Light Pollution

Light pollution is the presence of anthropogenic and artificial light in the night environment. It is exacerbated by excessive, misdirected or obtrusive use of light, but even carefully used light fundamentally alters natural conditions. As a major side-effect of urbanization, it is blamed for compromising health, disrupting ecosystems and spoiling aesthetic environments. Light pollution is the presence of artificial light in otherwise dark conditions. The term is most commonly used in relation to in the outdoor environment, but is also used to refer to artificial light indoors. Adverse consequences are multiple; some of them may not be known yet. Light pollution competes with starlight in the night sky for urban residents, interferes with astronomical observatories, and, like any other form of pollution, disrupts ecosystems and has adverse health effects. Read all..


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Satellite view of Paris at night
This time exposure photo of New York City at night shows skyglow.
This nighttime look at Earth, dubbed the Black Marble, provides researchers with a unique perspective of human activities around the globe.
Light pollution impact on visibility of sky
Impact of light pollution on a starry night, as seen from a 4200 m altitude on Mount Damavand in Iran.



Light pollution is the presence of artificial light in otherwise dark conditions.[1][2][3][4]


Light pollution is a side-effect of industrial civilization. Its sources include building exterior and interior lighting, advertising, outdoor area lighting (such as car parks), offices, factories, streetlights, and illuminated sporting venues. It is most severe in highly industrialized, densely populated areas of North America, Europe, and Japan and in major cities in the Middle East and North Africa like Tehran and Cairo, but even relatively small amounts of light can be noticed and create problems. Awareness of the deleterious effects of light pollution began in the second half of the 19th century,[8] but efforts to address effects did not begin until the 1950s.[9] In the 1980s a global dark-sky movement emerged with the founding of the International Dark-Sky Association (IDA). There are now such educational and advocacy organizations in many countries worldwide.


Energy conservation advocates contend that light pollution must be addressed by changing the habits of society, so that lighting is used more efficiently, with less waste and less creation of unwanted or unneeded illumination.[citation needed] Several industry groups also recognize light pollution as an important issue. For example, the Institution of Lighting Engineers in the United Kingdom provides its members with information about light pollution, the problems it causes, and how to reduce its impact.[10] Although, recent research[11] point that the energy efficiency is not enough to reduce the light pollution because of the rebound effect.


Disputes are still common when deciding appropriate action; and differences in opinion over what light is considered reasonable, and who should be responsible, mean that negotiation must sometimes take place between parties. Where objective measurement is desired, light levels can be quantified by field measurement or mathematical modeling, with results typically displayed as an isophote map or light contour map. Authorities have also taken a variety of measures for dealing with light pollution, depending on the interests, beliefs and understandings of the society involved.[citation needed] Measures range from doing nothing at all, to implementing strict laws and regulations about how lights may be installed and used.


A light pollution source, using a broad spectrum metal halide lamp, pointing upward at Uniqema factory, Gouda, the Netherlands


Light trespass



The city of Phoenix, seen from 55 miles away in Surprise, Arizona

U.S. federal agencies may also enforce standards and process complaints within their areas of jurisdiction. For instance, in the case of light trespass by white strobe lighting from communication towers in excess of FAA minimum lighting requirements[13] the Federal Communications Commission maintains an Antenna Structure Registration database[14] information which citizens may use to identify offending structures and provides a mechanism for processing citizen inquiries and complaints.[15] The U.S. Green Building Council (USGBC) has also incorporated a credit for reducing the amount of light trespass and sky glow into their environmentally friendly building standard known as LEED.

A few styles of light with their effect on the surrounding area shown in scale form.



An office building is illuminated by high pressure sodium (HPS) lamps shining upward. Much light goes into the sky and neighboring apartment blocks, causing light pollution.

