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Environmental scientists sometimes forget that the dose determines the response. A well recognized toxic material can be desparately needed by living creatures at very low concentration. It may, in fact, be an essential nutrient.
|For instance: A remarkably small amount of selenium determines|
|the survival of daphnids.|
0.1 ppb permits reproduction amidst gross deformity. 1.0 ppb allows apparent health, but all die of premature old age. 2.0 ppb allows normal life IF sufficient Zn and Cu are available. 5 ppb assures their normal life. 60 ppb begins to limit fecundity.
|Adding organic anti-oxidants spares the damages due to selenium deprivation||Nickel improves life in old age|
|Zn is far more available via the water than via solid foods.|
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