Can Water Reduce CO₂ Inside a Car? Our Real Cabin Test

Carbon dioxide dissolves in water.

That made us curious about a simple question:

If we put a large amount of open water inside a car, could it noticeably reduce cabin CO₂?

Instead of debating it theoretically, we tried it ourselves.

We placed a large open container of water inside the vehicle and monitored the cabin with our EVO-CO2V.

At the beginning of the measurement sequence, the monitor showed approximately:

792 ppm CO₂

As the test continued, the reading increased through approximately:

818 ppm → 955 ppm → 1,194 ppm → 1,375 ppm → 1,417 ppm → 1,464 ppm → 1,516 ppm

So in our test, the open water did not prevent cabin CO₂ from continuing to rise.

Quick Answer

Our real test showed:

Test stage	Cabin CO₂
Early reading	~792 ppm
Later	~818 ppm
Later	~955 ppm
Later	~1,194 ppm
Later	~1,375 ppm
Later	~1,417 ppm
Later	~1,464 ppm
Highest reading shown	~1,516 ppm

We are not saying that water cannot absorb carbon dioxide.

It can.

Our observation is narrower:

A large open container of ordinary water did not stop cabin CO₂ from increasing under the conditions of our test.

Watch Our Real Water-and-CO₂ Test

The video below shows the actual open container of water inside our vehicle and the live EVO-CO2V readings as cabin CO₂ continued to rise.

These measurements were recorded by our team during a real-world test. Despite the large open container of water, cabin CO₂ rose from about 792 ppm to about 1,516 ppm during the sequence shown.

Why We Tried This Experiment

We know that carbon dioxide can dissolve in water.

That raises an intuitive idea:

If we increase the amount of water inside a confined cabin, perhaps some of the CO₂ in the air will dissolve into it.

Chemically, the first part is correct.

CO₂ does transfer between air and water.

But the more practical question is:

Can an ordinary tub of water absorb enough CO₂, quickly enough, to make a meaningful difference inside an occupied vehicle?

That is what we wanted to explore.

How We Ran the Test

We placed a large open container of water inside the car.

We then monitored the cabin CO₂ concentration with the EVO-CO2V while remaining in the vehicle.

An early reading shown in the video was approximately:

792 ppm

The concentration subsequently continued upward.

Later readings included:

818 ppm

then:

955 ppm

then:

1,194 ppm

then:

1,375 ppm

and eventually:

1,516 ppm

The video is edited, so we do not use the apparent video length to calculate an exact ppm-per-minute rise.

Our conclusion is based on the direction and values of the readings shown.

What We Actually Observed

The result was straightforward.

Even with a substantial open surface of water in the cabin, the CO₂ concentration did not stabilize at the initial level.

Instead, it continued rising.

From approximately:

792 ppm

to approximately:

1,516 ppm

That is an increase of roughly:

724 ppm

during the measurement sequence shown.

Again, this does not mean that zero CO₂ entered the water.

It means that any absorption occurring was not sufficient to prevent the cabin-air concentration from rising under our test conditions.

But CO₂ Really Does Dissolve in Water

Yes.

This is established physical chemistry.

NIST reports Henry’s Law data for carbon dioxide in water. Around 25°C, a representative solubility constant is approximately 0.034 mol/(kg·bar). NIST Henry’s Law data for carbon dioxide in water

Henry’s Law means that, at equilibrium, the amount of dissolved gas depends partly on the gas’s partial pressure above the liquid.

So as the CO₂ concentration in cabin air rises, somewhat more CO₂ can dissolve into the water.

But that does not automatically mean that an open tub of water becomes an effective CO₂ removal system.

Why the Amount Dissolved Can Still Be Small

We can use the NIST Henry’s Law value to make an illustrative estimate.

At about 25°C, if we simplify the situation and treat the water as plain water at equilibrium:

• air at 792 ppm CO₂ corresponds to roughly 1.2 mg of dissolved molecular CO₂ per liter of water;
• air at 1,516 ppm CO₂ corresponds to roughly 2.3 mg/L.

The difference is only around:

1.1 mg of additional dissolved molecular CO₂ per liter of water

for that change in gas-phase concentration.

This is only an idealized calculation—it does not include all pH, alkalinity and carbonate chemistry—but it helps explain the scale.

An ordinary tub of water may absorb some CO₂, but that is very different from continuously removing the CO₂ being generated inside an occupied cabin.

Surface Area and Mixing Also Matter

Gas does not instantly equilibrate with all the water in a container.

CO₂ first has to cross the air-water interface.

How quickly that happens depends on factors such as:

• exposed water surface area;
• air movement;
• water movement;
• water temperature;
• CO₂ partial pressure;
• time.

A deep container holding more water does not automatically mean all of that water is immediately available to absorb CO₂ at the same rate.

