What can happen when relative humidity exceeds 60%?

When relative humidity exceeds 60%, the environment becomes conducive to the growth of mold, mildew, and other types of fungi. High humidity levels can also lead to corrosion of materials, especially metals, as moisture in the air creates a more conducive environment for oxidation and deterioration. Additionally, excess moisture can contribute to the proliferation of dust mites and other allergens, which can degrade indoor air quality.

In contrast, the other options do not accurately reflect the consequences of high humidity. For instance, improved air quality and enhanced comfort are typically associated with moderate humidity levels, while reduced energy costs don't align with the effects of high humidity, as it may actually increase the demand for air conditioning and ventilation to maintain comfort and control moisture levels.