A button bed gives off about 10 grams of carbon dioxide per square metre every hour during the first two flushes, when the mycelium is most active.
During spawn run that gas is wanted. ICAR-DMR keeps 10,000 to 15,000 parts per million with the whole air recirculated and no fresh air entering, because high carbon dioxide speeds the spawn through the compost. Case run continues above 7,500 parts per million with no fresh air introduced.

