Light pollution’s far-reaching effects
When artificial light shines in otherwise dark areas outdoors, it is considered to be a pollutant, similar to chemicals that contaminate soil, air, and water. Like chemicals, it can have detrimental effects on all kinds of species, including humans. However, unlike other types of pollution that might contaminate soil, air, and water, light pollution is remarkably easy to clean up because once the light source is switched off, it’s gone. Nevertheless, light pollution is a significant problem in urban areas since light is essential for urban life, such as for safe navigation, and used for perhaps less essential uses, such as advertising. Light inevitably travels beyond the area in which it is needed and pollutes spaces by disrupting the normal rhythm of daylight. This can occur anywhere artificial light is used and even in remote areas, as light can be scattered for long distances in the atmosphere (known as skyglow). It has been estimated that 99% of the Netherlands, a particularly densely populated area, is affected by light pollution. Furthermore, more than 99% of people in the US and Europe see skyglow rather than stars when they look to the sky at night. This level of exposure to constant light can disrupt light’s normal functions in humans, animals, and plants, causing all kinds of problems. Some of them may be familiar and widespread, such as difficulty sleeping, but others are specific to different organisms and their lifestyle. Much is still unknown about the effects of light pollution but luckily the solutions to it can be as simple as flicking a switch.
What is light for?
For many organisms, light can function as a resource, such as for photosynthesis, or as information, such as when looking around for food or a mate. Light can therefore become damaging, disruptive, or useless if there is too much of it or it’s in the wrong place. Light as a way to navigate the world is highly relevant to many animals, including humans, so the issue of light pollution is largely balancing the need to illuminate the world for our own purposes and the need to mitigate its negative effects.
Effects of light pollution
Light pollution has effects reaching across space, across time, and across the light spectrum. That is, light in places it shouldn’t be, light at the wrong time of day, and light that’s the wrong colour. These different aspects can cause different problems and require different solutions. The effects of light pollution act directly on humans, for example, by disrupting sleep, but since it affects entire ecosystems, humans will suffer those effects indirectly as well via the plants and animals we coexist with. This encourages viewing humans as part of the ecosystem, which is important when contending with any ecological issue. In turn, the solutions to light pollution that help us will also help other species.
We are all familar with the image of newly hatched sea turtles flapping their way towards the lights of a city skyline like wind-up toys. Similar disruptions of navigation can occur in many animal species. Specific examples include insects attracted to light reflected off a wet pavement, thinking it is a body of water, and fireflies attracted to nighttime lights, believing they are potential mates. However, light pollution can affect many more animals and in less dramatic, more persistent ways when navigating the world. Some animals may change their window of activity, the time when they hunt and forage, in response to lighter conditions at night. These changes may be slight, with animals treating skyglow like a moonlit sky and choosing to hunt for a bit longer. However, over time, these slight changes in behaviour cause changes in the amount of energy each animal uses.
Not only can the intensity and timing of light change animal behaviour but its spectrum may also have an effect. For example, LEDs have a broad spectrum of light and therefore may allow animals to see more colour, which, in turn, could allow them to hunt or find mates more effectively at night. It’s not really possible to classify these changes as good or bad, as the knock-on effects could vary significantly. Of course, the same effect could also be perceived as good or bad depending on which way you look at it. Increased light at night might be good for an owl that can catch more prey but the rodents it feeds on would probably have the opposite opinion. In the human world, a relatively new, and commonly complained about, type of light pollution comes from extremely bright LEDs, particularly in car headlights. These lights may reduce road safety due to glare preventing others drivers from seeing the road and obstacles. This glare may also be experienced differently by drivers and pedestrians and may therefore need to have multiple solutions.
Finally, the heterogeneity of lighting across space, especially in urban and suburban areas intermittently lit by street lights, may be a problem for animals that only move short distances in a night, such as insects. They may struggle to avoid a bright area lit by a single street light as it forms a significant obstacle to their usual movement or they may be irresistably attracted to it and become trapped by its glare.
