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Bright lights have obscured the views of the stars people have cherished for millennia. Many don’t take light pollution seriously, but it can have serious implications on plant, animal, insect, and even human health. For a look at what you don’t know about light pollution, read the 15 facts and statistics below.

  • Authors Amedee Guillemin and Sir John Herschel were among those who mentioned light pollution in their literature. By 1866, people were noticing that dim stars were no longer visible from cities such as London and Paris. At first, smoke from gas lights were creating haze. Skywatchers and astronomers had to find new ways to observe the stars by 1909, which could no longer be done effectively from cities. Political activists emerged in the 1960s and 1970s in the United States, resonating mostly amongst professional and amateur astronomers.

  • Artificial light has affected female sea turtles that lay eggs on beaches. They may return to the same location decades later, where development has created illumination that can prevent them from nesting. The lights can also disorient the turtles so much they end up on roads and getting hit by vehicles. Young hatchlings typically find the ocean by moving away from the dark land. With all the lights coming from human sources, they may travel in the opposite direction they’re meant to.

  • Light pollution also affects the migration patterns of birds. In North America, 200 species migrate at night, but bright buildings and other man-made structures can disorient them, especially when there is low cloud cover. The Fatal Flight Awareness Program has estimated that anywhere from 100 million to 1 billion migrating birds are killed every year because they collide with buildings. In Toronto, it estimates 1-10 birds die, per building, in any given year.

  • The New York City Audubon Society estimates 10,000 migratory birds collide with skyscrapers and high-rises each year, and are injured or killed. It also estimates up to a billion birds may die from collisions across the North American continent. Up to 50 million might perish from hitting communications towers, according to the U.S. Fish and Wildlife Service.

  • The circadian clock is present in most organisms. Brain wave patterns, cell regulation, and hormone production are just a few physiologic processes that can be affected. Sleep disorders can be caused by disruptions to these processes, and occur in people who work at night, rotate shifts, and who use computers at night. Some experts have suggested the increase in artificial light has coincided with an increased risk of obesity, early-onset diabetes, and breast and prostate cancers.

  • Darkness stimulates the production of melatonin. A lack of this hormone can disrupt sleep patterns, and lead to depression, fatigue, headaches, stress, and cardiovascular disease. Too much light exposure during the nighttime hours can, therefore, be detrimental to health.

  • Artificial lighting can disrupt the response of plants to the changes in seasons. In the fall, trees under street lights may retain green leaves for longer. The lights can also affect a plant that relies on natural day/night cycles, not only posing implications for plant life, but for wildlife that depends on it.

  • Aside from fertilizers, pesticides, and habitat losses, light pollution is thought to be a cause for a decline in firefly populations. They’re not often seen where the ambient light is bright, even from a full moon. The lights produced by the insects are used to signal one another to find mates, communicate, claim territory, and deter predators. Exterior lights, such as those in yards and gardens, may be especially harmful.

  • Light pollution consists of glare, which makes things excessively bright and visually uncomfortable; skyglow, which illuminates the sky over populated regions; and light trespass, or light that reaches where those at the source don’t intend it to. Groups of sources can create light clutter, a bright and confusing assortment of light from interior and exterior sources, factories, offices, sporting venues, and advertising.

  • According to the U.S. Department of Energy, 13 percent of the electricity used by homes is for outdoor lighting, and 0.5 kilowatt-hours of energy per house, per night, is wasted by bad lighting. In a year, about 120 terawatt-hours of energy are used in the United States, just for lighting up streets, parking lots, and other things outdoors.

  • Outdoor light pollution is costly in other ways. The International Dark-Sky Association has estimated 30 percent of it is wasted. In terms of cost, this adds up to $3.3 billion. It is also equivalent to 21 million tons of carbon dioxide being released into the air each year, and this is just from lights in the United States. Each year, 875 million trees would have to be planted to offset all those emissions.

  • More than one-third of people on Earth cannot view the Milky Way from their location because it is obscured by light. In North America, 80 percent of humans cannot see the galaxy, while in Europe, 60 percent of the population is unable to. Also, 98 percent of people living in Israel cannot see it, nor can anyone in Kuwait, Malta, or Qatar. The largest areas with enough light to obscure the Milky Way include Boston to Washington, D.C., London to Liverpool, and Beijing to Hong Kong.

  • A University of Hong Kong study analyzed data from 200 locations, finding that the sky over the city is more than 500 times brighter than over darker, non-urban areas. City lights can obscure almost all stars even on a clear, cloudless night.

  • Light pollution is also created by oil and gas production facilities; for example, the Bakken oil fields in North Dakota look nearly as bright as Chicago and Minneapolis from NASA’s space images of Earth. Gas flares and machinery can take away the views of clear night skies as much as urban lights.

  • Light pollution is reversible. It can be abated directly by changing human habits. Shielded lights for outdoor fixtures direct light down, instead of up, while lower wattage bulbs help too. Indoor lighting can be kept inside at night by closing drapes and blinds.

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