What can be a possible effect of high humidity levels indoors?

High humidity levels indoors can lead to an increased risk of mould growth, which is the correct answer. Mould thrives in damp environments, typically where humidity is above 60%. High moisture levels provide the ideal conditions for mould spores to germinate and grow on various surfaces, such as wood, drywall, and in carpets. This can create not only structural problems within a building but also contribute to health issues for occupants, as mould can release allergens and irritants.

To further understand why the other options do not pertain to high humidity effects: improved air quality is generally associated with lower humidity levels, as high humidity can trap pollutants and exacerbate indoor air quality issues. Enhancement of building aesthetics is not directly influenced by humidity; rather, it is dependent on design and maintenance rather than moisture levels. Lastly, rather than reducing dust mite populations, high humidity tends to support them, as dust mites thrive in moist environments. Thus, the presence of high humidity is linked to a range of negative outcomes, particularly the risk of mould growth.