Humidity is the amount of water vapor present in the air, commonly measured as Relative Humidity (RH). RH is expressed as a percentage, representing the moisture currently in the air compared to the maximum amount the air can hold at that temperature. When RH drops below the optimal range (generally below 30 to 40 percent), the air becomes exceptionally dry, affecting both the human body and the physical environment. Low humidity is common in winter when heating systems warm cold, dry outside air, significantly decreasing the overall indoor RH. This article explores the consequences of an arid indoor environment and outlines strategies for restoring a healthy moisture balance.
How Low Humidity Affects the Body
Dry indoor air increases the rate of evaporation, significantly affecting the body’s natural protective barriers. This primarily impacts the skin, respiratory system, and eyes, leading to discomfort and increased susceptibility to illness. The skin suffers from an increased rate of transepidermal water loss (TEWL) when the surrounding air is dry. This rapid moisture loss leads to dry, flaky, and itchy skin, and can exacerbate chronic conditions such as eczema and psoriasis.
The respiratory system is particularly vulnerable to arid conditions because it relies on moist mucous membranes as a first line of defense. Low humidity causes these membranes in the nose and throat to dry out, making them less effective at trapping airborne irritants and pathogens. Within the respiratory tract, tiny hair-like structures called cilia sweep mucus and trapped particles away from the lungs in a process called mucociliary clearance. When the air becomes too dry, the mucus thickens and the cilia’s movement becomes impaired, leaving the body more susceptible to colds, flu, and sinus infections.
The eyes are also affected as the tear film, a protective layer, evaporates more quickly in dry air. This accelerated depletion results in irritation, a gritty sensation, and general eye fatigue. Contact lens wearers often experience intensified discomfort because the hydrophilic lenses lose moisture to the surrounding dry air, causing them to stiffen and irritate the cornea.
Damage to Structures and Possessions
The physical materials within a home, especially those of organic origin, are highly sensitive to moisture content. Wood is a hygroscopic material, meaning it naturally absorbs and releases moisture to stay in equilibrium with the surrounding atmosphere. When the relative humidity drops sharply, wood releases this internal moisture and begins to shrink and contract.
This shrinkage is often first observed in hardwood floors, where noticeable gaps appear between planks during the dry winter months. Furniture, cabinetry, and wooden trim are also susceptible, resulting in surface cracks, warping, and the weakening of glued joints. For complex, high-value wooden items like pianos and stringed musical instruments, low humidity poses a significant threat. As the wood dries, necks can warp, soundboards can shrink, and fret ends can protrude, permanently affecting the instrument’s playability and tonal quality.
Low humidity increases the risk of electrostatic discharge (ESD). Dry air lacks the conductive water molecules needed to dissipate static charges. Walking across a carpet generates static electricity, and in very dry conditions, the charge can build up to thousands of volts. This static charge can be released as an uncomfortable shock or discharged into sensitive electronic components, causing damage or failure in computers and other devices.
Methods for Increasing Indoor Moisture
Restoring indoor moisture generally aims for a relative humidity between 40 and 60 percent, balancing human comfort and structural preservation. The most direct solution is a humidifier, which comes in two primary forms: portable units and whole-house systems. Portable humidifiers are single-room appliances that require daily refilling and frequent cleaning to prevent the growth of mold and bacteria.
Whole-house humidifiers integrate directly with the HVAC system, connecting to the plumbing to automatically supply moisture to the entire house. These systems require minimal maintenance, often just an annual or bi-annual replacement of a water panel, and provide a consistent humidity level. For minor adjustments, passive methods can introduce moisture, such as leaving the bathroom door ajar after showering, placing bowls of water near heating vents, or boiling water on the stovetop.
It is important to monitor moisture levels with a hygrometer. Maintaining control is crucial because over-humidification (levels consistently above 60 percent RH) can create new problems. Excessive moisture can lead to condensation on windows and walls, encouraging the growth of mold, mildew, and dust mites, and potentially causing structural damage.