Light pollution is excessive or inappropriate artificial—usually outdoor—lighting that has negative effects on the quality and safety of the nighttime environment. Light pollution decreases visible starlight, causes behavioral changes in animals, adversely impacts ecosystems, negatively affects immune-system function and human health, wastes energy, decreases safety, and interferes with astronomical research.
Life on Earth evolved over several billion years according to a light-dark rhythm controlled solely by the sun, moon, and stars. Artificial lighting and the nighttime glow of urban areas has disrupted this cycle, upsetting ecological balance. Loss of the sight of thousands of stars on every clear night may be adversely affecting human health and culture in ways that are not well understood.
There are four categories of light pollution, which often occur to together and can overlap:
A mere 100 years ago, virtually everyone on Earth could appreciate the beauty of stars in the night sky, a sight that has influenced human evolution and history in complex ways. Before significant light pollution in the nineteenth and twentieth century, everyone was familiar with the Milky Way, which—along with the moon, stars, and planets—occupies a prominent place in mythologies and cultures worldwide. Although astronomers were the first to recognize the impact of light pollution, it is now affecting most humans in ways that are only beginning to be understood.
Between 1947 and 2000, North American light emissions increased by an estimated 6% or more annually. Between 2002 and 2008, emissions from Las Vegas, Nevada, increased at a rate of 8% annually. These increases exceeded population growth, suggesting that more light was being emitted per person, and more light was being directed upward.
Major sources of light pollution include streetlights, advertising, and interior and external lights on buildings, offices and other commercial properties, factories, and sports fields, arenas, and stadiums. At least 25% of worldwide electricity consumption goes to lighting. Over-illumination—especially upward-directed nighttime light—wastes about two million barrels of oil daily, costing untold billions of dollars and increasing carbon emissions that contribute to climate change. Light pollution also may be worsening air pollution by destroying nitrate radicals that help disperse smog.
Over much of the United States, poorly designed streetlights beam up into the sky, rather than at objects and areas that require illumination on the ground. It has been estimated that at least 30% of street lighting is wasted in this way—some 22,000 gigawatt-hours annually at a cost of $1.7–2.2 billion, 3.6 tons of coal, or 12 million barrels of oil. Wasted lighting releases 38 million tons of atmospheric carbon dioxide annually in the United States, with unshielded outdoor lighting accounting for 1.2 million tons.
Sodium-vapor streetlights with incandescent light bulbs are gradually being converted to light-emitting diode (LED) lights that use solid-state technology. LEDs do not heat up much and lack filaments that burn out, thus improving quality and lowering cost. However, LEDs give off high amounts of blue light, which reaches significantly further, thereby contributing to light pollution. Blue-rich white lights increase glare that interferes with vision. Blue light poses potential human health hazards and interferes with wildlife behavior and reproduction. Like sunlight, LEDs emit all wavelengths of light and so may be even more disruptive to nocturnal animals than older outdoor lights that emit fewer wavelengths.
Bright days and dark nights regulate circadian rhythms that are essential for normal hormone production and cellular functioning in plants and animals, including humans. Light-dark cycles also regulate brain activity and behaviors, including feeding, mating, migration, competition, and predator-prey interactions in animals. Artificial lighting may be at odds with hundreds of thousands of years of human evolutionary history, but humans can use blackout curtains and turn off electronic screens. Animals have no such options. Light pollution is especially devastating to nocturnal animals. For example, it confuses newly hatched sea turtles that rely on starlight reflecting off ocean waves to navigate their way from the beach to the sea, causing them to travel in the wrong direction.
Light pollution also affects plants and insects, potentially affecting crop production. Plants and insects respond differently to different wavelengths of light, and their complex interactions are not well understood. However, artificial light can completely disrupt plant developmental and flowering patterns by interfering with circadian rhythms, and it significantly reduces interactions between plants and nocturnal insect pollinators by confusing and misdirecting the insects.
