In April, the agency proposed not changing the standards for particle pollution. It announced on Monday it also doesn't plan to tighten the standards for ozone -- commonly known as smog.

With sunshine and warmer temperatures, ground-level ozone values are expected to trend upward beginning Monday afternoon.

Maryland environmental officials filed a petition Wednesday asking the EPA to crack down on 19 coal plants in Indiana, Kentucky, Ohio, Pennsylvania and West Virginia.

A lawsuit could add Indiana to the Ozone Transport Region, a group of 11 states required by the EPA to reduce smog-causing pollutants.

They want a federal judge to force the EPA to add nine more states, including Indiana, to the group required by law to act together to reduce smog in the Northeast.

The air quality may become unhealthy for individuals, especially active children, the elderly, and anyone suffering from lung disease or other serious health problems.