Humidity is generally discussed in terms of relative humidity (RH), which is the amount of water vapor currently in the air compared to the maximum amount the air can hold at that specific temperature. This ratio is expressed as a percentage, and a low percentage indicates dry air. Low indoor humidity typically occurs during colder months because cold outside air holds very little moisture. When this dry air enters a heated building and is warmed, its capacity to hold water increases, causing the RH percentage to drop significantly.
Immediate Effects on Human Health and Comfort
The immediate consequence of low indoor humidity, often falling below the recommended 40% level, is the rapid dehydration of the body’s surfaces. The skin, which acts as a barrier, loses moisture to the surrounding dry air, often resulting in a tight, itchy feeling. This moisture loss can exacerbate existing skin conditions, frequently leading to flare-ups of chronic issues like eczema, characterized by cracking and flaking.
This evaporative effect impacts the body’s internal defenses, particularly within the respiratory system. Low humidity dries out the mucous membranes lining the nose and throat, which are the body’s first line of defense against airborne particles. When this protective layer dries, it impairs mucociliary clearance, a mechanism that sweeps away inhaled dust, irritants, and pathogens. This compromised defense makes the respiratory tract more susceptible to irritation and infection.
Research indicates that low relative humidity can increase the transmission and survival of certain airborne viruses, including influenza and coronaviruses. When an infected person exhales in dry air, the respiratory droplets rapidly evaporate, forming smaller, lighter aerosol nuclei. These smaller particles remain suspended in the air for longer periods and travel farther, increasing the risk of exposure for others. Maintaining indoor relative humidity between 40% and 60% is the optimal range for minimizing both respiratory discomfort and viral spread.
The eyes are similarly affected by the increased evaporation rate in a low-humidity environment. The tear film, a thin layer of fluid that protects the eye’s surface, dries out more quickly, causing a gritty, scratchy sensation often referred to as dry eye. The overall physical stress from irritated skin, eyes, and respiratory passages can contribute to fatigue and negatively impact sleep quality.
Impact on Household Materials and Structure
Beyond the biological effects on humans, low humidity stresses the physical materials within a home, particularly those of organic origin. Wood is a hygroscopic material, meaning it readily absorbs and releases moisture to match the surrounding environment. When the air becomes excessively dry, wood loses internal moisture and begins to shrink. This contraction can lead to visible gaps between floorboards, cracking in solid wood furniture, and the separation of glued joints.
The structural integrity of finely crafted wooden objects, such as musical instruments and artwork, is especially vulnerable to moisture loss. Instruments like guitars, violins, and pianos are precisely constructed; shrinkage of the wood can cause internal bracing to fail or glued seams to separate. This permanently affects both the instrument’s playability and its acoustic quality. The ideal relative humidity for preserving wood and these sensitive items is between 40% and 60%.
Low humidity increases the risk of electrostatic discharge (ESD) in the indoor environment. Water vapor in the air is conductive and helps to dissipate static electric charges that build up on surfaces and clothing. When the air is dry, this natural pathway for charge dispersal is removed, allowing static charges to accumulate to high levels. A person walking across a carpet in 20% relative humidity can generate up to 35,000 volts of static electricity.
While static shock is typically annoying to humans, it poses a threat to sensitive electronic components. An electrostatic discharge can damage the delicate integrated circuits found in computers and other high-tech devices, often causing irreversible harm that is not immediately apparent.
Counteracting the Effects of Low Humidity
The most direct approach to mitigating the effects of dry air is by introducing moisture back into the environment using a humidifier. Portable units are effective for single rooms, while whole-house humidifiers integrate with the central heating system to regulate moisture throughout the home. Warm mist humidifiers boil the water before releasing steam, which can reduce the presence of mineral dust or bacteria in the output.
Cool mist humidifiers are safer for households with children because they eliminate the scalding risk associated with boiling water, although they require diligent cleaning to prevent mold growth. Regardless of the type, maintaining the humidifier with distilled or demineralized water helps prevent the dispersion of white dust, the residue of minerals in tap water. The goal is to maintain the indoor relative humidity within the ideal 40% to 60% range.
Simple household practices can provide a localized boost to air moisture levels. Air-drying laundry inside the home allows the evaporating water to passively increase the room’s humidity. Placing trays or bowls of water near a heat source, such as a radiator, facilitates evaporation, although this method is less controlled than using a dedicated device. Certain houseplants, like ferns and palms, naturally release moisture into the air through transpiration.
Addressing the body’s internal hydration is a complementary step, as dry air draws moisture from the respiratory tract. Increasing overall water intake throughout the day is advisable to compensate for the higher rate of evaporative loss from the skin and breathing. Consuming water-rich foods and minimizing the consumption of beverages containing diuretics, such as excessive caffeine, can help the body retain fluid.