NATURALLY OCCURRING ASBESTOS IN SOILS, SOUTHERN NEVADA: INTERPRETATIONS FOR WIND DISTRIBUTION AND HUMAN EXPOSURE

Naturally occurring amphibole asbestos has been found in rock, soil, and dust in both urban and rural areas of southern Nevada and northwestern Arizona. In southern Nevada, although rates of malignant mesothelioma are similar to the mean USA rate, there are unusually high percentages of malignant mesothelioma cases in young individuals and women, compared to the USA as a whole. These data are strongly suggestive for environmentally-caused asbestos-related disease. The primary exposure route is through inhalation. The asbestos occurs in areas frequently disturbed by activities such as off-road driving, hiking, horseback riding, and construction, which create dust and cause fibers to become airborne. Rock, soil, dust and clothing were analyzed using scanning electron microscope and energy dispersive spectroscopy; additional rock samples were analyzed using wavelength dispersive electron probe microanalysis; additional soil samples were analyzed using polarizing light microscopy and transmission electron microscopy using the Fluidized Bed Asbestos Segregator preparation method. The asbestos is sourced from plutonic rocks with fibrous Ca-amphiboles occurring primarily in Nevada (actinolite, magnesiohornblende), and Na-amphiboles occurring primarily in northwestern Arizona (winchite, magnesioriebeckite, richterite). Wind erosion, transport, and deposition have resulted in soils containing mixed Ca- and Na-amphiboles. Erionite/offretite, which has not previously been reported in this area, was a common soil component found in 5 of 6 soil samples. The source of the erionite/offretite is currently unknown. Additionally, winds have transported the amphibole and erionite/offretite particles into the Nellis Dunes Recreation Area, located 35 km north of Boulder City that otherwise would not be geologically predicted to contain fibrous amphiboles. In Boulder City, wind directions are primarily bimodal N-NE and S-SW with the strongest winds in the spring coming from the S-SW. The arid climate in this part of the Mojave Desert greatly increases the potential for wind erosion and human exposures. These results suggest that the entire Las Vegas Basin has, at times, received mineral fibers through wind transport. The Las Vegas metropolitan area currently has a population of over 2 million people.