OK, I'm going to give some of the physiological background on CO poisoning. 

First, a disclosure: I'm not a doctor.  This is not medical advice. This is just a description of why "low" CO levels can be fatal over time.

When you're breathing, you're taking in O2 and giving off CO2. The CO2 is a result of cellular respiration in your body.  The CO2 comes off the hemoglobin in your lungs, and O2 attaches, and then the oxygenated blood is carried to the right side of your heart, where it's pumped over to the left side of your heart and then outwards to the rest of your body. It's really quite a elegant system.

When you breath in carbon monoxide, it binds to the hemoglobin in your erythrocytes (aka "red blood cells"). The problem is, CO binds to the heme group in the erythrocyte more tightly than O2 will/does - CO has an affinity for the hemoglobin at something like 200 times what O2 has. There's a limit to how quickly the CO can be displaced off the hemoglobin, and that limit is affected by the fact that room air is only 21% oxygen. The blood continues to circulate, but the erythrocytes that are saturated with CO are now effectively useless for taking CO2 out of your tissues and they're also useless at transporting O2 back to your tissues. In effect, your erythrocytes have become non-operative as far as tissue perfusion is concerned.

Further, CO attached to the heme group makes the erythrocytes undergo cellular death at an accelerated rate. Your body produces more erythrocytes from the marrow in your large bones (femur, etc), but at a rate of perhaps 1% of your red blood cell count per day. In other words, it takes quite a while to re-generate your erythrocytes compared to how fast they're being bound up and dying as a result of CO exposure.

So even in small(er) concentrations of CO (eg, 50ppm) with extended exposure, the situation can be severe or possibly fatal.The key issue is the length of exposure. If you're consistently exposed for a long time (eg, eight or more hours), this becomes more of an issue for people who already have issues with their oxygenation, eg, COPD, emphysema, etc.


Two tips I can give from EMS training about CO poisoning:

1. If you put a pulse oximeter on someone with CO poisoning, you will typically see that the person is "fully saturated" with O2. This is a bogus reading. PulseOx meters/widgets work by projecting a beam of red light from a LED into your tissues and seeing the level of color that is transmitted through the tissue (eg, a finger, toe or earlobe). When your blood has been affected by CO binding, your blood is a brilliant red color - almost too bright a red. PulseOx meters look at how red your blood is with CO and say "Great news! Your blood is 100% saturated with O2!"

Wrong. Oh, so very wrong. This is why you have to take medical instruments as only ONE indication of what is going on. Don't put your complete faith in instruments - you must use the signs and symptoms you see right in front of you as well. Changes in mentation (thought processes), irrational behavior, stumbling, slurred speech, splitting headache, etc - are signs the brain is being deprived of either O2 or glucose. The problem in less-than-fatal CO poisoning is that the symptoms and signs resemble flu-like symptoms. The symptoms could also be interpreted as hypoglycemia in people who have diabetes and who have taken on too much insulin for their level of glucose. The brain is a pig: it needs huge amounts of O2 and glucose (the brain uses something like 40% of the glucose in your body... because the brain can use almost nothing but glucose for energy). Deprive the brain of O2 or glucose, and you get people acting with decreased mentation.

Bright pink tissues, or unnaturally bright red blood is a sign that confirms a lack of O2 due to CO.

EMT's/Medics will likely evacuate the patient from the building,  put the patient on 100% O2 in the field, and assess how they respond. 

2. In more severe cases of CO poisoning, you will find the opposite of cyanosis. When tissues are not being perfused properly because someone isn't getting enough oxygen, they will turn "blue." You'll see this as the nail beds at the ends of fingers turning blue-ish, the inside of the lower lip turning blue-ish. Well, when tissues are being perfused with CO-contaminated blood, they turn a bright pink. If you look at the inside of someone's lower lip and it is an un-naturally bright pink, there you go, there's a field indication of CO poisoning. A severe case of CO poisoning leads to a loss of consciousness.

A severe CO poisoning case must get to the ER, fast. EMTs/Medics will put the patient on 100% O2 at high flows and transport. The ER will have to pull an arterial blood gas draw (where they stick a needle into your radial artery in your wrist, typically, and draw some blood from an artery instead of a vein) and send it to the ER lab to see how bad the CO poisoning is. 

People who are found in time, but who have severe CO poisoning, often have to go into hyperbaric chambers on high-pressure 100% O2. Because their blood has been compromised with CO and the erythrocytes are now going to die and have to be replaced, there are only two solutions to keep the patient alive:

- change out their blood with new blood (which an ER probably won't do)
- put them into into a hyperbaric chamber, which does the same thing with forcing O2 into your blood as deep diving does with N2: It forces the gas into the plasma, and bypasses the hemoglobin as the carrier of O2. Literally, what is happening is that the high-pressure O2 environment is putting oxygen into the patient's blood plasma the way carbonation puts CO2 into soda pop. NB what I just said there: The O2 is being dissolved into the plasma - not being carried in the erythrocytes. 

Patients will have to spend days (or more) in the hyperbaric chamber while their body makes new erythrocytes to replace those that will die off from CO poisoning.

One last note: A level of CO poisoning that an adult female can withstand and recover from promptly from may be a level that would kill a fetus. The sensitivity of a fetus to CO is significantly higher than the mother's sensitivity.