When we talk about indoor humidity, we often focus on how the air feels. But humidity also affects the environment we live in: it influences moisture on surfaces, conditions for mould and dust mites, and the behaviour of some microorganisms in the air.

That is why maintaining balanced humidity is about more than personal comfort. It is part of caring for the space around you.

What does the chart show?

The highlighted 40–60% relative humidity range sits between two different sets of concerns.

On the dry side, humidity can influence how some airborne microorganisms behave. For example, a laboratory study found that influenza virus remained more infectious in very dry air than at moderate humidity. Research also suggests that dry air can affect the airways’ natural defences. These findings help explain why the chart draws attention to viruses and respiratory infections at lower humidity levels. They do not mean that raising humidity will prevent infection.

On the humid side, persistent moisture creates better conditions for mould and dust mites. Mould may grow where walls, windows or other surfaces remain damp. Dust mites can thrive in more humid environments, and both mould and mite allergens may aggravate asthma or allergies in susceptible people. The humidity of a cold wall can also differ from a reading taken in the middle of the room, which is why condensation and damp patches matter.

Does every factor have an ideal humidity level?

No. Bacteria and viruses do not all react to humidity in the same way. The shapes in the chart illustrate broad relationships; they are not measurements of risk or boundaries at which an organism suddenly appears or disappears. Humidity alone cannot make indoor air free of germs.

The 40–60% range is therefore a useful guide to balance, rather than a promise that every indoor-air concern is at its minimum throughout that band. For many homes, around 40–50% is a sensible starting point. If condensation or dampness appears, a lower level may be appropriate and the moisture source should be addressed.

A humidity meter helps you see what is happening in your home. Used alongside ventilation, cleaning and moisture control, it can help you maintain an indoor environment that avoids both persistent dryness and excess dampness.

Better indoor air starts with knowing when to add moisture - and when to let it out.