When deciding which countries in the world qualify for the title of "Most Polluted," the number of credible candidates is so high that the task rapidly becomes unmanagable. Levels of air and water pollution are dangerously high in many countries, from China and Pakistan to Botswana and India. Having said that, what follows is an attempt at answering the question "which is the most polluted."
According to the World Health Organization, the most polluted cities in the world, with a preponderance in Asia, include Karachi (Pakistan), New Dehli (India), Katmandu (Nepal), and Beijing (China), along with Lima, Peru, and Cairo, Egypt. Not surprisingly, a list of the most polluted countries includes all of those listed. The extent of China's air pollution is a matter of record, as is the air and water pollution in India, both being countries with rapidly developing economies.
Because they are the two most populated countries in the world, each with over one billion people (the United States, in contrast, has a population of around 300 million), and are also putting tremendous emphasis on rapid industrial growth, they would get this educator's vote for the most polluted in the country. While China is a global leader in the development of "green" technologies, 40 years of massive industrialization with little regard for environmental contamination has resulted in enormous levels of air pollution throughout much of the country -- a problem greatly exacerbated by the explosion in the number of cars on China's roads as its population becomes more prosperous and mobile. The World Bank, which tracks such matters, lists 16 of the 20 most polluted cities in the world as being in China.
India, similarly, has seen its levels of air pollution rise dramatically with the growth of its domestic industries, particularly chemical manufacturing facilities in cities like Bhopal, Sukinda, and Kasargod.
If the Soviet Union still existed, it would make this list by virtue of the astonishing levels of air and water pollution it created across its once-great expanse. Today, with the Soviet Union only a memory, its former territories and now-independent countries like Kazakhstan, Azerbaijan, and Ukraine suffer very high levels of pollution associated with the haphazard manner in which the Soviet regime built its industrial infrastructure.
This, then, is one short list of the most polluted countries in the world. It is not, however, definitive. Mongolia, Pakistan, Botswana, Egypt, Nigeria, and many others sit atop of lists of most polluted countries in terms of particulate matter in their air. I chose India and China, however, because of the great expanse of pollution in those two geographically and demographically large countries.