<p>Photo: Fresno skyline beneath a haze of smog</p>

A cloak of smog gives Fresno, California, a hazy look. Smog, a hybrid of the words "smoke" and "fog," is caused when sunlight reacts with airborne pollution, including ash, dust, and ground-level ozone.

Photograph by Sarah Leen

The promise of jobs and prosperity, among other factors, pulls people to cities. Half of the global population already lives in cities, and by 2050 two-thirds of the world's people are expected to live in urban areas. But in cities two of the most pressing problems facing the world today also come together: poverty and environmental degradation.

Poor air and water quality, insufficient water availability, waste-disposal problems, and high energy consumption are exacerbated by the increasing population density and demands of urban environments. Strong city planning will be essential in managing these and other difficulties as the world's urban areas swell.


  • Intensive urban growth can lead to greater poverty, with local governments unable to provide services for all people.
  • Concentrated energy use leads to greater air pollution with significant impact on human health.
  • Automobile exhaust produces elevated lead levels in urban air.
  • Large volumes of uncollected waste create multiple health hazards.
  • Urban development can magnify the risk of environmental hazards such as flash flooding.
  • Pollution and physical barriers to root growth promote loss of urban tree cover.
  • Animal populations are inhibited by toxic substances, vehicles, and the loss of habitat and food sources.


  • Combat poverty by promoting economic development and job creation.
  • Involve local community in local government.
  • Reduce air pollution by upgrading energy use and alternative transport systems.
  • Create private-public partnerships to provide services such as waste disposal and housing.
  • Plant trees and incorporate the care of city green spaces as a key element in urban planning.


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