Light pollution—what is it and why is it important to know?

Light pollution is excessive and inappropriate artificial light. The four components of light pollution are often combined and may overlap:

  • Urban Sky Glow—the brightening of the night sky over inhabited areas.
  • Light Trespass—light falling where it is not intended, wanted, or needed.
  • Glare—excessive brightness which causes visual discomfort. High levels of glare can decrease visibility.
  • Clutter—bright, confusing, and excessive groupings of light sources, commonly found in over-lit urban areas. The proliferation of clutter contributes to urban sky glow, trespass, and glare.

For most of Earth’s history, our spectacular universe of stars and galaxies has been visible in the darkness of the night sky. From our earliest beginnings, the vast spectacle arrayed across the dark sky has inspired questions about our universe and our relation to it. The history of scientific discovery, art, literature, astronomy, navigation, exploration, philosophy, and even human curiosity itself would be diminished without our view of the stars. But today, the increasing number of people living on earth and the corresponding increase in inappropriate and unshielded outdoor lighting has resulted in light pollution—a brightening night sky that has obliterated the stars for much of the world’s population. Most people must travel far from home, away from the glow of artificial lighting, to experience the awe-inspiring expanse of the Milky Way as our ancestors once knew it.

The negative effects of the loss of this inspirational natural resource might seem intangible. But a growing body of evidence links the brightening night sky directly to measurable negative impacts on human health and immune function, on adverse behavioral changes in insect and animal populations, and on a decrease of both ambient quality and safety in our nighttime environment. Astronomers were among the first to record the negative impacts of wasted lighting on scientific research, but for all of us, the adverse economic and environmental impacts of wasted energy are apparent in everything from the monthly electric bill to global warming.

In refreshing contrast to some of today’s complex and lingering environmental problems, many existing solutions to light pollution are simple, cost-effective, and instantaneous. Recognizing when outdoor lighting no longer serves its function and becomes a pollutant is the first step toward choosing appropriate solutions.

Increased urban sky glow is responsible for the disappearance of the Milky Way from our night skies. For professional astronomers, the increasing distance to prime observing sites, well away from sources of air pollution and urban sky glow, becomes more problematic as economic and environmental energy costs continue to rise. Amateur astronomers, meanwhile, find prime observing spots eradicated by commercial and residential development and must travel farther from home for a clear view of the skies. Increasingly, the most important equipment needed to enjoy the wonders of the night sky is an automobile with a full tank of gas and a map.

Sky glow over Istanbul, Turkey. Image Credit: Davis Doherty

The adverse effects of light pollution extend well beyond astronomy. New research suggests that light at night may interfere with normal circadian rhythms—the 24-hour cycle of day and night that humans have used to maintain health and regulate their activities for thousands of years. Light trespass, occurring when streetlights or a neighbor’s security light directs unwanted lighting onto our property or into our homes, contributes to a loss of natural darkness. Wildlife, too, is harmed by the unnecessary brightening of the night. From newly hatched sea turtles to migrating birds, fish, frogs, salamanders, and lightning bugs, artificial night lighting disrupts the cycles of nocturnal creatures in potentially devastating ways. While research is still ongoing, it is becoming apparent that both bright days and dark nights are necessary to maintain healthy hormone production, cell function, and brain activity, as well as normal feeding, mating, and migratory behavior for many species, including humans.

Light trespass, Washington, DC. This “street lighting” directs very little light onto the street. Instead, it sends wasted light into the sky and the sides of residential buildings where it may not be needed or wanted. Image Credit: International Dark-Sky Association

Paradoxically, in addition to wasting resources, a nighttime environment that is over-lit results in lowered visibility: direct glare from improperly shielded fixtures is often blinding. Light spilling into the sky does not light the ground where we need it. The redundant lighting found in many urban centers results in a clutter of lights that contribute to sky glow, trespass, and glare while destroying the ambiance of our nighttime environment. Our eyes, when dark-adapted, have good natural capacity in lowlight situations. But when nightscapes are over-lit, eyes never have a chance to become dark-adapted, and areas adjacent to brightly lit areas become impenetrable, reducing safety. Some communities have experienced a decrease in crime by reducing or eliminating nighttime lighting in appropriate areas.

Glaring lights can actually reduce visibility. Here the brightest most visible objects in the area are the lighting fixtures, not the roadways, walkways or parking areas which a driver or pedestrian would expect to be lighted. Atlanta, Georgia. Image Credit: International Dark-Sky Association

Light pollution wastes money and energy. Billions of dollars are spent on unnecessary lighting every year in the United States alone, with an estimated $1.7 billion going directly into the nighttime sky via unshielded outdoor lights. Wasted lighting in the US releases 38 million tons of carbon dioxide into the atmosphere annually; unshielded outdoor lights are directly responsible for 1.2 million tons of carbon dioxide waste. Simply reducing and removing unnecessary lighting saves money and energy, often at minimal expense. Over-lighting the night neither improves visibility nor increases nighttime safety, utility, security, or ambiance.

Effects of two different types sports lighting: Partially shielded lighting directed downward in Tucson, Arizona, USA (above) Image Credit: International Dark-Sky Association; stadium lighting contributes to sky glow Portsmouth, Hampshire UK (below). Image credit: R Arbour

Light pollution affects every citizen. It is a serious environmental concern that wastes money and resources while jeopardizing wildlife, our environment, health, and human heritage. Each of us can implement practical solutions to combat light pollution locally, nationally, and internationally. Find out how by exploring this Web site and others in the “links” section.

The article is quoted from the International Dark-Sky Association’s Practical Guide 1: Introduction to Light Pollution. For a full color handout visit