The Lower Flammable/Explosive limit of CO is 12 ppm which means that in terms of volume CO must be at least 12 ml in 1000 L (1 cubic meter) of air if CO has to burn in fire. Also, it must not exceed 75 ppm above which it will not burn in fire because of insufficient reaction with oxygen.
Kitchen CO detectors go off after 15ppm.
At 400 ppm, CO becomes life threatening after 3 hours of continuous exposure.
If the CO concentration is 12,800 ppm death can occur in 1-3 minutes. 12,800 ppm means CO is 12.8 litres in 1 cubic meter of air. This is very very high concentration and it's not easy to attain in daily life situations.
So we see that the combustion range of CO (12 - 75 ppm) is below the fatal level. Hence, in the presence of spark/fire CO will inflame before the lethal level is reached.
This is all chemistry and some arithmetic but the fact is that in practical circumstances the concentration is not easy to measure. Hence, how CO will affect and what it will do is a matter of chance.
CO is incorrectly touted as painless way to commit suicide. In reality, CO is often adulterated with other gases such as air, hydrogen, nitrogen or inert Argon and Helium. This mixture makes the body's physiological reaction painful, slow and unpredictable. Intense headache and vomiting are typical symptoms. It deprives the blood from carrying oxygen and the body goes into convulsions and muscular spasm as it fights to get oxygen.
Also, because it is a gas it is harder to control its flow and inhalation, hence higher chance to fail. Other factors such as surrounding temperature and pressure may affect its lethality. Moreover, it can harm other people nearby, like other family members or potential rescuers.
Problems are far worse in case the suicidal person survives. Failure to die will result in chronic cardiac problems, long-term brain damage, issues with memory and low functioning mental processes.
Consider your options wisely. Seek help regarding your issues. Best wishes. *hugs*