Humidity refers to the amount of water vapor present in the air. Indoors, humidity is usually measured as relative humidity, which compares the moisture in the air to the maximum amount the air can hold at a specific temperature. In simple terms, it tells us how dry or moist the air feels.
Most experts agree that healthy indoor relative humidity should be maintained between 30% and 50%. When humidity drops below this range, indoor air becomes dry and uncomfortable. When it rises above this range, it can lead to problems such as mold growth and increased allergens. Achieving the right balance is vital for comfort, health, and the long-term protection of buildings.
In many regions, especially in colder climates, indoor air becomes very dry during winter because heating systems warm the air without adding moisture. This review article explores the importance of indoor humidity, its effects on health and buildings, and effective ways to improve and manage humidity in homes.
2.1 What Is Relative Humidity?
Relative humidity (RH) is expressed as a percentage. For example, 40% RH means the air contains 40% of the moisture it could hold at that temperature. Warm air can hold more moisture than cold air, which is why indoor air becomes dry in winter when cold outdoor air is heated indoors.
2.2 Factors That Affect Indoor Humidity
Several factors influence indoor humidity levels:
- Outdoor climate: Cold air contains less moisture than warm air.
- Heating systems: Furnaces and heaters dry the air.
- Ventilation: Excessive ventilation removes moist indoor air.
- Building design: Poor insulation and air leaks allow dry air to enter.
- Household activities: Cooking, bathing, and laundry add moisture.
Understanding these factors helps homeowners choose the most effective strategies to control indoor humidity.
- Health Effects of Low Indoor Humidity
3.1 Respiratory Health
Dry air dries out the slippery membranes in the nose and throat. These membranes normally trap dust, viruses, and bacteria. When they dry out, the body becomes weaker to respiratory infections such as colds and flu. People with asthma, allergies, or sinus problems often experience worsening symptoms in dry indoor environments.
3.2 Skin and Eye Problems
Low humidity removes moisture from the skin, leading to dryness, itching, flaking, and cracking. Conditions such as eczema and psoriasis may worsen. Dry air can also irritate the eyes, causing redness, burning, and uneasiness, especially for people who wear contact lenses.
3.3 Sleep Quality
Dry indoor air can cause throat dryness, nasal congestion, and coughing during sleep. These issues may disturb sleep and reduce overall sleep quality. Maintaining proper humidity can improve nighttime comfort and breathing.
3.4 Increased Spread of Viruses
Research suggests that many viruses, including influenza, survive and spread more easily in dry air. Maintaining moderate humidity levels may help decrease the survival and communication of airborne viruses indoors.
- Impact of Low Humidity on Homes and Materials
4.1 Damage to Wooden Materials
Wood is sensitive to changes in moisture. Low humidity causes wood to lose moisture, leading to shrinking, cracking, and warping. Wooden floors, furniture, doors, musical instruments, and cabinetry are especially weak.
4.2 Static Electricity
Dry air increases static electricity. This results in common electric shocks, clothes sticking together, and electronic device discomfort. While usually harmless, static electricity can be irritating and sometimes damaging to sensitive electronics.
4.3 Comfort and Energy Efficiency
Air with proper humidity feels warmer than dry air at the same temperature. In dry conditions, people may increase device settings to feel comfortable, which raises energy costs. Balanced humidity improves comfort without increasing heating expenses.
- Recommended Indoor Humidity Levels
Most health and building experts recommend keeping indoor relative humidity between 30% and 50%. Some key guidelines include:
- Winter: 30–40% (to reduce condensation on windows)
- Summer: 40–50% (to maintain comfort and reduce dryness)
Humidity should be adjusted based on outdoor temperatures, building insulation, and ventilation conditions.
- Natural Ways to Improve Indoor Humidity
6.1 Household Activities
Everyday activities can naturally increase indoor moisture:
- Cooking on the stovetop
- Boiling water
- Taking warm showers and baths
- Air-drying laundry indoors
Allowing moisture from these activities to remain indoors for a short time can help increase humidity levels.
6.2 Indoor Plants
Plants release moisture into the air through a process called transpiration. Having multiple indoor plants can slightly increase indoor humidity while also improving air quality and aesthetics.
6.3 Water Containers and Evaporation
Placing bowls of water near heat sources or sunny windows allows water to evaporate slowly into the air. This is a simple and low-cost method, though its effect is limited.
- Using Humidifiers to Improve Indoor Humidity
7.1 Portable (Room) Humidifiers
Portable humidifiers are commonly used in bedrooms and living spaces. They are reasonable and effective for small areas. Regular cleaning is important to prevent bacterial and mold growth.
7.2 Whole-House Humidifiers
Whole-house humidifiers are installed directly into HVAC systems. They provide reliable humidity throughout the home and need less daily maintenance. These systems are perfect for large homes or regions with long winters.
7.3 Proper Use and Maintenance
Nevertheless, of type, humidifiers should be:
- Cleaned regularly
- Used with clean or distilled water
- Monitored with a hygrometer
Over-humidification should be avoided to prevent mold growth.
- Monitoring Indoor Humidity
A hygrometer is a simple device used to measure indoor humidity. Many modern thermostats and smart home systems include built-in humidity sensors. Regular monitoring helps maintain healthy humidity levels and stops problems related with excess moisture.
- Ventilation and Humidity Balance
While increasing humidity is important, proper ventilation is still necessary. Controlled ventilation systems such as Heat Recovery Ventilators (HRVs) and Energy Recovery Ventilators (ERVs) help exchange indoor and outdoor air while retaining moisture and heat. These systems are especially useful in tightly sealed modern homes.
- Seasonal Humidity Management
10.1 Winter
Indoor air is driest during winter. Humidifiers, sealed windows, and reduced air leakage are most important during this season.
10.2 Summer
Air conditioning can sometimes dry indoor air. Monitoring humidity ensures it stays within a comfortable range without becoming too dry or too humid.
- Risks of Excess Humidity
Too much humidity can be harmful. High indoor humidity can cause:
- Mold and fungus growth
- Increased dust mites
- Moldy odors
- Damage to walls and insulation
Therefore, humidity control should always aim for balance.
- Conclusion
Indoor humidity is a key but often ignored factor in creating a healthy and comfortable living environment. Low humidity can negatively affect respiratory health, skin condition, sleep quality, and the durability of household materials. Luckily improving indoor humidity does not always require expensive solutions. A combination of natural methods, proper use of humidifiers, good insulation, and regular monitoring can help maintain ideal humidity levels. By understanding the importance of indoor humidity and applying practical strategies, homeowners can improve comfort, protect their health, and reserve their homes throughout the year.
References
- World Health Organization (WHO). WHO Guidelines for Indoor Air Quality: Dampness and Mould. WHO Press.
- S. Environmental Protection Agency (EPA). Indoor Air Quality and Humidity. EPA Publications.
- American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE). ASHRAE Handbook—HVAC Applications.
- Centers for Disease Control and Prevention (CDC). Indoor Environmental Quality and Health.
- Mayo Clinic. Dry Air: Health Effects and Prevention.
- National Institutes of Health (NIH). Humidity and Respiratory Health.
- Health Canada. Residential Indoor Air Quality Guidelines.
