Mushroom CO2 and fresh air: targets by species and stage

Fruiting mushrooms need CO2 below about 1,000 parts per million (ppm), and oysters lower still. One extension guide keeps oysters under 600 ppm. Colonizing blocks are the opposite: guides put the spawn run at 10,000–20,000 ppm, and one lists no fresh air exchange at all.

The targets below cover culinary species only: oysters, pink oyster, lion's mane, shiitake and button mushrooms. Every target names its source.

How much CO2 is OK at each stage?

CO2 can run high while blocks colonize. Most sources here want it below about 1,000 ppm once you want pins, and kept there while mushrooms grow. Oysters want the lowest levels. The table gives each source's figure in ppm.

"No figure given" means none of these sources gives a figure for that species at that stage.

UC is the University of California Cooperative Extension guide Cultivating Mushrooms on Small Farms, which gives one set of oyster figures for Pleurotus spp. as a group (UC Cooperative Extension). Penn State is its Extension guide to button mushroom farming (Penn State Extension). Cornell is the Cornell Small Farms and Fungi Ally farm guide (Cornell Small Farms and Fungi Ally). GroCycle covers lion's mane in its growing guide (GroCycle), and CO2Meter gives its figures in a blog post (CO2Meter).

North Spore lists its figures on each species' grain spawn page:

• Blue oyster (North Spore — affiliate link: we may earn a commission at no extra cost to you)
• Pink oyster (North Spore — affiliate link: we may earn a commission at no extra cost to you)
• Lion's mane (North Spore — affiliate link: we may earn a commission at no extra cost to you)
• Shiitake (North Spore — affiliate link: we may earn a commission at no extra cost to you)

Where do the sources disagree?

Sources disagree most about how low CO2 has to go. CO2Meter says most varieties need 1,200–1,500 ppm early in pinning. GroCycle's Martha tent guide also starts a CO2 controller at 1,200–1,500 ppm. It warns that below 800 ppm mushrooms come out too small and numerous, and above 2,000 they deform (GroCycle). UC and North Spore set pinning at or below their fruiting ceilings, and Cornell warns that fruiting can be inhibited above 1,200 ppm. So CO2Meter and GroCycle run higher than UC, Cornell and North Spore.

Those lower sources sit closer together on fruiting. Oysters sit lowest: UC says under 600 ppm, and Cornell sees deformed oysters above 800. Lion's mane and shiitake sit near 1,000, and Cornell says they tolerate up to about 1,300. If you want one rule, keep oysters under about 600–800 ppm and the others under about 1,000. If mushrooms then come in small and crowded, as GroCycle warns, raise the setpoint in steps.

Why can colonizing blocks sit in high CO2?

Colonizing mycelium tolerates high CO2, and UC says oyster mycelium grows faster in it, up to about 28,000 ppm. GroCycle says mycelium "tolerates, and sometimes even needs, higher levels of CO2 while colonizing a substrate, and exposing it to fresh air helps to trigger fruiting" (GroCycle). UC lists zero air exchanges an hour during the oyster spawn run. Monotub holes, likewise, stay fully covered during incubation and get filters only for fruiting.

The drop in CO2 is part of what tells a block to fruit. So keep blocks out of fresh air until they're fully colonized, then move them into it.

What does too much CO2 look like?

Too much CO2 shows up as long, stringy stems and small caps. North Spore adds that yields drop, and growth can halt. Cornell notes that dim light gives the same long stems and little caps, so check the light before you add air.

A 2025 lab study shows the effect in numbers. Enoki grown at 10,000 ppm had caps less than half as wide, on average, as enoki grown at 1,000 ppm (PMC). Our Martha tent guide has a symptom table that separates CO2 problems from humidity ones. If blocks sit white and won't pin at all, our pinning guide checks fresh air alongside the other causes.

How much fresh air does each chamber need?

A monotub gets by on filtered 2 in holes, and a shotgun chamber on its holes plus hand fanning at least twice a day. A Martha tent starts at 2 minutes of exhaust fan an hour, and a small room at 4–6 air changes an hour for oysters. These are published starting points for fresh air exchange (FAE), not final settings.

Monotub. A monotub breathes through a few filtered holes, so GroCycle places them to make air flow through it. Air comes in through the low holes on the long sides and leaves through the high ones on the short sides. The holes stay fully covered while the tub colonizes. For fruiting, filters go on. GroCycle adds that a monotub benefits from a fan in the room, as long as it isn't aimed at the tub.

