Carbon Dioxide (CO2) Monitors

In our COVID Statement, we declare that carbon dioxide (CO2) monitors may be used to establish ventilation rates of occupied rooms.  The idea is straightforward: people exhale CO2, and the concentration of CO2 will rise if more is produced than can leave the room.

The relevance to COVID safety, then, is when an infectious person is present in a room.  With several caveats, CO2 concentration may be used as a surrogate for the total volume of exhales in a room, including any exhaled aerosols containing SARS-CoV-2 virions.  This is the main mode of transmission (airborne) for COVID-19.

Shift Sight completed evaluating low-cost monitors for use in classrooms.  We did not find a low-cost (non-NDIR / non-photoacoustic type sensor) monitor that reliably measured CO2.  We are designing, building, and selling one at cost.

As with any tool, it's important to understand how it works to also understand its limitations.

We need to ask three questions:

1. Is the monitor accurate?
2. Is the monitor in the right place?
3. Is the monitor solving the right problem?

Use a button to jump ahead, or just keep reading.

The Sensor

The Right Place

The Right Problem

The Sensor

CO2 monitors are becoming a popular way to get piece of mind in an indoor setting.  It's understandable, too: you turn it on, and you get an instant CO2 ppm reading along with (commonly) a red, yellow, or green indicator.

How does it arrive at a number and an indicator?  We usually don't worry about this piece, but we should!

The CO2 monitor contains a sensor.  If the sensor is not accurate, the remaining questions posed above (right place? right problem?) are meaningless.

Shift Sight proceeded to look at a few types of sensors — the actual component — that would go into a final product.  (As a company that provides these types of solutions, this is what we do!)  We found several options via white-market distributors and priced them at production quantities:

Type	Description	Sensor Cost (@ qty 1000)	Range, Accuracy (ppm)
CO2 Equivalent (CO2eq)	This part measures H2 and ethanol (via MEMS) as a CO2 surrogate.  Others may rely on different compounds.	$7.10	CO2eq: 400 to 60,000 H2: Max ±40 Ethanol: Max ±50
CO2 NDIR	Measures CO2 IR absorption directly via an emitter / receiver in a chamber.	$41.08	CO2: 400 to 10,000 Accuracy ±(30ppm + 3%)
Photoacoustic CO2	MEMS IR emitter; microphone captures CO2 vibration.	$29.04	CO2: 400 to 5,000 Accuracy ±(40ppm + 3%)

Immediately, this tells us that CO2 monitors selling for $50 or less are likely using a CO2 equivalent sensor.  (There are exceptions, such as NDIR models that use gray-market and counterfeit parts.)  The photoacoustic sensors are relatively new and may be in a few mass-produced products.  This table is an anecdotal sample from one sensor supplier, but others are comparable in price.

We've evaluated data from several monitors that use CO2-equivalent sensors.  Unfortunately, none of these units track CO2 reliably against higher-end monitors.  There are a number of bioeffulents that influence these sensors; notably, and unfortunately, environmental concerns (such as gaseous concentration of cleaners) influence their measurements substantially.

Monitors using CO2 NDIR sensors typically sell for between $150 to $250 and will proudly advertise this sensor type as a selling point.

The Right Place

CO2 is produced in one container from vinegar and baking soda, then poured into a second container (only the gas).  There is enough CO2 in the second container to extinguish the fire when the gas is poured (!) on top of it.  This is an incredibly high concentration of CO2!

Where is your CO2 monitor?  Where are your emitters?

Let's make some basic assumptions about how CO2 spreads in a classroom.

• Children can be treated as CO2 emitters.  (Your not-so-little bundle of joy is an aerosol generator with each breath.)
• CO2 does not propagate like bioaerosols when emitted: CO2 has no dipole.  Aerosols are charged and expand rapidly.
• However, considering convection and the continuous disturbances in a room of emitters, these two dissimilar things can be approximated to disperse equally throughout a room as time passes.

Ideally, any CO2 monitor would be placed at the same height as the emitters to catch the highest localized concentrations.  If students remain seated most of the day, this would be about their head level.  Most NDIR-based monitors advise against placing the sensor in the direct path of airflow.  If possible, monitors should not be placed within 4 feet of students; their near-field exhales will artificially skew the CO2 (which should be an ambient measurement).

Note that most CO2 monitors will not update quickly.  Their gas sensors are usually in an enclosed chamber and rely on diffusion.  (Along those lines, if you are a parent that gives your child a CO2 monitor, ensure that they know to not cover the sensor chamber opening.  Inside a desk or a backpack may give marginally different readings than what your child is inhaling.  Your child should also not exhale directly at the aperture.)

Measurements of relevance are typically a 15-minute short-term exposure limit, while OSHA looks at an 8 hour exposure limit.  The OSHA scale is less helpful when thinking about virion-laden aerosols, since a COVID infection with the Delta / Omicron / BA.2 variant can happen in seconds if a mask / respirator seal fails.

Although CO2 is a heavier gas, it does not categorically settle near the floor; this discussion is out of scope for this page.  (This is not true in mines, where convection is barely present.)  There may also be hundreds, if not thousands, of different gases and particulates in the air of any classroom that are heavier than CO2.

The Right Problem

Now that we have the right sensor in the right place, it's important to determine if the right problem is being solved.

1. For general health considerations, CO2 levels should always remain below 1,000 ppm.  Ideally, this would be below 600 ppm.  ASHRAE suggests 700 ppm over outdoors (currently over 400 ppm) which would be 1,100 ppm.
   
2. If a CO2 monitor shows that levels climb beyond this limit, the room requires additional ventilation.  Windows may be opened to ensure the room is suitable for the occupancy or current activity.  For example, exercise produces more CO2 per occupant per minute.
   
3. An occupied classroom with no mechanical ventilation or windows that open will have CO2 continuously climbing.  However, if that same room HEPA air cleaners operating, there is now a disconnect between the level of potentially infectious aerosols and measured CO2: the HEPA removes the aerosols but does not influence the gas.
   
4. Indoor CO2 should be measured against outdoor levels.  Fresh air contains a little less than 430 ppm CO2 most days in most areas.  This changes according to elevation, but respiration rate (ppm per person per hour) is relatively the same.
   
5. Trends across time are also important: a room with continuously increasing CO2 is more concerning than a room that stays constant at 600 ppm.
   

As discussed in the introduction and revisited above, a CO2 monitor in the right place can be used to assess ventilation in a building and correlate a degree of safety for airborne SARS-CoV-2.

Building ventilation and filtration are both NPIs and address one part of safety.  PPE, such as properly-fitting respirators (N95 or equivalent, or better) are the other piece.  They complement each other.  See our COVID Statement for more.

Page updated 2022-06-06