Three years of data link dry, overheated rooms to more winter respiratory infections — and most UK homes spend the season sitting well below the healthy humidity range.Ask anyone about humidity in the home and they’ll say mould. Fair enough – Britain has a real damp problem. But new research points at the opposite risk hiding in plain sight: when indoor air gets too dry in the colder months, infections go up.
Hi, Chris here. As a parent to asthmatics – indoor air quality is super important to me for obvious reasons, and anything I can learn that can create a better environment for my family is of utmost importance. I knew that drier air caused issues, but wasn’t aware of the relationship between dry air and illness. This is certainly something that I will incorporate into my own indoor air quality sensor to keep on top of!
Summary: A long-term study following older adults found that when indoor air got too dry during the colder months, respiratory infections rose. It's the mirror image of Britain's usual damp-and-mould worry: heating a sealed home can push humidity down into the 25–30% range, drying out the body's first line of defence against infection. The healthy window is roughly 40–60%, and a cheap hygrometer tells you which side of it you're on.Researchers followed 285 older adults for three years and found that lower indoor humidity during cool seasons was linked to a higher chance of catching a respiratory infection. The same pattern didn’t show up in warmer months, which fits the mechanism: it’s the heating season that dries everything out.
DATA SOURCE: 2020 study, Journal of Infectious Diseases: 285 older adults in Hong Kong (2016–2019); 
Here’s why that matters. Crank the central heating in a well-sealed home in January and the air can drop to 25–30% humidity — desert territory. Very dry air dries out the lining of your nose and throat, which is your first line of defence against the bugs going round. So the goal isn’t “low humidity good, high humidity bad.” There’s a window — roughly 40 to 60% — and most UK homes spend winter sitting below it.
“Few studies have explored the impact of indoor environmental factors in detail”
The Journal of Infectious Diseases. Source data: described as among the first prospective cohort studies to examine the indoor-environment-to-infection link at the individual household level in a subtropical setting. News-Medical
The fix is cheap. A hygrometer costs a few pounds and tells you which way you’re erring. Below 40%, a bowl of water near a radiator or a small humidifier nudges it up. Above 60%, the answer is ventilation and gentle heat, not a humidifier. The only way to know which problem you’ve got is to measure it.
One caveat worth keeping in mind: this was an observational study, so it shows a link between dry air and infection rather than proving one causes the other.
Data source, study date and how much weight it carries
The study is in The Journal of Infectious Diseases (Oxford Academic):
https://academic.oup.com/jid/article/222/6/967/5820877
PubMed record: https://pubmed.ncbi.nlm.nih.gov/32297941/
Source data
The figures in this post come from "Indoor Environmental Factors and Acute Respiratory Illness in a Prospective Cohort of Community-Dwelling Older Adults," published in The Journal of Infectious Diseases by Lefei Han, Lin Yang and colleagues at Hong Kong Polytechnic University. Source: a prospective cohort study of 285 older adults in Hong Kong, tracking indoor humidity and infections across 2016–2019, with 168 respiratory-infection episodes recorded over the period.
How much to trust this study? a prospective cohort (following the same people forward over time) is a reasonably solid design — stronger than a one-off snapshot or a lab study. But two honest caveats belong in any post built on it. First, it’s observational, so it shows an association between dry air and infection, not proof that one causes the other. Second, the group was older adults in a specific climate, so it points the way for UK homes rather than settling the question
Topics covered: indoor humidity · dry air · respiratory infections · winter health · central heating · 40–60% sweet spot · hygrometers · damp vs dry · older adults · indoor air quality