Why is Pop unable to work in the novel, Towers Falling?

Pop is a 9/11 survivor who worked in the North Tower as a security guard. He helped get an elderly woman out of the tower and to an ambulance just as the tower crumbled behind them. The trauma scars him mentally and emotionally, in addition to leaving him with a respiratory illness from all the ash and concrete dust he inhaled that day. He is unable to work because of these medical issues.