باب زويلة
Air and water pollution in Cairo have a destructive effect on the many monuments in the city. The Sultan Al-Ghuri Complex, for example, is one of many buildings in the center of the city which is covered in a gray and black crust from the air pollution. Al-Ghuri is representative of issues of decay of historic buildings in Cairo, both because it is an important medieval site including a funerary complex and mosque, and because the encrustation that appears on it has been studied in detail. Other sites examined in detail in the Greater Cairo area include the pyramids at Giza, Bab Zuweila, Al-Azhar Mosque, and the Cairo Citadel.