With rapid urban development, the densely built high-rises block the airflow and trap the heat emissions from vehicles and industrial activities, while building materials, including concrete, absorb and retain some of the heat. These contribute to the temperature rise in the urban area. When the urban cooling rates are slower than rural areas, the Urban Heat Island effect develops. In areas of severe Urban Heat Island effect, when the mean temperature over a five-day period exceeded 29°C, the natural mortality increased by 4.1% with every 1°C increase.