I was chatting to my neighbour comparing how Canada differs from South Africa. I mentioned that triple glazing is very uncommon in South Africa. He answered that I should remember to sleep with the bedroom door open – because otherwise the CO₂ levels can get too high while you sleep and make you sleep badly and we aren’t used to efficiently sealed ventilation.

I have no idea how true this is. It sounds plausible: you are cooped up for hours in a sealed room doing nothing (give or take) but breathing. I want to buy a CO₂ logger and plot this to see if it really does rise to extreme levels. I also need to find out exactly what those extreme levels are.

But accuracy of the theory aside, his comment got me searching on the web, and it’s a fascinating rabbit hole to go down.

Some people who feel they have sleep disturbances due to CO₂ buildup put plants in their rooms to help absorb the exhaled CO₂. But I thought plants normally absorb CO₂ during the daytime, and release small amounts of CO₂ at night, photosynthesis and all that.

 

Crassulacean Acid Metabolism

Certain plants are adapted to arid and hot regions and keep their stomata closed during the daytime to minimize water loss through transpiration.

They have adapted to open their stomata at night when it’s cooler and absorb CO₂ during the dark hours. They then use this stored CO₂ during the day without losing too much water.

There are arid regions in South Africa (Karoo, Fynbos regions) where there is a distinct herbaceous “early evening smell” around sunset – I wonder if it isn’t these plants that open up their stomata and we smell their “exhalation”.

The “absorption” of carbon dioxide and conversion into usable carbon compounds is called carbon fixation, and doing so in the dark is also called dark fixation. Dark fixation would also make an OK band name.

Carbon fixation as part of photosynthesis comes in three flavours: C3 and C4 (stomata open during daylight) and CAM (Crassulacean Acid Metabolism, stomata open at night).

The Crassulacean in CAM comes from the genus of succulent plants first used in the modelling of the CAM carbon fixation mechanism. Crassula is a genus of succulent plants common to South Africa. Most of them are beautiful. So by choosing the right South African plants you can absorb CO₂ in your Canadian room at night.

 

How much CO₂ do you breathe out per night?

An “average” (whatever that means) person will exhale roughly 0.013 cubic meters of CO₂ per hour while sleeping.

Activity	CO2 exhaled per hour, m3/hour
Sleep	0.013
Resting, awake	0.020
Normal awake, light activity	0.080 – 0.130
Working, vigorously active	0.330 – 0.380

If you sleep eight hours let’s say the first hour counts as resting, giving us 0.020m³, and the other seven hours adds up to 0.091³, giving us a rough total of 0.111m³ CO₂ per person per night.

For interest, (if I did my math right) that means 0.203 kg of CO₂ (at 101kPa and 21⁰C), or since carbon dioxide weighs 44.009g/mol, about 4.6 mol of CO₂ per person per night. That feels like a lot, especially considering how many of us there are.

Let’s call 0.111m³ (i.e. 111 liters) of CO₂ one night-person of CO₂.

 

How many plants do you need?

We need to know how much CO₂ these plants can absorb on average. This is where I started running into Elsevier’s unethical paygates, so I will not directly link to these studies. One study on CO₂ removal from hospital rooms did give me a rough indication of how much CO₂ a plant can absorb.

The test room was 8m x 4.5m x 4m (i.e. 144m³). For the test it was filled to 400ppm CO₂. Since 400ppm is 0.4% (or 1/2500) that gives us about 0.0576m³ of CO₂ to begin with.

Five plants of various types and sizes were then left overnight during three separate tests and CO₂ levels were measured afterward.

	Bryophyllum pinnatum (Air Plant)	Astroloba congesta	Sedum rubrotinctum (Jelly Bean Plant)
Number of pots	5	5	5
Plant height, cm	30	25	30
Leaf area per plant (cm2)	864	384	400
CO2 after 10 hours in dark, ppm	388	263	368
“ppm” CO2 absorbed	12	137	32
Percentage drop	2.9%	34.3%	7.9%
m3 CO2 absorbed	0.00167	0.0198	0.00455
Night-persons absorbed after 10 hours, per 5 plants	0.015	0.178	0.041
Night-persons adjusted to 8 hours, per 5 plants	0.012	0.143	0.033

Conclusion:
You need a fuckload of plants.

You’ll need 35 Astroloba congesta pots filled with 25cm high plants to absorb one night-person in eight hours. The study also highlighted that there are big variances in CO₂ absorption between the different species. Since CO₂ uptake would logically be related to the plant’s dry weight it would make sense to choose plants that grow fast.

Other CAM plants that are pretty too are Haworthia (the highly efficient Astroloba congesta is part of this group), Kalanchoes, Portulacaria afra (AKA Spekboom, these are wonderful for carbon absorption), Sedum (Stonecrops), Jade Plants, Cotyledon, Echeveria, some Bromeliads (including pineapples), Aloe, and even Agave.

To be fair we don’t really have to absorb every last CO₂ molecule – just enough to keep it from rising to levels that disturb our sleep. Do we really know that it will improve our sleep? No – but I think aiming for 70 – 100 plants per room is a noble pursuit, so keep going. Why not live in a succulent forest?

Further reading

Wikipedia on Crassulacean acid metabolism

The Dark Fixation of CO2 by Succulent Leaves

Carbon dioxide uptake by succulents

Hypercapnia, abnormally elevated levels of CO₂ in the blood