Light pollution


Light pollution is considered wasted light, or more light than is needed for proper illumination. Light pollution is usually classified as stray light emitted from poorly designed lighting fixtures. Stray light illuminates the areas not intended to receive light, including the area above and around light fixtures, and often impedes visibility of the night sky.

Light pollution is relatively new, and has been steadily increasing since the invention and widespread use of electrical lighting systems. Light pollution and its effects were first highlighted in the 1950s, when astronomers around the world became concerned with the stray light being emitted into the night sky.

It unnecessarily contributes to greenhouse emissions, and wastes money. Light pollution is an environmental concern because it represents wasted energy that is used to generate the light. Because a large proportion of electricity generated relies on fossil fuels, light pollution may also increase carbon dioxide emissions and possibly contribute to global climate change. The global cost of light pollution may be significant in terms of economic costs, as well as in greenhouse emissions.

Glare, light trespass and skyglow are common terms used to describe various types of light pollution. The adverse effects of light pollution include light clutter (excessive groupings of light) and decreased visibility.

Skyglow (or sky glow), also known as uplight, is light that is aimed upwards and scatters in the atmosphere. Skyglow is typically seen as an orange /yellow glow (or haze) above cities and towns. The light from skyglow makes it difficult to see stars at night with the naked eye. Skyglow may even decrease the sensitivity of sophisticated astronomy equipment.

Glare is the discomfort and sometime dangerous effect lighting can have on vision, when eyes cannot adapt to sudden differences in light. This effect is similar to when a bright light shines into the eyes at night. The effects of glare can occur from numerous lighting sources, particularly from road and street lights, as well as security lights. The sharp contrast between dark and light often results in glare, which hinders night vision because eyes are unable to adjust quickly enough to various light emissions and levels. Glare can result from any light source, including the sun, and can be direct or reflected.

Light trespass refers to stray light that illuminates outside of the intended area. Sources vary from streetlights, security lights and brightly lit advertisements that are poorly shielded or aimed. Outdoor high intensity security lighting may infiltrate homes, resulting in unwanted light that may be difficult to block by blinds or shades.

Stray and excessive light may negatively affect the health of animal and human populations, particularly by upsetting natural circadian rhythms. Circadian rhythm refers to the day and night patterns that affect various biological processes. In animals, circadian rhythm affects the timing of sleeping, waking up, body temperature, and hunger. Studies have shown that both the flowering and the scent emission of some flowers are regulated by daylight and the onset of dusk.

Excessive light does not necessarily contribute to safety nor enhance living. Instead, it may create a nuisance. While humans need outdoor lighting at night, there are alternative light sources that save energy and improve the quality of nighttime lighting, while reducing skyglow and preserving the visibility of the night sky.

Light pollution can be controlled by turning off lights and replacing poor lighting fixtures with better designed models that do not cause stray light. Light pollution is relatively cheap to correct, compared with other types of pollution. However, because few lighting standards exist, new technologies that address the problem of light pollution are still being developed.

Municipal light ordinances address light pollution, although currently there is no international or national standard limit of light pollution. Ordinances aim to reduce the amount of light pollution by allowing fewer lights or banning the use of the most polluting lighting fixtures.

One of the first efforts to control light pollution was in Flagstaff, Arizona. Because of the many astronomical observatories in the state, Arizona is considered the most vigilant in the U.S. with regards to light pollution. Regulations were put in place to protect the view from Lowell Observatory 50 years ago. In 2001, Flagstaff was declared the first International Dark Sky City.

In 2002, the Czech Republic enacted the world's first national light pollution law, known as the Protection of the Atmosphere Act. The Act specifies measures and rules that limits the occurrences of light pollution, and the authority to impose fines for non-compliance.


Humans are diurnal creatures (resting at night, active during the day) and have used artificial light to counteract poor night vision. Modern electrical lighting systems have created more light than ever possible before. However, poor lighting designs have resulted in light that does not only focus downward where it is needed, but also escapes upwards and out. Such systems diffuse the darkness and alter light levels, which in turn affects natural animal behaviors, such as migration, feeding and reproduction patterns. Studies have found that some birds migrate early when exposed to artificial light and animals have shortened their hunting and feeding times because of the "delayed" onset of dusk.

Much of the developed world lives in light-polluted areas. Many developed nations are flooded with light at night, obscuring the starry night sky, and creating a nebula of light that nature did not supply. The effects on animals and humans have become the focus of current research.

