Earthjustice has sued EPA to force it to comply with the Clean Air Act, and review and act on state plans to reducing Particulate Matter. Their clients are the Sierra Club and Physicians for Social Responsibility – Los Angeles.
For an excellent overview, I recommend the blog post The Audacity To Breathe by Adrian Martinez, staff attorney for Earthjustice and MFN member.
For more information:
Audio Interview with Adrian Martinez (linked), KPCC
The court filing, Earthjustice