Above is a plot of the average relative humidity (RH) inside a house, for each of the 12 months of the year. The relative humidity is the amount of water in the air, as a % of the maximum amount of water that the air can hold. So 0% RH is completely dry air – no water – while 100% RH is air completely saturated with water. The RH is basically what people call the humidity, in the sense that when they say it is very humid, they mean a RH that is close 100%. The data above was collected in Sweden by Engvall and coworkers.
The humidity is pretty much always in the range 30 to 50%. This is comfortable, humidities of above around 60% can be too humid, while humidities close to zero are unpleasant, as we don’t like air that is too dry. But one thing that surprised me – when I learnt it a week ago – is that the RH indoors is actually higher in summer than in winter. I think I assumed that in winter the RH indoors would be higher than in summer. We close windows against the cold, and often dry clothes indoors in winter. But as you can see above that is not true. The RH is actually a little lower in winter than in summer.
Now, if you do close all the windows for very long periods, and dry clothes etc, then of course the RH will be high. But if you air your home properly the RH will be relatively low. And the reason is that in winter when you let air in, you heat it up. Key point here is that RH is the relative humidity, it is humidity relative to the maximum amount of water that air can hold – before water condenses out of the air. And this maximum amount steeply increases with increasing temperature, it approximately doubles between 10 C and 20 C.
So if you open windows, let air in from outside at 40% RH and 10 C, then heat it up to 20 C, then you half the RH to about 20 %. Then if you close the windows, do some clothes drying, etc then, although the RH will increase, it may still be below the value in summer when you just open a window and let in air at 40% RH at 20 C.
One reason why indoor RH may matter, is that it may affect COVID-19, flu and other respiratory diseases. According to a review of Moriyama et al., we have known that some diseases are seasonal since the time of Hippocrates in 400 BC. By seasonal I mean that flu, for example, is much more prevalent in the winter than in summer. But 2,400 years of thinking about it and research have not produced a convincing picture of why flu, for example, is seasonal.
It is probably complex – like almost all of biology and medicine – i.e., there are probably several reasons why flu spreads more easily in winter than in summer. We do spend more time indoors in winter, and most transmission occurs indoors. So part of the reason that flu (and COVID-19) spreads more easily in winter than summer, may be a difference in human behaviour. Maybe this is the dominant reason, maybe not, we don’t know. But we do know that RH affects how long flu, COVID-19 and similar, viruses survive in the air. So it is possible low RH may be helping viruses spread, but this is speculation, the affect of RH seems quite complex.