Built on what is, in effect, a peninsula, surrounded on three sides by water, and with the backdrop of the hills of the Western Ghats, Mumbai occupies a site of natural scenic beauty. However, sheer numbers of people and rapid population growth have contributed to some serious social and environmental problems. Mumbai attracts immigrants from rural areas seeking employment and a better life. Despite government attempts to discourage the influx of people, the city's population grew at an annual rate of more than four percent a year. Many newcomers end up in abject poverty, often living in slums or sleeping in the streets. An estimated 42 percent of the city's inhabitants live in slum conditions. Some areas of Mumbai city have population densities of around 46,000 per square kilometer—among the highest in the world.
As a result of Mumbai's size and high growth rate, urban sprawl, traffic congestion, inadequate sanitation, and pollution pose serious threats to the quality of life in the city. Automobile exhausts and industrial emissions, for example, contribute to serious air pollution, which is reflected in a high incidence of chronic respiratory problems among the populace. Breathing Mumbai's air has been likened to smoking more than 20 cigarettes a day! The scale of such environmental problems, however, pales in light of a United Nations (UN) report that projects Mumbai's population to reach 27.4 million by the year 2015.