Over-illumination is the excessive use of light. Specifically within the United States, over-illumination is responsible for approximately two million barrels of oil per day in energy wasted.[citation needed] This is based upon U.S. consumption of equivalent of 18.8 million barrels per day (2,990,000 m3/d) of petroleum.[16] It is further noted in the same U.S. Department of Energy (DOE) source that over 30% of all primary energy is consumed by commercial, industrial and residential sectors. Energy audits of existing buildings demonstrate that the lighting component of residential, commercial and industrial uses consumes about 20–40% of those land uses, variable with region and land use. (Residential use lighting consumes only 10–30% of the energy bill while commercial buildings' major use is lighting.)[17] Thus lighting energy accounts for about four or five million barrels of oil (equivalent) per day. Again energy audit data demonstrates that about 30–60% of energy consumed in lighting is unneeded or gratuitous.[18]

An alternative calculation starts with the fact that commercial building lighting consumes in excess of 81.68 terawatts (1999 data) of electricity,[19] according to the DOE. Thus commercial lighting alone consumes about four to five million barrels per day (equivalent) of petroleum, in line with the alternate rationale above to estimate U.S. lighting energy consumption. Even among developed countries there are large differences in patterns of light use. American cities emit three to five times more light to space per capita compared to German cities.[20]

Over-illumination stems from several factors:

  • Consensus-based standards or norms that are not based on vision science;[21]
  • Not using timers, occupancy sensors or other controls to extinguish lighting when not needed;
  • Improper design, by specifying higher levels of light than needed for a given visual task;[22]
  • Incorrect choice of fixtures or light bulbs, which do not direct light into areas as needed;[22]
  • Improper selection of hardware to utilize more energy than needed to accomplish the lighting task;
  • Incomplete training of building managers and occupants to use lighting systems efficiently;
  • Inadequate lighting maintenance resulting in increased stray light and energy costs;
  • "Daylight lighting" demanded by citizens to reduce crime or by shop owners to attract customers;[23]
  • Substitution of old lamps with more efficient LEDs using the same electrical power; and
  • Indirect lighting techniques, such as illuminating a vertical wall to bounce light onto the ground.




Glare can be categorized into different types. One such classification is described in a book by Bob Mizon, coordinator for the British Astronomical Association's Campaign for Dark Skies, as follows:[24]

  • Blinding glare describes effects such as that caused by staring into the Sun. It is completely blinding and leaves temporary or permanent vision deficiencies.
  • Disability glare describes effects such as being blinded by oncoming car lights, or light scattering in fog or in the eye, reducing contrast, as well as reflections from print and other dark areas that render them bright, with significant reduction in sight capabilities.
  • Discomfort glare does not typically cause a dangerous situation in itself, though it is annoying and irritating at best. It can potentially cause fatigue if experienced over extended periods.



View of the Phoenix metro area from the top of Goldmine Trail in the San Tan Mountains

Light clutter

The Las Vegas Strip displays excessive groupings of colorful lights. This is a classic example of light clutter.


From satellites

Another source of light pollution are artificial satellites. With future increase in numbers of satellite constellations, like OneWeb and Starlink, it is feared especially by the astronomical community, such as the IAU that light pollution will increase significantly, beside other problems of satellite overcrowding.[26][27][28]

Public discourse surrounding the anticipated growth of satellite constellation, like OneWeb and Starlink, includes multiple petitions by astronomers and citizen scientists,[29][30] and has raised questions about which regulatory bodies hold jurisdiction over human actions that obscure starlight.[31][32][33][34][35][36]

Measurement and global effects

False colors show intensities of skyglow from artificial light sources




The amount of airglow and zodiacal light is quite varied (depending, amongst other things on sunspot activity and the Solar cycle) but given optimal conditions the darkest possible sky has a brightness of about 22 magnitude/square arc second. If a full moon is present, the sky brightness increases to about 18 magnitude/sq. arc second depending on local atmospheric transparency, 40 times brighter than the darkest sky. In densely populated areas a sky brightness of 17 magnitude/sq. arc second is not uncommon, or as much as 100 times brighter than is natural.

To precisely measure how bright the sky gets, night time satellite imagery of the earth is used as raw input for the number and intensity of light sources. These are put into a physical model[37] of scattering due to air molecules and aerosoles to calculate cumulative sky brightness. Maps that show the enhanced sky brightness have been prepared for the entire world.[38]

Inspection of the area surrounding Madrid reveals that the effects of light pollution caused by a single large conglomeration can be felt up to 100 km (62 mi) away from the center.[39] Global effects of light pollution are also made obvious. The entire area consisting of southern England, Netherlands, Belgium, west Germany, and northern France have a sky brightness of at least two to four times normal (see above right). The only places in continental Europe where the sky can attain its natural darkness are in northern Scandinavia and in islands far from the continent.