Shotgun chamber. A shotgun chamber relies on many small holes and damp perlite. Warmth from the mycelium draws fresh air in through the bottom holes, up through the perlite, and out the top. GroCycle still says the holes aren't enough on their own. Take the lid off and fan it lightly at least twice a day, then mist the walls and perlite. Don't run a fan right next to it: GroCycle says that dries everything out.

Martha tent. A Martha tent uses an exhaust fan, and our Martha tent guide covers the fan, timer and controller in detail. Start with GroCycle's timer settings above, or let a CO2 controller run the fan when CO2 passes your setpoint. North Spore notes that "higher temps and more growth mean more CO2", so a full, warm tent needs more air than a half-empty, cool one (North Spore — affiliate link: we may earn a commission at no extra cost to you).

Small room. A room's fan size follows from its volume and the air changes you want. UC gives fruiting oysters 4–6 air changes an hour and shiitake 2–4. Cornell recommends exchanging all the air in a grow room at least every 10 minutes. It warns that a closed room's CO2 gets too high for fruiting in about 10 minutes.

To size a fan, multiply the room's volume in cubic feet by the air changes an hour you want, then divide by 60 to get CFM. Cornell's example room is 8 x 16 ft. With an 8 ft ceiling, that's 1,024 cu ft. Six air changes an hour, one every 10 minutes, works out to about 100 CFM of steady airflow.

Some room plans size the fans larger and let a controller decide when they run. A Transfarmation fruiting-room plan says all the air should ideally exchange within 2–3 minutes. It calls a CO2 meter set at 700–800 ppm more efficient than a timer (Transfarmation). For the 1,024 cu ft room, that means fans of roughly 340–510 CFM, running only when CO2 climbs. For comparison, Penn State's small button-mushroom rooms moved 6 to 9 air changes an hour of mixed fresh and recirculated air. CO2 there generally stayed below the study's 3,000 ppm maximum target (Penn State).

Why do fresh air and humidity pull against each other?

Fresh air and humidity pull against each other because room air is much drier than a fruiting chamber. Every bit the fan pulls in has to be brought up to 85–95% RH. North Spore says it plainly about its tent: "Higher fan speed or colder temps mean more Myco-Mister refills."

So add fresh air in short runs, keep the mist and the exhaust at opposite ends, and size the humidifier for the airflow you actually run. Our Martha tent guide works through the numbers in its "Size the fan and humidifier together" section. Our humidifier guide covers sizing a humidifier for each chamber type. A CO2 controller helps here too, because it adds only as much air as the CO2 reading calls for.

Which CO2 sensor works at fruiting humidity?

The sensor to use in a fruiting chamber is an NDIR CO2 sensor rated to at least 95% RH, non-condensing, mounted out of the mist. Avoid "eCO2" or "CO2eq" sensors: they estimate CO2 from other gases.

NDIR or eCO2?

An NDIR (non-dispersive infrared) sensor measures CO2 itself, with infrared light in a small optical chamber. Aranet calls NDIR sensors "highly stable, selective, and the industry standard for accurate indoor air monitoring" (Aranet). Sensirion's SCD4x is a compact version that uses a photoacoustic NDIR principle (Sensirion).

An eCO2 sensor doesn't measure CO2 at all. Sensirion's SGP30, a metal-oxide gas sensor, outputs a "CO2eq" value "calculated from Ethanol and H2 measurements" (Sensirion). An estimate like that can't tell you whether a tent is at 600 ppm or 1,200.

Read the humidity rating

The humidity rating on a CO2 sensor's datasheet matters more than its accuracy. Here is what makers say about a few options. Prices are from the sellers' own pages on 19 October 2026.

The sources are Inkbird's product page (Inkbird), Aranet's product page and handling notes, and the Sensirion datasheets above. Aranet's handling notes say plainly: "do not use it in high humidity environments (greater than 85%)". That rules out a room monitor like it inside most fruiting chambers.

"Non-condensing" means water must never form on the sensor. Fog and dripping tent walls are exactly where it forms, so even a 95% rating assumes you mount the sensor out of the mist. Some industrial probes heat the sensor head to keep condensation off (Vaisala).

Accuracy is rarely the weak point. Inkbird rates the ICC-500T at ±(30 ppm + 3% of the reading), about ±60 ppm at 1,000 ppm. Sensirion rates its SCD41 at ±(50 ppm + 2.5% of the reading) up to 1,000 ppm, about ±75 ppm. Either is far tighter than the gap between 600 and 1,000 ppm.

