Mushrooms breathe out carbon dioxide, CO2, and a closed room fills with it. The manual wants less than 600 ppm during cropping; the folder allows up to 800 ppm. Both are DMR; without a meter, work to the stricter one.
The folder's method is to open windows or run exhaust fans two or three times a day. DMR describes the symptom of too much CO2 exactly: long stalk, small cap, a mushroom like the mouth of a trumpet. The mushroom is your meter.

