What is fresh air exchange (FAE) and why does it matter?

Growing mycelium releases carbon dioxide, and too much of it makes mushrooms grow long stems, small caps, or abort at the pin stage. Fresh air exchange means swapping that stale air for fresh air by fanning, venting, or running a small fan on a timer. Oysters are sensitive and want frequent air, while shiitake and lion's mane tolerate more carbon dioxide. Balance air exchange with humidity so surfaces stay damp.