Environment and climate · CO2

CO2 enrichment: a safe supply of carbon to the plant

A plant changes carbon dioxide (CO2) into sugar. In strong light, a crop in a building can use much more CO2 than the air supplies. Added CO2 can increase the dry yield by approximately one third. The plant uses CO2 only in the light period, and in a ventilated room the exhaust air removes the CO2. At an incorrect concentration, CO2 can kill a person in the room. This paper gives the target range, the method to seal a room, and the safety limits and alarms to install before a person goes in.

Environment and climate7 diagrams38 sources~30 min to read

01 · Read this first

Purpose and scope

A plant makes its structure from air. The carbon in each leaf, stem and flower is from carbon dioxide (CO2). The plant absorbs the CO2 from the room air. It uses the energy of light to make sugar from the CO2 and water.

If you give a canopy in bright light more CO2 than the approximately 420 ppm of usual air, the canopy can increase its rate of growth. Thus growers add CO2. But CO2 enrichment is one of the tasks in the room in which it is easiest to make a dangerous error.

The five primary facts of this paper

The plant uses CO2 only in the light period. In the dark period, the CO2 moves in the opposite direction. The plant releases CO2. CO2 that you add at night is waste, and it is a risk to safety.
CO2 increases the yield only in strong light. CO2 and light are each a limit for photosynthesis. In weak light, more CO2 has almost no effect^[1].
The target range is approximately 1,000–1,500 ppm. If the concentration is less than this range, the yield is less than the maximum. If the concentration is more than this range, the gas that you add is waste. It does not increase the yield^[6].
You must seal the room. In a ventilated room, the exhaust air removes the CO2 that you add, and the CO2 goes directly out of the room. The cost of this CO2 is waste^[12].
CO2 can kill a person. You cannot smell the gas. A sealed room with CO2 enrichment, or a drying room, can have a high concentration that kills a person^[16].

The remaining sections of this paper give more information about these five facts. Add CO2 in the light period. Seal the room. Install a CO2 alarm on the wall. These three instructions are the most important.

02 · The terms

Definitions

CO2 (carbon dioxide)The gas that plants change into sugar with the energy of light. At this time, usual outdoor air contains approximately 420 ppm of CO2^[3].

ppmOne part in a million parts. 1,000 ppm is one part of CO2 in a thousand parts of air (0.1%). All the CO2 values in this paper are in ppm. 10,000 ppm is 1%.

PhotosynthesisThe reaction that changes CO2 and water into sugar with the energy of light. It is the only reaction that moves CO2 into the plant. The reaction cannot operate without light.

RespirationThe reaction in which the plant uses the sugar as energy for growth and for its other functions. This reaction releases CO2. It operates day and night in each living cell^[8].

C3 plantCannabis is a C3 plant. In a C3 plant, the CO2 concentration of usual air is the limit for photosynthesis. Thus CO2 enrichment helps cannabis.

PhotorespirationA side reaction that is a waste of energy. In this reaction, the enzyme that makes sugar from CO2 absorbs oxygen and not CO2. When the CO2 increases, this waste decreases. A lower leaf temperature also decreases this waste^[2].

Compensation pointThe CO2 concentration (approximately 50 ppm for a C3 plant) at which the rate of photosynthesis is equal to the rate of respiration. At this concentration, the plant does not make more sugar than it uses^[2].

Saturation pointThe CO2 concentration at which more CO2 does not help the plant. For CO2 enrichment, this value is approximately 1,000–1,500 ppm.

NDIR sensorNon-dispersive infrared. It is the only type of low-cost sensor that measures CO2 correctly. Low-cost ‘MQ’ gas sensors do not measure CO2 correctly^[22].

TWA / STEL / IDLHLimits for the safety of workers. TWA is the limit for the average CO2 concentration for 8 hours. STEL is the limit for a short period of 15 minutes. IDLH is the concentration that is immediately dangerous to life or health^[18].

Quality of the information

Evidence and limitations

Three types of information

We want each paper to be correct. Thus we examine each paper for information that is not solid. This information comes from the view of one grower, from a small number of sources, or from grower methods that have no controlled test. We identify this information, and we do not show it as solid.

Frequently there is no paper for the decision that you must make. Then we use the reports of other growers and the methods that work in the rooms that we know. This information can help you, but it is not the result of a laboratory test. Use the methods that work for your plants, for your room and for your meters. If a table does not agree with your crop, use the data from your crop and record the difference.

Solid

Tests of plants, specifications or many sources that agree show that the information is correct

In strong light, a higher CO₂ concentration increases the photosynthesis of C3 plants, for example cannabis.
Inject CO₂ only when the light is on. Measure the CO₂ with NDIR sensors. The occupational limits are approximately 5,000, 30,000 and 40,000 ppm.
A sealed room keeps the CO₂ enrichment. Exhaust removes it.

Grower method

Many growers use this method in their rooms. Start with it, then change it for your room.

