What's the Right Humidity at Home?

For most homes, aim for a relative humidity of about 30 to 50 percent, and keep it below 60 percent: that’s the US Environmental Protection Agency’s (EPA) advice for preventing mould. A widely cited 1986 review put the range that minimises most humidity-related health effects a little higher, at 40 to 60 percent. Either way, the point is the middle: air that stays damp feeds mould and dust mites, and very dry air is linked to more respiratory infections.

This article is general information, not medical advice.

What Relative Humidity Means

Relative humidity is how much water vapour the air holds, as a percentage of the most it could hold at its temperature. Warm air holds more, so the same moisture reads lower in a warm room and higher in a cool one. That’s why a cold window or an outside wall can reach 100% at its surface and gather condensation while the room reads 50%, and why humidity drops indoors in winter, when cold outdoor air is heated up.

Too Damp

• Mould. The World Health Organization’s (WHO) guidelines on dampness and mould concluded that the main health effects of damp buildings are more respiratory symptoms, allergies, and asthma, and effects on the immune system. They’re about preventing persistent damp and mould growth, whatever the source.
• Dust mites and fungi. Arundel and colleagues’ 1986 review found that mite populations are smallest when relative humidity is below 50% and largest at 80%, and that most fungi can’t grow below 60%.
• Signs: condensation on windows, walls, or pipes, a musty smell, and visible mould. The EPA’s advice is to fix the source of moisture; vent clothes dryers and other appliances that make moisture to the outside; run exhaust fans or open windows while showering, cooking, or washing dishes; use a dehumidifier or air conditioner when needed; and dry anything wet within 24 to 48 hours.

Too Dry

The same review found experimental studies in which airborne bacteria and viruses survived least between 40% and 70% relative humidity, and epidemiological studies in which people in offices, homes, and schools with mid-range humidity had fewer respiratory infections or absences than those with low or high humidity. The evidence on how much humidifying helps against infections is weaker than the evidence on damp; if you use a humidifier, keep it clean, since the review warns that cool-mist humidifiers can spread contaminated aerosols.

Measuring It

Humidity sensors are among the more reliable parts of low-cost air monitors: a 2024 study of 87 low-cost indoor monitors found their humidity and temperature sensors met their makers’ specifications. Place a monitor away from kettles, showers, and windows, and read the pattern over days. A bathroom’s spike after a shower is normal; a bedroom that stays above 60% is worth acting on.

Following It in Kaizora

Relative Humidity and Air Temperature are air quality metrics on Air Quality, imported from an Aranet4’s CSV, with CO₂ and pressure, or logged by hand, each reading at a known position. Your exposure joins them with your location history: at each of your location fixes, your value is the newest reading within 100 metres in the hour before, so a bedroom monitor counts only while you’re there. A reference range you add on Relative Humidity, such as a mild band above 60%, flags damp readings on its chart and on Home. See Importing From an Aranet4 Monitor and Add a Reference Range.

The Short Version

• Aim for about 30 to 50% relative humidity, and keep it below 60%.
• Persistent damp feeds mould and dust mites and is linked to respiratory symptoms, allergies, and asthma.
• Very dry air is linked to more respiratory infections, on weaker evidence.
• Cold surfaces read damper than the room; watch for condensation.

Sources

• US Environmental Protection Agency, A Brief Guide to Mold, Moisture and Your Home.
• World Health Organization, WHO guidelines for indoor air quality: dampness and mould (2009).
• Arundel AV et al., “Indirect health effects of relative humidity in indoor environments,” Environmental Health Perspectives 65 (1986): 351–361. doi:10.1289/ehp.8665351
• Manu S et al., “A co-location study of 87 low-cost environmental monitors: assessing outliers, variability, and uncertainty,” Buildings 14, no. 9 (2024): 2965. doi:10.3390/buildings14092965