Ancient Boiler Issues

[Jeremy Pace]

General Question:
Several of my rentals have boilers across the spectrum of quality and age.  My oldest boiler is in a 5 BR 2 BA house.  It's been producing a LOT of carbon monoxide, so much that I feel like it's a hazard to go into the basement to work on it.

Does anyone have any ideas about how I can 1) reduce the amount of carbon monoxide produced or 2) better mitigate?