The problem is not population itself; the unregulated, organic growth of the city is. People have arrived by the millions to Mexico City since the last half of the 20th century, but many of them settled on poor, irregular settlements known as ciudades perdidas (lost cities). These settlements lacked many basic services such as water, electricity or police patrolling and strained already limited resources for the rest of the city's population such as the schooling, transportation and sanitary (garbage disposal) services.
As for pollution, the city sits in a valley surrounded by mountains. Smog from manufacturing and chemical plants as well as vehicle exhaust fumes get trapped by the surrounding mountains due to lack of any powerful-enough winds. This makes up the layer of gloom that sits over the valley due to a "cook-pot effect".
a
yes
Geography.
Poverty and pollution.
The cities that have the worst air pollution is Pittsburgh and Los Angeles
Mexico City does, due to its large population (21.16 million) and its countless industries and vehicles.
It has a very large population.
The pollution in Mexico has been an issue for several years. The Mexican government has worked on several plans to fix the problem and the most recent is to eliminate the emissions that contribute to the climate.
It is very difficult to put a blame for Mexico City pollution problems on a specific type of worker or economic class. I would say 'everyone', as the most important issue related to Mexico City is overcrowding.
hazardous air pollution and liter happen in New Mexico.
The same you can find in other parts of the world: air pollution, noise pollution, litter, polluted ground and underground water sources, deforestation. It is not specially high, at least when compared with other countries in the world.
The city is built in a bowl-shaped crater of an extinct volcano.