Understanding Pollutants
Understanding Humidity
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11 mins
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Published on
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Reviewed by the BREATHE editorial team
Quick Answer
Relative humidity measures how much moisture is in the air compared with the maximum it could hold at that temperature. It's one of the most important comfort factors in a home, and it also directly affects several health-related risks: air that's too dry can irritate the airways and skin, while air that's too humid encourages mould growth and dust mite populations. Most guidance places a comfortable, low-risk range at roughly 40 to 60 percent relative humidity, though some bodies cite a slightly lower 30 to 50 percent. Humidity changes constantly with cooking, showering, weather and ventilation, so continuous monitoring gives a far more complete picture than an occasional check, particularly for spotting a slow, sustained rise that could point to a damp problem.
TL;DR
- What it is: A measure of how much moisture is in the air relative to what it could hold at the current temperature.
- Why it matters: Both very dry and very humid air carry health and comfort risks, and sustained high humidity is a leading driver of mould and dust mites.
- Main factors: Cooking, showering, drying laundry indoors, occupancy, ventilation, and outdoor weather.
- How to interpret them: Compare readings against the 40–60 percent comfort range and watch for sustained trends rather than brief spikes after cooking or showering.
- What to do: Ventilate or extract during and after moisture-generating activities, and use a dehumidifier or humidifier if levels stay outside the comfortable range.
What is Humidity?
Relative humidity describes how much water vapour is in the air compared with the maximum amount the air could hold at that particular temperature, expressed as a percentage. Because warmer air can hold more moisture than cooler air, the same amount of water vapour will produce a different relative humidity reading depending on temperature, which is why humidity and temperature are always considered together.
Relative humidity is different from absolute humidity, which measures the actual quantity of water vapour in the air regardless of temperature. Consumer devices almost always report relative humidity, since it's the figure most directly linked to comfort, mould risk and how “sticky” or “dry” a room feels.
Where does it come from?
Indoors, the biggest contributors to humidity are everyday activities: cooking, showering and bathing, drying laundry indoors, and simply breathing all add moisture to the air. A single hot shower can release a significant amount of water vapour into a bathroom within minutes, which is why extractor fans are so effective at managing humidity at the source.
Building factors also play a role. Poor ventilation traps moisture indoors rather than letting it escape, while cold surfaces, such as single-glazed windows or poorly insulated walls, encourage condensation, which can concentrate dampness in specific areas even when the average humidity in a room seems reasonable. Outdoor weather and season have a strong seasonal effect too: humid summer air can push indoor readings up, while cold winter air, once heated indoors, often becomes quite dry.
| Category | Examples |
|---|---|
| Natural sources | Outdoor weather and season, soil moisture and groundwater near a building, plant transpiration |
| Human-made (anthropogenic) sources | Cooking, showering and bathing, drying laundry indoors, breathing and occupancy, humidifiers, poor ventilation and building condensation |
Health and comfort effects
Both very low and very high humidity carry measurable risks. Dry air, generally below around 30 percent relative humidity, can irritate the eyes, throat and airways, dry out skin, and increase static electricity. A well-known 1986 review of indoor humidity research found that airborne bacteria and viruses generally survive for the shortest time at moderate relative humidity, roughly 40 to 70 percent, with both very dry and very humid conditions extending their survival. More recent research on influenza specifically has found that low humidity increases both the airborne survival of the virus and its transmission between hosts, one reason some public health guidance recommends maintaining moderate indoor humidity during cold and flu season.
High humidity, generally above around 60 to 70 percent, creates favourable conditions for mould growth and dust mite populations. The same 1986 review found that dust mite populations are minimised below 50 percent relative humidity and reach their maximum size around 80 percent. Mould and dust mites are both well-established triggers for allergy and asthma symptoms, and sustained damp conditions can also damage building materials over time.
