Take a flooded building in steam New Orleans, and within days dark mold blooms on every surface, bringing the stench of decay to much of the hurricane-hit city. Authorities encourage residents to wear masks and protective clothing when clearing their homes, especially during dusty work like removing drywall. They say the risk depends on the person\’s overall health and exposure to mold. But an environmental group, alarmed by reading that show mold spores at extraordinarily high levels, said that approach is not enough.