Light Pollution, Explained

Updated: Jun 5

Most people are familiar with pollution in the form of air- and water-borne contaminants, both chemical and biological, that adversely affect the natural environment; not as many are familiar with pollution that takes the form of energy, such as noise, heat, or light. "Light pollution" refers to the presence of inappropriate and/or excessive artificial lighting, during the day and especially at night, primarily outdoors. It's often considered a side-effect of industrialization, and is most severe in highly industrialized, densely populated areas — Chicago, for example.

LED streetlight on Catalpa Ave. Credit: Burton DeWilde.

Light pollution is mostly caused by interior and exterior building lighting, advertising, streetlights and other outdoor area lighting, and illuminated, open-air spaces like sporting venues. It’s not that artificial lighting is intrinsically bad, but that much of this lighting is too bright, too diffuse, too blue, and downright unnecessary. These issues take various visible forms:

  • Glare: bright light that temporarily blinds or otherwise causes visual discomfort, such as when an oncoming car fails to switch off its high-beams

  • Trespass: light falling where it’s not wanted or needed, such as when street lighting enters one’s home through a window

  • Skyglow: brightening of the night sky from below by ground-based lights, commonly observed as a glowing “dome” over towns and cities

  • Clutter: confusing and excessive groupings of light sources, such as when bright advertising and storefronts surround roadways (think: the Magnificent Mile)

Components of light pollution. Credit: Anezka Gocova in “The Night Issue”, Alternatives Journal 39:5 (2013).

Unfortunately, light pollution has serious, negative impacts on public health and safety, wildlife, and the environment. Artificial lighting disrupts the circadian rhythms of humans and animals that evolved in response to regular day/night cycles. Notably, it suppresses production of the hormone melatonin, which leads to sleep deprivation, headaches, anxiety, and has even been linked to increased incidence of cancer. Blue light, as commonly emitted by LEDs and phone/computer screens, is particularly detrimental here; this issue prompted the widespread introduction of “night shift” functionality that automatically dims and warms the colors of our screens at night. (So why not follow suit with our outdoor lighting?)

Animals suffer similar health effects, in addition to other behavioral changes around navigation, predation, and habitat formation. For example, Chicago lies on a prominent migration path for birds, which rely on natural light to navigate, and the bright city lights confuse and disorient thousands of them to their deaths each year; in fact, Chicago is considered the most dangerous U.S. city for migratory birds! Here’s another example: Since many nocturnal insects are drawn to light, light pollution impairs their ability to feed, pollinate, reproduce, and rest, and this is a significant driver of the decline in insect populations over the last few decades.

Light pollution is also a safety hazard. Glare makes it harder for us to see potential dangers, while clutter disorients and confuses — a dangerous combination for drivers and pedestrians. Aging eyes are particularly affected. Furthermore, studies have found that outdoor lighting doesn’t actually deter crime, and can in fact increase crime! For instance, the April 2000 Chicago Alley Lighting Project observed a positive correlation between bright alleyway lighting and crime rates. Though outdoor lighting can make us feel safer, it doesn’t necessarily make it true.

Light pollution represents a massive waste of energy, not to mention the money needed to pay for it. If fossil fuels are burned to generate that energy, excess light can be said to needlessly contribute to global heating and the environmental harms that arise from it. Last but not least, light pollution separates people — astronomers and otherwise! — from the night sky, which has inspired so much awe and creativity throughout human history. It’s an intangible but meaningful loss for us all.

Old (left) and new (right) streetlights on Broadway. Credit: Burton DeWilde.

Fortunately, there are straightforward ways to reduce light pollution without adversely affecting our safety and enjoyment in nocturnal, outdoor spaces. According to the International Dark Sky Association, we should light only where it’s needed, when it’s needed, in the amount needed — and no more. In practice, this often means choosing lower-wattage, warmer bulbs, fitted into downward-facing enclosures with proper shielding, optionally connected to light/motion sensors or timers. To go one step further, re-design lighting plans so that fewer lights are used to meet the particular use case.

Unlike most other forms of pollution, light pollution is immediately reversible: Just turn down the lights! And once your eyes adjust… look up.

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