How can carbon monoxide (CO) enter the cockpit?

Carbon monoxide (CO) can enter the cockpit primarily through the cabin heater and defrost vents, which is a significant safety concern in aviation. This gas can be produced by incomplete combustion of fuels in engines or heating systems. When the aircraft's heating system is used, it can inadvertently draw in exhaust gases that contain CO, especially if there are leaks or issues with the exhaust system. This is particularly dangerous because CO is colorless and odorless, making it difficult for pilots and passengers to detect it without specialized equipment.

In contrast, the other options are less relevant to the way CO typically infiltrates the cockpit. While external environmental factors can introduce CO, such as pollution from other aircraft or ground vehicles, they are not the primary means of entry into the cockpit during flight. The aircraft's fuel system does not directly allow CO into the cockpit as the fuel is not inherently a source of CO unless combusted. Lastly, cabin windows are designed to prevent airflow into the cockpit in a way that would allow CO to enter, as they are sealed for pressurization and to maintain cabin climate. Thus, the heating and ventilation system is the most direct route for CO to pose a risk inside the cockpit.