What is a common problem in many urban areas

In most urban areas, it is very common that there is a lot of pollution as a result of all the cars and pollutant producing things like vehicles. Thus making it quite a problem as it can have many health implications (if on a large scaled (see cities like China where people have had to wear masks to prevent any toxication)). Pollution also contributes to global warming which is an ongoing issue that affects the entire world.