Carbon monoxide poses a danger if a vehicle is:

Carbon monoxide is a colorless, odorless gas that can be extremely dangerous, particularly in enclosed spaces. When a vehicle is running, it produces exhaust that contains carbon monoxide. If this occurs in an area that is not properly ventilated, the concentration of carbon monoxide can build up quickly, creating a serious health risk for anyone present.

In a situation where the vehicle is running and the shop lacks ventilation, the toxic gas has no means of dissipating, leading to potentially hazardous levels in the air. Symptoms of carbon monoxide poisoning can include headaches, dizziness, confusion, and even loss of consciousness, highlighting the importance of ensuring adequate airflow in any environment where vehicles are operating.

In contrast, when a vehicle is not running, it does not produce exhaust fumes, dramatically reducing the risk of carbon monoxide exposure, regardless of ventilation. This is why the statement that a running vehicle in a non-vented area is the most dangerous scenario is the correct one to understand.