Nearly all cars, urban offices, schools, and houses are air-conditioned if the owner can afford one. About 90% of urban indoor environments are air-conditioned. However, people are concerned with the rising costs of electricity and fuel, especially when they have to pay utility bills for their own residences. In suburban houses, air-conditioning is used during the hot hours of the days. During the cooler hours, most people are still willing to open their windows and let fresh air in. Therefore, almost all windows in suburban houses can be opened.