Relative humidity (RH) describes the amount of water vapor in the air compared to the maximum amount it can hold at that temperature. Warmer air holds more moisture, meaning the same amount of water vapor results in a lower RH on a warm day. Maintaining consistent indoor RH is important, as levels that are too high or too low can negatively affect both your well-being and the physical structure of your home.
The Ideal Range for Human Health
The most comfortable and healthful environment for human occupants falls within a relative humidity range of 40% to 60%. Within this range, the survival and spread of many airborne pathogens, including viruses, are minimized. Maintaining these mid-range levels supports the body’s natural defenses and reduces the viability of viruses that thrive in extreme conditions.
Too Low (Below 40%)
When the RH dips below 40%, the air becomes dry, causing physical discomfort and increasing susceptibility to illness. Low moisture levels irritate the mucous membranes in the nasal passages and throat, which defend against inhaled particles. This irritation can lead to symptoms such as dry skin, chapped lips, scratchy throat, and nosebleeds.
Dry air allows viruses to remain infectious longer because respiratory droplets evaporate quickly, leaving concentrated viral particles that float readily. Prolonged low humidity can also worsen existing respiratory conditions and increase the severity of allergic reactions.
Too High (Above 60%)
Humidity levels consistently above 60% create a warm, moist environment conducive to the growth of biological contaminants. This moisture is a breeding ground for dust mites, which commonly trigger asthma and allergies. The droppings of these microscopic creatures become airborne and worsen respiratory symptoms in sensitive individuals.
High humidity also encourages the proliferation of mold and mildew, which release spores into the air and exacerbate respiratory issues. High humidity can make the air feel heavy and “stuffy,” making it difficult for the body to cool itself through sweat evaporation, leading to discomfort and fatigue.
Protecting Your Home and Belongings
Improper humidity levels pose a threat to the structural integrity of your home and the condition of your furnishings. Wood, textiles, and electronics are highly sensitive to the amount of moisture present in the air.
When the air is too dry, wood components throughout the home lose moisture and shrink. This contraction results in visible gaps between hardwood floorboards, cracking in furniture, and joint separation in trim and doors.
For sensitive items like musical instruments and artwork, this rapid moisture loss can cause irreparable damage, such as warping or fracturing. Low humidity also facilitates the buildup of static electricity, which can damage delicate electronic components.
Conversely, high moisture in the air causes porous materials like wood, drywall, and textiles to absorb water and swell. This expansion can lead to peeling paint, bubbling wallpaper, and the decay of wood framing and structural beams. High moisture levels also accelerate the corrosion of metal fixtures and appliances, such as hinges, pipes, and internal air conditioning components.
Tools for Measurement and Adjustment
To effectively manage indoor RH, the first step is to accurately measure the current levels. A tool for this is a hygrometer, which measures moisture. Digital hygrometers are preferred over analog models for their greater accuracy and ease of reading. These devices should be placed in the main living areas, away from direct sources of moisture or heat, to get a representative reading.
When the RH is too low, the solution is to add moisture using a humidifier. Humidifiers come in various types, including cool mist and warm mist models, and can be portable units or whole-house systems integrated into the HVAC system. Simple methods like placing a bowl of water near a heat source or leaving the bathroom door open after a shower can also provide a small boost in moisture.
If the humidity is too high, a dehumidifier is the most direct way to remove moisture from the air. These appliances draw in humid air, cool it to condense the water vapor, and then release drier air into the room. Improving ventilation is also important, particularly in moisture-generating areas like kitchens and bathrooms, by using exhaust fans during and after activities such as cooking or showering.