Which source accounts for the largest share of human-caused mercury emissions in the United States?

• A. Vehicles
• B. Industrial solvents
• C. Natural sources
• D. Coal-burning power plants

Answer: (D)

Mercury released to the atmosphere from human activities comes primarily from burning coal for electricity and heat. Coal contains mercury, and when it is burned, mercury vapor is emitted in stack gases. The enormous scale of coal-fired power production means this source contributes more mercury to the air than any other single human-caused source. Emission controls have reduced releases over time, but no other activity releases as much mercury overall. Other options—vehicles, industrial solvents—do emit mercury, but their total contributions are much smaller. Natural sources exist, but they are not human-caused. Understanding this helps explain why mercury ends up in waterways and bioaccumulates in fish, affecting human exposure through diet.