In North America the situation is comparable. There is a significant problem with light pollution ranging from the Canadian Maritime Provinces to the American Southwest.[39] The International Dark-Sky Association works to designate areas that have high quality night skies. These areas are supported by communities and organizations that are dedicated to reducing light pollution (e.g. Dark-sky preserve). The National Park Service Natural Sounds and Night Skies Division has measured night sky quality in national park units across the U.S. Sky quality in the U.S. ranges from pristine (Capitol Reef National Park and Big Bend National Park) to severely degraded (Santa Monica Mountains National Recreation Area and Biscayne National Park).[40] The National Park Service Night Sky Program monitoring database is available online (2015).[41]

The Bortle scale is a nine-level measuring system used to track how much light pollution there is in the sky. Five or less is the amount required to see the Milky Way whilst one is "pristine", the darkest possible.[42]

Light pollution in Hong Kong was declared the 'worst on the planet' in March 2013.[43]

In June 2016, it was estimated that one third of the world's population could no longer see the Milky Way, including 80% of Americans and 60% of Europeans. Singapore was found to be the most light-polluted country in the world.[44][39]

A worldwide cloud-free mosaic from the Suomi NPP satellite, showing the extent of visible lights in 2016. The effects of light pollution—especially skyglow—spread far beyond the light sources visible here.


Effects on human health and psychology

Streetlights at the ski resort Kastelruth in South Tyrol, Italy

Medical research on the effects of excessive light on the human body suggests that a variety of adverse health effects may be caused by light pollution or excessive light exposure, and some lighting design textbooks[45] use human health as an explicit criterion for proper interior lighting. Health effects of over-illumination or improper spectral composition of light may include: increased headache incidence, worker fatigue, medically defined stress, decrease in sexual function and increase in anxiety.[46][47][48][49] Likewise, animal models have been studied demonstrating unavoidable light to produce adverse effect on mood and anxiety.[50] For those who need to be awake at night, light at night also has an acute effect on alertness and mood.[51]

In 2007, "shift work that involves circadian disruption" was listed as a probable carcinogen by the World Health Organization's International Agency for Research on Cancer. (IARC Press release No. 180).[52][53] Multiple studies have documented a correlation between night shift work and the increased incidence of breast and prostate cancer.[54][55][56][57][58][59] One study which examined the link between exposure to artificial light at night (ALAN) and levels of breast cancer in South Korea found that regions which had the highest levels of ALAN reported the highest number of cases of breast cancer. Seoul, which had the highest levels of light pollution, had 34.4% more cases of breast cancer than Ganwon-do, which had the lowest levels of light pollution. This suggested a high correlation between ALAN and the prevalence of breast cancer. It was also found that there was no correlation between other types of cancer such as cervical or lung cancer and ALAN levels.[60]

A more recent discussion (2009), written by Professor Steven Lockley, Harvard Medical School, can be found in the CfDS handbook "Blinded by the Light?".[61] Chapter 4, "Human health implications of light pollution" states that "... light intrusion, even if dim, is likely to have measurable effects on sleep disruption and melatonin suppression. Even if these effects are relatively small from night to night, continuous chronic circadian, sleep and hormonal disruption may have longer-term health risks". The New York Academy of Sciences hosted a meeting in 2009 on Circadian Disruption and Cancer.[62] Red light suppresses melatonin the least.[63]


A new 2021 study published in the Southern Economic Journal indicates that light pollution may increase by 13% in preterm births before 23 weeks of gestation.[64]

Disruption of ecosystems


Light pollution has caused animals sea turtles hatchlings and Puffins to move away from the sea at birth, mistaking the artificial light for moonlight. Some cities are starting to turn off nearby lights during hatching.

