The term 'POISONOUS GAS' can be summarized by two crossword solutions: RADON (5 letters) and ARSINE (6 letters). RADON is a naturally occurring radioactive gas that is colorless, odorless, and tasteless. It is found in soil and rocks and can enter buildings through cracks in the foundation or other openings. Arsine is an extremely toxic gas that is colorless and has a garlic-like odor. It is produced from the reaction of arsenic compounds with acids and is used in the semiconductor industry.