Harmful gas can refer to two different gases: MIASMA (6 letters) and RADON (5 letters). MIASMA is a type of air pollution that is caused by decaying organic matter, such as sewage or rotting vegetation. It is known to cause a variety of illnesses, including cholera and malaria. RADON is a naturally occurring radioactive gas that is found in soil and rocks. It is odorless and colorless, and can be harmful when it accumulates in enclosed spaces, such as homes and buildings. Long-term exposure to RADON can lead to lung cancer.