Indoor air quality can be poor due to several problems. Common reasons include inadequate ventilation, which traps pollutants indoors; indoor sources of pollution like tobacco smoke, household cleaners, and cooking fumes; allergens like pet dander, dust mites, and mold spores; and outdoor pollutants that enter through open windows and doors. Poorly maintained HVAC systems can also contribute to indoor air quality issues. To improve indoor air quality, ensure proper ventilation, maintain HVAC systems, use air purifiers or filters, and minimize indoor pollutants through regular cleaning and proper ventilation practices.