I’m living in a carbon bubble. Literally.

I’ve been making myself stupid for the last 2 years without knowing it. You might be too.

Joel Jean
11 min readDec 3, 2016

Most of us live with high carbon dioxide (CO2) levels in our offices, bedrooms, classrooms, and cars without ever thinking about it. But recent double-blind studies suggest that CO2 exposure can reduce cognitive and decision-making performance dramatically — by 50% or more at common indoor levels. This is one of the most under-discussed impacts of climate change: It makes us all stupid. I monitored CO2 levels in my bedroom for the last 2 years, and the results are stunning.

When I think about living healthier, I think about sleeping more, exercising more, and eating less. I never think about the air I breathe.

It turns out I should.

The Earth’s atmosphere is about 0.04% carbon dioxide (CO2)—400 parts per million or ppm. But the air we breathe out is 100 times more concentrated in CO2 — around 4% or 40,000 ppm. Every time we exhale, we make the air around us a little less hospitable. Every place we go, we raise CO2 levels.

And where there are lots of people — in our cities, our classrooms, our offices, our homes— CO2 levels rise. A lot. Several studies have measured CO2 concentrations in the human-dense and enclosed places where we spend most of our lives, and the numbers are striking:

  • City centers can have outdoor CO2 levels above 500 ppm due to the “urban CO2 dome” effect [Idso 2001, Jacobson 2010].
  • Offices often have CO2 levels of 600 ppm or higher. Only 5% of US offices have average CO2 concentrations above 1000 ppm [Persily 2008], although one study suggests that a typical meeting room can reach up to 1900 ppm CO2 during 30- to 90-minute meetings [Fisk 2010].
  • Classrooms often reach average CO2 levels above 1000 ppm, as observed across elementary school classrooms in Texas, Michigan, Washington, Idaho, Texas, South Carolina, Sweden, and England [Stafford 2015]. In a 2002 study, 21% of classrooms in Texas had CO2 concentrations over 3000 ppm [Corsi 2002].
  • Public housing units in Boston have average CO2 concentrations of 810 ppm in conventional apartments and 1200 ppm in new LEED Platinum apartments (good insulation reduces air flow) [Colton 2014].
  • Passenger aircraft have an average CO2 concentration of around 1400 ppm during flight, with peak concentrations up to 4200 ppm [NRC 2002].
  • Bedrooms in dorms at the Technical University of Denmark reach CO2 levels of 2400 ppm without ventilation and 840 ppm with ventilation [Strøm-Tejsen 2016].
  • Cars with one occupant reach an estimated steady-state CO2 concentration of 4100 ppm, with the windows closed and the air recirculating [Satish 2012].
Typical CO2 levels in everyday places. Many of the places we live have CO2 concentrations far above the recommended “safe” levels.

Many of us live with high CO2 levels — all day, every day. In a study in Singapore, people carried around CO2 monitors for a week [Gall 2016]. They logged time-averaged CO2 levels of 840 ppm. Nearly all were exposed to above 1100 ppm at least 5% of the time (1.2 hours per day).

So CO2 levels above 1000 ppm are fairly common—so what?

A few hundred parts per million can’t make that big a difference, right?


Before the last few years, most people thought that high indoor CO2 levels reduce perceived air quality and impair work performance only because CO2 is associated with other indoor pollutants. Both are exacerbated by poor ventilation.

But recent studies suggest that CO2 itself is an indoor pollutant. To be specific, a pollutant — according to Wikipedia — is a substance introduced into the environment that has undesirable effects. And CO2 has many undesirable effects, even if we ignore climate change.

CO2 on the Brain

High CO2 concentrations reduce our cognitive performance, our health, and our comfort*. High CO2 levels during sleep are associated with low comfort based on subjective measures of air freshness, mental state, restedness, and mouth/skin dryness [Strøm-Tejsen 2016]. In schools in Washington and Idaho, elevated CO2 levels are associated with increases in student absences: A 1000 ppm increase in CO2 concentration leads to a 0.5–0.9% reduction in annual average daily attendance [Shendell 2004].

Even astronauts suffer: On the International Space Station, the odds of a crew member reporting a headache double for every 1300 ppm increase in CO2 concentration [Law 2014]. And it’s hard to keep CO2 levels low in an airtight box filled with people.

