Indoor Humidity: Where the 40–60% Figure Comes From and How to Measure It

The address of this page says 40–60%, a figure repeated in shop listings and articles. It comes from a 1986 review that laid several risk curves side by side (infectious agents lowest at 40–70%, dust mites below 50%, fungi above 60%) and took the overlap as a compromise, not a measured band (Arundel, Sterling et al.). Agencies draw the lines differently, but most put the upper end somewhere around 50–60%, above which dust mites and mould have an easier time. A hygrometer is the cheap way to find out where your home actually is, as long as you know how far to trust it. How humidity fits with ventilation and air purifiers is covered in the overview.

What is happening

Agencies give different ranges.

• US, EPA: 30–50%, and below 60% in any case; when humidifying, no higher than 50%.
• US, CDC (mould guidance): as low as you reasonably can, no higher than 50%.
• Germany, UBA: 40–60%.
• Russia, GOST 30494-2011: living rooms in the heating season, optimum 30–45%, acceptable up to 60%.
• ASHRAE Standard 55 (the US thermal comfort standard): no humidity limit for comfort at all.

Too dry. Dry indoor air is linked to complaints of irritated eyes and airways and a feeling of “dry air”; raising humidity can improve how the air is perceived and eye symptoms (Wolkoff review). Many people also notice dry skin and static. Better sleep in healthy people from humidifying has not been shown.

Too damp. Dust mites survive and multiply above about 45–50% relative humidity, and humidity is the main factor in how many live in a home (WHO). In US homes in a temperate climate, keeping humidity below 51% for 17 months cut mite numbers and mite allergen in dust many times over (Arlian et al.).

Mould follows the surface, not the room. Common building materials stay clean while the humidity right at their surface stays below roughly 75–80% (WHO). Surfaces colder than the room, such as outside walls behind a wardrobe, window reveals and cold pipes, push that local humidity up. The WHO's example: air at 20°C and 58%, cooled to 15°C, reaches about 75%; cooled to about 11°C, it hits 100% and water condenses (WHO). So a room reading 55% can still have a wet corner, and a cool bedroom has colder walls at the same humidity.

What to do

Measure properly.

• Expect an error of a few points. In a lab test most low-cost monitors read humidity within about ±5 points; individual units can be worse (Building and Environment, 2020).
• Check it with salt. Salt with a little water, still with undissolved crystals, keeps the air in a closed container at about 75% (Greenspan, NIST). Put the hygrometer and a small dish of it in a sealed box or bag and wait several hours until the reading stops changing. This checks one point only; if it is off by 5 points or more there, trust the device less.
• Place it in the middle of the room, away from windows, radiators, the kettle and any humidifier, and look at readings at different times of day.
• Check the problem spots: behind large furniture on outside walls and under beds, where cold surfaces collect condensation (UBA; UK government).

Act on the reading.

• Dry in winter: a humidifier can help; while humidifying, the EPA's line is no higher than 50% (US), and cut back if windows mist up. See the humidifier page.
• Damp: air rooms (how to air out your home), use extractor fans, dry laundry outdoors, consider a dehumidifier, and move furniture a little away from outside walls (UBA; UK government).
• Condensation on windows every morning is a signal to find the cause, whatever the hygrometer says: checking for mould.
• Dust mites: humidity is half the job, bedding the other: wash pillows and duvets.

When this is not the case

• Asthma or dust-mite allergy: aim for the lower part of your range; above 50% favours mites (WHO; Arlian et al.).
• Bathroom and kitchen: spikes after a shower or cooking are normal, and the Russian standard does not set humidity for these rooms at all (GOST). What matters is how quickly the level falls with the fan on or a window open.
• A good reading does not rule out a leak. A stain, a musty smell or peeling paint outrank the hygrometer (WHO).

Sources

• Arundel, Sterling et al. — Indirect health effects of relative humidity in indoor environments, 1986
• EPA — The inside story: a guide to indoor air quality
• EPA — A brief guide to mold, moisture and your home
• EPA — Use and care of home humidifiers
• CDC — About mold and your health
• Umweltbundesamt — Wie lüfte ich richtig?
• GOST 30494-2011 — Indoor climate parameters (in Russian)
• ASHRAE — Technical FAQ on Standard 55 and humidity
• WHO — Guidelines for indoor air quality: dampness and mould
• Arlian et al., J Allergy Clin Immunol, 2001
• Wolkoff, Int J Hyg Environ Health, 2018
• Greenspan — Humidity fixed points of binary saturated aqueous solutions, NBS, 1977
• Evaluation of low-cost indoor air monitors, Building and Environment, 2020
• UK government — Damp and mould: understanding and addressing the health risks

Checked: 24 September 2026