Keeping the air you breathe at home reasonably fresh in winter

On Wednesday 14th December 2022 it was around freezing (0C) all day, and night. I worked at home in the morning to around midday, then was in work from midday-ish to five-ish. The plot shows the CO2 levels in my home for the 24 hours of that day, midnight to midnight. When I was home I kept small windows in my bedroom and kitchen open a few centimetres to keep some air turning over. As you can see from the plot, when I was at home the CO2 level was mostly between 800 and 1000 ppm (parts per million).

When I was out at wok the CO2 level slowly dropped, even though the windows were then all closed. Even with all windows closed, the flat leaks a bit and the only CO2 source (me) was not present. The CO2 concentration in the atmosphere is a bit over 400 ppm (shown as green line), so with no CO2 source inside, the CO2 concentration tends to relax to the 400 ppm of outside air. The slope suggests that with all windows closed the air in the flat turns over very roughly once a day.

There is no standard for ventilation and CO2 levels in homes as far as I know. Maybe there should be, the tragic and avoidable death of the two-year-old child Awaak Ishak due to mould is terrible evidence that what we breathe in while we are at home can in extreme cases kill us. But for workplaces, the Health and Safety Executive says: “CO2 levels consistently higher than 1500 ppm in an occupied room indicate poor ventilation and you should take action to improve it.” The CO2 concentration never reached 1500 ppm but did nudge of 1000 ppm a few times (those measurements are the orange points in the plot). But with windows closed it can pretty easily far exceed 1500 ppm, see plot in an earlier blog post where overnight it exceeded 2500 ppm if the bedroom window was closed.

So I think that a couple of windows open a little is the way to go in winter, it keeps the ventilation as measured by the CO2 level OK (i.e., mostly well below 1500 ppm) without creating noticeable drafts, and hopefully without adding much to the heating bill – but I haven’t checked that. I do worry that especially in cold weather and for those struggling to pay their heating bills, the temptation is to keep all the windows tightly closed all or almost all of the time to try and keep as much as possible of the heat in, but this looks like a recipe for breathing poor quality air.