"My health is not good," is how he decides to start. "A lifetime of less-than-ideal living conditions has damaged my lungs beyond repair." He pauses briefly, flicking his gaze away from Venom before returning it for his next admission. "I'm breathing on borrowed time."

Hence the respirator. Venom strikes him as an intelligent man. He can no doubt put two and two together without Thane explicitly stating that he's dying.