The effects of light pollution can also be classified by their biological impacts. An obvious one is the disruption of sleep, in both humans and other animals. In an experiment, great tits (Parus major) woke up earlier and slept less when exposed to light at night than in normal dark conditions. Similar effects have been seen in humans, for example, in rural areas where shale gas drilling increased the level of light pollution in the US, residents were more likely to report insufficient sleep compared to areas with less drilling. However, other evidence from the US suggests that while people in areas with more light pollution get less sleep, the effect is only small. These effects caused by excess light outdoors may be exacerbated by excess light indoors from mobile phones and a lack of natural light during the day, for example, when working in a windowless room, which has been shown to reduce sleep quality.
Similarly, light pollution can cause disruptions of hormones and subsequent changes in circadian rhythm, which encompasses the physiological responses to the cycling of day to night and season to season. Animals track these cycles for feeding, reproducing, migrating, and anticipating changes in temperature or humidity from day to day or season to season depending on the species. The lunar cycle may also be relevant for some species. As might be expected, light pollution mimics the natural light that organisms’ circadian rhythms respond to and can delay or advance them, for example, by disrupting the production of melatonin, which is involved in the sleep/wake cycle in many animals, including humans.
Light pollution could cause physical ailments such as retinal degeneration. Excess light exposure can cause oxidative stress in the eye, during which chemicals in the retina become more active and cause damage. This damage occurs after exposure to bright light but even low light can be harmful. In the brain, excess light has been shown to cause a loss of dopamine neurons, which may be related to the onset of Parkinson’s disease.
Although most research on light pollution is based on land, it can also affect the ocean. For example, it was estimated that in 2010 around 22% of the world’s coastlines were exposed to artificial light. As on land, light is involved in regulating biological processes in the ocean, such as feeding behaviour, navigation, and reproduction. It is already known that animals such as fish and squid are attracted to light at night, as light is used as a lure for catching them. Perhaps unique to the marine environment, some organisms use light to adjust their depth in the absence of other cues in open water. Mass spawning, when many members of a species release eggs and sperm into the water at the same time, is also common in the ocean and may be synchronised with the lunar cycle as part of a combination of cues. Unfortunately, since there has been relatively little research on light pollution in the ocean, its exact effects are poorly known. However, solutions for removing light pollution from coastal and marine areas are much the same as those on land: dimming or switching off of lights, using lights that produce a spectrum that causes less disruption, etc.
Light pollution may even affect plants. Deciduous trees near street lights may produce leaves earlier in spring and not fully shed their leaves in autumn. Additionally, since plants and animals are constantly interacting, it could be expected that light pollution will affect these interactions, for example, the timing of the emergence of caterpillars and leaves, which may become desynchronised in the presence of light pollution.
Light pollution can even prevent astronomers from studying the night sky but, more universally, light pollution prevents everyone from seeing the stars and planets and so the opportunity to witness the beauty of the solar system, sometimes known as the “astonomical sublime”, is taken from us. Other cultural effects associated with light pollution include the loss of Indigenous traditions and knowledge of the night sky. Indeed, in this context, light pollution has been considered a type of “cultural genocide”, for example, for Aboriginal and Torres Strait Islander communities, who use the stars for navigation, telling origin stories, forecasting weather and seasonal changes, and as a way to memorise information to be passed to future generations. Celestial bodies even inform moral and social values. Thus, light pollution appears to go hand in hand with other forms of theft and loss from Indigenous cultures, such as those of land and language. The solutions to light pollution in these areas must include Indigenous people with a transdisciplinary approach that doesn’t merely reflect the dominant Western perspective.
Using the level of brightness, it is possible to estimate the contribution of different sectors to light pollution. For example, in a study in China, farmland contributed the least light while commercial areas, streets, and airports contributed the most. This information is important because it can help to pinpoint which sectors can reduce their light levels and which are more likely to be affected by light pollution rather than produce it.
Solutions to light pollution and their reception
The solutions to light pollution are obvious: switch lights off, dim them, or replace them with less harmful bulbs as much as possible. For example, street lights that are shaded on the top and sides, only illuminating the ground directly below, prevent light from shining up into the sky and into the windows of surrounding buildings. Perhaps bright billboards and signs should be dimmed or switched off altogether too, especially since they are designed to emit light horizontally. A positive aspect of cleaning up light pollution is that it doesn’t have to be all or nothing. Since it can be removed with the flick of a switch, bright light could be permitted at certain times or dimmed, which would reduce the level of pollution while still retaining the light’s usefulness. These dimming periods could be aligned with the activities of wildlife, such as breeding or migration.
