The most common causes include inadequate ventilation, which leads to a buildup of pollutants. These pollutants come from everyday activities like cooking, cleaning, and using certain products. Indoor air can also be contaminated by allergens like dust mites, pollen, and pet dander. Mold and mildew growth due to moisture issues can also release spores that worsen air quality. Volatile organic compounds (VOCs) emitted by building materials, furniture, and cleaning products contribute to poor air quality, too.