Former New York City Mayor Rudy Giuliani's respiratory illness was exacerbated by his exposure to toxins and dust from the terrorist attacks on the World Trade Center on Sept. 11, 2001, his spokesperson said. 

Giuliani, 81, remains in critical but stable condition, the Associated Press reported, after suffering a bout of pneumonia. He's being monitored as a precautionary measure. 

He began coughing on his nightly online talk show on Friday and told his viewers that he wasn't feeling well. He was taken to the hospital near Palm Beach, Florida, sometime after the show aired. The condition he developed after the terrorist attacks, his spokesperson said, complicates respiratory illnesses, allowing viruses to quickly overwhelm him. 

He required mechanical ventilation to maintain adequate oxygen, but he was breathing on his own on Monday.