Singapore
Pittsburgh is clean for sure.
They have no method to clean it well enough to drink it safely.
The cities in the Indus Valley were very clean because of the meticulous way they were built in. The city's sanitation and waste water technologies had substantially developed.
surat,chandighgarh, jamshedpur
Asmara
millions of dollars
to clean or give water to the animals
They transported clean and fresh water to the cities like Rome.
The modern ones are very efficient and clean. The old ones need to go to the dump and can be dangerous.
Growing food and getting clean water for mega cities
For the most part, the air in desert is clean air, much cleaner than that found in most of the major cities of the world.