I recalled from my biophysics/biochemistry grad study that one needs to have a large consumption of heavy water to be detrimental. While I could not find that reference, I did find the one below.
Barbour, H. G. and J. Trace, "The pharmacological action of deuterium oxide: I. Toxicity and symptoms; metabolic rate; water exchanges," J Pharmacol Exp Ther, 58 (4): 460-482 (1936) <http://jpet.aspetjournals.org/cgi/reprint/58/4/460>
In the summary, the author mentions that one needs 40% or greater saturation to cause death in mice, if I read it in enough detail.
Richard
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Richard L. Bowman
Dept. of Physics
Bridgewater College, VA, USA