CO₂ is no joke

Carbon dioxide is commonly known for its macro effects as a greenhouse gas, but most people are not aware of what elevated indoor CO₂ can tell you about ventilation and shared indoor air. Let's explore.

Carbon dioxide molecule diagram with bond and oxygen callouts.

What is normal?

Outside air contains anywhere between 400-500 ppm of CO₂. ppm is parts per million, or how many units of something exist within a million units.

At these levels, CO₂ itself is not the story. This is roughly the outdoor baseline most indoor readings are compared against.

CO₂ and airborne infection risk

The correlation between CO₂ concentration and risk of airborne diseases is well-established.

CO₂ is mainly a ventilation and rebreathed-air indicator, not a complete picture of air quality. High indoor CO₂ usually means that a higher fraction of what you are breathing was already in someone else's lungs.

Humans exhale CO₂ at around 40,000 ppm. As indoor air gets stale, that pushes up the odds of transmission for respiratory infections between humans.

We can calculate the percentage of rebreathed air in the room using the formula below.

                    (indoor_co₂ - outdoor_co₂)
% rebreathed air =  ―――――――――――――――――――――――――
                    (exhaled_co₂ - outdoor_co₂)

If we plot this for various CO₂ concentrations, the exposure risk climbs fast as CO₂ accumulates.

In fact, there are studies showing that CO₂ is a useful proxy for SARS-CoV-2 risk in shared indoor air↗pubs.acs.orgExhaled CO2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and ActivitiesShows how exhaled CO2 can act as a practical proxy for shared rebreathed air and airborne infection risk indoors.. Keeping CO₂ as low as possible reduces the amount of rebreathed air you share with everyone else in the room.

Okay, but how do you make sense of what 5,000 ppm means? Read on, anon.

What does CO₂ physiologically do?

CO₂ is a normal part of breathing, and at very high concentrations it clearly affects blood flow, acidity, and symptoms. But the lower concentrations people usually encounter indoors are less dramatic than a lot of internet infographics suggest.

At ordinary indoor levels, CO₂ is often more useful as a marker of low ventilation and rebreathed air than as a fully proven explanation for every headache, sleepy feeling, or drop in performance.

A newer crossover study also found that exposure near 5,000 ppm shortened sleep latency and increased subjective sleepiness in healthy volunteers↗sciencedirect.comImpact of carbon dioxide exposures on sleep latency among healthy volunteers: A randomized order, paired crossover study, evidence from the multiple sleep latency testA controlled sleep study reporting shorter sleep latency and higher subjective sleepiness after elevated CO2 exposure..

Ventilation, occupancy and exposure duration

CO₂ is not just about the number on the meter. It is about how many people are sharing a space, how much fresh air is entering it, and how long you stay there. Those three variables decide how quickly rebreathed air accumulates.

A crowded bedroom overnight, a packed classroom for an hour, and a bus commute with shut windows can all land in a similar ppm range, but the path there is different. Higher occupancy raises CO₂ faster, weak ventilation clears it more slowly, and longer exposure gives your body more time to feel the effects and your infection risk more time to compound↗pubs.acs.orgExhaled CO2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and ActivitiesShows how exhaled CO2 can act as a practical proxy for shared rebreathed air and airborne infection risk indoors..

That is why the same 1,500 ppm reading feels much worse in a tiny sealed room than in a large space that only spiked briefly. The reading matters, but the shape of the exposure matters too.

Below are ballpark numbers for CO₂ levels in some common places you breathe in. Remember that CO₂ levels and their effects vary with ventilation, occupancy, and duration of exposure.

Outdoor air CO₂ levels for reference

400-500 ppm

Clubs

2500-9000+ ppm

A crowded club can soon get hot and humid with elevated levels of CO₂, sometimes reaching 9,000+ ppm.

Closed meeting rooms

1500-2500 ppm

Depending on the size of the room, long meetings can push CO₂ into a range where the room starts to feel stale and heavy.

Gyms

1500-2500 ppm

A crowded gym with dozens of people exhaling at elevated heart rates. Ventilation matters fast in spaces like this.

Closed bedrooms

1500-2500 ppm

Overnight, CO₂ can rise steadily if there is no fresh air circulating in the room, disturbing your sleep.

Airplanes

1500-2500 ppm

Flights usually have abundant fresh air mixed with filtered air in the cabin. Crowding can still increase CO₂, but usually only to higher levels on long flights. Cabin humidity is also very low, which is why your mouth and lips dry out.

Buses (decent crowd)

1500-2500 ppm

Short bus commutes are usually fine, but overnight bus journeys can be much more problematic.

Cars

1500-2500 ppm

Long car rides with closed windows, full recirculation, and multiple passengers can become surprisingly risky. Read on.

Verified over multiple measurements made using an Aranet4 sensor.

CO₂ and cognition

You're in a packed meeting room. Someone closed the door 30 minutes ago. Humans exhale CO₂ at 40,000 ppm. With little fresh air entering the room, you're gradually inhaling what others have exhaled, about 1% of what you breathe.

% rebreathed air = (indoor_co₂ - outdoor_co₂)/(exhaled_co₂ - outdoor_co₂)

The evidence for direct cognitive effects from typical indoor CO₂ levels is mixed. Some controlled studies reported lower decision-making scores as CO₂ rose, but the literature is not settled and it is hard to separate CO₂ itself from heat, ventilation, odors, sleepiness, and the general misery of a stale room.

What people often notice first is the room itself feeling bad. You might feel:

Sleepy or drained

A mild headache

That stale-room heaviness

CO₂ and car rides

Cars are a nasty tradeoff. Recirculation helps keep traffic particles and fumes out, but it also lets exhaled CO₂ build very quickly inside a small cabin.

Experimental vehicle studies found cabin CO₂ can reach roughly 1,500 ppm within about 20 minutes with one occupant, and around 3,000 ppm within the same time with two occupants. Separate recirculation studies found full recirculation can drive cabin CO₂ to around 3,000 ppm, while allowing some fresh air keeps CO₂ much lower without giving up all the particle protection↗pubmed.ncbi.nlm.nih.govCarbon dioxide accumulation inside vehicles: The effect of ventilation and driving conditionsModeling and measurements showing how recirculation, trip duration, speed, and passenger count drive cabin CO2 higher..

The practical rule is simple: full recirculation is useful briefly in heavy traffic, tunnels, or smoky air, but it is a bad idea for long rides with passengers and closed windows.

My god, what can I do about it?

Fresh outdoor air is the fix. If a room feels stale, crack a window, open a door, step outside for a few minutes, or reduce how many people are packed into the space. If you sleep in a closed bedroom, leave a window or door slightly open when you can.

Open windows for cross-ventilation whenever possible

Avoid long stays in small crowded rooms

Use car recirculation briefly, not for whole rides

Use a CO₂ monitor if you spend time indoors for work

Personal fixes help, but the real solution is ventilation that matches the occupancy of the room↗pubs.acs.orgExhaled CO2 as a COVID-19 Infection Risk Proxy for Different Indoor Environments and ActivitiesShows how exhaled CO2 can act as a practical proxy for shared rebreathed air and airborne infection risk indoors..