Quick answer
Indoor relative humidity between 40 and 60 per cent is the healthy range. Below it, dry air irritates eyes, skin and sinuses and helps viruses spread; above it, condensation feeds mould, dust mites and bacteria. High-performance homes hold the range year-round with balanced ventilation and humidity control.
Key takeaways
- Target 40-60 per cent relative humidity year-round.
- Both extremes carry health costs: dryness irritates airways; excess moisture feeds mould and dust mites.
- Condensation on windows is the early warning sign of excess humidity.
- HRVs and right-sized HVAC do the heavy lifting; habits like venting while cooking help.
- A simple hygrometer tells you where you stand.
5 Ways Indoor Relative Humidity Impacts Health & Thermal Comfort
When the level of relative humidity is too low or too high, achieving indoor comfort is easier said than done. Low humidity levels cause issues, from joint stiffness and sinuses irritation to dry eyes and itchy skin and lower immune response. On the other hand, high humidity can cause negative health effects and discomfort due to reduced indoor air quality.
Common Signs of High Relative Humidity
When the indoor relative humidity is too high, your house will generally feel warmer than it is. Here are the common signs of high humidity levels.
- Foggy mirrors and windows
- Moist, clammy indoor air
- Musty, foul odor
- Disturbed sleep
- Chronic discomfort
- Mold growth
- Wet insulation
- Rotting wood
- Peeling wallpaper
- Blistering paint
- Dust mites
High humidity levels indoors produce muggy conditions that often result in lesser comfort. For instance, home occupants may experience a disruption to their sleeping patterns and asthma aggravation. Also, allergies and other respiratory issues may indicate high humidity levels indoors.
Ideal Range of Indoor Humidity
Experts recommend relative humidity for comfort and better health within a home is between approximately 40% to 60%. Maintaining a humidity level below 60% prevents mold and mildew growth and dust mite infestations and inhibits the spread of bacteria.
How Indoor Relative Humidity Impacts your Health and Thermal Comfort
The air quality inside your home can impact your health in many ways. The amount of humidity or water vapor in the air can be especially important. Here are different ways indoor humidity affects your health and comfort.
1. Condensation
Condensation is a common problem in most households, especially during the winter months. When warm, moist air meets a cold surface, water droplets can form. This can lead to dampness and mold growth, both of which can be damaging to your home. Remember, warm air and vapor are often generated from various human activities such as bathing, cooking, and more.
The relative humidity of the air plays a big role in how much condensation will occur. The higher the humidity, the more likely it is that water droplets will form. In general, you want to keep the relative humidity in your home below 60%.
2. Spread of Viruses
According to Joseph G. Allen, the director of Harvard University’s Healthy Buildings program, maintaining indoor relative humidity at 40% to 60% helps reduce virus transmission. Based on scientific evidence, dry air supports the movement of viruses in the air. If an infected patient coughs, breathes, talks, or sneezes, they emit tiny respiratory droplets into the indoor air.
Dry air, or low relative humidity levels, causes these droplets to easily evaporate quickly. This forms virus-carrying particles that have been shown to stay in the air for an extended period. Their smaller size isn’t caught by many furnace filters and these particles can stay in suspension for hours, even days.
One study showed that the influenza virus was more stable and infectious at low relative humidity levels (RH). This means that the virus can remain suspended in the air for longer periods of time and be easily inhaled by people in the same room. Another study showed that rhinoviruses (the cause of the common cold) are also more stable and infectious at low RH levels. These findings suggest that maintaining a higher indoor RH level can help to reduce the spread of respiratory viruses.
3. Fungi, Mold, and Bacteria Growth
Fungi, mold, and bacteria are all common types of microorganisms that can be found in indoor environments. All three thrive in moist conditions, and as the relative humidity levels rise, so does their growth. High humidity levels also provide an ideal environment for dust mites, which are known to cause asthma and other respiratory problems.
In addition to causing health problems, these microorganisms can also damage building materials and lead to musty odors. To prevent the growth of these organisms, it is important to keep indoor relative humidity levels below 60% but not below 30% to prevent excessive air dryness, according to the EPA. This can be done by using a dehumidifier or by increasing ventilation.