Enrichment ranges of 1,000 to 1,500 ppm, and CO₂ from bottles, as the standard procedure in a sealed room.
Methods of air exchange in the dark period and of ethylene control.

Weak

The view of one grower, a small number of sources, one test, or a method that works in one room

One percentage of yield increase for all rooms. The genetics, the light and the quality of the seal have the largest effect.
Maximum CO₂ values in percent for a drying room, as facts with no measurement. Measure your room.
Percentage increases for tomato in a greenhouse, with the same values (1:1) for cannabis flower.

If you find an error, make a report. We will correct it. Make the report on GitHub, with the name of the paper and the information that is incorrect. If you have a source, include it: Make a report of an accuracy problem. The regulations, labels and licenses of your area always override each procedure in these papers. The label weak in a paper shows the points where the risk of an error is highest.

03 · How the plant uses CO2

Photosynthesis in the leaf and the effect of the CO2 concentration

In a leaf in the light, an enzyme with the name Rubisco absorbs CO2 from the air and changes it into sugar. Rubisco does not absorb only CO2. It also absorbs oxygen. When Rubisco absorbs oxygen and not CO2, photorespiration starts. Photorespiration is a reaction that is a waste of energy and carbon. It does not make a product that the plant can use.

When the ratio of CO2 to oxygen increases, photorespiration occurs at a lower rate^[2].

At this time, the air contains approximately 420 ppm of CO2^[3]. This concentration is not sufficient for Rubisco to operate at the maximum rate. In a C3 plant, for example cannabis, the CO2 concentration is the limit for photosynthesis. When the CO2 increases, photosynthesis increases.

In the one primary investigation of gas exchange in cannabis, the CO2 increased from 350 to 750 ppm. The net photosynthesis of the leaf increased by approximately 50%^[1]. A subsequent investigation of four cultivars with high THC found that the net photosynthesis increased by 38–48% when the CO2 increased from 390 to 700 ppm^[4].

Figure 1. Net photosynthesis of a cannabis leaf at different CO2 concentrations. If the CO2 decreases to 250 ppm, the net photosynthesis decreases by approximately 50%. If the CO2 increases to 750 ppm, it increases by approximately 50%^[1]. At first, the net photosynthesis increases quickly with the CO2. Then the curve becomes horizontal, because other limits become the maximum for the rate.

Photosynthesis has a maximum value. When the CO2 increases, the rate at which the leaf can use the light becomes the limit for photosynthesis. Thus the curve becomes horizontal at a saturation point. For most C3 crops in greenhouses, the horizontal part of the curve starts at approximately 1,000–1,300 ppm^[7].

04 · Day and night

Photosynthesis and respiration in the light cycle

Persons who start CO2 enrichment frequently make an error with this fact. Photosynthesis is the reaction that uses CO2. It operates only in the light. Respiration is the reaction that releases CO2. It does not stop, day and night, in each living cell^[8].

In the light period, photosynthesis absorbs much more CO2 than respiration releases. Thus the canopy has a net CO2 uptake. When the lights go off, photosynthesis stops and only respiration continues. Thus the same canopy has a net CO2 emission. At night, the rate of CO2 emission is much lower than the rate of CO2 uptake in the day.

Figure 2. The net CO2 flux of the canopy in one day. In the light period, the canopy absorbs much CO2. In the dark period, it releases a small quantity of CO2. The trough in the day is much larger than the peak at night. The next section gives more information about this difference.

The rate of respiration also increases when the temperature is higher. When the temperature increases by 10 °C (18 °F), the rate becomes approximately two times higher (a Q10 of approximately 2)^[10]. In soil or coco, microbes and roots in the root zone also make more CO2. But these facts do not change the primary fact: the plant absorbs CO2 only while the lights are on. Because of this one fact, you add CO2 in the light period and do not add CO2 in the dark period.

05 · The mass balance

CO2 uptake in the day and CO2 emission at night

Growers frequently think that the plant uses CO2 in the day and releases the same quantity of CO2 at night. The two quantities are not the same. The cause of this difference is the cause of the effect of CO2 enrichment.

The plant does not release again all the carbon that it absorbs in the light. In one growing season, respiration releases only approximately half of the carbon that photosynthesis absorbs. The remaining carbon, approximately 46%, stays in the plant as sugar, stem, leaf and flower^[9]. Review papers on crop physiology give a range of 30–60% for the fraction that respiration releases^[8].

This carbon in the plant is the crop. Thus the CO2 that the plant absorbs in the day is always more than the CO2 that it releases at night. The difference is the quantity of carbon that became plant tissue.

You can find this difference in a sealed room. When the lights are on and you do not add CO2, the canopy quickly decreases the CO2 in the air. The concentration becomes lower than the concentration of outdoor air. In a greenhouse with good seals, the CO2 can decrease to approximately 200 ppm in some hours. This low concentration decreases the photosynthesis of the canopy by approximately 26%^[11].

During the night, the CO2 in the same room increases again, because the respiration of the plants releases CO2. But the CO2 increases slowly. The CO2 does not increase to a high value, because the room has leaks.

