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Paul R. Sheppard,¹ Robert J. Speakman,² Gary Ridenour,³ and Mark L. Witten⁴

¹Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona, USA; ²Museum Conservation Institute, Smithsonian Institution, Suitland, Maryland, USA; ³Internal Medicine, Fallon, Nevada, USA; ⁴Department of Pediatrics, University of Arizona, Tucson, Arizona, USA

Abstract
Background: Since 1997, Fallon, Nevada, has experienced a cluster of childhood leukemia that has been declared "one of the most unique clusters of childhood cancer ever reported." Multiple environmental studies have shown airborne tungsten and cobalt to be elevated within Fallon, but the question remains: Have these metals changed through time in correspondence with the onset of the leukemia cluster?

Methods: We used dendrochemistry, the study of element concentrations through time in tree rings, in Fallon to assess temporal variability of airborne tungsten and cobalt since the late 1980s. The techniques used in Fallon were also tested in a different town (Sweet Home, OR) that has airborne tungsten from a known source.

Results: The Sweet Home test case confirms the accuracy of dendrochemistry for showing temporal variability of environmental tungsten. Given that dendrochemistry works for tungsten, tree-ring chemistry shows that tungsten increased in Fallon relative to nearby comparison towns beginning by the mid-1990s, slightly before the onset of the cluster, and cobalt has been high throughout the last ~ 15 years. Other metals do not show trends through time in Fallon.

Discussion: Results in Fallon suggest a temporal correspondence between the onset of excessive childhood leukemia and elevated levels of tungsten and cobalt. Although environmental data alone cannot directly link childhood leukemia with exposure to metals, research by others has shown that combined exposure to tungsten and cobalt can be carcinogenic to humans.

Conclusion: Continued biomedical research is warranted to directly test for linkage between childhood leukemia and tungsten and cobalt.

Key words: childhood leukemia, cobalt, dendrochemistry, Fallon, Nevada, tungsten. Environ Health Perspect 115: 715–719 (2007) . [Online 20 February 2007]

Address correspondence to P.R. Sheppard 105 W. Stadium, University of Arizona, Tucson, AZ 85721 USA. Telephone: (520) 621-6474. Fax: (520) 621-8229. E-mail: sheppard@ltrr.arizona.edu

A. Clemens, C. Welch, and C. Fastje assisted this research. ICP-MS measurements of tree-ring samples were conducted at the University of Missouri Research Reactor Center.

This project was funded in part by the Gerber Foundation, the Cancer Research and Prevention Foundation, and a grant from the U.S. Environmental Protection Agency (AM-96963201 Fallon Leukemia Studies) . These funding agencies are not otherwise responsible for any content of this paper.

Although the information in this document has been funded in part by the U.S. Environmental Protection Agency to M.L.W. and P.R.S., it does not necessarily reflect the views of the agency and no official endorsement should be inferred.

The authors declare they have no competing financial interests.

Received 22 June 2006 ; accepted 20 February 2007.