Comment on ‘‘Elevated tungsten and cobalt in airborne particulates in Fallon, Nevada: Possible implications for the childhood leukemia cluster’’ by

Sheppard et al. (2006) measured W and Co loading in air samples from Fallon and other neighboring towns in northern Nevada. They observed that W and Co loading was higher in Fallon, the location of a childhood leukemia cluster, than in the neighboring towns, and that W and Co loading decreased with distance from a hard-metal processing facility in Fallon. They concluded the temporal covariance of W and Co indicated a single source for the two, and that the heavy-metal processing facility was the likely source because no specific deposits of W and Co are known near Fallon. They hypothesized that emissions from the facility could be causally linked to the leukemia cluster because Co and W-carbide together are probably carcinogenic. It is this author’s opinion that the article contained a significant error in how data were interpreted, at least two alternative hypotheses to explain their results were not evaluated, and the authors did not collect essential data necessary to test their hypothesis. The fundamental error in interpretation made by Sheppard et al. (2006) is that comparisons are