Figure 3. Room CO2 in a sealed flower room without CO2 enrichment. When the lights come on, the CO2 decreases quickly to a very low value. At night, the CO2 increases slowly and by a small quantity. CO2 enrichment replaces the CO2 that the canopy removes in the day^[11].

The numbers for a room of medium size

When the lights come on. A canopy of approximately 45 m² (484 ft²) absorbs 15–25 µmol CO2 m²/s. In a sealed room of approximately 160 m³, the CO2 decreases at first by approximately 350–600 ppm for each hour. The CO2 decreases from 420 to approximately 200 ppm in 20–40 minutes. Then the rate becomes lower, because the CO2 is low.
When the lights go off. In the dark period, the respiration of the same canopy adds only approximately 25–70 ppm for each hour. During the night, the CO2 increases by two hundred ppm or more. Then the CO2 stays at that value because of the leaks of the room.
The difference is the crop. Approximately half of the carbon that the plant absorbs stays in the plant, and respiration does not release it again. Thus the CO2 uptake in the day is many times the CO2 emission at night^[9].

This balance gives the primary information about the quantity of CO2 that is necessary. CO2 enrichment is not for the night. It keeps the CO2 high in the light hours. Without CO2 enrichment, the canopy removes most of the CO2 from the room.

06 · The dose

CO2 targets and the effect on the yield

The best yield data for cannabis come from controlled tests at Utah State University. In these tests, the CO2 concentration of a sealed room increased from the concentration of outdoor air (approximately 420 ppm) to approximately 1,200 ppm. The dry flower yield increased by approximately 40%. The concentration of 1,200 ppm gave approximately 95% of the possible yield gain. Thus a higher concentration gives almost no more yield^[6].

Other crops in greenhouses show the same result. Many C3 crops in greenhouses have a yield gain of ten percent or more at approximately 800–1,200 ppm (the very high yield gains in reports change with the control treatment, which can have a low CO2 concentration, and with the system)^[7]. For most C3 crops, the saturation point is approximately 1,000–1,300 ppm^[7].

Figure 4. The range for operation. Keep the CO2 at approximately 1,000–1,500 ppm in the light period. The curve is horizontal at more than approximately 1,500 ppm. Thus, if you increase the CO2 to 2,000 ppm, most of the gas that you add is waste. Also, the risk to safety and the cooling load are higher^[6].

07 · Supply of CO2

CO2 supply and dose

There are four usual methods to put CO2 into a room. One method is clearly the best for a sealed cannabis room:

In a sealed room with a license, the standard selection is a cylinder of CO2 and a controller that uses a sensor.
Method	Operation	Information
Compressed CO2 in a cylinder	Cylinder → regulator → solenoid valve. An NDIR controller operates the valve.	This method is more clean than the other methods. It makes no heat and no unwanted products, and it is accurate. We recommend that you use it for sealed rooms. A cylinder becomes empty, thus you must replace it.
Propane or gas burner	The burner burns fuel to make CO2 (approximately 3 lb of CO2 for each lb of fuel)	The cost of the gas is low, but the burner adds heat and water vapor. If the fuel does not burn fully, the burner makes carbon monoxide (CO), ethylene and NOx. These gases cause damage to plants and injury to persons. A CO alarm is necessary.
Fermentation with yeast	Sugar + yeast → CO2 + alcohol	The output is very small, and you cannot control it. This method is only for hobby growers.
Dry ice	Solid CO2 changes directly into gas	You cannot control it. It continues for a short time. It also causes a risk of cold burn and a risk in a confined space.

Time and airflow. Add CO2 only in the light period. Typically, start approximately one hour after the lights come on, and stop before the lights go off^[12]. Use an NDIR CO2 sensor at the height of the canopy to control the CO2 at a setpoint. Do not use a timer without a sensor.

CO2 is heavier than air, thus it flows down. Install the supply pipe above the canopy. Use horizontal airflow (HAF) fans to mix the CO2 down. The fans decrease the thickness of the boundary layer at the leaf (a layer of air without movement and with a deficit of CO2)^[14].

Figure 5. A CO2 enrichment loop that uses a sensor. The controller adds CO2 to hold the setpoint in the light period. In the dark period, the controller stops the CO2 supply.

08 · Keep the CO2 in the room

Sealed rooms and ventilated rooms

In a ventilated room, most of the cost of the CO2 is waste. An exhaust fan replaces the air of the room with outdoor air of approximately 420 ppm. Thus the CO2 that you added decreases to the concentration of the outdoor air. The CO2 decreases in a curve. When the CO2 in the room is higher and the air changes at a higher rate, the CO2 decreases more quickly. If the exhaust fan and the CO2 supply operate at the same time, the exhaust fan removes the CO2 that you add.

A room with the fans off also has leaks. Measurements in commercial greenhouses show that more than half of the CO2 that the growers added went out through leaks in the structure. The CO2 use efficiency was less than 50–60% at a setpoint of 1,000 ppm^[12].

