Acoustic box Fans are used to move air into your grow room or extract from your grow room. By doing so this will be replenishing co2 around your plants is an essential part of the process of photosynthesis. As carbon dioxide is used by plants and levels in the atmosphere become depleted, air in the room needs to be replaced. If stale air is not removed from the grow area, moisture quickly builds, raising humidity and triggering problems with mould and rot.