Sulfur hexafluoride (SF6) is a synthetic chemical that plays a critical role as an electrical insulator in high-voltage equipment. The gas is odorless, invisible, non-toxic and non-flammable. But it is also a climate killer, 24,300 times more effective at warming the planet than carbon dioxide on a pound-for-pound basis. Once released, sulfur hexafluoride remains in the atmosphere for 1,000 years. Emissions come primarily from the electric power sector, where the gas is used in circuit breakers and other high-voltage equipment to prevent electricity from arcing. Nearly indestructible, SF6 serves as a critical backstop for any modern electric grid, ensuring that electricity flows only when and where it is needed. However, when electrical equipment in China reaches the end of its useful life, much of the gas is likely vented into the atmosphere.