Light pollution, often called “skyglow,” is artificial light that illuminates the night sky, altering the natural cycle of light and dark. It is primarily caused by inefficient or excessive outdoor lighting, ranging from streetlights and commercial signs to residential security fixtures. Satellite data indicates that the light-polluted area of the Earth is expanding each year. This change disrupts both natural ecosystems and human biological processes.
Identifying the World’s Most Light-Polluted Regions
Identifying the “most light-polluted city” is complex because rankings vary based on the specific metric used, such as brightness per square kilometer or total light output. Global analyses consistently point to several heavily urbanized regions as the most saturated with artificial light. Studies show that more than 80% of the world’s population lives under light-polluted skies, a figure that rises to over 99% for residents in the United States and Europe.
The most intense concentrations of light pollution are found in the major metropolitan corridors of the Eastern United States and Western Europe, where population and infrastructure density is highest. In Asia, regions like South Korea, Singapore, and parts of China also exhibit extreme levels of skyglow. Singapore, for instance, has been cited as the most light-polluted nation, with its entire population living under skies too bright for the human eye to fully adapt to night vision.
When comparing metropolitan areas based on their brightness, cities like Moscow and Riyadh have been measured as having some of the highest light intensity worldwide. North American cities such as Chicago and Toronto also rank highly due to vast, densely illuminated infrastructure. These cities rank high due to a combination of high population density, which requires extensive public and commercial lighting, and a high concentration of infrastructure like highways and commercial centers.
Quantifying Light Pollution
Scientists use various methods to measure and classify light pollution, ranging from satellite observation to visual surveys. Satellite instruments, such as the Visible Infrared Imaging Radiometer Suite (VIIRS) Day-Night Band, capture the radiance emitted from the Earth’s surface. This space-based data is essential for producing global maps, such as the World Atlas of Artificial Sky Brightness, by measuring the light directed upward toward the atmosphere.
A limitation of VIIRS satellite data is its reduced sensitivity to the blue wavelengths of light, which are prominent in modern white LED fixtures. This spectral gap means that the true extent of skyglow, especially in areas using blue-rich LED lighting, is often underestimated by satellite measurements. Ground-based measurements provide a necessary check on the satellite data.
The Sky Quality Meter (SQM) is a common ground-based tool that directly measures the brightness of the night sky at the zenith. This objective reading is correlated with the Bortle Dark-Sky Scale, a nine-level classification system used by astronomers to rate the darkness of a location. The Bortle scale ranges from Class 1 (pristine dark sky) to Class 9 (inner-city sky where only the brightest stars are visible). Light pollution manifests in two forms: “sky glow,” the diffuse brightening over populated areas, and “light trespass,” which is light falling where it is not intended.
Impacts on Human Health and Wildlife
The presence of excessive artificial light at night (ALAN) has measurable consequences for human biology, primarily through the disruption of the circadian rhythm. This internal biological clock regulates the sleep-wake cycle and hormone production. Exposure to light during nighttime hours, especially blue-rich light common in modern fixtures, suppresses the production of the hormone melatonin.
Melatonin regulates sleep and triggers biological processes that occur during darkness. Chronic suppression of this hormone can lead to poor sleep quality, fatigue, and increased risk of long-term health issues. Studies associate light pollution exposure with higher risks for metabolic and cardiovascular conditions, including obesity and diabetes. For shift workers, who experience chronic circadian disruption, the World Health Organization has classified this work schedule as a probable carcinogen.
Ecological Effects
The ecological effects of ALAN are extensive, impacting the behavior and survival of many nocturnal species.
- Migratory birds rely on the moon and stars for navigation, but become disoriented by city lights, often leading to fatal collisions with buildings.
- Sea turtle hatchlings instinctively crawl toward the brightest natural horizon, but artificial light confuses them, causing them to move inland instead of toward the ocean.
- Insect populations are disproportionately affected, as artificial lights act as traps that disrupt foraging and reproduction, contributing to population decline.
- Plant life cycles are also disrupted, as light exposure can affect the timing of seasonal events like leaf budding and flowering.
Global Efforts to Preserve Dark Skies
An international movement focuses on mitigating light pollution through policy, technology, and conservation. Policy solutions involve implementing municipal lighting ordinances that mandate responsible outdoor lighting practices. These regulations often require the use of full cut-off fixtures, which shield the light source and direct all illumination downward, preventing light from spilling upward into the sky.
Technological improvements center on using light more intelligently and efficiently. This includes adopting warmer-colored LED lights, which have a lower Kelvin temperature and emit less of the blue-rich light spectrum. Adaptive controls, such as dimming schedules and motion sensors, ensure that lights are only at full brightness when necessary.
Conservation efforts are spearheaded by organizations like the International Dark-Sky Association (IDA), which certifies Dark Sky Places around the world. These certified locations, including parks, reserves, and communities, commit to a management plan to minimize light pollution and promote public education. Communities aim to restore the natural nighttime environment for both human health and ecological integrity.