title: Temporal variability of tungsten and cobalt in Fallon, Nevada.
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 approximately 15 years. Other metals do not show trends through time in Fallon.
  
  
  CONCLUSIONS
  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.
  
  
  CONCLUSIONS
  Continued biomedical research is warranted to directly test for linkage between childhood leukemia and tungsten and cobalt.
authors.affiliation: Laboratory of Tree-Ring Research, University of Arizona, Tucson, Arizona 85721, USA. sheppard@ltrr.arizona.edu
authors.forename: Paul R
authors.initials: PR
authors.lastname: Sheppard
authors.forename: Robert J
authors.initials: RJ
authors.lastname: Speakman
authors.forename: Gary
authors.initials: G
authors.lastname: Ridenour
authors.forename: Mark L
authors.initials: ML
authors.lastname: Witten
mesh_terms: D002648:Child; D016000:Cluster Analysis; D003035:Cobalt / Q000032:analysis / Q000633:toxicity*; D004781:Environmental Exposure; D004784:Environmental Monitoring / Q000379:methods*; D004785:Environmental Pollutants / Q000032:analysis / Q000633:toxicity*; D062665:Epidemiological Monitoring; D006801:Humans; D007938:Leukemia / Q000139:chemically induced / Q000453:epidemiology*; D009505:Nevada / Q000453:epidemiology; D018547:Plant Stems / Q000737:chemistry; D032107:Populus / Q000737:chemistry; D013997:Time Factors; D014414:Tungsten / Q000032:analysis / Q000633:toxicity*
chemical_list: D004785:Environmental Pollutants; D003035:Cobalt; D014414:Tungsten