One-third of world
Light pollution, most common in densely populated urban areas, is the result of the artificial light humans create to illuminate roads, buildings and neighborhoods. Nearly everyone who lives in the U.S. and Europe experiences some level of light pollution.
North America in New World Atlas of Artificial Sky Brightness, as seen in Google Earth.
Researchers are still working to understand the implications of light pollution for humans. Researchers say it may affect the human ability to see at night in normal conditions.