*Conventionally, indoor CO2 levels below 1000 ppm are considered to be acceptable. Much higher CO2 levels can have direct health effects: 10,000 ppm (1% CO2) causes increases in respiratory rate, 50,000 ppm (5%) causes dizziness and confusion, 100,000 ppm (10%) causes visual disturbances, tremors, vomiting, disorientation, hypertension, and loss of consciousness, and 250,000 ppm (25%) can cause death [Lipsett 1994][Rice 2003].

To me, the most compelling studies — the studies that spurred me to write this — are a pair of recent double-blind experiments looking at how CO2 levels affect human cognitive performance.

In both cases, the researchers put their guinea pigs — college students in one case, working professionals in the other — in simulated office environments with controlled CO2 levels for several hours at a time. They measured the participants’ cognitive function using a standard, well-validated test of decision-making performance with 9 metrics, including basic activity level (number of actions taken), initiative (development of new or creative activities), information usage (ability to use information effectively), breadth of approach (flexibility in approach), and basic strategy (number of strategic actions taken). All of these metrics are probably important for performing well at work — and in life.

Both studies reached similar conclusions. The first study (Satish et al. 2012) found that decision-making performance of college students was massively impaired at high CO2 levels. Averaged across all metrics, performance was reduced by 12% at 1000 ppm and by 51% at 2500 ppm compared to the 600 ppm control scenario. At 2500 ppm, the participants’ cognitive function — initially above-average compared to a reference population of 20,000 U.S. adults — dropped to marginal or dysfunctional levels on 5 of the 9 metrics*. Keep in mind that these metrics cover things like the ability to use information, to be creative, to take initiative, and to make strategic decisions. No big deal.

The degree to which cognitive function is impaired with increasing CO2 concentration seems to be close to linear within the examined range of 600 ppm to 2500 ppm. Data from Satish 2012.

*People actually got better at focused activity with higher CO2 levels. Focused activity also improves with alcohol consumption, due to “overconcentration”—the tendency to focus on small details at the expense of the big picture. This is consistent with my experience with drunk people.

High CO2 levels reduce cognitive performance dramatically. Figure from Satish 2012.

The second study (Allen et al. 2015) found that the decision-making performance of working professionals—managers, engineers, programmers, and designers—was also impaired at high CO2 levels. Averaged across all metrics, performance was reduced by 15% at 945 ppm and 50% at 1400 ppm compared to the 550 ppm control*. The study reports that a 400 ppm increase in CO2 concentration is associated with a 21% decrease in cognitive function (averaged across all domains)**, after controlling for volatile organic compound (VOC) levels and differences between participants.

*945 ppm is the CO2 level expected at the ASHRAE-recommended minimum ventilation rate of 20 cubic feet of outdoor air per minute per person. 1400 ppm is a higher, but not uncommon indoor CO2 concentration, corresponding to the maximum observed 8-hour time-weighted-average CO2 concentration across many U.S. buildings.

**This result is consistent with the findings of the first study, which examined CO2 levels up to 2500 ppm. It’s not clear why—physiologically—such small increases in ambient CO2 levels impair cognitive performance so dramatically.

Let me repeat that: Increase CO2 by 400 ppm, and you decrease cognitive function by over 20%.

Remember, CO2 levels in our classrooms, bedrooms, and cars are often above 1000 ppm, 2000 ppm, and 4000 ppm, respectively—600–3600 ppm above outdoor levels. That means we could be handicapping our brains—as we learn, sleep, and drive—by 50% or more!


My Carbon Bubble

Out of curiosity, I’ve been monitoring the CO2 levels in my bedroom (10.5 ft x 14 ft x 8.5 ft) in Cambridge, MA, continuously over the last 2 years, using a scale with a built-in CO2 sensor (Withings WS-50). I’m a grad student at MIT, so this isn’t such unusual behavior (I think). I’ve also recorded my sleep schedule every day for the past year, so I can correlate changes in CO2 concentration with time spent sleeping (yes, grad students sleep just like normal people — most nights).

In my bedroom, the average CO2 concentration is around 1500 ppm when I’m in town and 890 ppm when I’m out of town. When I’m sleeping, the average CO2 level is around 2500 ppm, and it often peaks above 3000 ppm.

