Each passenger or crew compartment must be suitably ventilated. Carbon monoxide concentration may not be more than one part in 20,000 parts of air, and fuel fumes may not be present. In any case where partitions between compartments have louvres or other means allowing air to flow between compartments, there must be a means convenient to the crew for closing the flow of air through the partitions, when necessary.
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14 CFR · PART 121
§ 121.219 Ventilation.
Official text · Current as of Oct 19, 2026 · from eCFR
Not legal advice. This is the official text as published in the eCFR, which is updated daily but is not the legal edition. For legal purposes, use the annual Code of Federal Regulations or ask an aviation attorney.