That is another reason why simply placing a large volume of still water inside the cabin is very different from an engineered gas absorber.

Why Industrial CO₂ Absorption Is Different

There are real systems that remove CO₂ using liquids.

But they generally improve gas-liquid contact or use chemical solutions that react with CO₂.

For example, industrial scrubbers may use:

• packed columns;
• sprays;
• forced gas flow;
• alkaline solutions;
• amine solutions;
• high pressure.

Our test used none of those things.

We simply put ordinary open water inside a passenger vehicle.

So our result should not be interpreted as:

“Water cannot absorb CO₂.”

A better interpretation is:

Ordinary standing water was not an effective substitute for ventilation in our car test.

Why Cabin CO₂ Can Still Rise

People inside a vehicle continuously exhale carbon dioxide.

In a confined cabin, CO₂ can accumulate when its production exceeds the rate at which it is removed.

Vehicle research has found that ventilation settings are a major determinant of cabin CO₂ accumulation, particularly when outside-air exchange is restricted. vehicle cabin CO₂ and ventilation study

That is fundamentally different from relying on passive absorption by a container of water.

Fresh-air ventilation continuously replaces some cabin air with outdoor air.

A bowl or tub of water does not do that.

Water Is Not a Substitute for Fresh-Air Ventilation

This is probably the most useful practical takeaway from our experiment.

If cabin CO₂ is increasing, placing water in the vehicle should not be treated as a substitute for increasing air exchange.

We have directly observed the effect of ventilation in several of our other tests.

Related Test: Recirculation vs Fresh Air

In one real driving test, cabin CO₂ dropped from approximately 2,150 ppm toward 500 ppm after we switched from recirculation to outside-air ventilation.

See our real recirculation vs fresh-air CO₂ test.

Related Test: Fan Speed in Fresh-Air Mode

We also tested four occupants while keeping outside-air mode selected but changing the blower setting.

At fan level 1, CO₂ rose from about 524 ppm to 866 ppm.

At fan level 4, readings remained around the high-400 ppm range during the later segment.

See our four-person fresh-air fan-speed CO₂ test.

Related Test: Inside vs Outside the Same Car

We directly measured approximately:

1,500 ppm inside

versus:

450 ppm outside

during another test.

See our real inside-vs-outside car CO₂ test.

About the 1,400 ppm Alert

During this test, the reading eventually moved above the EVO-CO2V’s configured alert point of approximately:

1,400 ppm

That value is the monitor’s alert setting.

We are not presenting 1,400 ppm as a universal boundary between “safe” and “unsafe” air.

The purpose of the alert is to give us a practical indication that cabin CO₂ has risen and ventilation may deserve attention.

CO₂ Is Not Carbon Monoxide

This experiment measured:

Carbon dioxide — CO₂

It did not measure:

Carbon monoxide — CO

The EVO-CO2V is a CO₂ monitor.

It does not determine whether carbon monoxide, NO₂, VOCs or other pollutants are present.

What This Test Does Not Prove

This was one real-world experiment.

We did not measure:

• the exact volume of CO₂ absorbed by the water;
• dissolved inorganic carbon in the water;
• water pH before and after;
• gas-transfer rate;
• exact cabin air-exchange rate;
• exact CO₂ generation rate of the occupants.

So we cannot say:

“The water absorbed zero CO₂.”

We also cannot calculate an exact absorption efficiency from this video.

What we can say directly from what we measured is:

Despite the large open container of water, cabin CO₂ continued rising from roughly 792 ppm to roughly 1,516 ppm during our test sequence.

How We Would Improve This Experiment

If we repeated this as a more controlled experiment, we would run two matched tests:

Test A: same vehicle, occupants and ventilation conditions, without the water.

Test B: identical conditions, but with the water container present.

We would also record:

• exact water volume;
• water temperature;
• water surface area;
• starting cabin CO₂;
• outside CO₂;
• elapsed time;
• ventilation setting;
• vehicle speed;
• water pH before and after.

Then we could compare the CO₂ curves directly.

That would allow us to answer a more precise question:

How much, if anything, did the water change the rate of cabin CO₂ accumulation?

Our current video does not answer that quantitatively.

Our Takeaway

The chemistry is real:

CO₂ dissolves in water.

But our real car test showed that this fact alone does not make a container of ordinary water an effective way to control cabin CO₂.

With the water present, our monitor still rose from approximately:

792 ppm

to:

1,516 ppm

For us, the practical lesson is simple:

Passive water absorption is not a replacement for adequate vehicle ventilation.

When we want to understand what is actually happening inside the cabin, we would rather measure the CO₂ directly and adjust ventilation accordingly.

Related Resources

• EVO-CO2V Car CO₂ Monitor — View the product and its cabin CO₂ monitoring features.
• Vehicle Air Quality — Explore more real-world cabin CO₂ tests and measurement guides.