FILE - In this Sept. 11, 2001 file photo, firefighters work beneath the destroyed mullions, the vertical struts which once faced the outer walls of the World Trade Center towers, after a terrorist attack on the twin towers in New York. New research released on Friday, Sept. 6, 2019 suggests firefighters who arrived early or spent more time at the World Trade Center site after the 9/11 attacks seem to have a greater risk of developing heart problems than those who came later and stayed less. (AP Photo/Mark Lennihan)