Even in the biggest and most industrialized cities, indoor air pollution can be worse than outdoor air pollution. The effects are frequently most pronounced in those who may be exposed to indoor air pollutants for the most extended periods. The primary source of poor IAQ in homes is indoor pollution that emits gasses or particles into the air. Inadequate ventilation can raise indoor pollutant levels by failing to carry indoor air outside the house and bringing in enough outside air to offset emissions from inside sources.