Thus a sealed room is necessary for CO2 enrichment. Do not remove the heat and the humidity with exhaust air. Control them in the room. Use a recirculating mini-split air conditioner (AC) for the heat and a dehumidifier for the moisture. Thus the door stays closed and the CO2 stays in the room.

CO2 enrichment and exhaust airflow have opposite effects. Select a sealed room before you get the first gram of CO2.
	Ventilated room	Sealed room
Heat and humidity	Exhaust air removes them from the room	The AC and the dehumidifier remove them in the room
CO2 enrichment	The exhaust air removes the CO2, and the enrichment has no effect	The room keeps the CO2. Thus the enrichment has an effect.
Air exchange	High. There are many air changes for each hour.	Low. The air changes only through leaks.
When to use it	Use when you add no CO2, or only to prevent low CO2 in the day	Use for all CO2 enrichment where the result is important

09 · Outdoor air

Air exchange in sealed rooms

You seal the room to keep the CO2 in the room. Then you must let outdoor air come in. The two instructions have opposite effects. Carbon filters and HEPA filters clean the air all the time. Thus a grower can think that an air exchange is not necessary.

Filters clean the air, but they do not replace it. The waste gases that the plants make cause most of the stale air. Heat and moisture also cause stale air. A carbon filter and a HEPA filter remove almost no part of the waste gases, the heat or the moisture.

The table shows the items that these filters remove and the items that they do not remove:

The two filters do not replace the air. They do not remove the gases that collect in a sealed room. They also do not remove the heat or the moisture.
Filter	Removes	Does NOT remove
HEPA	Particulates ≥0.3 µm: mold spores, dust and pollen (99.97%)^[25]	All gases, water vapor, heat, CO2, ethylene and oxygen
Activated carbon	Odor, and heavy VOCs and terpenes. Adsorption on the carbon removes them.	CO2, humidity, heat and oxygen. Carbon holds ethylene only weakly, and it becomes full^[26].

The gas that makes an air exchange necessary is ethylene (C2H4). It is a plant hormone. The plants make it and release it. They release a small quantity all the time. They release much more when they have stress or wounds, after defoliation, or in the flowering stage^[30].

Ethylene has an effect on plants at very low concentrations. Sensitive crops show damage at 10 ppb (0.01 ppm). In greenhouses, growers keep the ethylene at less than approximately 20 ppb^[28]^[29].

In a sealed space, the ethylene increases because of the plants only. NASA sealed a chamber with wheat, soybean, lettuce and potato plants. The ethylene increased to 40–120 ppb. The plants made all this ethylene. This concentration was sufficient to cause a change in the shape of the wheat that you can see^[30].

In a controlled test on tomato, a stable 20 ppb decreased the fruit yield to approximately half, and 40 ppb decreased it by approximately 90%. At the same time, the foliage showed almost no damage^[31]. Ethylene also has an effect on cannabis. Cannabis has a full ethylene signaling system. The system is very sensitive. Thus ethylene changes the sex of the flower^[32].

Figure 6. The scale of ethylene damage. The unit is ppb, one part in a billion parts. In a sealed room, the ethylene that the plants make increases to the range of slow damage^[30]^[31].

Incorrect information about oxygen

Growers frequently think that an air exchange gives oxygen to the plants. It is not necessary for you to supply oxygen to the plants from the room air. The air in the room contains approximately 21% oxygen (approximately 209,000 ppm). For workers, the minimum oxygen concentration is 19.5%. This concentration is approximately 14,500 ppm less than usual air. A plant canopy removes much less oxygen than this quantity^[33].

Oxygen in the room air is not a problem. The oxygen that is important is in the root zone. The oxygen is in the water (dissolved oxygen) and in the air-filled porosity of the substrate. Irrigation and the substrate change this oxygen. Air exchange does not change it. The only cause that decreases the oxygen and increases the CO2 of a room is the persons in the room.

Ethylene is the primary cause for an air exchange, but stale air also has these items:

Humidity. The roots of a plant move water up through the stems, and the water moves out of each leaf as vapor through small pores (stomata). This movement is transpiration. Transpiration moves most of the water that you irrigate out of the plant^[34]. A dehumidifier removes this vapor, and a filter does not. After the lights go off, the vapor continues to come if the transpiration is more than the dehumidifier can remove.
Heat. The air conditioner removes the heat. The carbon filter does not remove it.
The terpenes of the plants and other VOCs. Carbon catches them for some time. When the carbon is full, they go through it.
Mold spores. A HEPA filter on the recirculation loop decreases the number of spores in the air^[38], but it cleans only the air that flows through it. In the inner part of a thick canopy, the humidity is 15–25% higher than the reading of your room sensor^[35]. This area of air without movement and with high humidity is where bud rot starts. An air exchange, and air movement through the canopy, decrease the disease. A filter on the average air of the room is not sufficient^[36] (the mold paper and the airflow paper give more information).

