Damage to the exhaust system a danger for which reason?

A broken exhaust system can let poison fumes into the cab or sleeper berth, look for:
• Loose, broken, or missing exhaust pipes, mufflers, tailpipes, or vertical stacks.
• Loose, broken, or missing mounting brackets, clamps, bolts, or nuts.
• Exhaust system parts rubbing against fuel system parts, tires, or other moving parts of the vehicle.
• Exhaust system parts that are leaking. . [California Commercial Driver Handbook]