Big cities are always densely populated. You will never find a city without any plant or factory. They may produce necessary goods, but at the same time they are the main air pollutors in a city. This can become a global ecological problem if we don't invent the way of cleaning the air.
Plants and factories also pollute water in rivers and lakes. A polluted river loses its "population": fish and water grass, and becomes unavailable for rest and fishing.
While I am speaking about factories I don't except that people are the city polluters too. Nowadays every fourth person owns a car, which also pollutes the air. There are some people who drop litter in parks and even in streets. This litter will not disappear immediately.