestiform minerals because they are not one of the criteria pollutants identified by the Environmental Protection Agency for monitoring air quality. DAQ has no jurisdiction over naturally occurring asbestos, DAQ told Eos in an email exchange.
Buck, Metcalf, and Pfau all say their major goal in researching the amphiboles is public education.
“We’re trying to help people,” Pfau said.
The ironic thing is that scientists don’t even know how much the people of Clark County are being exposed to asbestiform minerals, Buck said. “We don’t know how serious of a problem this may be, because we don’t know the dose people are being exposed to or how reactive anyone should be: Is it a huge health threat or something less serious?”
It’s all about knowing and taking “acceptable risks,” Metcalf added.
Look, he said, your chances of being killed in a car crash are much higher than dying of mesothelioma. But when you get out of the car, your risk goes to zero. “With asbestos, once you’re exposed, you take that risk with you for life.”
—Megan Sever (@MeganSever4), Science Journalist