It can cause sudden loss of consciousness which seems like it could have been the case here.
Carbon Monoxide Poisoning Basics
From the attached article:
Symptoms
The most common symptoms of CO poisoning are headache, dizziness, weakness, upset stomach, vomiting, chest pain, and confusion. CO symptoms are often described as “flu-like.”
If you breathe in a lot of CO, it can make you pass out or kill you. People who are sleeping or drunk can die from CO poisoning before they have symptoms.
Risk factors
Everyone is at risk for CO poisoning. Infants, the elderly, and people with chronic heart disease, anemia, or breathing problems are more likely to get sick from CO.
Each year, more than 400 Americans die from unintentional CO poisoning not linked to fires, more than 100,000 visit the emergency room, and more than 14,000 are hospitalized.