Carbon monoxide poisoning

Carbon monoxide poisoning happens from breathing in carbon monoxide (CO) at large levels. 
Symptoms are often described as "flu-like" and commonly include headache, dizziness, weakness, vomiting, chest pain, and confusion. Large exposures can result in loss of consciousness, arrhythmias, seizures, or death.
Carbon monoxide poisoning can be accidental, like if the oven's gas is left on by accident or a gas leak. However, it can often be used as a suicide method such as keeping the car on while in a closed garage.