4. Thermal Comfort
Indoor relative humidity (RH) is related to indoor temperature and thus can have a significant impact on thermal comfort. Generally, the higher the RH, at typical room temperatures, the warmer the air feels. This is explained by scientists by dividing the air temperature into sensible and latent heat or what we know more commonly as the “humidex” in hot weather.
It is important to strike a balance, as too high of an RH can be uncomfortable and lead to condensation on surfaces. An RH of between 40-60% is considered ideal for thermal comfort and health. Of course, other factors, such as air temperature and clothing, also play a role in determining thermal comfort. But by keeping indoor RH at a comfortable level, you can help to ensure that your home is a comfortable and healthy place to be.
5. Sleep Quality
The quality of sleep can be affected by many factors, including the environment in which you sleep. One factor that is often overlooked is indoor relative humidity. This is the amount of water vapor present in the air, and it can have a significant impact on sleep quality. When the air is too dry, it can cause nasal congestion, mouth drying, and skin irritation.
On the other hand, if the air is too humid, it can create an uncomfortable sleeping environment.
The ideal relative humidity for sleeping is between 40 and 60 percent. By maintaining this level of humidity, you can help to help with a comfortable and uninterrupted night’s sleep.
Final Thoughts
Overall, it is important to maintain indoor relative humidity levels within a comfortable range for both health and thermal comfort. Monitoring the RH level can help avoid problems such as static electricity shocks, respiratory infections, mold growth, and musty odors. Taking into account the season and individual preferences can ensure that everyone enjoys a comfortable indoor environment.
FAQs
How does indoor humidity affect health?
Although the human body is adapted to a wide range of environmental conditions, indoor humidity levels that are too high or too low can lead to a variety of problems. For example, excessively dry air can cause respiratory problems such as nosebleeds and bronchitis, while excessive moisture can lead to the growth of mold and dust mites, which can trigger allergies and asthma. In addition, indoor humidity levels that are out of balance can also cause skin irritations, headaches, and fatigue.
How humidity affects your thermal comfort?
Humidity is the measure of the water vapor content in the air and plays an important role in our thermal comfort. When the air is saturated with water vapor, we feel sticky and uncomfortable. This is because our bodies rely on evaporation to cool down, and when the air is already full of water vapor, there is nowhere for our sweat to go. On the other hand, very dry air can also be uncomfortable as it can lead to static electricity discharges and makes our mucous membranes dry out.
Frequently asked questions
What should indoor humidity be in an Ontario winter?
Aim for the lower end of the 40-60 per cent band in winter – around 40 per cent, sometimes slightly less in deep cold snaps. That keeps air comfortable while limiting condensation on windows, which appears whenever warm moist air meets cold glass.
Does low humidity make you sick?
Low humidity dries eyes, skin and sinuses, stiffens joints for some people and weakens the airway’s natural defences, which helps viruses spread. If winter brings static shocks and morning sore throats, your home is likely running too dry for comfort and health.
What humidity level prevents mould?
Mould growth accelerates when relative humidity sits above roughly 60 per cent. Keeping indoor air in the 40-60 band – and fixing condensation spots like cold corners and window frames – removes the moisture mould needs. The healthy-air band and the anti-mould band are the same range.
How do I lower indoor humidity?
Run ventilation when cooking and bathing, fix leaks and drainage first, and let a properly sized HVAC system dehumidify in summer. Persistently high humidity usually signals a ventilation or envelope issue – worth diagnosing at the source rather than masking with portable dehumidifiers.
Do HRVs control humidity?
An HRV exchanges stale indoor air for filtered fresh air while recovering most of the heat. In winter it expels humid indoor air, which helps hold the target range; in summer its effect is smaller and the cooling system carries the load. It is a core part of the strategy, not the whole answer.
What humidity is best for sleep?
The same 40-60 per cent range serves sleep: enough moisture that airways do not dry out overnight, not so much that the bedroom feels close or dust mites thrive in bedding. Stable temperature plus mid-range humidity is the recipe for comfortable nights.
