Domestic air cargo flights typically have 2-3 crew members on board. They're inside the same pressure hull as the cargo; there's no barrier or filter. That's why dry ice must be declared and requires special handing for air transport.
Does someone crawl into the cargo compartment to unload it the moment its door is opened? If that person got caught in a CO2 pool they could plausibly fall unconscious & at least start suffocating before coworkers noticed they were absent for too long.