How to do an air exchange and keep the CO2

Do the air exchange at lights-off. The plants do not use CO2 in the dark period. Thus an air exchange at specified times in the dark period removes the ethylene, the VOCs and the humidity that collected. It does not remove CO2 that you supplied for the plants.
Or use a scrubber and not an air exchange. A scrubber that uses potassium permanganate, or a photocatalytic unit, lets a fully sealed room keep its CO2. At the same time, it removes the ethylene^[27].
Circulation is not the same as air exchange. HAF fans mix the air of the room. They decrease the thickness of the boundary layer at the leaf, which helps the CO2 to go to the stomata^[14]. But the fans move only the same air. Only outdoor air (or a scrubber) changes the gases that are in the air.

Do an air exchange that is sufficient to keep the ethylene and the humidity low. It must not remove too much CO2 or dry the media. Closed plant environments have from less than 1 to approximately 15 air changes for each hour. One investigation of cannabis micropropagation found that approximately 4.4 air changes for each hour gave the best result. A higher rate dries the substrate and causes stress to the plants^[37] (that investigation was at plantlet scale. Thus the value is an indication, not a setpoint for a flower room). A sealed room is not a room without air exchange.

10 · Other effects of CO2

Room conditions with CO2 enrichment

A change of the CO2 concentration causes changes in the other conditions of the room. The grow-room systems paper gives the same fact: the inputs of a room change at the same time. Three other conditions change with the CO2:

When you increase the CO2, find the other conditions that must change. Growers frequently do not know about the changes of temperature and water.
Lever	Effect of high CO2	Procedure
Temperature	The best temperature for the plant increases by some degrees^[13]	If the light is strong, use approximately 28–30 °C (82–86 °F) and not approximately 25 °C (77 °F). The photosynthesis of cannabis has its peak at approximately 30 °C (86 °F)^[1]
Stomata and water	High CO2 closes the stomata in part (the pores of the leaf for gas exchange). It decreases the stomatal conductance (how easily vapor moves through the pores) by approximately 42%. It decreases the leaf transpiration by approximately 29% for each leaf^[1]	Each leaf uses less water. The total quantity of water that the crop uses can decrease, stay the same or increase. It changes with the leaf area, the PPFD, the temperature and the VPD (VPD is the capacity of the air to absorb moisture. When the temperature is higher and the humidity is lower, the air removes water from the leaf more quickly.) Measure the quantity of water that the crop uses.
Light	No effect if the light is not high^[5]	Use CO2 enrichment only in rooms with strong light. In other rooms, the CO2 is waste.
Feed	When there is less transpiration, the concentration of salts at the root can increase	Monitor the runoff EC and adjust the feed strength

11 · A risk that you cannot see

High CO2 concentration in drying rooms

A drying room has conditions that are opposite to the conditions of the flower rooms in this paper. You fill a sealed room with a large mass of biomass that you cut, from ten kilograms to many hundred kilograms. The room has no light and low airflow. The biomass continues respiration for days after the harvest. It releases CO2 all the time, and there is no photosynthesis to absorb the CO2 again^[15].

Cut leaf and flower have a high rate of respiration. After the harvest, they are in the same group as leafy greens and cut herbs. A high temperature increases the rate (the same Q10 of 2 again)^[15].

A drying room at 15–18 °C (59–64 °F) with almost no air movement is a CO2 trap that holds almost all the CO2. In sealed greenhouses with plants, the CO2 concentration increases at night to 600–1,000 ppm or more because of the living plants^[11]. A room with much cut biomass and almost no airflow can have a much higher concentration. The concentration can be many thousand ppm. In the worst conditions, in a room without airflow, it can possibly increase to a concentration that you measure in percent.

12 · The risk that can kill a person

CO2 exposure limits and worker safety

If you read only one section of this paper, read this section. Your CO2 enrichment setpoint of 1,000–1,500 ppm is much less than each worker limit. It is safe for a person who stays in the room for a short time.

The risk is in these conditions: a regulator with a leak, or a room with too much CO2 from the supply. Other conditions are a burner fault and a sealed drying room. Each of these conditions can cause a CO2 concentration that kills a person, and you get no indication of it.

Figure 7. The usual enrichment range and the worker exposure limit. When you operate enrichment correctly, the CO2 is much less than the dangerous values. Thus it is easy to ignore a leak that increases the CO2 without control, until the CO2 is dangerous^[17].