Understanding measurements
Relative humidity is reported as a percentage. Because it's a relative figure, it should always be interpreted alongside temperature.
| Relative humidity | Interpretation |
|---|---|
| Below 30% | Dry; can irritate airways, dry skin and eyes, and increase static electricity |
| 30–40% | On the low side; generally comfortable but may still feel dry in centrally heated homes during winter |
| 40–60% | Widely cited healthy comfort range; minimises dust mites, mould risk and irritation from dry air |
| 60–70% | Elevated; increasing risk of mould growth and dust mite proliferation, particularly with poor ventilation |
| Above 70% | High; significant mould and dust mite risk, often accompanied by condensation and a damp, musty smell |
Some bodies cite a slightly lower comfort range of roughly 30 to 50 percent rather than 40 to 60 percent; either is a reasonable practical target, and the exact figure matters less than avoiding the extremes at either end for a sustained period.
How to reduce levels
A short list of practical steps helps keep humidity in a comfortable, healthy range:
1. Run extractor fans during and after showering, bathing and cooking to remove moisture at the source.
2. Avoid drying laundry indoors where possible, or ventilate the room well if there's no alternative.
3. Use a dehumidifier in persistently damp rooms, particularly basements or rooms with poor natural ventilation.
4. Use a humidifier in very dry conditions, such as a centrally heated home in winter, keeping levels within the 30–50 percent range to avoid overcorrecting.
5. Improve ventilation generally, since trapped moisture is a major driver of both high humidity and condensation.
6. Address condensation and damp promptly, since they concentrate moisture problems in specific spots even when overall room humidity seems reasonable.
| Source | Impact | Recommended action |
|---|---|---|
| Showering and bathing | Sharp, short-lived humidity spikes in bathrooms | Run the extractor fan during and after use |
| Cooking | Steady humidity increase in kitchens | Use an extractor hood, especially when boiling or steaming food |
| Drying laundry indoors | Sustained humidity increase over hours | Ventilate the room well or dry laundry outdoors or in a vented space |
| Poor ventilation | Trapped moisture builds up over time | Ventilate regularly, especially after moisture-generating activities |
| Cold, poorly insulated surfaces | Localised condensation and damp | Improve insulation and address cold spots |
| Centrally heated homes in winter | Air can become uncomfortably dry | Use a humidifier, keeping levels within 30–50 percent |
Monitoring and sensors
Most consumer and professional humidity sensors use a capacitive sensor, which measures changes in electrical capacitance as a thin polymer film absorbs or releases moisture from the air. These sensors are compact, reasonably accurate, and respond quickly to changes, making them well suited to continuous monitoring.
An older but still-used alternative is the resistive sensor, which measures changes in electrical resistance across a moisture-sensitive material; these tend to be less accurate and more prone to drift over time than capacitive sensors. For applications needing very high accuracy, chilled mirror hygrometers directly measure the dew point by cooling a mirrored surface until condensation forms, offering laboratory-grade precision but at a cost and complexity unsuitable for everyday consumer devices.
| Method | How it works | Best for |
|---|---|---|
| Capacitive sensor | Measures changes in electrical capacitance as a polymer film absorbs moisture | Compact, accurate consumer and professional monitoring |
| Resistive sensor | Measures changes in electrical resistance across a moisture-sensitive material | Low-cost devices; less accurate and more prone to drift |
| Chilled mirror hygrometer | Measures the dew point directly by cooling a mirror until condensation forms | Laboratory-grade precision measurement |
| Psychrometer (wet-and-dry bulb) | Compares the temperature difference between a dry and a water-moistened thermometer | Simple manual measurement without electronic components |
Common myths
Myth: Higher humidity always feels warmer and lower humidity always feels cooler. Reality: humidity affects perceived temperature, but the direction depends on the actual temperature; high humidity makes warm air feel hotter, while in cold conditions it can make a room feel colder and damper.
Myth: A dehumidifier is the only fix for a damp home. Reality: a dehumidifier treats the symptom; identifying and addressing the source, such as poor ventilation, a leak, or condensation on cold surfaces, is usually more effective long-term.
