Which of the following can contribute to allergic reactions indoors?

High indoor humidity levels can contribute to allergic reactions indoors because they create an environment that promotes the growth of mold, dust mites, and other allergens. When humidity levels exceed the recommended range, typically above 50%, it allows these allergens to thrive and proliferate. As a result, occupants may be exposed to higher concentrations of indoor allergens that can trigger allergic reactions, such as sneezing, nasal congestion, and respiratory discomfort.

In contrast, while low humidity levels can lead to discomfort and other issues like dry skin or respiratory irritation, they are less associated with the growth of the biological contaminants that commonly cause allergies. Outdoor air quality can also affect indoor environments, but it is not a direct factor that determines allergy triggers indoors. Temperature variations alone do not directly contribute to allergic reactions without the presence of moisture or other allergens. Therefore, maintaining optimal indoor humidity levels is crucial for minimizing allergen presence and improving indoor air quality.