Each primary safety authority gives the same limit. The limit for the 8-hour average CO2 concentration that a worker can breathe is 5,000 ppm (0.5%). These authorities are OSHA in the US^[17], NIOSH and ACGIH^[18], and WorkSafe New Zealand^[19]. When the concentration is more than this limit, the effects increase quickly:

The effects of CO2 at different concentrations. The difference is large: the enrichment range is safe, but the concentrations that a leak can cause in a sealed room are not safe.
CO2 concentration	Effect	Reference
approximately 420 ppm	Usual outdoor air	Baseline^[3]
1,000–1,500 ppm	Usual enrichment target. It is safe for short tasks.	3 to 5 times less than the worker limit
1,000–2,500 ppm	Some reports show small effects on cognition. The reports do not agree with each other.	An effect on cognition. It is not dangerous^[20]
5,000 ppm (0.5%)	8-hour worker exposure limit (TWA)	OSHA / NIOSH / ACGIH / WorkSafe NZ^[19]
30,000 ppm (3%)	The 15-minute short-term limit (STEL). A person has a headache and breathes quickly.	NIOSH / ACGIH / WorkSafe NZ^[18]
40,000 ppm (4%)	IDLH, immediately dangerous to life or health	A person cannot easily go away from the area^[18]
50,000 ppm (5%)	Hypercapnia and respiratory acidosis in approximately 30 min	The CO2 is directly poisonous. It is not only low oxygen^[16]
70,000–100,000 ppm (7–10%)	Unconsciousness occurs in some minutes	A person cannot do self-rescue^[16]
>300,000 ppm (>30%)	Loss of consciousness occurs in seconds	In accidents that killed persons, the CO2 was 14–26%^[16]

13 · Measurement and alarms

CO2 sensors, alarms and the limits of MQ sensors

You cannot control CO2, and you cannot be safe from CO2, if you do not measure it. Many sensors that you can get do not measure CO2. The correct sensor type is NDIR (non-dispersive infrared). It reads the CO2 from its infrared signature^[22].

Vendors give the name ‘CO2 sensors’ to low-cost ‘MQ’ metal-oxide sensors. These sensors are not correct. The table shows this:

If a ‘CO2 node’ uses an MQ-135 or MQ-5 sensor, it does not measure CO2. Use only NDIR sensors for control and for the safety of persons.
Sensor	Does it measure CO2?	Result
NDIR (SCD30, SCD41, MH-Z19 and other models)	Yes. It uses infrared light for CO2 only.	Correct for control and for safety^[22]
MQ-135	No. It calculates an ‘equivalent CO2’ from a mixture of VOCs.	The reading is not a correct CO2 value. Do not use it^[23]
MQ-5	No. It is for LPG and for gases that can burn.	It is for a different gas^[23]

Two sensors, two tasks. Install a control sensor at the height of the canopy. This sensor controls the CO2 at the setpoint. Install a second sensor low on the wall, approximately 30 cm (12 in) from the floor. This sensor is for the safety of persons. CO2 collects near the floor.

For larger systems, the International Fire Code (Section 5307) makes this mandatory. Each installation with more than 100 lb of CO2 must have gas detection. The system gives an alarm at 5,000 ppm and a strong alarm at 30,000 ppm. It also stops the CO2 supply automatically and starts the exhaust airflow^[21]. Also for installations with less than this quantity, an alarm and an occupancy interlock give protection at a low cost.

14 · The facts

Expected results and limitations

In a sealed room with strong light, CO2 enrichment is one of the changes that give the largest yield gain for the cost. In all other rooms, the cost is waste^[24]. The cost of the gas is low. The payback changes with the condition of the room.

recommended

Use CO2 enrichment when:

The room is sealed (AC and dehumidifier, not exhaust)
The light is strong (high PPFD and DLI)^[5]
The temperature, the water and the feed are correct
The room has a CO2 alarm and interlock

not recommended

Do not use CO2 enrichment when:

The room has an exhaust fan. The exhaust air removes the CO2^[12]
The light is low. The plants cannot use the CO2^[1]
You want to increase the CO2 to more than approximately 1,500 ppm to get more yield^[6]
You cannot monitor the CO2 safely at this time

The usual CO2 problems. Almost always, the cause is the room, the light or the sensor, not the CO2.
Problem	Possible cause	Correction
More CO2, but the yield is not higher	The light is too low, or the room is not sealed	Increase the PPFD. Seal the room. Do this before you think that the gas is the cause.
The CO2 does not stay at the setpoint	The room has leaks, or the exhaust fan operates when you add CO2	Seal the leaks. Use the exhaust only at lights-off.
The plants show wilt, or the substrate stays wet	Plants with CO2 enrichment have a lower transpiration rate. The irrigation quantity that you used before the CO2 enrichment is too high at this time.	Adjust the VPD. Decrease the irrigation volume^[1]
Damage to the leaves when you use a burner	The fuel does not burn fully. The burner makes CO, ethylene and NOx.	Do maintenance on the burner. Install a CO alarm. Or use CO2 from a cylinder.
The sensor reading becomes lower in some weeks	ABC causes an incorrect calibration in a sealed room	Do not use ABC. Calibrate the sensor manually^[22]

Read the grow-room systems paper, the airflow paper and the harvest and dry paper with this paper. CO2 enrichment gives a good result only as part of the full system.

