Plan CO2 — A Thing or Two About Monitoring CO2

Internet of Things
Plan CO2 — A Thing or Two About Monitoring CO2

This is the third in a series of articles on using CO2 devices to monitor ventilation indoors and a look at several DIY projects for building these devices. As the pandemic began, we covered the maker’s civic response in a series of articles and videos that we called Plan C. As we now expect a gradual return to normal activity, approaching the end of the pandemic, we call this new series Plan CO2 — how we might get back together safely.

During the lockdown, Tim Dye and a group of friends who share an apartment complex in Petaluma, California, decided that they weren’t going to ring in the New Year alone.  Together they figured out which protocols would support them having a party — masks, open windows with fans blowing fresh air into the room, and a handheld CO2 monitoring device. “Everyone got a little bundled up,” said Tim.  As more people showed up to the party, Tim checked his commercial CO2 monitor. “Sure enough, this is reading 600,” he said. “If that’s running 600 and we’re all partying in here, it’s not a problem. I feel comfortable. I feel comfortable doing that.”

I asked Tim what level would start to make him uncomfortable.  He said if it was starting to get over 800-1000, he’d begin to be concerned.  “Readings of 430 to 450, that’s for ambient air outside, clean air.” he said.  “600 means that you’ve got some CO2 in there —  it’s building up, but still ok.”

When I talked to Tim via Zoom, he was in his one-room office and he looked at a CO2 monitor on his desk.  “It’s 1300 in here now.  I haven’t opened the door in a couple hours and I’ve been talking to people (remotely). It can get even higher, “ he said, reaching to open the door.  Within a minute or two, the CO2 levels went down by half.  CO2 levels are a relative indicator, he explained, not an absolute.  “A CO2 sensor is super-responsive,” he said. “That’s one of the Interesting and unique angles here.  This is not something you need to wait four hours to figure out. You can see the numbers change pretty rapidly.” If you have a CO2 sensor, breathing directly on it will cause the numbers to rise.

“For Covid-19, if there was someone there at the party who was coughing directly on me and spreading droplets that have a lot of virus in them, that could get into me. There was a chance that I could get it.”  To be clear, measuring CO2 doesn’t tell you if there are virus particles in the air. It can indicate, however, that the air inside is more like the air outside, and the room is well-ventilated. This isn’t a risk-free strategy for people gathering in a room, only a way to mitigate risk by providing feedback.

Sensing Changes in Air Quality

Tim Dye of TD Environmental Services is a leading expert in air quality sensors. He knows a thing or two about CO2.  When I first started thinking about how we might use CO2 to measure the proper ventilation of indoor spaces, I thought of Tim and his work on air quality monitors, both in the professional and government sector on projects such as as well as the citizen deployment of a distributed network of sensors to monitor air pollution. 

Tim in backyard in 2012 testing air quality sensors

Tim has spoken at Maker Faire and he wrote a 2014 article for us, “Air Quality Sensors: How Good is Good Enough” comparing expensive, professional-grade sensors to cheaper, off-the-shelf DIY sensors.  (He did this in his garage, by starting a fire in a trash can and immediately putting it out, and then gathering side-by-side readings from a range of devices.)  Now he spends time consulting with organizations to use air sensors effectively and gave a recent talk about the State of Air Sensors.

YouTube player

Last fall 2020, when there was a lot of smoke in the air from wildfires in California, the air outdoors was at times unhealthy.  “I took a time lapse of that sensor, when it was smoky outside and when I was opening and closing the window.  I put little sticky notes on it saying what I was doing.”

“You can see numbers changing, going up and down. Like when I opened the door, the particulate matter indicator would go up, but the CO2 would go down until I closed the door.”

Seeing that change is interesting and a sensor can be thought of as an extension to our limited senses.  “I personally think we as humans are a lot smarter than maybe people think we are. We can get used to indicators and changes if we can sense it and if we can see it.  We can sense things with our body like temperature, but our bodies can’t detect air pollution, so sensors let us see the invisible, harmful air pollution.  All of a sudden we begin to get an understanding of how air pollution changes on a minute-to-minute and day-to-day basis.”

Using air sensors in the classroom or any indoor environment, you allow the occupants of the room to sense what is going on and importantly, they can take small, appropriate actions to change conditions by opening doors and windows or reducing the number of people in the space.  “What does it mean to have not just smarter students but more alert students?” Tim asked.

CO2 and Airborne Transmission

Tim shared a citation to a 2019 article from researchers in Taiwan that looked at the effect of room ventilation on the spread of tuberculosis during an outbreak on a university campus.

Tuberculosis is an airborne disease which spread through infectious aerosol generated by patients during cough. In an indoor environment, infectious aerosol progressively accumulates and put everyone in the room at risk unless the indoor air is continuously replaced with the fresh outdoor air by ventilation.

This study provides the first empirical data showing that improving indoor ventilation to levels with CO2 <1000 ppm is highly effective in controlling a TB outbreak which occurred in poorly ventilated indoor environment.

* Chun‐Ru Du, Effect of ventilation improvement during a tuberculosis outbreak in underventilated university buildings

The researchers found that poorly ventilated rooms at the university were registering levels above 3000 ppm but when the CO2 in the room was lowered below 600 ppm, the tuberculosis outbreak stopped.

