The mushroom mycelium builds up carbon dioxide. This carbon dioxide will spread over the whole growing area. As this area is normally quite small, it gets filled with carbon dioxide very fast. Mushrooms find it very difficult to handle high concentrations of carbon dioxide. The growing of the mushrooms will also rapidly come to a stop.
To prevent this from happening it's important to refresh the air in the growing area regurlarly. When the substrate has been exposed to the fruiting conditions and the first pinheads come out, it's very important to refresh the air very often.
The amount of fresh air for an optimum growing environment varies per type of mushroom. At the same time this depends on differents factors, like for instance the size of the growing area.. In general the mushrooms from the Panaeolus family have a bigger need for fresh air then the mushrooms from the Psilocybe family.
Psilocybe: refresh the air 3 to 4 times per day (minimum)
Panaeolus: refresh the air 6 to 8 times per day (minimum)