# Is it best to sleep with a window open at least a little?

Above is a plot of the concentration of carbon dioxide (C02) in a bedroom in which one person is getting about 6.5 to 7 hours of sleep. Plot is as a function of time from when they go to bed to when they get out of bed, and is from a 2013 paper by Batog and Badura. The units are parts per million (ppm). For context, the atmospheric CO2 concentration is about 400 ppm, i.e., in the Earth’s atmosphere (i.e., outside our homes) out of every 1 million molecules, 400 are carbon dioxide*. In our homes and offices, the CO2 concentration is higher because we are all breathing out CO2. We breathe in oxygen (O2), use it to burn our food for energy, then breathe out the CO2 this produces. As far as I know, there are basically no set limits on CO2 concentrations but guidance for workplaces, schools, etc is typically that it should be no more than a 1000 ppm. As you can see, this concentration is exceeded for most of the night.

An adult breathes out about half a cubic metre of air in one hour at rest, maybe a bit less when you are asleep, but it will also depend on how big you are. Over seven hours of sleeping, you breathe in about 7/2 = 3.5 m3. In the case above the bedroom had a volume 21 m3, so this is a fraction 1/6th of the total bedroom air.

Our breath has about 40,000 ppm CO2, i.e., about 100 times the concentration in the Earth’s atmosphere. So, with zero ventilation (i.e., no air in or out), at the end of the seven hours, the CO2 concentration in the bedroom would be 40,000/6 = 7000 ppm, or about 18 times the concentration outside. This is higher than above, because the room is not airtight.

A typical bedroom will not be airtight, there will little leaks around windows and doors in particular. Note that the different nights show different CO2 levels, if the weather outside varies this can affect airflow, eg if pressure drops outside this will tend to suck air out, so increasing the airflow.

As Batog and Badura note, the lower set of CO2 profiles are consistent with air in the bedroom turning over at about 2 litres/second or 7 m3/h**. For a 21 m3 bedroom this airflow turns over the air in the room once per 3 hours. The steady state concentration (that the profiles are tending to) is then just volume of air breathed out per hour times the CO2 concentration in breath times time to turnover the air in the room, divided by volume of the room. This is about 3,000 ppm. The blue, gold and black profiles above look they are tending towards about 2,500 ppm, over a timescale not far from 3 hours, so 2 litres/s looks about right.

My bedroom is a bit bigger, at about 35 m3, so the rise in CO2 will be slower, but still it is a bit worrying that with all windows and closed the CO2 levels could easily clear 1,500 ppm by morning*** – which is a bit above what most building engineers seem to be recommending. Maybe it is best to keep a small window open a little, even in winter.

I also worry a bit about people in small bedrooms in winter, particularly if they are trying to keep any airflow to a minimum, to try and keep and warm – energy poverty is real and getting worse. If the room is well insulated against airflow and smaller than 21 m3, or there there two people, not one person as in the data above, then the air is not going to be very fresh when they wake up in the morning.

Apart from some civil engineers, we don’t seem to worry about the quality of the air we breathe, when we are indoors, as we are for most of the time, even in summer, and especially in winter. Maybe we should worry a bit more****, and be a bit more careful about what we are sucking into our lungs every minute, day and night.

* Now the Earth’s atmosphere has 400 ppm of CO2, before the industrial revolution it was below 300 ppm. The extra 100 ppm plus is due to us burning, coal, oil, gas, etc.

** I don’t have a good feel for what are the residual air flows into and out of a room with windows and doors closed. 2l/s sounds high to me. Eg under a door which is say 50 cm wide and for a 2 mm high gap, that is an area of 10 cm2. For air moving at a speed of 1 cm/s, that is a flow rate of 0.01 l/s. This is very rough, so I can see it could out by a factor of ten, but 2 l/s is 200 times larger.

*** Assuming similar airflow in my bedroom to that in the bedroom studied by Batog and Badura.

**** Interest in how ventilated our rooms are, has been sparked by the COVID0-19 pandemic, as poorly ventilated rooms are where COVID-19 easily spreads. When you are sharing poorly ventilated room with someone infected with COVID-19, you may not only be breathing in CO2 they breathed out, but also virus they breathed out. Arguably, it should not be taking a global pandemic to make us think of how well ventilated are our homes and workplaces, but there you are.