Measurement range can be the weak point if you watch a spawn run. The ICC-500T and the Aranet4 both read only up to 9,999 ppm (Inkbird, Aranet). So both max out in a 10,000–20,000 ppm spawn run.

Watch automatic calibration

Automatic calibration can quietly pull a CO2 reading low in a grow space. NDIR sensors with automatic calibration reset their baseline by assuming the lowest reading they see is outdoor air. Sensirion's SCD4x assumes it was "exposed to outdoor fresh air at 400 ppm CO2 concentration at least once for >3 minutes after every week of operation" (Sensirion).

Aranet gives a worked example for its monitor: "If the lowest recorded measurement was 460 ppm, then this will be considered 420 ppm" (Aranet). A sensor in an incubation room or a stuffy tent may never see outdoor air. It then treats its lowest reading as outdoor air, and reads low from then on.

You have three ways around it:

• Give it fresh air on schedule. Sensirion's SCD4x needs more than 3 minutes of outdoor air each week. Aranet's automatic mode wants about 8 hours in fresh air every 30 days.
• Calibrate by hand. Aranet ships its monitor with automatic calibration off (Aranet). For a manual calibration, it wants fresh air that stays steady for 30 minutes, ideally outdoors, with nobody within a meter of the sensor (Aranet). The ICC-500T takes a manual offset.
• Check it against outdoor air. Outdoor air averaged about 426 ppm worldwide in July 2026 (NOAA), and Aranet calibrates to about 420. If a sensor reads well off that outdoors, calibrate it by hand.

Altitude matters too. An NDIR reading depends on air pressure. Aranet's monitor has a barometer to correct for it, and Aranet still says to calibrate it manually at high altitude (Aranet).

A disclosure about our own sensor

We're building a CO2 sensor of our own. Hypha Aeris is a CO₂, temperature and humidity sensor in a closed, application-only beta. It isn't for sale; reservations and production are planned for 2027, with no ship month set. It's built on Sensirion's SEN66 module.

Where should the CO2 sensor go?

The CO2 sensor goes inside the chamber, at the height of the mushrooms, out of the mist and away from where fresh air comes in. A sensor anywhere else measures some other air.

• At canopy height, mid-chamber. That's the air the mushrooms grow in. In a Martha tent, GroCycle puts a CO2 controller's sensor on the top shelf (GroCycle).
• Out of the mist and off the walls. Fog and condensation break the non-condensing rating. The humidity probe has the same needs, and our humidifier guide covers where it goes.
• Away from the intake, the door and your face. Aranet notes that readings near a window or door run slightly low because of the airflow. Breathing on a sensor pushes it up (Aranet).
• Inside the tub, not beside it. A monotub's sensor belongs above the substrate and below the lid. One sitting on the shelf next to the tub reads the room.
• Checked at more than one spot. In larger rooms, compare positions before you fix the sensor in place. CO2Meter describes farm workers walking grow rooms with portable meters to find spots with higher or lower CO2.

A simple way to dial in fresh air

Dial in fresh air by measuring a full fan cycle, then changing one thing at a time. A routine like this works for any chamber:

1. Start from the published setup for your chamber type in the table above.
2. Log CO2 across a full cycle, from just before the fan or fanning to just after. Note the peak and the low.
3. Compare the peak with your species' ceiling. For oysters, use UC's 600 ppm; for most others, about 1,000.
4. Change one thing. Run the fan longer or more often, open filter holes, or fan a shotgun chamber more often. Then log another full cycle.
5. Recheck humidity after every change. More air usually means lower RH, so confirm the humidifier keeps up.
6. Watch the mushrooms. Long stems and small caps mean more air, or more light. Dry, cracked caps mean too much air movement or too little humidity.

Keep a note of each setting and what the next flush looked like. That record, not a single number from a guide, is what tells you where your own chamber should sit.

Other questions

Does an incubation room need fresh air?

An incubation room needs moving air, but not fresh air. Cornell says incubation room air "does not need to be actively exchanged with the outdoors". It adds that oscillating fans help keep the air from going stagnant. It also spaces bags a palm's width apart, because tightly packed bags can overheat and contaminate.

Do any mushrooms fruit well in high CO2?

Maitake and enoki can, according to Cornell: they "can fruit in high CO2 environments and still be a high quality mushroom". Commercial enoki growers use the effect on purpose. The 2025 enoki study above says the longer stems and slower cap growth that high CO2 brings are "commonly leveraged in commercial cultivation to optimize fruiting body appearance" (PMC).