WASHINGTON (AP) — An appeals court is siding with environmental groups that had challenged Environmental Protection Agency regulations on soot as too weak.

The three-judge panel ruled Friday that the EPA regulated soot of a certain size under weaker cleanup requirements than it should have.

The environmental groups, including the Natural Resources Defense Council, had challenged two rules dating back to the George W. Bush administration. The court sent the rules back to the EPA with instructions to strengthen them.

Soot, or fine particulate matter, is microscopic pollution released from smokestacks, diesel trucks and other sources. Breathing it can cause lung and heart problems, contributing to heart attacks, strokes and asthma attacks.

Two of the three judges were appointed by Republican presidents, the third by a Democrat.