"Air is drawn out of the compression section of the engine and cooled. It then enters the cabin, where it mixes with recirculated air that has passed through filters designed to remove bacteria and viruses. These “recirculated air” filters do not remove any fumes or vapours from the engine. So if engine oil or hydraulic fuel leaks, because of poorly designed or faulty seals, or even over-filled tanks, toxic chemicals can contaminate the air supply. "
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