I am working with a cardboard manufacturing plant that uses exhaust fans to remove small trim scrap and particles from the cutting machines. Typically, a mechanical conveyor carries scrap from the machine to the opening of a round 10 to 16 inch duct where it is drawn into the duct and conveyed...
I suspect CO2 or CO - probably from either a surface (vehicle exhaust) or subsurface source (biological or chemical activity in surrounding soil). An autopsy should identify the mechanism and agent. Once the mechanism and agent is identified, the source should be fairly straight forward.