Ideal humidity for home: where the 40–60% figure comes from and what it covers

Search for the ideal humidity for home and almost every answer says the same thing: 40 to 60%. This article looks at where that figure came from and what it does and does not cover. It draws on a 1986 review, two overviews by Wolkoff, a laboratory study with influenza virus and two meta-analyses on dampness and mold.

It is informational material, not medical advice. If dry air, a stuffy nose or a cough keeps coming back, that is a question for a doctor, not for a hygrometer.

Ideal humidity for home: a range assembled from different risks

The 40–60% range goes back to a review by Arundel and colleagues, published in 1986. It did not measure one outcome and find a threshold. It collected several effects of humidity, and each has its own best level (Arundel et al., 1986):

• Survival and infectivity of airborne bacteria and viruses in the experimental studies were lowest at relative humidity of 40 to 70%.
• Populations of allergenic mites were lowest below 50% and highest at 80%.
• Most species of mold fungi did not grow until humidity rose above 60%.
• Humidity also affects the release of formaldehyde from building materials, the formation of acids and salts from sulfur and nitrogen dioxides, and the formation of ozone.

The review also covered nine epidemiological studies, in offices, homes and schools. Absences and respiratory infections were less frequent among people who worked or lived at medium humidity than at low or high humidity.

The authors state the logic themselves: at any given humidity some adverse effects are at their maximum and others at their minimum. The range that minimizes as many of them as possible "appears to lie between 40 and 60%". So 40–60 is a compromise between mites (lower is better), mold (lower is better) and viruses (somewhere in the middle). It is not a measured threshold at which people stop getting ill.

The authors add a practical conclusion: in a cold climate, reaching that range in winter requires humidification. They prefer evaporative or steam humidifiers, because ultrasonic and cool-mist ones can spread aerosols contaminated with allergens.

Indoor humidity levels in winter: why the air gets dry

Heating is why the question comes up in the cold season. When cold air is warmed, its relative humidity falls. That physics and what is known about influenza and humidity are covered in why we get sick in winter, so they are not repeated here. If you want a rough idea of what your heated rooms look like, the site has a calculator for indoor humidity from outdoor weather. It was built for the heating season, so read it as an estimate for heated rooms.

Dry air in house: what low humidity is linked to

A 2021 overview of the office environment by Wolkoff, Azuma and Carrer describes what low humidity does to the body: it dries the eyes and the airways, and mucociliary clearance works less well. That is why complaints about dry and tired eyes are common. Epidemiological, experimental and clinical studies support the idea that humidifying dry air eases dryness of the eyes and airways, symptoms of fatigue and complaints about dry air. For health, productivity and a lower risk of infection, relative humidity between 40 and 60% "appears optimal" in that review (Wolkoff et al., 2021). Note the setting: it is about offices.

Wolkoff's 2018 overview adds caution. The link between indoor humidity and health has long been disputed. Raising humidity may improve perceived air quality, eye symptoms and possibly office productivity. In experiments on mice, low humidity did not increase irritation of the airways. No clear improvement of the voice was found, except in people with voice fatigue (Wolkoff, 2018).

One more point from that overview: a person has no sense organ for humidity. The feeling of "dry air" may reflect odor, dustiness and the drying effect of low humidity. The overview also asks readers to separate air humidity from dampness in the building structure and the emissions from it, and to look at homes and public spaces separately.

Humidifier benefits: what the virus study shows and what it does not

The strongest lab result is a 2013 study by Noti and colleagues. They used a model of an examination room with coughing and breathing manikins, at constant temperature and humidity from 7 to 73%. Virus collected over 60 minutes kept 70.6–77.3% of its infectivity at humidity of 23% or below, and only 14.6–22.2% at 43% or above. Most of the loss happened in the first 15 minutes after the cough. The authors conclude that keeping humidity above 40% would significantly reduce the infectivity of influenza virus in aerosol (Noti et al., 2013).

What this is: manikins and influenza virus in a laboratory. What it is not: sick people. Wolkoff's overview says both low and high relative humidity favor the transmission and survival of influenza virus in many studies (and the absolute humidity may be the better measure), but that the relation of temperature, humidity, virus and aerosols is complicated and depends on the type of virus.

This is why the list of humidifier benefits is shorter than advertising suggests. The sources support easing dryness of eyes and airways in offices and show a laboratory drop in virus infectivity. They do not show that a humidifier at home lowers your own chance of falling ill. No study in this set randomly assigned people to different home humidity and counted who got sick. The humidifier type also matters, as the 1986 review noted.

Indoor humidity at 60 and above: where dampness and mold come in

Mold is where the upper end of the range matters. In the 1986 review most mold species did not grow below 60%, and mites peaked at 80%. Here a distinction is needed: a reading on a hygrometer is not the same as damp walls. Wolkoff separates the humidity of air from dampness of the building structure and what comes out of it.

Dampness and mold in homes have their own research. A meta-analysis by Fisk and colleagues found that dampness and mold are associated with roughly 30–50% higher odds of a range of respiratory and asthma-related outcomes (central odds ratios 1.34 to 1.75) (Fisk et al., 2007). A second meta-analysis looked at residential dampness and mold and respiratory infections and bronchitis, in studies adjusted for the main confounding factors. The odds ratio was 1.44 (1.31–1.59) for respiratory infections and 1.45 (1.32–1.59) for bronchitis (Fisk et al., 2010).

These are observational data. The authors write that "if these associations were confirmed as causal", controlling dampness would avert a significant share of respiratory infections. That condition is still open. It is also a statement about visible dampness and mold, not about a humidity of 55 or 65% on a gauge.

What nobody knows yet

• Whether 40–60% is the best range for a particular person. It is a compromise built in 1986 from risks with different optima, and it is almost 40 years old.
• Whether humidifying your home changes your own risk of getting ill. No trial in these sources assigned home humidity at random.
• How far a laboratory result with manikins carries over to sick people.
• Whether the feeling of dryness tells you anything about the actual humidity. The sense organ does not exist.
• Anything about infants and children, or about sleep and humidity: these sources do not cover them. For a child, ask a pediatrician.

What you can check in your own data

MeteoHealth records outdoor weather, including outdoor relative humidity, next to your own wellbeing entries and looks for correlations in your own history. The app does not measure humidity in your home, and it does not diagnose. Its symptom list has headache, fatigue and insomnia but not dry eyes or a sore throat, so if those three get worse on certain days, the entries will show whether that lines up with outdoor humidity in your history. For the rest, the calculator for indoor humidity from outdoor weather gives a rough estimate for heated rooms. A related question, the bedroom itself, is covered in bedroom temperature for sleep.