Air that stays in one area for a period of time is called stagnant air. Stagnant air can occur indoors or outdoors, and it can be caused by several factors, such as lack of ventilation, air pollution, or weather conditions. Stagnant air can be harmful to health, as it can contain higher levels of pollutants and allergens than the surrounding air. This can cause respiratory problems and other health issues in people who breathe in stagnant air.