Teaching is now almost finished for this academic year. On Friday I had my last meeting with one of my project students. It was nice, he has learnt a lot and we said we’d next meeting at graduation in July. So I have a bit more time, and a new toy: a CO2 meter*. This measures the carbon dioxide aka CO2 in the air. The concentration of CO2 in the Earth’s atmosphere is about 400 ppm** – where ppm = parts per million. Out of every million molecules in the atmosphere, 400 are CO2. There is some evidence – although I am not sure it is very strong – that at CO2 levels above 1000 ppm your brain functions a little less well. In any event, CO2 levels allow you to estimate how well ventilated a room is.
We burn our food and oxygen to power our bodies and brains, and this produces CO2 (just like cars burning petrol do). We vent this CO2 in our breathe (as cars vent CO2 via their exhaust pipes). Our breath has a CO2 concentration about 100 times higher than in the atmosphere. So if there is one or more of us in a room that is not well ventilated, the CO2 level will increase. But if air is moving in and out of the room from the atmosphere outside faster than we breathe out CO2, then the CO2 will remain not much above 400 ppm.
Anyway, that is the theory. With my new CO2 meter I can test it. In the plot above, the blue circles are CO2 measurements every five minutes from a CO2 meter in my bedroom overnight on Friday night, while I slept (green dashed line is approximately the concentration outside). I left a small window open just a little bit; the window is 50 cm wide and opening was very roughly a couple of centimetres. It is not easy to see from above plot (sorry!) but overnight CO2 levels rose from between 600 and 700 to over 900 ppm. I was clearly breathing out CO2 a bit faster than the air was coming in and out of the small opening. During the day I had windows open so CO2 was about 600 ppm.
That was Friday night. In the interests of science, on Saturday I closed the window, and also closed the door (it was ajar on Friday), and repeated the experiment. I have to say, I was a bit shocked by the results – in orange above. I woke up to CO2 at over 2600 ppm, in the eight hours it had gone up by 2000 ppm.
But this number is very possible. My bedroom is about 35 m3 and at rest we breathe out about 0.5 m3 of air with 40,000 ppm of CO2. So in an eight-hour night that is 4 m3 of air, which diluted in 35 m3 gives an increase in CO2 of almost 5,000 ppm. So a bit more than half the air in the bedroom turned over in the night (via cracks under the door and elsewhere I guess).
So leaving a window ajar seems to have increased the rate of turnover of air in the bedroom by almost a factor of ten.
I am not sure that there are limits on CO2 at home (but am not an expert so could have missed them). But there are for the workplace. The website of the Health and Safety Executive (HSE) says
Long-term exposure limit (8-hr reference period) of 5000 ppm
HSE workplace exposure limit
Now, 2600 ppm is maximum, average must be less than 2,000 ppm, but even so I did get almost within a factor of 2.5 of being unlawful (for a workplace not bedroom). I worry that a couple sleeping in a smaller bedroom could at least get close to 5,000 ppm. Two people in a box room half the size of my bedroom would be producing four times as much CO2 per unit volume of the bedroom.
The HSE also say
CO2 levels consistently higher than 1500ppm in an occupied room indicate poor ventilation and you should take action to improve it.
HSE guidance on ventilation in the workplace
And I think with window and door closed, it could easily be consistently above 1,500 ppm. Maybe I, and you?, should take the HSE’s advice. It is for the workplace not the home but it does not seem sensible to spend eight hours in an office at CO2 kept far bellow 1,500 ppm, then go home and spend 8 hours in CO2 above 1,500 ppm.
* I am pleased with the Aranet4 CO2 meter I got. You can download an app to your phone, that display plots and allows you to download data. The CO2 meter uses bluetooth to communicate with the phone.
** It is worth noting that a bit more than a quarter of the 400 ppm is our fault, the Earth’s atmosphere used to have a bit less than 300 ppm. But since the industrial revolution we have burnt a lot of coal, oil and gas, which has increased the level from less than 300 ppm to over 400 ppm.
Chance & Necessity 
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