The fungus breathes. It takes oxygen and gives out carbon dioxide, CO2. During spawn run high CO2 is tolerated and even useful: oyster up to 2 percent, button 10,000 to 15,000 ppm, with no fresh air at all.
At fruiting, CO2 must fall. DMR's manual asks for under 600 ppm for oyster cropping and the Hindi folder under 800; button pinning wants 800 to 1,000 ppm; milky needs 3 to 4 air changes an hour. Too much CO2 shows on the mushroom: long stems and small caps, oysters shaped like a trumpet mouth, onion-shaped buttons.