Even slightly more ingestion of selenium than is required, more than the recommended 50-150 micrograms a day, can have side effects. Unfortunately, it is unclear at what level it causes toxicity. Through a process called bioaccumulation, it becomes more concentrated the higher it is ingested on the food chain. This factor has resulted in a ban on eating certain species of diving water birds that have been exposed to high levels of selenium. The seriousness of the effects depends on how much extra and how often selenium excesses are eaten to the point of being life-threatening.
Selenium is five times more poisonous than arsenic.
Selenium poisoning at steady, low levels deforms nails, produces brittle hair, leads to dental cavities and inflammation of the skin. Respiratory distress may result from inhalation or oral ingestion. In extreme cases liver damage and loss of feeling and control of arms and legs may result. Contrarily, not enough selenium in one's diet may increase the risk of cancer. Selenium has been shown to slow down the advancement and development of cancer.