References

Chandra S, Lata H, Khan IA, ElSohly MA (2008). Photosynthetic response of Cannabis sativa L. to variations in photosynthetic photon flux densities, temperature and CO2 conditions. Physiol. Mol. Biol. Plants 14(4):299-306. https://pmc.ncbi.nlm.nih.gov/articles/PMC3550641/
Tolbert NE, Benker C, Beck E (1995). The oxygen and carbon dioxide compensation points of C3 plants: possible role in regulating atmospheric oxygen and carbon dioxide concentrations. Proc. Natl. Acad. Sci. USA 92(24):11230-11233. https://doi.org/10.1073/pnas.92.24.11230
NOAA Global Monitoring Laboratory (2024). Trends in atmospheric carbon dioxide, global monthly/annual means (2024 global mean 422.8 ppm). (source not from a journal, for example from a manufacturer). https://gml.noaa.gov/ccgg/trends/global.html
Chandra S, Lata H, Khan IA, ElSohly MA (2011). Photosynthetic response of Cannabis sativa L., an important medicinal plant, to elevated levels of CO2. Physiol. Mol. Biol. Plants 17(3):291-295. https://doi.org/10.1007/s12298-011-0066-6
Rodriguez-Morrison V, Llewellyn D, Zheng Y (2021). Cannabis yield, potency, and leaf photosynthesis respond differently to increasing light levels in an indoor environment. Front. Plant Sci. 12:646020. https://pmc.ncbi.nlm.nih.gov/articles/PMC8144505/
Westmoreland FM (2023). Environmental Physiology of Medical Cannabis. PhD dissertation, Utah State University (Bugbee Crop Physiology Lab): CO2 enrichment to 1,200-1,400 ppm raised dry flower yield ~40%. (source not from a journal, for example from a manufacturer). https://digitalcommons.usu.edu/etd2023/243/
Doddrell NH, Lawson T, Raines CA, Wagstaff C, Simkin AJ (2023). Feeding the world: impacts of elevated [CO2] on nutrient content of greenhouse-grown fruit crops and options for future yield gains. Horticulture Research 10(4):uhad026. https://doi.org/10.1093/hr/uhad026
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Agriculture & Horticulture Development Board (AHDB). CO2 enrichment: best-practice guide (a sealed glasshouse can fall to ~200 ppm, cutting canopy photosynthesis ~26%). (source not from a journal, for example from a manufacturer). https://horticulture.ahdb.org.uk/knowledge-library/co2-best-practice-guide-background
Wang X, Lv J, Shi X, et al. (2022). CO2 enrichment in greenhouse production: towards a sustainable approach. Frontiers in Plant Science 13:1029901 (CO2 use efficiency below ~50-60% from leakage). https://doi.org/10.3389/fpls.2022.1029901
Lv Z, Zhu Y, Liu X, et al. (2022). Elevated [CO2] raises the temperature optimum of photosynthesis and thus promotes net photosynthesis of winter wheat and rice. Physiologia Plantarum 174(6):e13757. https://doi.org/10.1111/ppl.13757
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Kader AA, Saltveit ME (2002). Respiration and gas exchange. In Postharvest Technology of Horticultural Crops, 3rd ed. Univ. of California ANR Publication 3311. (source not from a journal, for example from a manufacturer). https://escholarship.org/uc/item/8q37j80t
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US OSHA. Annotated Permissible Exposure Limits, Table Z-1, carbon dioxide (CAS 124-38-9): PEL 5,000 ppm 8-hour TWA. (source not from a journal, for example from a manufacturer). https://www.osha.gov/annotated-pels/table-z-1
US NIOSH / CDC. Pocket Guide to Chemical Hazards and IDLH documentation, carbon dioxide: REL 5,000 ppm TWA, STEL 30,000 ppm, IDLH 40,000 ppm; 1 ppm = 1.80 mg/m3. (source not from a journal, for example from a manufacturer). https://www.cdc.gov/niosh/idlh/124389.html
WorkSafe New Zealand (2025). Workplace Exposure Standards and Biological Exposure Indices, 15th ed., carbon dioxide: WES-TWA 5,000 ppm, WES-STEL 30,000 ppm. (source not from a journal, for example from a manufacturer). https://www.worksafe.govt.nz/topic-and-industry/monitoring/workplace-exposure-standards-and-biological-exposure-indices/all-substances/view/carbon-dioxide/
Azuma K, Kagi N, Yanagi U, Osawa H (2018). Effects of low-level inhalation exposure to carbon dioxide in indoor environments: a short review on human health and psychomotor performance. Environment International 121:51-56. https://doi.org/10.1016/j.envint.2018.08.059
International Code Council (2021). International Fire Code, Section 5307, carbon dioxide systems: gas detection alarming at 5,000 ppm (low) and 30,000 ppm (high) for installations over 100 lb CO2, with automatic shutoff and ventilation. (source not from a journal, for example from a manufacturer). https://codes.iccsafe.org/s/IFC2021P2/part-v-hazardous-materials/IFC2021P2-Pt05-Ch53-Sec5307.3.2