Light pollution poses a serious threat in particular to nocturnal wildlife, having negative impacts on plant and animal physiology. It can confuse animal navigation, alter competitive interactions, change predator-prey relations, and cause physiological harm.[65] The rhythm of life is orchestrated by the natural diurnal patterns of light and dark, so disruption to these patterns impacts the ecological dynamics.[66]

Studies suggest that light pollution around lakes prevents zooplankton, such as Daphnia, from eating surface algae, causing algal blooms that can kill off the lakes' plants and lower water quality.[67] Light pollution may also affect ecosystems in other ways. For example, lepidopterists and entomologists have documented that nighttime light may interfere with the ability of moths and other nocturnal insects to navigate.[68] Night-blooming flowers that depend on moths for pollination may be affected by night lighting, as there is no replacement pollinator that would not be affected by the artificial light. This can lead to species decline of plants that are unable to reproduce, and change an area's longterm ecology.[69] Among nocturnal insects, fireflies (Coleoptera: Lampyridae, Phengodidae and Elateridae) are especially interesting study objects for light pollution, once they depend on their own light to reproduce and, consequently, are very sensitive to environmental levels of light.[70][71][72] Fireflies are well known and interesting to the general public (unlike many other insects)[73] and are easily spotted by non-experts, and, due to their sensibility and rapid response to environmental changes[citation needed] good bioindicators for artificial night lighting.[74] Massive insect declines have been suggested as being at least partially mediated by artificial lights at night.[75]

A scorpion hides under rocks.
Birds flying trace and star trail near Rio de Janeiro beach at night time in light pollution
Brazil star trails and birds in light pollution in Rio beach at night

A 2009 study[76] also suggests deleterious impacts on animals and ecosystems because of perturbation of polarized light or artificial polarization of light (even during the day, because direction of natural polarization of sun light and its reflection is a source of information for a lot of animals). This form of pollution is named polarized light pollution (PLP). Unnatural polarized light sources can trigger maladaptive behaviors in polarization-sensitive taxa and alter ecological interactions.[76]

Lights on tall structures can disorient migrating birds. Estimates by the U.S. Fish and Wildlife Service of the number of birds killed after being attracted to tall towers range from four to five million per year to an order of magnitude higher.[77] The Fatal Light Awareness Program (FLAP) works with building owners in Toronto, Ontario, Canada and other cities to reduce mortality of birds by turning out lights during migration periods.


Birds migrate at night for several reasons. Save water from dehydration in hot day flying and part of the bird’s navigation system works with stars in some way. With city light outshining the night sky, birds (and also about mammals) no longer navigate by stars.[79]

Sea turtle hatchlings emerging from nests on beaches are another casualty of light pollution. It is a common misconception that hatchling sea turtles are attracted to the moon. Rather, they find the ocean by moving away from the dark silhouette of dunes and their vegetation, a behavior with which artificial lights interfere.[80] The breeding activity and reproductive phenology of toads, however, are cued by moonlight.[81] Juvenile seabirds may also be disoriented by lights as they leave their nests and fly out to sea.[82][83][84] Amphibians and reptiles are also affected by light pollution. Introduced light sources during normally dark periods can disrupt levels of melatonin production. Melatonin is a hormone that regulates photoperiodic physiology and behaviour. Some species of frogs and salamanders utilize a light-dependent "compass" to orient their migratory behaviour to breeding sites. Introduced light can also cause developmental irregularities, such as retinal damage, reduced juvenile growth, premature metamorphosis,[85] reduced sperm production, and genetic mutation.[65][86][87][70][88][89]



Effect on astronomy


Astronomy is very sensitive to light pollution. The night sky viewed from a city bears no resemblance to what can be seen from dark skies.[92] Skyglow (the scattering of light in the atmosphere at night) reduces the contrast between stars and galaxies and the sky itself, making it much harder to see fainter objects. This is one factor that has caused newer telescopes to be built in increasingly remote areas.

