Tuesday 10 February 2015

The dangers of naturally occurring asbestos in southern Nevada.

Asbestos refers to a number of forms of naturally occurring fibrous amphibole minerals known to be highly carcinogenic, and also linked to a number of other health problems, including cardiovascular illnesses and immune system deficiencies. Exact definitions of asbestos vary, as they were originally intended to classify commercially useful asbestos minerals before the health risks were understood, which has caused problems for US regulators, as some minerals are classed as asbestos and subject to strict regulations, whereas others have been called ‘asbestos-like minerals’, which can potentially escape legislation. In one famous case in Montana lawyers representing a mining company tried for some years to claim that the company could not be held responsible for cases of asbestosis in the local community as the minerals involved were not technically asbestos, even though a direct link between dust from the mine and the health problems had been established.

In a paper published in the Soil Science Society of America Journal on 14 October 2013, Brenda Buck of the Department of Geosciences at the University of Nevada-Los Vegas, Dirk Goossens of the Department of Geosciences at the University of Nevada-Los Vegas and the Department of Earth and Environmental Sciences at the University of Leuven, Rodney Metcalf, also of the Department of Geosciences at the University of Nevada-Los Vegas, Brett McLaurin of the Department of Environmental, Geographical and Geological Sciences at Bloomsburg Univsity of Pennsylvania, and Minghua Ren and Frederick Freudenberger, again of the Department of Geosciences at the University of Nevada-Los Vegas describe the first discovery of naturally occurring asbestos minerals in Clark County in southern Nevada.

While such minerals have not previously been reported from southern Nevada, they are known from Mohave County in Arizona, which shares a long border with Clark County, where they are formed by hydrothermal alteration of minerals in Miocene granite intrusions. The Mohave granite outcrops which produce these minerals are located within the unpopulated Lake Mead National Recreation Area, and are not thought to present any immediate hazard to public health. However similar granite outcrops also exist in Clark County, and some of these outcrops are close to the population centres of Boulder City, Henderson and Los Vegas.

Buck et al. collected rock samples from seventeen rock outcrops in Boulder City, Black Hill and the McCullough Range, they also collected soil samples from desert surfaces and dirt roads, as well as samples of airborne sediments captured in traps at several locations, and clothing and a vehicle mounted spare tyre from an outdoor recreation centre in Boulder City.

All of the samples contained asbestos particles, and all of the particles found away from the granite outcrops were sufficiently similar to minerals collected at these sites that these sites are considered to be the sources of the minerals.

Scanning electron microscope (SEM)/field-emission scanning electron microscope (FESEM) images of minerals from the Clark County study: (a) amphibole fiber from soil in an alluvial fan along the western edge of Black Hill; (b) amphibole fiber bundles from rock at Martha P. King Elementary School, Boulder City; (c) FE-SEM image of curling fibers from rock at Martha P. King Elementary School; (d) amphibole fiber bundle from rock at Martha P. King Elementary School. Buck et al. (2013).

The asbestos fibres collected from Clark County appear consistent with fibres collected from other locations known to be carcinogenic, and are sufficiently airborne to be inhaled easily. Clark County had a population of over 2 million at the time of the 2012 census, mostly within the Las Vegas–Henderson–North Las Vegas conurbation. Populations in Boulder City and eastern Los Vegas are deemed to be at risk of exposure to wind-blown asbestos dust.

Landsat image showing the study area in southern Nevada. Granitic plutons are shown in red. Estimated areas of alluvium deposited from erosion of those plutons are shown in yellow. All sampling locations are shown as white circles and contained fibrous amphiboles. Buck et al. (2013).

The areas of highest risk of wind erosion are considered to be to the south and southwest of Boulder City, particularly the Eldorado Dry Lake and the sandy surfaces east of Highway 95. This area, while largely unoccupied, as used for a variety of human activities, and is crossed by a number of dirt roads. Winds in the area tend to blow from the south and southwest in spring and summer, and from the north and northeast in winter and autumn. Winds from the southeast are rarer, but do occur, blowing dust towards the more densely populated areas of Henderson and Los Vegas.

Dust storm entering Henderson–Las Vegas from the south; the McCullough Range is obscured by dust. Buck et al. (2013).

Boulder City is considered to be at risk year round, as it is bounded by asbestos-producing outcrops to the north and east, and by areas of asbestos containing sediment which are easily activated by the wind to the south and west. As such Buck et al. recommend that urgent action is taken to better assess the health risks presented by wind-blown asbestos dust within the city.

Dust generated by a single off-road vehicle driving on a dirt road in the McCullough Range. Buck et al. (2013).

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