There are three mains sources of light pollution: lights that are simply unnecessarily bright, lights that are unshielded, and lights that are greater in number than is necessary to illuminate areas. All sources can cause light trespass, skyglow and glare. For example, business lights after hours are often bright, parking lots are fully illuminated when closed, and residential and roadside lights are frequently unshielded, resulting in light pollution. Unnecessary light is a hazard to motorists, causing glare and visual impairment. Light pollution is often intrusive to people and animals, and flashy advertising lights can be distracting.

Skyglow creates a yellow/orange glow in the sky that results in a persistent haze of light, especially above cities. Skyglow also interferes with sensitive astronomical instruments, disrupting their ability to capture light from distant galaxies.

In the United States, as much as $2 billion is wasted on energy from excess light. Higher wattage bulbs are used in non-shielded lighting fixtures, which both increase monetary costs and energy usage.

The International Dark-Sky Association (IDA) suggests that utility companies want to keep profits (income from customers) at the same levels, and so they will not replace municipal bulbs to lower wattages, or recommend lower output bulbs unless mandatory. The IDA was founded in 1988, and is a nonprofit organization dedicated to protecting and preserving the nighttime environment. IDA researches and publishes reports and resources about light pollution and its prevention. Additionally, IDA provides educational materials to educators, children and adults.

Solutions: Trends in lighting show a movement towards more energy-efficient bulbs, like metal-halide, high pressure sodium (HPS), fluorescent and low pressure sodium (LPS), away from mercury and incandescent lights.

Fully-shielded (full-cutoff) outdoor lighting points light down to the ground where it is needed, and full-cutoff lenses direct light uniformly to the ground. These types of lighting use lower wattage (lower lumen bulbs), both saving money and energy. Some researchers, however, say that full-cutoff lenses require high lumen bulbs to achieve the correct lighting levels, and thus the same amount of energy is used.

Studies suggest that an average town in Massachusetts, with a population of 25,000-30,000, will pay up $500,000 per year to keep streetlights lit all night. Some cities, such as Concord, California, are electing to turn off lights at night to save money. IDA also states that in towns where streetlights have been turned off or removed, there may be an initial period where residents complain about having not enough light, yet these conceptions decrease once residents become used to having less glare and light trespass.

Many municipalities now have policies requiring the installation of full-cutoff lights, including cities in California and Texas. Policies can be enacted at the state and/or local (city) level.

The solutions to light pollution include less lighting, and more effective lighting. Measures to reduce stray light emissions may include installing full-cut off fixtures, which use activated light sensors instead of lights that are on all night. For areas of "over lighting," lights may be reduced or turned off. Outdoor lighting ordinances save energy and improve the efficiency of light. However, even when ordinances are in place, private businesses do not always comply, and may be fined for non-compliance

Lighting ordinances across the United States and worldwide, particularly in Europe, are in place to mitigate light pollution. In 2002, the Czech Republic enacted the world's first national light pollution law, known as the Protection of the Atmosphere Act.


General: Urban sprawl is said to have contributed to light pollution. Nighttime images from the Defense Meteorological Satellite Program (DMSP) show the world at night, and since 1998, has allowed for quantitative measurements of the upward light flux. The data does not however detail the effects of light pollution. Measurements of the upward light flux are used to compute the effects on the night sky by modeling the light propagation in the atmosphere. This illustrates and measures how much light is in the sky, and identifies the areas that emit the most light pollution.

Lighting fixtures: Full-cutoff fixtures direct light down to the ground, not outwards or upwards. Flat-lens Cobra head fixtures are a type of full-cut off fixture that can improve the quality of lighting. They are considered excellent roadway lights because they reduce glare and cause no uplight. Box design lights illuminate down without any uplight.

Semi-cutoff lights are fitted with drop-lens cobra heads or refractors, which increase glare and emit light outward and up. Mercury type lights are thought to emit the most glare. Acorn style ornamental lights cause light pollution by emitting up and outward. Globe type and upward aimed lights, often used in advertising, scatter light up and outwards. They greatly contribute to wasted light and skyglow. These types of fixtures are often found outside. Unshielded lights cause the most light pollution; however, just aiming light fixtures correctly can reduce light pollution.

Low sodium pressure lighting emits monochromatic yellow light that is easily filtered out by astronomical instruments, and is the preferred source of lighting near observatories. In contrast, the white light from metal halides is considered to be very disruptive.

