When Congress enacted the landmark Clean Air Act in 1970, the thinking was that aging, and dirtier, coal-fired power plants would be shuttered soon enough. Now, four decades later, the moment for the closings has arrived. On Thursday, FirstEnergy announced that it will send into retirement six power plants, four in Ohio, with an average age of 55 years. The Akron-based power company is doing so as part of complying with new federal rules for reducing emissions of mercury and other toxic pollutants.

No question, this move and similar steps elsewhere in the country will bring degrees of disruption and hardship. FirstEnergy noted that 529 workers will be affected, some eligible for retirement, others possibly moving within the company, many certain to lose their jobs. School districts and other public entities face reduced tax revenues.

What deserves emphasis is the reason behind the closings, the Clean Air Act placing the national priority on public health. Mercury is a neurotoxin, science revealing its capacity to cause developmental problems for children, not to mention contributing, along with other toxic pollutants, to heart disease, asthma, cancer and premature death. More, emissions fall to the ground, and carried by rain and snow flow into rivers and lakes, even attaching to the food chain.

For years, federal officials have had the task of curbing mercury pollution. George W. Bush attempted to implement rules, but they were found insufficient by the courts. In that way, the effort of the Obama White House comes as no surprise, the Environmental Protection Agency finally issuing rules that reflect the mandate of the law.

The cry quickly sounded about the excessive cost, especially in a fragile economy. What the critics push aside are the far greater benefits, long the outcome of such environmental legislation. The EPA calculates that the mercury rules will prevent 11,000 premature deaths and 4,700 heart attacks per year. They will reduce childhood asthma cases by 130,000 and result in fewer hospital visits and fewer missed days at school and work.

The agency projects that an estimated $11 billion in implementation costs will translate into $37 billion to $90 billion in benefits by 2016.

All told, FirstEnergy has invested more than $10 billion in environmental protection the past four decades. It now must route another $2 billion to $3 billion to comply with the mercury rules. These are no small sums. Yet, true, too, is that the power company has performed well, growing stronger in recent years, playing an important civic role.

Such perspective is useful. The mercury rules, long in the works, were coming sooner rather later. Now that they have arrived, FirstEnergy has moved smartly to make the transition work.