The U.S. Environmental Protection Agency plans to regulate the largest remaining source of uncontrolled toxic air pollution in the United States: coal- and oil-fired power plant emissions. The new rules — 20 years in the making — will affect 1300 power plants, half of which lack modern air-pollution controls.

Most power plants affected by the rules were built in the 1970s, before clean-air technologies were developed that capture, or “scrub,” smokestack pollutants, notably mercury from burning coal, before they are released into the air.

Mercury is a neurotoxin that can cause severe birth defects and impair cognitive and fine motor skill development in children.