There are three commonly used streetlights: low pressure sodium (LPS), high pressure sodium (HPS) and high pressure mercury (HPHg). The International Dark-Sky Association (IDA) claims LPS lamps are the most efficient and cause less light pollution, no matter what the fixture, at 135 watts and the cost of $50 annually. HPS lamps are the most used lamp in the US currently, and cost about $94 for a 250-watt lamp per year. HPHg lamps are commonly found in cities, with an average cost at about 400 watts and $150 each year. The use of HPHg streetlamps has declined partly because of the costs associated with mercury waste; however, they are still often used in security lighting.


General: Recent studies have shown that almost two-thirds of the world's population can no longer see the Milky Way because of the haze of light pollution. Light sent upwards from the ground is reflected back down to earth. Urban city dwellers, in particular, can rarely see many of the star constellations, including the planet Saturn.

The loss of the night sky is the subject of numerous books and studies. Some state that many people have never seen the night sky, and if they do get the chance, it is a profound experience. This experience has been lost for many people. The stars have been the subject of tales for centuries, and have been used for navigation. Researchers fear that if light pollution increases, even less of the universe will be visible on earth, and the loss of the stars may impact human appreciation and interest in astronomy.

Lights used in the fishing industry have contributed to over-fishing and the decline of fish around the world. Some species of fish are attracted to light. Often, light is used to catch more fish than just bait alone.

Biologists worry about the ultimate impact of the loss of darkness. Designating a dark sky as a natural resource may prevent ecological disturbances, as well as save energy. The International Dark-Sky Association (IDA) has estimated that one-third of all lighting in the U.S. goes to waste, at the annual cost of $2 billion. This amounts to about 30 million barrels of oil and 8.2 million tons of coal.

Sources of light pollution vary enormously from region to region: in some areas, the worst offenders may be street lighting; in others, it could be billboards and parking lots.

Effects on humans: Humans need both darkness and light for internal clocks to function properly. Waking and sleep is a circadian (daily) rhythm that is fundamental to well-being. By creating artificial light, humans have extended the daytime and shortened nights, consequently affecting the body's response to dark and light.

One recent study suggests a correlation between higher rates of breast cancer in women with the brightness of neighborhoods at night. Higher rates of breast cancer and colorectal cancers have been documented with lower levels of melatonin. In humans, studies have shown a correlation that the exposure to light at nighttime suppresses melatonin levels. Melatonin is a natural hormone that helps regulate circadian rhythms. Research on shift workers demonstrates indirect links between light exposure at night to breast and colorectal cancers. Experiments in rats support the research: constant light suppressed melatonin and increased cancer cell growth rates.

Evidence also indicates that a bright day environment (strong natural sun) vs. a dim day (indoor artificial light) affects melatonin production: bright day time coupled with dark night times produce stronger melatonin rhythms.

Effects on birds: For wildlife, light pollution interferes with the behavior of nocturnal animals. Researchers have addressed the effects of artificial lighting on numerous species of wildlife, including birds.

For example, nocturnal birds navigate using the moon and stars as guides during their bi-annual migrations. The Fatal Light Awareness Program (FLAP), based in Toronto, Canada, suggests that when birds fly through brightly lit areas, they become confused and disorientated. About 450 species of birds that migrate during the night are at risk, and there have been documented cases of entire flocks crashing into brightly lit structures. In 1981, more than 10,000 birds collided with flood-lit smokestacks near Kingston, Ontario, and in 1954, 50,000 birds died after hitting the flood-lit ground at Warner Robins Air Force Base, Georgia.

Sea birds that are attracted by light, such as the Leach's Storm petrel, are at risk from lighthouses, off-shore drilling platforms, and even the high-intensity lamps used by fishermen. The European lesser horseshoe bat began to disappear once streetlights were installed. Other mammals, such as fruit bats, opossums, badgers and desert rodents, have become easier targets for predators under the constant artificial light.

Light and darkness have powerful biological affects that researchers are only beginning to understand. Migrating birds have been captivated by bright lights, circling them until they literally drop from the sky with exhaustion. Artificial light can disrupt nocturnal singing patterns in birds such as blackbirds and nightingales, and cause them to sing at unusual times of the day or night.

Scientists have discovered artificially short nights induce early breeding in many bird species and disruptions in migration patterns have been documented: Bewick's swans in England have primed themselves to migrate early by putting on fat early. However, early migration can mean the swans arrive too early for optimal nesting conditions.