CO2 and Human Performance

Higher CO2 levels are known to impair cognition, another reason for keeping track of it in enclosed spaces. If humans can’t detect carbon dioxide (CO2) or its more deadly cousin, carbon monoxide (CO), we aren’t often aware of its effects on us. If you’ve ever been in a closed conference room after lunch, and felt sleepy, it’s not digestion that makes you feel that way. It’s the rising CO2 levels in the room.

While federal guidelines use a number of 5000 ppm as the threshold of harm, researchers have found that the effects of lower levels of CO2 can affect decision making. Look at the chart below to see where levels at 1000 ppm begin to have impact different kinds of decision making.

Elevated Indoor Carbon Dioxide Impairs Decision-Making Performance.*

In the real world, CO2 concentrations in office buildings normally don’t exceed 1,000 ppm, except in meeting rooms, when groups of people gather for extended periods of time.

In classrooms, concentrations frequently exceed 1,000 ppm and occasionally exceed 3,000 ppm. CO2 at these levels has been assumed to indicate poor ventilation, with increased exposure to other indoor pollutants of potential concern, but the CO2 itself at these levels has not been a source of concern.


Data from elementary school classrooms has found CO2 concentrations frequently near or above the levels in the Berkeley Lab study. Although their study tested only decision making and not learning, Fisk and Mendell say it is possible that students could be disadvantaged in poorly ventilated classrooms, or in rooms in which a large number of people are gathered to take a test. “We cannot rule out impacts on learning,” their report says.

*Julie Chao. October 17, 2012 Elevated Indoor Carbon Dioxide Impairs Decision-Making Performance. Retrieved from

An Educational Program for Air Quality Monitoring

Tim showed me a different Air Quality Monitoring (AQM) device that measures particulate matter, CO (carbon monoxide), CO2,  temperature and humidity. It sends reports in real-time to a cell phone. 

2BTech PAM Sensor

This device is part of an educational program, called Air Actions, that Tim’s company developed.  It is a curriculum for developing “a multidisciplinary program at the intersection of air pollution, citizen science, and civic action for middle and high school students.”  The program “empowers students to learn, measure, and take action.” In fall of 2019, he took this device into Petaluma schools and taught it with the teacher

“The teacher gave this to the students, and they went around and started making measurements,” he said. And what did they find? “They found high carbon monoxide levels in the men’s locker room, due to a malfunctioning boiler.  Then they found high CO2 levels in the classroom, not surprisingly because there’s kids in the classroom breathing and talking. The teacher said how are we going to fix that? The kids said open the windows, and instantly CO2 decreased. They were they able to figure that out.”

Build vs. Buy 

I asked Tim about building your own devices such as the early DIY AQM kits that were developed up in the maker community.   “DIY was big in the maker space,” he said.  “You saw folks at Maker Faire doing it. There were articles about it.  It was really new. Then what happened is a bunch of startups emerged and you saw this transition from maker experimental projects to startups. Now there’s a bunch of companies that are building products and doing a really good job at it.”  

If you just want a CO2 monitor, you can find a commercial device on Amazon and elsewhere.  They range from just under $100 up to several hundred dollars.  Tim declined to recommend specific devices because he’s an independent thought leader in the space.

Tim adds: “People call me and they say, ‘Hey, I want to make my own air sensor.’ The first thing I say is, do you really want to do that? Yeah. If you wanna, if you want to play with electronics by all means, go do it. But if you’re after something serious, you’re probably better off buying a device.”

However, while students can learn from just using a CO2 monitor, there’s more to learn from making one. By building the device, students can gain a better understanding how the device works (it’s not magic!), how the sensor detects particles, and how hardware and software work closely together.  They are building tools for sensing the world in new ways. Most importantly, the data they collect might be seen as public health information and can be used to initiate action.

Ventilation Matters

“The thing I noticed is that ventilation matters,” said Tim, speaking of his own experience getting people together during the pandemic. “We’ve been successfully able to have a bunch of parties with 13 or 14 people in a 1300-square-foot apartment and we had a good time. We celebrated the New Year with a party and we were able to feel safer. I was able to feel comfortable with the sensor device there.”

“I think there are whole bunch of benefits to using air sensors in everyday life.”

If you have deployed a CO2 monitoring device to monitor ventilation in a classroom, office or retail business, please let me know.  (dale at

Credits: Photos provided by Tim Dye.

Discuss this article with the rest of the community on our Discord server!

DALE DOUGHERTY is the leading advocate of the Maker Movement. He founded Make: Magazine 2005, which first used the term “makers” to describe people who enjoyed “hands-on” work and play. He started Maker Faire in the San Francisco Bay Area in 2006, and this event has spread to nearly 200 locations in 40 countries, with over 1.5M attendees annually. He is President of Make:Community, which produces Make: and Maker Faire.

In 2011 Dougherty was honored at the White House as a “Champion of Change” through an initiative that honors Americans who are “doing extraordinary things in their communities to out-innovate, out-educate and out-build the rest of the world.” At the 2014 White House Maker Faire he was introduced by President Obama as an American innovator making significant contributions to the fields of education and business. He believes that the Maker Movement has the potential to transform the educational experience of students and introduce them to the practice of innovation through play and tinkering.

Dougherty is the author of “Free to Make: How the Maker Movement Is Changing our Jobs, Schools and Minds” with Adriane Conrad. He is co-author of "Maker City: A Practical Guide for Reinventing American Cities" with Peter Hirshberg and Marcia Kadanoff.

View more articles by Dale Dougherty


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