A federal health official’s ruling has cleared the way for 50 different types of cancer to be added to the list of sicknesses covered by a $4.3 billion fund set up to compensate and treat people exposed to the toxic smoke, dust and fumes in the months after the Sept. 11, 2001, terrorist attacks. ... It will allow not only rescue workers but also volunteers, residents, schoolchildren and passers-by to apply for compensation and treatment for cancers developed in the aftermath of the attacks.