Effects on other animals: There is evidence that light pollution endangers sea turtles. Florida beaches are nesting ground for loggerhead, leatherback and green turtles; however, the bright lights discourage females from coming ashore. Hatchlings (newly hatched turtles) need dark night skies to correctly orient themselves to the seas, but the artificial lights confuse them and they may crawl inland or along the beach instead. Some communities have passed ordinances requiring beachfront lights to be turned off during nesting season.

Studies are currently ongoing examining the movements of animals that typically hunt more on moonless nights, like some snakes, salamanders and frogs. Other creatures forage after dusk; however, artificial light often does not allow darkness to fall, consequently reducing their time to hunt and feed.

A recent experiment sheds light on the light pollution problem for salamanders. Ecologists from Utica College found that when lights are lit, salamanders remain hidden for about an hour longer, reducing the time for feeding. Results during the same research revealed that tree frogs did not perform the mating call in brightly lit areas, ultimately meaning they do not reproduce. Studies further illustrate negative impacts of light pollution on frogs: light exposure affects DNA synthesis and the production of hormones, which regulate fat stored for the winter and when eggs are produced. These studies suggest that because of artificial light, frogs are receiving incorrect biological signals.

Few studies have tested the effect of light pollution on nocturnal mammals. Nine hundred and eighty-six species of bats, many smaller carnivores and rodents, 20 percent of primates, and 80 percent of marsupials are nocturnal. One study though, showed that pumas avoid brashly lit areas at night. Other possible effects of light pollution have been seen in the reduced mating of fireflies, and in reducing the defenses of moths. Species such as fireflies and glowworms, which communicate with light, are affected by artificial light, as it reduces their visibility.


General: While urbanization may have contributed to the concentration of brightly lit areas, the lighting fixtures themselves are problematic. Ten years ago, about a third of the global population lived in urban areas; today approximately 50 percent of humans live in urban areas. Images taken by the Defense Meteorological Satellite Program Operational Linescan System (DMSP-OLS) records lights around the globe, and labels regions surrounding according to the density of lights, urban, peri-urban (low-density suburban areas or farmland) and non-urban.

LED lighting technology: In 2008, the U.S. Department of Energy (DOE), International Association of Lighting Designers and Illuminating Engineering Society (IES) hosted the Lighting Designer Roundtable on Solid-State Lighting. Here, lighting designers voiced concerns about light-emitting-diode (LED) technology, and the need for standards, and their environmental impact. LED technology is used in many forms of lighting, large and very small, including streetlights, vehicle lights, bar code scanners, and electronic billboard advertisements, Recently, in Grand Rapids, MI, city officials did not go ahead with new LED streetlights because prototypes were not producing the level of illumination required. Plus, the amount of energy being saved was much less than had been expected.

Lighting designers also have concerns about the inconsistency of color and brightness of LEDs and that the expectation that they will solve all "lighting problems" is unrealistic. One complaint designers have with Leeds is that they fade slowly over time, reducing the amount of light emitted, unlike incandescent bulbs, which burn out and simply stop working. This means that even though light is being emitted, it may not be strong as originally intended. Moreover, because LED technology is relatively new, there are variations of performance standards from manufacturer to manufacturer.

Even though many do have reservations over the efficiency and energy-saving capabilities of LED's, the U.S. Government's energy efficiency program, ENERGY STAR, has very recently qualified (late 2008) LED lighting and is proposing to qualify fully shielded LED street lights. If passed, this requirement will impact future design and marketability of LED streetlights, and may be deemed as an endorsement of LED technology. The criterion is a result of DOE-sponsored workshops emphasizing the need for fully shielded lights in luminaire (lighting) design.

Future lighting regulations: The ENERGY STAR guidelines propose that outdoor street and public lighting shall deliver a maximum of 10% of total lumens delivered within the 80©- 90© zone (bilaterally symmetrical), and 100% in the 0 to 90 degree zone, to minimize light trespass and skyglow. These specifications meet the International Dark-Sky Association (IDA) standards for dark sky compliance.

As of December 2008, a Model Lighting Ordinance (MLO) is being developed by lighting experts and lawmakers on a joint task force of the IDA and the Illuminating Engineering Society of North America (IESNA). Once the MLO is revealed, the regional lighting ordinances in the U.S. will likely end, as the MLO will be the standard dark sky legislation nationwide. Possible components of the MLO include lighting zoning and lighting design recommendations


This information has been edited and peer-reviewed by contributors to the Natural Standard Research Collaboration (

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