Even at apparent clear night skies there can be a lot of stray light that becomes visible at longer exposure times in astrophotography. By means of software the stray light can be reduced, but at the same time object detail is getting lost in the images. The following picture of the area around the Pinwheel Galaxy (Messier 101) with the apparent magnitude of 7.5m with all stars down to an apparent magnitude of 10m was taken in Berlin in direction close to the zenith with a fast lens (f-number 1.2) and an exposure time of five seconds at an exposure index of ISO 12800:

Some astronomers use narrow-band "nebula filters", which allow only specific wavelengths of light commonly seen in nebulae, or broad-band "light pollution filters", which are designed to reduce (but not eliminate) the effects of light pollution by filtering out spectral lines commonly emitted by sodium- and mercury-vapor lamps, thus enhancing contrast and improving the view of dim objects such as galaxies and nebulae.[93] Unfortunately, these light pollution reduction (LPR) filters are not a cure for light pollution. LPR filters reduce the brightness of the object under study and this limits the use of higher magnifications. LPR filters work by blocking light of certain wavelengths, which alters the color of the object, often creating a pronounced green cast. Furthermore, LPR filters work only on certain object types (mainly emission nebulae) and are of little use on galaxies and stars. No filter can match the effectiveness of a dark sky for visual or photographic purposes.

The Atacama Desert in northern Chile is far from any cities, and the night sky there is pitch-black. Photo by José Francisco Salgado.[94]

Light pollution affects the visibility of diffuse sky objects like nebulae and galaxies more than stars, due to their low surface brightness. Most such objects are rendered invisible in heavily light-polluted skies above major cities. A simple method for estimating the darkness of a location is to look for the Milky Way, which from truly dark skies appears bright enough to cast a shadow.[95]

In addition to skyglow, light trespass can impact observations when artificial light directly enters the tube of the telescope and is reflected from non-optical surfaces until it eventually reaches the eyepiece. This direct form of light pollution causes a glow across the field of view, which reduces contrast. Light trespass also makes it hard for a visual observer to become sufficiently adapted to the dark. The usual measures to reduce this glare, if reducing the light directly is not an option, include flocking the telescope tube and accessories to reduce reflection, and putting a light shield (also usable as a dew shield) on the telescope to reduce light entering from angles other than those near the target. Under these conditions, some astronomers prefer to observe under a black cloth to ensure maximum adaptation to the dark.

Increase in atmospheric pollution

A study presented at the American Geophysical Union meeting in San Francisco found that light pollution destroys nitrate radicals thus preventing the normal night time reduction of atmospheric smog produced by fumes emitted from cars and factories.[96][97] The study was presented by Harald Stark from the National Oceanic and Atmospheric Administration.

Reduction of natural sky polarization


In the night, the polarization of the moonlit sky is very strongly reduced in the presence of urban light pollution, because scattered urban light is not strongly polarized.[98] Polarized moonlight can't be seen by humans, but is believed to be used by many animals for navigation.


This kind of LED droplight could reduce unnecessary light pollution in building interiors.


  • Utilizing light sources of minimum intensity necessary to accomplish the light's purpose.
  • Turning lights off using a timer or occupancy sensor or manually when not needed.
  • Improving lighting fixtures, so they direct their light more accurately towards where it is needed, and with fewer side effects.
  • Adjusting the type of lights used, so the light waves emitted are those that are less likely to cause severe light pollution problems. Mercury, metal halide and above all first generation of blue-light LED road luminaires are much more polluting than sodium lamps: Earth's atmosphere scatters and transmits blue light better than yellow or red light. It is a common experience observing "glare" and "fog" around and below LED road luminaires as soon as air humidity increases, while orange sodium lamp luminaires are less prone to showing this phenomenon.
  • Evaluating existing lighting plans, and re-designing some or all the plans depending on whether existing light is actually needed.

Improving lighting fixtures

A flat-lens cobra luminaire, which is a full-cutoff fixture, is very effective in reducing light pollution. It ensures that light is directed only below the horizontal, which means less light is wasted through directing it outwards and upwards.


Full cutoff fixtures first became available in 1959 with the introduction of General Electric's M100 fixture.[99]








Adjusting types of light sources

Several different types of light sources exist, each having a variety of properties that determine their appropriateness for different tasks. Particularly notable characteristics are efficiency, and spectral power distribution. It is often the case that inappropriate light sources have been selected for a task, either due to ignorance or because more appropriate lighting technology was unavailable at the time of installation. Therefore, poorly chosen light sources often contribute unnecessarily to light pollution and energy waste. By updating light sources appropriately, it is often possible to reduce energy use and pollutive effects while simultaneously improving efficiency and visibility.

