Humidity is the amount of water vapor present in the air, influencing both human comfort and the integrity of a structure. Relative humidity (RH), expressed as a percentage, compares the moisture currently in the air to the maximum amount it can hold at that temperature. When RH climbs above the recommended range of 30% to 50%, the excess moisture creates negative effects, turning a comfortable environment into one that is taxing on the body and destructive to property. High humidity prevents the body from cooling itself effectively, creates ideal conditions for biological hazards, and causes materials within a home to degrade.
Hindrance of Natural Cooling and Comfort
High relative humidity directly interferes with the body’s primary mechanism for heat dissipation: the evaporation of sweat. When the air is saturated with water vapor, the rate at which sweat evaporates from the skin slows down significantly. Since evaporation is a cooling process that removes heat from the body, this reduced rate means the body cannot shed heat efficiently, leading to a rapid rise in core temperature.
The physiological strain causes the heart to work harder, pumping more blood to the skin’s surface in an attempt to cool down, which can be taxing for individuals with pre-existing heart conditions. This impaired thermoregulation is why humid air feels much warmer than the actual temperature, a sensation quantified by the heat index. Difficulty in cooling can lead to a spectrum of heat-related illnesses, ranging from heat exhaustion to heat stroke. The unevaporated sweat also collects on the skin, resulting in the characteristic “sticky” feeling and general discomfort.
Fostering Biological Hazards and Allergens
Excessive indoor moisture creates a nourishing environment for various biological organisms that negatively impact respiratory health. Mold, mildew, and other fungi need relative humidity consistently above 60% to begin colonizing surfaces like drywall, wood, and insulation. These organisms reproduce by releasing microscopic spores into the air, which act as potent allergens and respiratory irritants when inhaled. Exposure to these spores can exacerbate existing conditions like asthma and allergies, leading to symptoms such as chronic coughing, wheezing, and irritated eyes.
A persistently high relative humidity also provides ideal breeding conditions for house dust mites, which are a common trigger for indoor allergies. These microscopic arthropods absorb moisture directly from the air, and their populations thrive when humidity levels reach the 70% to 80% range. It is not the mites themselves but their fecal pellets and decaying body parts that become airborne and cause allergic reactions. Maintaining an RH below 51% has been shown to significantly reduce dust mite and allergen levels, highlighting the direct link between moisture control and allergen management.
Physical Deterioration of Structures and Possessions
Beyond the biological threats, high humidity causes substantial, non-biological harm to a home’s structure and contents through moisture absorption. Wood, a common building material in framing, flooring, and furniture, readily absorbs airborne moisture, causing it to swell and warp. This can lead to structural issues like cupping in hardwood floors, doors and windows that stick in their frames, and wood rot that compromises structural integrity.
Moisture also damages surface finishes and metallic objects through different mechanisms. High humidity weakens the adhesive bond in materials like wallpaper, causing it to bubble and peel away from the wall. Similarly, it can cause paint to blister and detach from surfaces. Metal components, including tools, appliances, and internal electrical systems, are susceptible to accelerated corrosion and rust when exposed to moisture-laden air, which can lead to premature failure of sensitive electronics.
Strategies for Reducing Excessive Moisture
Controlling indoor humidity requires a multi-pronged approach focusing on both mechanical removal and improved ventilation. The use of a dehumidifier is a highly effective mechanical solution, especially in damp areas like basements or crawl spaces, as it actively pulls moisture from the air and collects it as water. Air conditioning systems also help to dehumidify the air as a secondary function while cooling, making regular maintenance of the HVAC unit important for moisture control.
Proper ventilation is necessary to exhaust moisture generated by everyday activities directly to the outdoors. Exhaust fans in bathrooms should be run after a shower, and range hoods should be used every time the stove is on to remove steam and cooking vapors. Simple home maintenance steps, such as promptly fixing any leaky pipes and ensuring that attic and crawl spaces have adequate venting, prevent external moisture sources from contributing to the internal humidity load.