Why is carbon monoxide so harmful?

Because if you have a lung condition it can make it worse. When I was 15 I had a lung biopsy and I was breathing thru my mouth, the nurses came in and said that I had to start breathing thru my nose as breathing thru your mouth brings in more carbon monoxide, you can get carbon monoxide posioning from faulty air conditioners as well which can lead to legionnaires (sp?) disease. Also when people kill themselves in the cars the fumes from the gas are carbon monoxide so that's why it kills them as well.