Why you need to take control of humidity

In the built environment, humidity issues can wreak havoc, harming buildings as well as their occupants. Here's why.
Bacteria in a petri dish

What is humidity?

Humidity describes the amount of water vapor contained in the air. As temperature increases, the air can hold more water vapor. The higher the humidity level, the wetter it feels.

Humidity is a crucial part of the water cycle: continuously created by evaporation from large bodies of water (eg lakes, seas, oceans) and removed via condensation.

The warmer the air, the greater the ability for it to carry water vapour. As an example, a densely saturated amount of air may contain 28g/m3 of water at 30°C, but contains just 8g/m3 of water at 8°C.

Understanding indoor humidity levels

Colder air cannot hold as much moisture. That's why temperature is such an important consideration for indoor humidity - particularly as humans spend around 90% of their time indoors.

On a typical winter's day, for example, the outdoor air may have 100% relative humidity at 5°C - containing 6.8g/m3 of water. That's quite a cold temperature, so we need to warm up that air. When we heat that outdoor air within a building to 23°C, the absolute amount of water in the air remain the same, but - since warm air can contain more moisture -relative humidity drops to 33%.

Now consider a warm summer's day. While the outdoor air has 80% humidity at 30°C - containing 24g/m3 of water, that would be quite uncomfortable indoors. Cooling this air below 26°C, for example, elevates relative humidity to 100% - resulting in condensation. This is why HVAC systems often feature built-in dehumidifiers. Without them, the building's walls would be drenched during the summer months.

As we can see, climate plays a significant role in managing humidity indoors. In addition to this, there are small, everyday actions which release moisture into the air indoors - causing indoor humidity levels to rise. These activities include: breathing, cooking, cleaning, washing and showering.

How do indoor humidity levels affect health?

Maintaining healthy humidity levels (between 30 and 60%) within the built environment is paramount. Here's why.

High humidity (>60%)

When humidity levels run higher than the recommended upper limit of 60%, the air starts feeling dense and damp - almost like living in a petri dish. The higher the levels, the greater the likelihood for problems to develop, including:

Bacteria and mould  

These thrive in damp, humid conditions. When this happens within the built environment, its impact on occupant health should not be underestimated. Studies show occupants are more likely to suffer from a range of issues, including: asthma, allergies and respiratory illnesses.  

Dust mites

These conditions also help dust mites to thrive. They prefer to live in places with moderate temperatures and high humidity, so they can absorb moisture. They may be microscopic, but these creatures are known to exacerbate asthma and allergies.

Low humidity (<30%)

When humidity levels run lower than the recommended lower limit of 30%, the air feels dry and uncomfortable for occupants. Maintaining low humidity increases the risk of issues including: 

Dry skin and eczema

Low humidity can lead to itching and dryness for occupants. Research indicates that low humidity can damage tear film in the eyes, as well as aggravate dermatological conditions, such as eczema and psoriasis.

Productivity and performance

Studies link changes in indoor humidity and temperature with measurable changes in occupants' ability to focus, perform and learn tasks.


Research demonstrates a significant link between low humidity and the spread of influenza.  In peak times of low absolute humidity, such as in winter, the transmission of the virus soars, according to one study.

Understanding humidity levels

Please note: you should always consider humidity in relation to temperature.


This indicates poor humidity. Action is needed, such as: running a dehumidifier or opening windows for up to two hours on dry days

≥60% and <70%

This is a fair humidity level. Keep monitoring!

≥30% and <60%

This a healthy level. Keep maintaining!

≥25% and <30%

This is a fair humidity level. Keep monitoring!


This indicates poor humidity. Action is needed, such as: decreasing indoor temperature (but keep it higher than 15°C) and ensuring good ventilation when undertaking high-humidity activities, such as showering or cleaning.

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