Some types of light sources are listed in order of energy efficiency in the table below (figures are approximate maintained values), and include their visual skyglow impact, relative to LPS lighting.[102][103]

Type of light source Color Luminous efficiency
(in lumens per watt)
Sky glow impact
(relative to LPS)
LED street light (white) warm-white to cool-white 120 4–8
Low Pressure Sodium (LPS/SOX) yellow/amber 110 1.0
High Pressure Sodium (HPS/SON) pink/amber-white 90 2.4
Metal Halide warm-white to cool-white 70 4–8
Incandescent yellow/white 8–25 1.1
PCA-LED amber 2.4

Many astronomers request that nearby communities use low pressure sodium lights or amber Aluminium gallium indium phosphide LED as much as possible, because the principal wavelength emitted is comparably easy to work around or in rare cases filter out.[104] The low cost of operating sodium lights is another feature. In 1980, for example, San Jose, California, replaced all street lamps with low pressure sodium lamps, whose light is easier for nearby Lick Observatory to filter out. Similar programs are now in place in Arizona and Hawaii. Such yellow light sources also have significantly less visual skyglow impact,[105] so reduce visual sky brightness and improve star visibility for everyone.



Re-designing lighting plans

In some cases, evaluation of existing plans has determined that more efficient lighting plans are possible. For instance, light pollution can be reduced by turning off unneeded outdoor lights, and lighting stadiums only when there are people inside. Timers are especially valuable for this purpose. One of the world's first coordinated legislative efforts to reduce the adverse effect of this pollution on the environment began in Flagstaff, Arizona, in the U.S. There, more than three decades of ordinance development has taken place, with the full support of the population,[107] often with government support,[108] with community advocates,[109] and with the help of major local observatories,[110] including the United States Naval Observatory Flagstaff Station. Each component helps to educate, protect and enforce the imperatives to intelligently reduce detrimental light pollution.

One example of a lighting plan assessment can be seen in a report originally commissioned by the Office of the Deputy Prime Minister in the United Kingdom, and now available through the Department for Communities and Local Government.[111] The report details a plan to be implemented throughout the UK, for designing lighting schemes in the countryside, with a particular focus on preserving the environment.

In another example, the city of Calgary has recently replaced most residential street lights with models that are comparably energy efficient.[112] The motivation is primarily operation cost and environmental conservation. The costs of installation are expected to be regained through energy savings within six to seven years.

The Swiss Agency for Energy Efficiency (SAFE) uses a concept that promises to be of great use in the diagnosis and design of road lighting, "consommation électrique spécifique (CES)", which can be translated into English as "specific electric power consumption (SEC)".[113] Thus, based on observed lighting levels in a wide range of Swiss towns, SAFE has defined target values for electric power consumption per metre for roads of various categories. Thus, SAFE currently recommends an SEC of two to three watts per meter for roads less than ten metres wide (four to six for wider roads). Such a measure provides an easily applicable environmental protection constraint on conventional "norms", which usually are based on the recommendations of lighting manufacturing interests, who may not take into account environmental criteria. In view of ongoing progress in lighting technology, target SEC values will need to be periodically revised downwards.



In the effort to reduce light pollution, researchers have developed a "Unified System of Photometry", which is a way to measure how much or what kind of street lighting is needed. The Unified System of Photometry allows light fixtures to be designed to reduce energy use while maintaining or improving perceptions of visibility, safety, and security.[115] There was a need to create a new system of light measurement at night because the biological way in which the eye's rods and cones process light is different in nighttime conditions versus daytime conditions. Using this new system of photometry, results from recent studies have indicated that replacing traditional, yellowish, high-pressure sodium (HPS) lights with "cool" white light sources, such as induction, fluorescent, ceramic metal halide, or LEDs can actually reduce the amount of electric power used for lighting while maintaining or improving visibility in nighttime conditions.[116]


See also


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