High levels of CO can be fatal, causing death within minutes. The concentration of CO, measured in parts per million (ppm) is a determining factor in the symptoms for an average, healthy adult. 50 ppm: No adverse effects with 8 hours of exposure. 200 ppm: Mild headache after 2-3 hours of exposure. 400 ppm: Headache and nausea after 1-2 hours of ...