Fresh Air Exchange, also known as FAE, is one of the fundamental parameters of mushroom cultivation. It represents the rate of fresh air intake and exhaust in an enclosed growing space. Essentially, it is synonymous with ventilation.
The term FAE is often utilized in the context of grow rooms/tents, but its significance is diverse and relevant for monotubs, fruiting chambers, or even the rate of air exchange through the filter of a growbag. FAE is generally described as being higher or lower, depending on the quantity of ventilation.
When FAE Is Needed... And When It’s Not
FAE is relevant because, in a similar manner to us, fungi “exhale” carbon dioxide and breath oxygen. While they do not utilize specialized organs like we do, they passively absorb and release these gases through their tissues
When left in an enclosed space, high amounts of carbon dioxide can accumulate. While this won’t necessarily kill them (as it might an animal), it can impede their growth, especially during fruiting.
Spawn Colonization: Low FAE levels are generally recommended when incubating spawn. This is usually done using an excellent 0.2 micron filter on the grow bag. This helps prevent contamination and the drying of the substrate.
Incubation: During incubation, you do not need to provide high levels of FAE. In fact, high levels of carbon dioxide (low FAE) have actually been shown to stimulate the growth of mycelium in some cases. Excessive FAE during incubation could also unintentionally trigger premature fruiting in some species, as is the case with Lion's Mane. High rates of FAE during incubation could also dry out the substrate. If you think about it, in a natural context, incubation happens inside a woody log or in the soil, where FAE is naturally limited.
Pinning: One of the environmental parameters that triggers pinning is the switch from low FAE to high FAE. This naturally happens when you cut into or open a growbag, but it can be further enhanced by increasing the rate of FAE in your grow space. This can be helpful to stimulate vigorous pinning in fully colonized substrates.
Fruiting: Good levels of FAE are most important for developing healthy fruiting bodies. Mushrooms grown under low FAE conditions tend to look leggy and unusual. Some mushrooms, like Oysters, require very high levels of FAE during fruiting, while others, like Shiitake, require less. Some growers may purposefully fruit certain mushrooms with low FAE to induce certain growth forms, as with long-stemmed Enoki and Reishi antlers.
A Note On Carbon Dioxide
Carbon dioxide is a byproduct of all living things. It’s produced by plants, animals, bacteria, and all kinds of fungi. This is because all of our metabolisms utilize carbon-based molecules (either in the form of sugars, cellulose, lignin, etc.) to produce energy, resulting in carbon dioxide as a waste product. This is why the constant metabolic activity of fungal mycelium consuming its substrate can significantly alter the levels of carbon dioxide in an enclosed space.
While fungi can’t utilize carbon dioxide directly, they do use it to sense and respond to the world around them. In nature, high carbon dioxide conditions can indicate to the fungus that they are still buried in the soil, below heavy leaf litter, or perhaps inside a piece of large woody debris. This is why mushrooms growing in high carbon dioxide conditions will grow elongated stems with small caps, or antler forms, which could help them “escape” from these conditions.
FAE Fruiting Preference By Variety
| High FAE (<1000 PPM) |
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| Medium FAE (1000-1500 PPM) |
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| Low FAE (>1500 PPM) |
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How To Control FAE
Introducing fresh air into your growing environment poses inherent challenges to controlling other environmental parameters like humidity and temperature. Essentially, the more FAE you provide, the more you will have to intervene by controlling these other parameters. This is why growers should take precise care when regulating levels of FAE as too much FAE could cause unfavorable growing conditions (too dry, inadequate temperatures, etc.) or unnecessary energy costs. There are two main approaches to managing FAE as discussed below.
Passive Fresh Air Exchange
This simply means providing enough breathability or permeability in an enclosure so that “stale” air will naturally diffuse into the environment and be replaced with fresh air. This would be the case in monotub, grow bags, or fruiting chambers, which usually contain breathable filters. In some cases, simply opening a door/window can be a form of passive fresh air exchange.
Active Fresh Air Exchange
Active Fresh Air Exchange entails providing FAE with an intake/exhaust fan. This is recommended for grow tents or grow rooms, where precise control is important.
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Martha tents and smaller setups can get away with a single exhaust fan while larger growers may want both an intake and an exhaust.
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Exhaust fans should have their intake near the bottom of the grow space, as carbon dioxide is relatively heavy and settles to the bottom.
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In many cases, it is recommended to connect your intake directly to a humidifier so that any fresh air is properly humidified.
- Growers often utilize controllers with CO2 sensors to automate the process, while others put their fans on a timer to ensure regular air replenishment.
Air Circulation
Air circulation is often confused with fresh air exchanged, despite them being fundamentally different. Air circulation refers to moving air inside an enclosed growing space. This can help regulate temperatures and avoid “pockets” of high carbon dioxide that can occur when there is no movement. Air circulation can be provided easily with an oscillating fan.
Symptoms Of Excessive Carbon Dioxide Levels (due to low FAE)
- Elongated stems
- Small or underdeveloped caps
- Fuzzy stem base
- Aerial mycelium from the substrate
- Delayed or no pinning
- Malformed or misshapen fruiting bodies
- Antler formation (in the case of Reishi)
- “Overlay,” which is when mycelium covers a casing layer
Monitoring and Regulating Fresh Air Exchange
A carbon dioxide sensor can easily monitor the quantity of fresh air in a grow room. These sensors read carbon dioxide levels in parts per million (PPM) and can help a grower know if they need more fresh air exchange in their grow room.
| Carbon Dioxide Level | Significance | |
|---|---|---|
| 280 PPM | This is the preindustrial (early 1900s) atmospheric level. | Very High FAE |
| 428 PPM | This is the average modern atmospheric level. | Very High FAE |
| 450 - 800 PPM | Excellent for stimulating primordia formation or the fruiting of all species. | Very High FAE |
| 800 - 1,000 PPM | Great for fruiting of all species and still suitable for pinning. | High FAE |
| 1,000 - 1,500 PPM | Good for less CO2 sensitive species like Shiitake. | Medium FAE |
| 1,500 - 2,000 PPM | It can still be fine for less sensitive species, but it is usually not optimal. Suitable for those with high CO2 tolerances. | Medium-Low FAE |
| 2,000 - 5,000 PPM | Generally not recommended for fruiting, but fine for colonization. | Low FAE |
| >5,0000 PPM | This is fine for long-stemmed Enoki or Reishi antlers. In fact, these can handle up to 20,000 ppm or more for fruiting. These levels are also helpful for colonization. | Very Low FAE |
Conclusion
Fresh air exchange is a fundamental parameter for mushroom cultivation. While it’s only necessary in high amounts during fruiting, its role and impacts are important to consider at every stage in the cultivation process. If you found this guide helpful and want to learn more, we recommend you check out our introductory series on mushroom cultivation to learn more about every step of the process!
