“But is it clean?” asked Elizabeth Berger, a 41-year-old mother of two. It was five months after the terrorist attacks on September 11, 2001, and Berger was testifying at a Senate field hearing on air quality in Lower Manhattan. Just blocks away, at Ground Zero, responders were still sifting through what they called “the pile”—eight stories of twisted steel and debris, spread over 16 smoldering acres—searching for bodies, bone fragments, even scraps of clothing. Outside, the air carried the acrid smell of burnt metal. Berger had lived 150 yards from the World Trade Center for nearly two decades. The complex was where her kids went roller skating and learned to ride their bikes. After the Twin Towers collapsed, increased levels of air pollution and particulates showed up across a wide area of New York and New Jersey. Lower Manhattan was covered in toxic dust. It had taken eight men in white suits and respirators five days to clean Berger’s apartment. She and her husband had thrown out trash bags full of toys and stuffed animals; …