Several groups are challenging an air monitoring plan, saying it doesn't do enough to address the health of Southern California residents living near freeways.
The Natural Resources Defense Council and others filed a lawsuit Tuesday against the Environmental Protection Agency, saying it violated the Clean Air Act by approving the plan by regional air regulators last year.
The groups want sensors placed within 300 yards of freeways to track pollution from cars and trucks.
Studies have suggested that people who live or work near major roads have an increased risk of health problems.
The EPA did not immediately comment.