The term 'Dangerous Element' can be summarized by two crossword solutions: RADON (5 letters) and Plutonium (9 letters). RADON is a naturally occurring radioactive gas that is colorless, odorless, and tasteless. It is produced from the decay of uranium and thorium in soil, rock, and water. Plutonium is a synthetic element created in nuclear reactors and is one of the most toxic substances known to man. It is used in nuclear weapons and nuclear power plants. Both RADON and Plutonium are considered dangerous elements due to their radioactivity and toxicity.