Sensirion. SCD30 / SCD4x NDIR CO2 sensor datasheets and SCD30 field-calibration application note (ASC/ABC assumes periodic exposure to ~400 ppm fresh air). (source not from a journal, for example from a manufacturer). https://sensirion.com/products/catalog/SCD30
Zhengzhou Winsen Electronics. MQ-135 and MQ-5 metal-oxide gas sensor datasheets (MQ-135 infers equivalent-CO2 from mixed VOCs; MQ-5 targets LPG / combustible gas). (source not from a journal, for example from a manufacturer). https://www.winsen-sensor.com/d/files/MQ-5.pdf
Oklahoma State University Extension. Greenhouse carbon dioxide supplementation (HLA-6723). (source not from a journal, for example from a manufacturer). https://extension.okstate.edu/fact-sheets/greenhouse-carbon-dioxide-supplementation.html
US EPA. What is a HEPA filter? A HEPA filter removes at least 99.97% of airborne particles at 0.3 um (particulates only, not gases). (source not from a journal, for example from a manufacturer). https://www.epa.gov/indoor-air-quality-iaq/what-hepa-filter
Souza et al. (2024). Ethylene elimination using activated carbons impregnated with copper oxide. Molecules 29(12):2717 (plain activated carbon held ~1,111 ug ethylene/g; ethylene kinetic diameter ~3.9 A). https://doi.org/10.3390/molecules29122717
Alvarez-Hernandez MH, et al. (2024). Postharvest handling of ethylene with oxidative and absorptive means (review). PMC10933227 (KMnO4 scrubbers ~40-80 mg ethylene/g, irreversible; activated carbon 11-78 mmol/kg; photocatalytic TiO2/UV). https://pmc.ncbi.nlm.nih.gov/articles/PMC10933227/
Abeles FB, Morgan PW, Saltveit ME (1992). Ethylene in Plant Biology, 2nd ed. Academic Press (response threshold ~10 ppb; half-maximal response 0.1-1 ppm).
Cornell University Greenhouse Horticulture. Ethylene in the greenhouse: symptoms, detection and prevention (chronic damage from ~10 ppb; acute epinasty/abscission/chlorosis above 0.1 ppm). (source not from a journal, for example from a manufacturer). https://greenhouse.cornell.edu/crops-culture/ethylene-in-the-greenhouse-symptoms-detection-prevention/
Wheeler RM, Peterson BV, Sager JC, Knott WM (1996). Ethylene production by plants in a closed environment. Advances in Space Research 18(4-5):193-196 (sealed NASA CELSS chamber reached 40-120 ppb ethylene from plant emission alone; visible wheat epinasty). https://doi.org/10.1016/0273-1177(95)00877-h
Hudelson TJ, et al. (2023). Elevated atmospheric ethylene and high temperature independently inhibit fruit set but not vegetative growth in tomato. HortScience 58 (continuous 20 ppb cut fruit yield to 37-51% of control; 40 ppb to 4-11%; vegetative growth reduced <10%). https://journals.ashs.org/hortsci/view/journals/hortsci/58/3/article-p247.xml
Monthony AS, et al. (2026). Sex-specific ethylene responses drive floral sexual plasticity in Cannabis sativa. The Plant Journal (cannabis has an intact, sensitive ethylene-signalling pathway).
US OSHA. 29 CFR 1910.146, permit-required confined spaces: an oxygen-deficient atmosphere is below 19.5% O2 (normal air is ~20.9%). (source not from a journal, for example from a manufacturer). https://www.osha.gov/laws-regs/regulations/standardnumber/1910/1910.146
HPAC Engineering. Latent loads matter: HVAC for cannabis grow facilities (transpiration returns most irrigation water to room air as vapour, the dominant dehumidification load; filters do not remove it). (source not from a journal, for example from a manufacturer). https://www.hpac.com/industrial/article/21270796/latent-loads-matter-hvac-for-cannabis-grow-facilities
Zhang D, et al. (2020). Substantial differences occur between canopy and ambient climate: quantification of interactions in a greenhouse-canopy system. PLoS ONE 15(5):e0233210 (in-canopy RH ~15-25% higher than surrounding air). https://doi.org/10.1371/journal.pone.0233210
Baptista FJ, et al. (2012). Effect of nocturnal ventilation on the occurrence of Botrytis cinerea in Mediterranean unheated tomato greenhouses. Crop Protection (ventilation that lowers humidity reduced grey mould).
Liang J, et al. (2026). CO2 and air-change-rate optimisation in photoautotrophic micropropagation of medicinal Cannabis sativa. Industrial Crops and Products (~4.4 air changes/hour optimal at 800 ppm CO2; higher exchange dried the substrate). Plantlet-scale.
Punja ZK, Collyer D, Scott C, Lung S, Holmes J, Sutton D (2019). Pathogens and molds affecting production and quality of Cannabis sativa L. Front. Plant Sci. 10:1120. https://pmc.ncbi.nlm.nih.gov/articles/PMC6811654/

The Cannabis White Papers · CC BY-NC 4.0 · version 1.2 · updated 2026-07-18