CO2 concentration in my bedroom in Cambridge, MA from July 2014 to November 2016. CO2 levels often spike to over 3000 ppm overnight.
Distribution of CO2 levels in my bedroom in Cambridge, MA from July 2014 to November 2016. CO2 levels are clearly higher when I’m in town.

The CO2 concentration in my room follows a predictable diurnal cycle — it rises during the night and falls during the day when I’m out of the house. When I’m out of town, the CO2 concentration is lower overall but still follows the same daily trend, presumably because I have 3 housemates who also breathe once in a while.

CO2 levels in my bedroom throughout the day when I’m in town (red) and out of town (blue)

On weekends, when I spend more time in my room, the daytime CO2 level is higher than on weekdays.

Comparison of daily CO2 levels in my bedroom on weekends (red) vs. weekdays (blue)

Most evenings, I keep my bedroom well-sealed, with the door and windows closed. The poor ventilation should lead to a steady increase in the CO2 concentration while I’m in my room. And that’s exactly what I find.

During a typical night, the CO2 level in my room rises by more than 1000 ppm—to over 2500 ppm—between when I get home in the evening (usually 4–6 hours before bedtime) and when I wake up in the morning.

The dangers of sleeping in: CO2 levels in my bedroom for every night from November 2015 to November 2016 (black). Each line begins 6 hours before I go to sleep and ends when I wake up. Red lines indicate mean values—the sharp rise after 8 hours asleep is an artifact due to limited data. Negative times are when I’m in my room before going to sleep.

It’s especially bad in late fall, when it’s too cold to leave windows open but not cold enough to need central heating. From October through December, the average CO2 level in my bedroom rises overnight by more than 1500 ppm. At other times of year, the overnight increase is less dramatic, as fresh air circulating through windows and around the house reduces CO2 levels.

CO2 levels in my bedroom through the night, averaged seasonally.

CO2 and You

To summarize, most of us are living with high CO2 concentrations in our offices, bedrooms, classrooms, and cars without even thinking about it. Double-blind experimental studies suggest that CO2 exposure can reduce cognitive and decision-making performance dramatically — by 50% or more at common indoor levels.

Sleeping in might be making me stupid.

You might be reducing your own brainpower if you spend a lot of time in rooms or buildings with tight insulation and limited air flow. For example, I added foam insulation around my bedroom door frame to dampen noise and reduce heat loss during the winter — not good for air circulation. I installed the insulation in May 2015, and CO2 levels in my room have been much higher ever since.

CO2 levels in my bedroom before and after adding insulation in May 2015

Luckily, you can mitigate high CO2 levels in your living environment without too much trouble. If you want to collect your own data, buy a CO2 logger or air quality monitor—they start at $99 (consider Netatmo, Awair, AZ Instruments, or the more hardcore CO2Meter)—then act if necessary.

If you want to solve the problem, you can usually keep CO2 levels low just by keeping your bedroom door or a few windows open [Strøm-Tejsen 2016]—especially if you live in a place with moderate temperatures year round (say, northern California).

To test the effect of ventilation on indoor CO2 levels, I alternated sleeping with my bedroom door open and closed for several days. With the door closed—no ventilation—average CO2 levels increased by over 1000 ppm during the night (overnight average of 2470 ppm ± 560 ppm). With the door cracked open—moderate ventilation—average CO2 levels actually decreased by roughly 500 ppm during the night (overnight average of 1840 ppm ± 860 ppm).

CO2 levels in my bedroom with the door closed (red) vs. open (blue)

One of the scariest and most underreported impacts of climate change — and climate change has plenty of scary and underreported impacts — is that rising CO2 levels in the atmosphere lead to higher CO2 levels in our workplaces and homes, which make it harder for us to work and think effectively. And unlike many other climate impacts, this one will be unavoidable and unadaptable*. Even the rich will suffer — i.e., get stupid.

*The consistency of results between studies with varying lengths of exposure suggests that people don’t get desensitized or adapt to high CO2 levels, at least over the time scale of days. Cognitive performance is hampered in all cases.

Even if the world meets all the national emissions reduction pledges under last year’s Paris Agreement—which Trump may axe—we’ll hit an atmospheric CO2 concentration of 695 ppm by 2100 [Climate Interactive]. That’s a 300 ppm increase from today’s outdoor levels. If indoor CO2 levels rise by the same amount, we can expect a 16% decrease in average brainpower — for every single person on the planet.


Time to get mitigating.


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