U.S. EPA air chief Gina McCarthy has a thick Boston accent, a shock of cropped white hair and a penchant for a good fight. "I cannot shy away from controversy," she told a panel of EPA advisers recently. "I don't know if it's my Irish blood, but I love it. I love disagreements. I love the democratic process. If I'm in a room where everybody agrees, I start to nod off." That's lucky for McCarthy, 55, whose job as the nation's top air regulator has her in what may be the world's hottest spot: the center of a political free-for-all over climate regulation and other air pollution policies.