Maryland is suing the Environmental Protection Agency, demanding it address air pollution that blows in from upwind states.
The action against President Donald Trump's administration would press the EPA to apply stricter pollution controls to dozens of power plants in Indiana, Kentucky, Ohio, Pennsylvania and West Virginia, something state officials argue is required to uphold the Clean Air Act.
The plants contribute much of the smog that fouls Maryland's air on hot summer days.
"Maryland has made significant progress in improving our air quality in recent years, and that progress is in jeopardy due to a lack of action by the EPA that dates back to the previous administration," Gov. Larry Hogan said in a statement announcing the lawsuit Wednesday morning.
Attorney General Brian Frosh said his office filed the complaint Wednesday afternoon in U.S. District Court for the District of Maryland.
"The law requires EPA to take action against the states and the power plants," he said. "I think we win this one."
The Maryland Department of the Environment estimates that 70 percent of the ozone pollution that often makes the Baltimore and Washington regions' air unhealthy to breathe blows in from outside the state, according to MDE Secretary Ben Grumbles. The lawsuit seeks to require 36 generating units at 19 plants in upwind states to install the same scrubbers and other air-cleaning technology that Maryland requires plants within its borders to install.
Ozone is a form of oxygen that is found naturally in upper levels of the atmosphere but acts as a pollutant when it forms close to the ground. It is created when industrial and vehicle emissions interact with compounds in the air in summer heat.
Such pollution can require "Code Orange" air-quality alerts — such as one that occurred across the Baltimore region Monday, and more than a dozen times this summer. The warning indicates pollution that can make it hard for children, the elderly and people with respiratory and heart conditions to breathe.
In July, state officials had warned the EPA that they planned to sue over the issue. The Clean Air Act requires that 60 days' notice be given ahead of citizen lawsuits.
Maryland's efforts to challenge the EPA on the matter go back nearly a year. Grumbles petitioned EPA to enforce what is known as the "good neighbor" provision of the air pollution law in November 2016. After the agency gave itself a six-month extension to respond to that petition, state officials argue, the federal agency was required to act by July 15 but did not, prompting the lawsuit.
Frosh said the "good neighbor" provision has been used successfully in court in the past and said he is confident the state will prevail in the case. While he acknowledged that the original petition dates to President Barack Obama's administration, he said he worries that "the EPA has changed its mission since a year ago."
EPA officials did not respond to a request for comment.
The Chesapeake Bay Foundation praised the lawsuit, and said it and partner groups are planning to file a similar action of their own in the coming weeks.
"We applaud and support Maryland in this effort to force EPA to act against out-of-state power plants whose emissions worsen Marylanders' health problems, and have a negative impact on Maryland waters," Jon Mueller, the foundation's vice president for litigation, said in a statement.
The lawsuit is one of many Frosh, a Democrat, has filed, joined or threatened in recent months to challenge the Republican Trump administration. Others have focused on concerns about chemical accidents, pesticide regulation, vehicle emissions, offshore drilling and energy efficiency.
It is a second example of Hogan, a Republican, challenging Trump in court. Earlier this month, he directed Frosh to sue the Federal Aviation Administration over new air traffic control patterns that have stirred complaints of unbearable noise from residents around Baltimore-Washington International Thurgood Marshall Airport.