But typically, a concentration of as little as 0.04% (400 parts per million) carbon monoxide in the air can be fatal.

Below is a chart to help elaborate on the toxicity levels of CO and its symptoms:
* 35 ppm (0.0035%) Headache and dizziness within six to eight hours of constant exposure
* 100 ppm (0.01%) Slight headache in two to three hours
* 200 ppm (0.02%) Slight headache within two to three hours
* 400 ppm (0.04%) Frontal headache within one to two hours
* 800 ppm (0.08%) Dizziness, nausea, and convulsions within 45 minutes. Insensible within two hours.
* 1,600 ppm (0.16%) Headache, dizziness, and nausea within 20 minutes. Death in less than two hours.
* 3,200 ppm (0.32%) Headache, dizziness and nausea in five to ten minutes. Death within 30 minutes.
* 6,400 ppm (0.64%) Headache and dizziness in one to two minutes. Death in less than 20 minutes.
* 12,800 ppm (1.28%) Unconsciousness after 2-3 breaths. Death in less than three minutes.

In addition, a recent report concludes that carbon monoxide exposure can lead to significant loss of lifespan after exposure due to damage to the heart muscle.