Reducing lighting has the added benefit of lowering energy use and therefore carbon emissions. Indeed, some reductions in street lighting have been prompted by the need to reduce carbon emissions. The switch to LEDs will help to reduce carbon emissions in both developed countries and developing countries that often rely on kerosene lamps that produce pollution that damages the environment and human health.
It is understandably believed by some that bright lighting increases road safety and reduces crime. In one study in England and Wales, public opinion on reduced street lighting was highly polarised due to the perception that reduced lighting is conducive to crime and road accidents. Many people agreed that there are benefits to reduced street lighting, such as being able to see stars, and many people had no opinion as switch offs tend to occur late at night when few people venture out. Regarding dimming of street lights, few people even noticed. However, there may be the perception that street lighting is a public good paid for by taxes and should therefore be provided to taxpayers. Some of these pro-street lighting attitudes hark back to views from the advent of public lighting in Victorian England, when a lack of light was seen as encouraging “sickness and depravity”, although it’s possible these attitudes will change with increased knowledge on the effects of light pollution.
However, there is evidence that street lighting may not actually have a consistent effect on crime. One study found that reduced lighting was associated with increased vehicle crime but reduced violent crime, while there was no difference in criminal damage. Additionally, the changes in crime appeared to be stronger over a short period after the reduction in lighting, with crime levels in areas with and without lighting becoming similar again after a few years. On the other hand, a study in which additional street lighting was deployed in New York City reported that crime was reduced. Overall, the evidence is mixed and more research is needed but it doesn’t appear to be as simple as reduced lighting always resulting in increased crime. This may be because the data are often difficult to analyse due to inconsistencies in the compared areas and between different types of crime or simply because people go out less when there’s less lighting. Nevertheless, people’s feelings of a lack of safety are important if they feel unable to do their usual activities, even if they are not in increased danger. There is clearly a tradeoff between making people feel safe, actually keeping people safe, and reducing light pollution.
Regarding traffic safety, bright lighting at pedestrian crossings seems to be important to ensure the safety of pedestrians, as LED lighting at crossings has been shown to reduce drivers’ speed even if no pedestrians are present. Therefore, it may be important to leave some brightly lit areas around high-traffic spots and focus on reducing lighting elsewhere.
Threats to public life and safety are also a concern in developing countries where there is a historical lack of electrical lighting. So, there is a focus on the benefits of the introduction of lighting rather than the detriments of reducing it. However, the efforts to increase lighting in developing countries could be partly an expression of the desire to “modernise” regardless of the need for lighting and its negative effects. Indeed, the amount of street lighting is considered to be a socioeconomic indicator in countries such as Ghana. The inequality between well-lit developed regions and developing regions should be removed while avoiding the negative effects of light pollution. For example, if these areas install new electric lighting, care can be taken to use the most efficient lights in a way that preserves darkness as much as possible from the start as long as they are accessible and affordable.
DarkSky (formerly the International Dark-Sky Association) advocates for reducing light pollution and certifies Dark Sky Places, areas in which light pollution is low and the night sky can be appreciated. It has produced a list of recommendations for individuals to help stop light pollution, however, individuals are responsible for very little light pollution (as demonstrated by the research showing that the brightest environments are commercial areas and streets rather than private households), so individual action cannot solve the problem. The recommendations for getting involved in public policy campaigns may have a more meaningful effect on light pollution and Dark Sky Places particularly so, as places around the world vie for recognition. Wildlife charities have also tried to spread the word about light pollution, such as Buglife in the UK, which focuses on invertebrate conservation. Buglife gives recommendations for individual action, although they are mostly based on nurturing a fascination with nocturnal insects rather than trying to stop light pollution itself. Therefore, these organisations have a role in raising awareness of light pollution, reintroducing people to the benefits of a dark night sky, and actually reducing light pollution.
There is a balance to be struck between reducing light pollution and people’s lighting needs, and more research is needed on how those needs can best be fulfilled. The reduction of light pollution largely falls to corporations, as they own the most polluting facilities, and local authorities that control street lighting. Individuals should get involved in campaigns to reduce street lighting where they can but shouldn’t forget to appreciate the night sky whenever possible.
