Light Pollution
Light Pollution

Photograph by Jim Richardson National Geographic November 2008

Light pollution in the broad sense refers to any nighttime artificial light that shines where it's not needed. This nocturnal brightness can disorient humans and a host of other animals, confounding eyes and biological rhythms that evolved in a world without such light.

One way stray rays pollute is sky glow, the scattering of light by clouds and atmospheric particles that makes it difficult to see stars and other features of the night sky—an increasing problem. These maps show the global extent of sky glow. More than just snapshots of the Earth at night, they account for the light-scattering effects of clouds and dust, and show how bright the sky is above a given point on the ground—revealing that most of the world's population lives under night skies fuzzy with the haze of reflected light.

Sky glow doesn't stop at city limits or even international borders. But a close look at this upward escaping luminosity can reveal cultural clues about the polluters. This video, which shows the view from the International Space Station, provides a tour of the light we send spaceward—and of the striking social and geographical signatures it carries with it, from the incandescent spiderwebs emanating from European cities to the rigid grids illuminating the western United States to the pinpoints sprinkled around the remote corners of Australia.

Klinkenborg, Verlyn. "Our Vanishing Night." National Geographic (November 2008), 102-123.

Cinzano, Pierantonio. The World Atlas of the Artificial Night Sky Brightness.

National Geophysical Data Center. "Nighttime Lights From the International Space Station."

Other Resources
Bower, Joe. "The Dark Side of Light." Audubon (March-April 2000).

Owen, David. "The Dark Side. Making War on Light Pollution." New Yorker (August 2007).

Nightscape. International Dark-Sky Association.

International Dark-Sky Association.

International Year of Astronomy 2009.

Ecological Light Pollution

Humans have been aggressively lighting up the night for just over a hundred years, and though scientists have spent only a fraction of that time exploring the impact of the unintended peripheral glow on the natural world, observational and experimental data show that it affects how animals move about, communicate, find food, and even select mates. The most famous example is newly hatched sea turtles that become disoriented by the light from brightly illuminated beach communities and have difficulty finding the ocean. But behavioral changes have been documented in a wide range of species, including birds, mammals, insects, reptiles, and amphibians.

Not all species respond to light in the same way. Many nocturnally migrating birds are drawn to the source and can wind up circling round and round lighted towers, often colliding with other birds or dropping from exhaustion. Some animals, such as mountain lions, avoid lighted areas at night; other species are able to exploit such areas—foraging longer or targeting prey that congregate near lights, as some bat species do. But the increase in foraging hours can have its downside, putting animals on the prowl at risk for predation. The overall effect on complex ecological relationships is not yet fully understood.

There's also evidence that our grand transformation of the night might have serious implications for our own health. The contrast between dark and light allows our bodies to calibrate our circadian rhythms, such as hormone levels and sleep schedules. Disrupting these can have a dramatic impact. Researchers think this phenomenon may help explain higher breast cancer rates in societies with brighter nights. Studies on shift workers exposed to nearly constant light during the night hours reveal a higher risk for the disease, perhaps because of altered levels of melatonin. Other research shows that blind women have a lower occurrence of breast cancer. One study that looked at a general population also found a correlation between neighborhood nighttime light levels and breast cancer incidence.

Longcore, Travis, and Catherine Rich. "Ecological Light Pollution." Frontiers in Ecology and the Environment. Vol. 2 , no. 4 (2004), 191-98.

Kloog, Itai, and others. "Light at Night Co-Distributes With Incident Breast But Not Lung Cancer in the Female Population of Israel." Chronobiology International (January 2008), 65-81.

Rich, Catherine, and Travis Longcore, eds. Ecological Consequences of Artificial Night Lighting. Island Press, 2006.

Stevens, Richard. "Artificial Lighting in the Industrialized World: Circadian Disruption and Breast Cancer." Cancer Causes and Control (May 2006), 501-07.

Got Milky Way?

A glance at the night sky might make you think the Milky Way has dried up. The glow of light pollution has hidden one of the night's most striking features from one-fifth of humanity. The phenomenon is worst in the U.S., where about two-thirds of residents can't see the galaxy, and in Europe, where about half are shut out.

One of the regions with the worst light pollution is the northeastern U.S. But a midsummer power outage in 2003 granted a short reprieve from the widespread light pollution. The contrast was stunning, as Canadian skygazer Todd Carlson illustrates with this pair of before and after images.

