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As early as 1989, there was evidence of a global decline of amphibian populations and numerous findings of frogs with deformities throughout several states and Canada. This trend led to a collaborative research effort to determine the cause of frog deformities by NIEHS, the EPA, University of Minnesota, Minnesota Pollution Control Agency, the National Wildlife Health Center, and the U.S. Geological Survey. Possible explanations for the deformed or missing limbs include: chemical contamination of water, increased ultraviolet radiation from ozone depletion, or parasitic infestation- or a combination of the three. One NIEHS study found high numbers of deformities occurring in lab specimens when exposed to Minnesota water where deformed frogs have been found. As of September 1997, the cause of the deformities and any potential impacts to human health, such as birth defects, remain unclear.