Myth: Mould only grows where you can see visible damp. Reality: mould can develop behind furniture, inside wall cavities or under flooring where humidity is locally elevated, even if the room's average reading looks acceptable.
Myth: Very low humidity is only an issue in dry climates. Reality: centrally heated homes in cold weather commonly experience low indoor humidity, even in naturally humid climates, because heating warms the air without adding moisture.
Myth: Humidity doesn't matter if the temperature is comfortable. Reality: humidity independently affects respiratory comfort, virus survival, and mould and dust mite risk, regardless of how comfortable the temperature feels.
BREATHE Recommendation:
Humidity changes constantly with cooking, showering, weather and the season, which makes an occasional check easy to miss the bigger picture. Continuous monitoring shows how humidity in your home actually trends over time, including whether a room is creeping toward the conditions that encourage mould and dust mites. The BREATHE Airmonitor Plus tracks humidity alongside temperature, CO₂, PM1, PM2.5, PM10, TVOCs and formaldehyde, so you can see comfort and air quality together.
Frequently asked questions
What is a healthy indoor humidity level?
Most guidance places a comfortable, low-risk range at roughly 40 to 60 percent relative humidity, though some bodies cite a slightly lower 30 to 50 percent.
What happens if humidity is too low?
Air below around 30 percent relative humidity can irritate the eyes, throat and skin, increase static electricity, and has been linked to increased survival and transmission of some airborne viruses.
What happens if humidity is too high?
Sustained humidity above around 60 to 70 percent creates favourable conditions for mould growth and dust mite populations, both common triggers for allergy and asthma symptoms.
Does humidity affect mould growth?
Yes, significantly. Mould generally needs sustained relative humidity above 60 to 70 percent, or localised condensation, to establish and spread.
Can a humidifier or dehumidifier fix humidity problems?
Yes, both can help, but it's worth addressing the underlying cause, such as poor ventilation or a leak, alongside using either device.
Does humidity affect dust mites?
Yes. Dust mite populations are minimised below roughly 50 percent relative humidity and reach their maximum size around 80 percent.
Why does my home feel dry in winter?
Heating systems warm the air without adding moisture, and since warm air can hold more water vapour than cool air, the relative humidity of heated indoor air often drops significantly compared with the cold air outside.
Are humidity sensors accurate?
Most consumer devices use capacitive sensors, which are generally reliable and respond quickly to changes, though accuracy can drift slightly over time and benefits from occasional calibration checks.
Can humidity affect how I sleep?
Yes. Very dry air can irritate the throat and airways overnight, while very humid air can feel stuffy and uncomfortable; a moderate, stable humidity generally supports more comfortable sleep.
Should I monitor humidity continuously?
It's useful, particularly for catching a slow, sustained rise that could indicate a ventilation or damp problem before it becomes visible as mould or condensation.
Sources:Arundel, A.V., Sterling, E.M., Biggin, J.H. and Sterling, T.D. (1986). Indirect Health Effects of Relative Humidity in Indoor Environments. Environmental Health Perspectives, 65, 351-361.
Lowen, A.C., Mubareka, S., Steel, J. and Palese, P. (2007). Influenza Virus Transmission Is Dependent on Relative Humidity and Temperature. PLoS Pathogens, 3(10), e151.
World Health Organization. WHO Guidelines for Indoor Air Quality: Dampness and Mould. Geneva: WHO, 2009.
ASHRAE Standard 55-2023: Thermal Environmental Conditions for Human Occupancy. Atlanta: ASHRAE, 2023.
At a Glance
- Relative humidity measures moisture in the air relative to what the air can hold at that temperature.
- Most guidance places a comfortable range at roughly 40–60 percent, though some bodies cite 30–50 percent.
- Air that's too dry can irritate the airways, skin and eyes; air that's too humid encourages mould and dust mites.
- Humidity is closely linked to temperature and should be interpreted alongside it.
- Continuous monitoring helps catch a slow, sustained rise that could indicate a damp or ventilation problem.