The majority of people worldwide no longer experience truly dark nights. More than 80% of the human population—and more than 99% in the United States and the European Union—live with some amount of skyglow, even in isolated regions over 60 mi (100 km) or more from large cities. People near Paris must travel 660 mi (900 km) to central Scotland, central Spain, or Corsica to escape light pollution. The Central African Republic, Madagascar, and most of Greenland are least affected. However, a 2017 analysis of five years of advanced satellite imaging reported that light pollution was increasing most rapidly in developing countries with new and growing cities and increased gross domestic product (GDP). Studies have found that countries use about as much artificial light as can be paid for with 0.07% of their GDP. Thus, South American, African, and Asian countries have increased artificial lighting as their GDP has grown. Exceptions to rapid increases in light pollution have occurred in the few locales with lighting regulations, as well as in regions undergoing economic downturns and government collapse, as in Eastern Europe in the 1990s.
Light pollution prevents one-third of humans from seeing the Milky Way, including 60% of Europeans, almost 80% of North Americans, and the entire populations of Malta, Kuwait, and Singapore. The galaxy is visually blocked across 14% of the United Kingdom, making it invisible to 77% of the population. It is invisible around Hong Kong, Beijing, and much of the East Coast of North America; in Qatar, the Netherlands, and Israel; and over 51% of Belgium. Millions of children worldwide will never experience this majestic sight.
Chronobiology—the effects of sleep/wake cycles on health—indicates that artificial light—especially blue light—can cause wakefulness during human sleep times and that excessive nighttime light is associated with a range of disorders from obesity to cancer. Most outdoor LED lighting, as well as compact fluorescent light bulbs (CFLs) and electronic displays, including televisions and computer screens, produce more blue light than do older lights. Light-dark cycles regulate production of the sleep-inducing hormone melatonin, which can be suppressed by light exposure during sleep. Melatonin is important for proper functioning of the thyroid, pancreas, adrenal glands, ovaries, and testes. It has antioxidant properties, boosts the immune system, and lowers cholesterol. Melatonin suppression can cause or contribute to various problems including the following:
Other effects associated with light pollution and disrupted circadian rhythms are:
Much of the response to light pollution has come from scientists and citizens.
Because of its effects on human health and the environment, the U.S. Environmental Protection Agency has the authority to regulate light pollution, but, as of 2018, regulation was considered highly unlikely. A few areas—such as Tucson and Flagstaff, Arizona, and parts of Northern Italy—have established and enforced lighting regulations. However, in the absence of action by national governments, it falls to nongovernmental organizations to address the problem. Potential actions include dark-sky certifications, insurance incentives, and encouragement of corporate initiatives.
According to the 2017 satellite-imaging study, light pollution was increasing in both brightness and area by about 2% annually, along with energy consumption for lighting and urban sprawl that extended light-pollution borders. Although light pollution from developed nations, including the United States, appeared to be more stable over time than that from developing countries, the satellite images do not detect blue light from LEDs, and photos from the International Space Station that reveal all visible wavelengths showed cities shifting from yellow to blue light. If all sodium lights were replaced with LEDs, light pollution across Europe could double due to increased blue light. Furthermore, although LEDs are more energy-efficient, cost savings tend to be reinvested in more outdoor lighting.
Individuals can minimize harm from blue light by choosing bulbs of 3000K or lower and downloading color temperature apps that set electronic screens for cool light in the daytime and warm light at night—f.lux for Mac OS/X, Windows, Linux, and jailbroken iPhones and iPads; Lux for Android devices; and Twilight for smartphones and tablets. The Night Shift app is preinstalled on Apple devices running iOS 9.3 and above. Packaging for new CFL and LED bulbs provides color temperature information. Higher-color temperatures mean more blue light. Low-color temperatures are warm white sources that are less harsh and less harmful to people and the environment.
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Margaret Alic, PhD