There are a few ways to figure out how your backyard measures up. Find your home on this map of the night sky created by the Light Pollution Science and Technology Institute in Italy, which uses satellite data and calculations of how particles in the air scatter the light, or this one from the International Dark-Sky Association. You can also count the stars in Orion to help the Globe at Night project build a new database of sky brightness estimates from amateur astronomers around the world. To rate the nighttime luminescence of your neighborhood, compare the view from it to these star-magnitude charts.

Cinzano, Pierantonio. The World Atlas of the Artificial Night Sky Brightness.

Globe at Night.

International Dark-Sky Association Dark-Sky Finder.

"Photo of the Week: The August 14, 2003 Blackout." SkyNews, 2006

Getting Back in the Black

To make dark-sky-friendly changes to their outdoor lights, homeowners, businesses, and communities can update existing fixtures using designs that minimize disorienting glare and focus light on the ground. The International Dark-Sky Association has created a light-fixture seal of approval to simplify the process. For help finding fixtures with the IDA seal, check out their online guide. Ensuring that a building is lit with minimal ecological impact can help building owners secure LEED certification, an environmental seal of approval offered by the U.S. Green Building Council for both new and existing construction.

U.S. Green Building Council. LEED Rating Systems.

International Dark-Sky Association.

Nightlight Savings Time

Wasted light is wasted money, and the switch to more conscientious lighting can put not just the night sky overhead in the black but also balance sheets. The International Dark-Sky Association figures that about a third of outdoor lighting is wasted because of poorly designed and inefficient fixtures. It estimates that in the U.S. alone, the annual price tag for the misdirected light is $10 billion.

Keeping the lights on in public spaces is a large expense for local governments, and dark-sky-friendly fixtures can lessen the strain on municipal coffers. Between 2002 and 2005 the Canadian city of Calgary replaced its older, drop-lens streetlights with flat-lens night-sky-friendly fixtures. With 37,500 lights to retrofit, the project was a major investment, but the estimated $1.7 million in annual energy savings means the project is expected to pay for itself by 2012.

The scale of its upgrade made Calgary a trailblazer, but scores of communities have mandated specific lighting designs, curfews, and other measures to preserve darkness. To find out what you might be missing, check out this ordinance-protected night sky over Flagstaff. For more inspiration, if you"re willing to grab a tent and sleeping bag for a dark-sky vacation, you can start with this study by NOAA scientist Steve Albers, which includes an index of the darkest national parks. You can also find tools and suggestions for a dark-sky vacation on the IDA website.

Albers, Steve, and Dan Duriscoe. "Modeling Light Pollution From Population Data and Implications for National Park Service Lands." National Oceanic and Atmospheric Administration.

"Astronomy Picture of the Day: A Protected Night Sky Over Flagstaff." NASA, April 16, 2008.

"EnviroSmart Streetlight Retrofit." City of Calgary.

"Dark Sky Observing Sites & Destinations." International Dark-Sky Association.

"Lighting Ordinances." International Dark-Sky Association.

Lights and Safety

In terms of public safety and crime deterrence, experts say that more light is not necessarily better. Fixtures that reduce light pollution can actually make lighting safer, by reducing what's known as disability glare. This type of glare occurs when bright light scatters within the eye, making it harder to detect contrast—and harder to see people lurking in the shadows. Everyone can experience this effect, but older eyes can take three times longer than younger ones to readjust after being exposed to intense light.

Motion-activated lights can also help reduce both light pollution and criminal activity. They're off when no one's around, reducing wasted light and energy, but turn on when someone moves past, alerting others to the activity.

Crime Prevention Through Environmental Design is a movement looking at ways that well-designed spaces—including thoughtfully lit ones—can assist in public safety. These principles are being adopted as part of official crime-prevention plans, such as the "National Guidelines for Crime Prevention through Environmental Design in New Zealand."

Schieber, Frank. ""Age and Glare Recovery Time for Low-Contrast Stimuli."" In Designing for an Aging Population, ed. W. A. Rogers. Human Factors and Ergonomics Society, 1997.

"National Guidelines for Crime Prevention Through Environmental Design in New Zealand." New Zealand Ministry of Justice, November 2005.

International Clearinghouse on Crime Prevention Through Environmental Design.

Other National Geographic Resources
"Earth at Night." National Geographic map supplement (November 2004).

Guynup, Sharon. "Light Pollution Taking Toll on Wildlife, Eco-Groups Say." National Geographic News, April 17, 2003.

Roach, John. "Dark Skies Initiatives Aim to Boost Stargazing." National Geographic News, May 16, 2006.

Tourtellot, Jonathan. "Travel Column: Hotels Cut Light Pollution, See Stars." National Geographic News, March 19, 2004.

Last saved: October 2, 2008